Structure and Function of the Neurologic System

A neuron (Fig. 15.2) has three components: a cell body (soma), the dendrites (thin branching fibers of the cell), and the axons. Cell bodies for most neurons, even those extending axons into peripheral nerves, are located within the CNS. Dense collections of cell bodies in the CNS are called nuclei. Those in the PNS usually are found in groups called ganglia (or plexuses—a group of relay nerves). The dendrites are extensions that carry nerve impulses toward the cell body. The dendritic zone is the receptive portion of a neuron that receives a stimulus. Axons are long, conductive projections that carry nerve impulses away from the cell body. The axon hillock is the cone-shaped process where the axon leaves the cell body. The first part of the axon hillock has the lowest threshold for stimulation, so action potentials begin there.

A series of illustrations show neuron structures. Illustration A. There are four types of neurons. • Unipolar neuron. The illustration shows a nucleus inside a cell body with an axon and two synaptic knobs (nerve terminals). • Pseudounipolar neuron. The illustration shows a nucleus inside a cell body with a branch that splits into a dendrite on the right and an axon with synaptic knobs on the left. • Bipolar neuron. The illustration shows a nucleus inside a cell body with a branch on either side; dendrite on the right and axon with synaptic knobs on the left. • Multipolar neuron. The illustration shows a nucleus inside a cell body. The nucleus is surrounded by Nissl (in rough endoplasmic reticulum). The cell body has multiple branches of dendrites and one axon with synaptic knobs. Illustration B shows the structure of a typical multipolar neuron. The cell body has multiple dendrites and a nucleus at the center. Other structures inside the cell body (soma) are Golgi apparatus, endoplasmic reticulum and ribosomes (Nissl substances), and mitochondrion. An axon hillock extends into the axon with Schwann cell inside a myelin sheath. The axon branches into an axon collateral and a node of Ranvier that terminates with synaptic knobs on telodendrion.

A typical neuron has only one axon, which may be wrapped with a myelin sheath—a membrane made of a lipid material called myelin. In the CNS, myelin is produced by oligodendrocytes. Regions of the brain and spinal cord with a high level of myelination constitute the white matter, whereas regions lacking significant myelination (typically primarily composed of cell bodies) are gray matter. In the PNS, myelin is produced by Schwann cells. The myelin sheaths are interrupted at regular intervals by the nodes of Ranvier. Nutrient exchange is not possible through the myelin sheath, although it can occur at the nodes of Ranvier. Axons can branch at the nodes of Ranvier, forming axon collaterals. Axons end with telodendria, branches that terminate with synaptic knobs, which are used in neurotransmission.

Where there is myelin, the velocity of nerve impulses increases. Myelin acts as an insulator that allows an action potential to leap between segments rather than flowing along the entire length of the membrane, yielding the increased velocity. This mechanism is referred to as saltatory conduction. Disorders of the myelin sheath (demyelinating diseases), such as multiple sclerosis, Guillain-Barré syndrome, and Charcot-Marie-Tooth disease, demonstrate the important role myelin plays in nerve conduction (see Chapter 17). Conduction velocities depend not only on the myelin coating but also on the diameter of the axon. Larger axons transmit impulses at a faster rate.

Neurons are structurally classified based on the number of processes (projections) extending from the cell body (see Fig. 15.2A). There are four basic types of cell configuration: (1) unipolar, (2) pseudounipolar, (3) bipolar, and (4) multipolar. Unipolar neurons have one process, an axon, which branches shortly after leaving the cell body. One example is found in the retina. Pseudounipolar neurons (unipolar) have one axon process and one dendrite process, but the dendrite and axon are fused near the cell body, and the axon then extends into the CNS. This configuration is typical of sensory neurons in both cranial and spinal nerves. Bipolar neurons have two distinct processes (one axon and one dendrite) arising directly from the cell body. This type of neuron connects the rod and cone cells of the retina. Multipolar neurons are the most common and have multiple processes capable of extensive branching. A motor neuron is typically multipolar.

Functionally, there are three types of neurons (their typical configuration is noted in parentheses): (1) sensory (mostly pseudounipolar), (2) associational (multipolar), and (3) motor (multipolar). Sensory neurons are afferent neurons, carrying impulses from peripheral sensory receptors to the CNS. Associational neurons (interneurons) transmit impulses from neuron to neuron, sensory to motor neurons. They are located solely within the CNS. Motor neurons are efferent neurons, transmitting impulses from the CNS to an effector (i.e., skeletal muscle or organs). In skeletal muscle, the end processes form a neuromuscular (myoneural) junction (see Fig. 15.14 and the Spinal Cord section later in the chapter).

An illustration shows a cross-section of a normal neuromuscular junction. The following structures are labeled on the illustration: motor neuron fiber, myelin sheath, Schwann cell, synaptic vesicles (containing A C h), A C h, synaptic cleft, sarcolemma, A c h receptors, motor end plate, sarcoplasm, and muscle. Synaptic vesicles pass through the motor neuron fiber. The synaptic vesicles release A C h into the synaptic cleft which bind to the A C h receptors on the motor end plate.

Neuroglia (“nerve glue”) is the general classification of nonneuronal cells that support the nervous system's neurons. They comprise approximately 50% of the total brain and spinal cord volume. Neuroglia are present in both the CNS and PNS. Astrocytes, oligodendroglia (oligodendrocytes), ependymal cells, and microglia are found in the CNS; Schwann cells, nonmyelinating Schwann cells, and satellite glial cells are found in the PNS (Fig. 15.3) (Emerging Science Box: Schwann Cells). The different types of neuroglia serve different functions (Table 15.1).

A series of illustrations shows central nervous system neuroglia and peripheral nervous system neuroglia. The central nervous system neuroglia shows the following structures. • Illustration A. The structure shows the foot processes with multiple astrocytes around a single capillary. • Illustration B. The structure shows multiple nerve fibers, each covered in a myelin sheath and connected by oligodendrocyte. • Illustration C. The structure shows a layer of ependymal cells covered in cilia. • Illustration D. The structure shows microglia. The cells are distorted and shapeless. The peripheral nervous system neuroglia shows the following structures. • Illustration E. The structure shows a Schwann cell with a nucleus at the center. • Illustration F. The structure shows the node of Ranvier and labels the following structures beneath the nucleus of the Schwann cell, from the inside: myelinated nerve fiber, myelin sheath, neurilemma. • Illustration G. The structure shows a neuron cell body with satellite cells. The structure also labels the axon inside the neurolemmocyte.

Mature nerve cells do not divide; therefore, injury can cause permanent loss of function. Wallerian degeneration occurs in the distal axon when an axon is severed, the portion cut off from the cell body, within hours of the injury. The morphologic and biochemical changes that occur in the distal axon include: (1) a characteristic swelling within the portion of the axon distal to the cut; (2) neurofilaments hypertrophy; (3) shrinkage and fragmentation of the myelin sheath; and (4) axon degeneration, and disappearance. The myelin sheaths reform into Schwann cells that align in a column between the severed part of the axon and the effector organ (Fig. 15.4B).

A series of illustrations show the peripheral nerve regeneration following injury. Illustration A. There are four stages of successful regeneration. Stage 1. The structure shows an injury along the axon of a normal neuron connected to a normal muscle. The cell body of the neuron comprises multiple Nissl substances. Stage 2, 2 weeks after injury. The structure shows fewer Nissl substances (chromatolysis) in the cell body. There are multiple macrophages along the degenerating fiber and myelin sheath. Stage 3, 3 weeks after injury. The structure shows an increase in Nissl substances in the cell body. Schwann cells proliferate around the axon penetrating the Schwann cells, connected to the atrophied muscle. Stage 4, 3 months after injury. The structure shows normal amount of Nissl substances in the cell body. The nerve has successfully regenerated and is connected to a regenerated muscle. Illustration B. An illustration shows the structure of a neuron in an unsuccessful nerve regeneration. The structure shows a cell body with Nissl substances. The site of disorganized axon growth along the axon is identified. A column of Schwann cells is connected to the atrophied muscle, but is disconnected from the axon.

At the proximal end of the injured axon, similar changes occur but only back to the next node of Ranvier. The cell body responds to trauma by swelling and dying by chromatolysis (dispersing the Nissl substance) or apoptosis. During the repair process, the cell increases protein synthesis and mitochondrial activity. Days to weeks after injury, new terminal sprouts project from the proximal segment and may enter the remaining Schwann cell pathway. (Fig. 15.4A contains a more detailed representation of these events.)

This process is very slow (about 1 mm/day) and is limited to myelinated fibers in the PNS. The regeneration of axonal constituents in the CNS is limited by an increased incidence of glial scar formation (gliosis) and the different nature of myelin formed by the oligodendrocyte.

Nerve regeneration depends on many factors, such as the location of the injury, the type of injury, the presence of inflammatory responses, and the process of scarring. The closer the injury is to the nerve's cell body, the greater chance that the nerve cell will die and not regenerate. A crushing injury allows for fuller recovery compared with a cut injury. Crushed nerves sometimes recover fully, whereas cut nerves often form connective tissue scars that block or slow regenerating axonal branches. Peripheral nerves injured close to the spinal cord recover poorly and slowly because of the long distance between the cell body and the peripheral termination of the axon.¹

Neurons are not physically contiguous with one another. The region between adjacent neurons is called a synapse (Fig. 15.5). The neurons that conduct a nerve impulse are named according to whether they relay impulses toward (presynaptic neurons) or away from (postsynaptic neurons) the synapse. The synapse is composed of a bulbous end of the presynaptic neuron (synaptic knob) that is separated from the postsynaptic neuron by a gap called the synaptic cleft. Four basic types of connections occur in regions of contact between the presynaptic and postsynaptic neurons. These are between axons (axo-axonic), from axon to cell body (axo-somatic), from axon to dendrite (axo-dendritic), and from dendrite to dendrite (dendro-dendritic).

An illustration shows neuronal transmission and synaptic cleft. The structure of a motor neuron cell body with axon of presynaptic neuron, synaptic knobs, and axon of motor neuron. An accompanying illustration shows and labels the structures in the postsynaptic neuron. The structure identifies the action potential feeding into a synaptic knob. There are sodium ion channels, potassium ion channels, and calcium ion channels on the knob. A matrix of vesicles storing neurotransmitter molecules are identified at the base of the synaptic knob. Neurotransmitters are released into the synaptic cleft that are attracted to the stimulus-gated sodium ion channels.

Impulses are transmitted across the synapse by chemical and electrical conduction (only chemical conduction is discussed here. Chapter 1 contains information on electrical conduction [see Fig.1.31].) When an impulse originates in a presynaptic neuron, the impulse reaches the vesicles, where chemicals (neurotransmitters) are stored in the synaptic bouton. Once released from the vesicles, the neurotransmitters diffuse across the synaptic cleft and bind to specific neurotransmitter (protein) receptor sites on the plasma membrane of the postsynaptic neuron, relaying the impulse (see Fig. 15.5). Brain synapses can change in strength and number throughout life, and this is known as synaptic plasticity or neuroplasticity.

Neurotransmitters are chemicals synthesized in the neuron and localized in the presynaptic terminal (synaptic bouton). Neurotransmitters are released into the synaptic cleft in response to the arrival of an electrical impulse and bind to a receptor site (binding site) on the postsynaptic membrane of another neuron or effector, where they affect ion channels (see Fig. 15.5). Each neurotransmitter is removed by a specific mechanism from its site of action. Neurons can synthesize more than one neurotransmitter, and postsynaptic membranes can contain more than one type of transmitter-specific receptor. Many substances are neurotransmitters, including norepinephrine, acetylcholine, dopamine, histamine, and serotonin. Many of these transmitters have more than one function. Neuromodulators are chemical messengers released from a neuron in the CNS or the PNS, and this affects a group of neurons that have receptors for that messenger. They may have excitatory or inhibitory effects. Neurotransmitter and neuromodulator substances are summarized in Table 15.2.

Because the neurotransmitter is normally stored on one side of the synaptic cleft, and the receptor sites are on the other side, chemical synapses operate in one direction. Therefore, action potentials are transmitted along a multineuronal pathway in one direction. The binding of the neurotransmitter at the receptor site changes the permeability of the postsynaptic neuron and, consequently, its membrane potential. Two possible scenarios can occur:

Chapter 1 reviews electrical impulses and membrane potentials.

Usually, a single EPSP cannot induce a neuron's action potential and the nerve impulse propagation. Whether this occurs, depends on the number and frequency of potentials the postsynaptic neuron receives—a concept known as summation. Temporal summation (time relationship) refers to the effects of successive, rapid impulses received from a single neuron at the same synapse. Spatial summation (spacing effect) is the combined effects of impulses from several neurons onto a single neuron simultaneously. Facilitation refers to the effect of EPSP on the plasma membrane potential. The plasma membrane is facilitated when summation brings the membrane closer to the threshold potential and decreases the stimulus required to induce an action potential. The effect that a chemical neurotransmitter has on the plasma membrane potential depends on the balance of these effects. The mechanisms of convergence (many neurons firing and converging on one neuron), divergence (one neuron firing and diverging on many neurons), summation, and facilitation allow for the integrative processes of the nervous system.

The cerebrum is the largest part of the brain and contains both gray matter and white matter. The three primary embryonic vesicles (structural divisions) of the brain are (1) the forebrain (prosencephalon), (2) the midbrain (mesencephalon), and (3) the hindbrain (rhombencephalon). These three vesicles then develop further into the secondary vesicles and their structures, which are summarized in Table 15.3 and Fig. 15.6. The midbrain, medulla, and pons comprise the brainstem, which connects the brain's hemispheres, cerebellum, and spinal cord. A collection of nerve cell bodies (nuclei) within the brainstem makes up the reticular formation. The reticular formation is a large network of diffuse nuclei that connect the brainstem to the cortex and control vital reflexes, such as cardiovascular function and respiration. It is essential for maintaining wakefulness and attention and is referred to as the reticular activating system (Fig. 15.7). Some nuclei within the reticular formation support specific motor movements, such as balance and posture.

An illustration shows the right lateral view of the brain and identifies the following structures, from the forebrain to the hindbrain: cerebrum (telencephalon), thalamus, hypothalamus (diencephalon), midbrain, cerebellum, hindbrain, pons, and medulla.

An illustration shows the left lateral view of the human brain and traces the paths through reticular activating system. Auditory and visual information and ascending sensory information pass through the reticular activating system and spread inside the brain.

Divisions of the brain are associated with different functions but attributing specific functions to definite brain regions is not entirely accurate. However, functional specificity is very useful for localizing pathologic conditions in various nervous system regions for clinical considerations. Brodmann areas are used to correlate functional activities to many regions of the cerebral cortex.2Fig. 15.8C illustrates these regions and describes some of the areas. The mapping of brain networks is also helpful in discovering how varying parts of the brain are interconnected when performing a specific function (see Emerging Science: Brain Networks).

Three illustrations of the left lateral and midsagittal views of the human brain label the cerebral hemispheres. Illustration A. The following structures are identified in the frontal lobe: inferior frontal gyrus, middle frontal gyrus, superior frontal gyrus, and precentral gyrus. The central sulcus (fissure of Ronaldo) separates the frontal lobe from the parietal lobe. The postcentral gyrus in the parietal lobe is identified. The lateral sulcus (Sylvian fissure) separates the frontal lobe from the temporal lobe. The following structures are identified in the temporal lobe: inferior temporal gyrus, middle temporal gyrus, and superior temporal gyrus. The occipital lobe is identified at the back of the brain. The cerebellum is labeled at the base of the brain, followed by the pons and the medulla oblongata. Illustration B. The following structures are labeled from above the midbrain to the hindbrain: thalamus, hypothalamus, subthalamus, corpus callosum, cingulate gyrus, parietooccipital sulcus, calcarine sulcus, and primary visual. The following structures are identified below the thalamus: epithalamus (pineal gland), midbrain, pons, cerebellum, and medulla. Illustration C. The following structures of the brain are labeled, clockwise from the frontal lobe: bronco area (motor speech area), prefrontal area, frontal eye field, premotor, precentral gyrus (primary voluntary motor), central sulcus, postcentral gyrus (primary somatic sensory), somatic sensory association area, visual association, visual cortex, Wernicke (sensory speech), primary taste area, primary auditory, auditory association, and Broca area (motor speech area).

Forebrain

Telencephalon

The telencephalon (cerebral hemispheres) consists of the cerebral cortex (the largest portion of the brain) and the basal ganglia (composed of several nuclei). The surface of the cerebral cortex is covered with convolutions called gyri (see Fig. 15.8A), which greatly increase the cortical surface area and the number of neurons. Grooves between adjacent gyrus are termed sulci; deeper grooves are fissures. The cerebral cortex contains an outer layer of cell bodies of neurons called gray matter, which is organized into columns perpendicular to the surface that receive, integrate, store, and transmit information. The cerebral cortex is located in the frontal, parietal, temporal, and occipital lobes. White matter lies beneath the cerebral cortex and is composed of myelinated nerve fibers (axons).

Lobes

The two cerebral hemispheres are separated by a deep groove known as the longitudinal fissure (see Fig. 15.10B). The surface of each hemisphere is divided into lobes named after the region of the skull under which each lobe lies: frontal, parietal, occipital, and temporal lobes.

Illustration A shows the left cerebral hemisphere and labels the following structures: hippocampus, fornix, basal ganglia (lentiform nucleus, caudate nucleus), limbic system (thalamus, hypothalamus, amygdala, and substantia nigra in midbrain). Illustration B shows a cross-sectional view of the coronal section of the brain shows and labels the following structures: longitudinal fissure, putamen, head of caudate nucleus, hypothalamus, substantia nigra, subthalamic nucleus, hippocampus, thalamus, internal capsule, and corpus striatum. Corpus striatum is made of body of caudate nucleus and lentiform nucleus. The lentiform nucleus is made of putamen and globus pallidus.

The posterior margin of the frontal lobe is on the central sulcus (fissure of Rolando), and it borders inferiorly on the lateral sulcus (Sylvian fissure, lateral fissure) (see Fig. 15.8A). The prefrontal area is responsible for goal-oriented behavior (e.g., ability to concentrate), short-term or recall memory, the elaboration of thought, and inhibition of the limbic areas of the CNS. The premotor area (Brodmann area 6) (see Fig. 15.8C) is involved in programming motor movements. This area contains the cell bodies that form part of the basal ganglia system. The frontal eye fields (the lower portion of Brodmann area 8), which control eye movements, are located on the middle frontal gyrus.

The primary motor area (Brodmann area 4) is located along the precentral gyrus forming the primary voluntary motor area. It has a specific correspondence between a body region and an area in the brain (somatotopic organization) that is often referred to as a homunculus (little man) (Fig. 15.9). Electrical stimulation of specific areas of this cortex causes specific muscles of the body to move. For example, stimulation of Brodmann area 4 in the medial longitudinal fissure affects the lower limb and foot, whereas stimulation of the superior lateral surface of the precentral gyrus affects the torso and arm, the middle third of the hand, and the lower third of the face and mouth/throat. The axons traveling from the cell bodies in and on either side of this gyrus project fibers (axons) that form the pyramidal system. This system includes the corticobulbar tract that synapses in the brainstem and provides voluntary control of muscles in the head and neck and the corticospinal tracts that descend into the spinal cord and provide voluntary control of muscles throughout the body. Cerebral impulses control function on the opposite side of the body, a phenomenon called contralateral control (see Fig. 15.15 later in the chapter). The Broca speech area is on the inferior frontal gyrus (Brodmann areas 44, 45). It is usually on the left hemisphere and is responsible for the motor aspects of speech. Damage to this area, commonly as a result of a cerebrovascular accident (stroke), results in the inability to form words or at least some difficulty in forming words (expressive aphasia) (see Chapter 17).

Illustration A shows the representations in the main motor area, clockwise from the top: toes, ankle, knee, hip, trunk, shoulder, elbow, wrist, hand, fingers (little, ring, middle, index, thumb), neck, brow, eyelid and eyeball, face, lips (vocalization), jaw (salivation), tongue, and swallowing (mastication). Illustration B shows the representation in the somaesthetic cortex, counterclockwise from the top: genitalia, toes, foot, leg, hip, trunk, neck, head, shoulder, arm, elbow, forearm, hand, fingers (little, ring, middle, index, thumb), eye, nose, face, upper lip, lips, lower lip, teeth, gums, and jaw, tongue, pharynx, and intra-abdominal.

Illustration A traces the somatic motor pathways from the motor cortex. Three pathways traced on the illustration are as follows • Motor cortex through internal capsule, vestibular nuclei (lateral and medial), pyramidal decussation (medulla), lateral corticospinal tract, corticobulbar tract (to head and neck), and thoracic (to arms and trunk), to lumbar (to legs). • Vestibular nuclei (lateral and medial) through rubrospinal tract and anterior corticospinal tract (decussates in spinal cord), to dorsal and ventral vestibulospinal tract. • Red nucleus through rubrospinal tract and anterior corticospinal tract (decussates in spinal cord), to dorsal and ventral vestibulospinal tract. Illustration B traces two sensory pathways from the cortex. 1. The tip of the index finger touches a feather, activating the ascending branches of dorsal root fibers. The tip of the big toe touches a feather, activating the dorsal root and spinal ganglion. The pathways are then traced through the following structures: ascending branches of dorsal root fibers, dorsal column nuclei in lower medulla oblongata, medulla oblongata, pons (medial lemniscus), midbrain, spino-mesencephalic tract in mesencephalon (midbrain), and internal capsule (pathway from foot) or ventrobasal complex of thalamus (pathway from hand). 2. The tip of the index finger and the big toe touch a candle flame each, activating the dorsal root and spinal ganglion. The pathways are then traced through the following structures: dorsal root and spinal ganglion, lower medulla oblongata, spinoreticular tract, medulla oblongata, lateral division of the anterolateral spinothalamic tract, pons, and internal capsule (pathway from hand) or ventrobasal and intralaminar nuclei of the thalamus (pathway from foot).

The parietal lobe lies within the borders of the central, parietooccipital, and lateral sulci. This lobe contains the major area for somatic sensory input, located primarily along the postcentral gyrus (Brodmann areas 3, 1, 2) (see Fig. 15.8), which is adjacent to the primary motor area precentral gyrus. Communication between the motor and sensory areas (and among other regions in the cortex) is provided by association fibers. Much of this region is involved in sensory association (storage, analysis, and interpretation of stimuli). Fig. 15.9 shows the distribution of functions associated with both the primary motor area and the primary sensory area of the cerebral cortex.

The occipital lobe lies caudal to the parietooccipital sulcus and is superior to the cerebellum. The primary visual cortex (Brodmann area 17) is located in this region and receives input from the retinas. Much of the remainder of this lobe is involved in visual association (Brodmann areas 18, 19). The temporal lobe lies inferior to the lateral fissure and is composed of the superior, middle, and inferior temporal gyri. The primary auditory cortex (Brodmann area 41) and related association area (Brodmann area 42) lie deep within the lateral sulcus on the superior temporal gyrus. The Wernicke area (posterior portion of Brodmann area 22) and adjacent portions of the parietal lobe constitute a sensory speech area. This area is responsible for reception and interpretation of speech, and dysfunction may result in receptive aphasia or dysphasia. The temporal lobe also is involved in memory consolidation and smell.

Another lobe, the insula (insular lobe), lies hidden from view in the lateral sulci between each hemisphere's temporal and frontal lobes. The insula processes sensory and emotional information and routes the information to other areas of the brain. Lying directly beneath the longitudinal fissure is a mass of white matter pathways called the corpus callosum (transverse or commissural fibers). This structure connects the two cerebral hemispheres through sensory and motor contralateral projection of axons and is essential in coordinating activities between hemispheres (see Fig. 15.8B).

Basal ganglia

Inside the cerebrum are numerous tracts (white matter) and nuclei (gray matter). The major subcortical nuclei are called the basal ganglia (basal nuclei) system. The basal ganglia system is a group of nuclei that includes the caudate nucleus, putamen, and globus pallidus (Fig. 15.10). The putamen and the globus pallidus together are called the lentiform nucleus (because they are shaped like a lentil). The caudate nucleus, putamen, and nucleus accumbens together are called the striatum. Functionally, the substantia nigra is a component of the basal ganglia. It synthesizes dopamine, a neurotransmitter and precursor of norepinephrine. The basal ganglia nuclei are important for voluntary movement and cognitive and emotional functions (i.e., the nucleus accumbens has pleasure and reward functions).

The internal capsule is a thick layer of white matter in which axons of afferent (sensory) and efferent (motor) pathways pass to and from the cerebral cortex through the center of the cerebral hemispheres and between the caudate and lentiform nuclei (see Fig. 15.10B). The basal ganglia, plus their direct and indirect interconnections with the thalamus, premotor cortex, red nucleus, reticular formation, and spinal cord, have been considered part of the extrapyramidal system. The extrapyramidal system is part of the motor control system that causes involuntary reflexes and coordinated movement and stabilizes motor control. Parkinson disease and Huntington disease are characterized by disruption of the extrapyramidal system and various involuntary or exaggerated motor movements (see Chapter 17).

Limbic system

The limbic system is a group of interconnected structures between the telencephalon and diencephalon and surrounding the corpus callosum. It comprises the amygdala, hippocampus, fornix, hypothalamus, and related autonomic nuclei (see Fig. 15.10A). It is an extension or modification of the olfactory system and influences the autonomic and endocrine systems. Its principal effects are involved in primitive behavioral responses, visceral reaction to emotion, motivation, mood, feeding behaviors, biologic rhythms, and the sense of smell. The limbic system mediates emotion and long-term memory through connections in the prefrontal cortex (limbic cortex).

Diencephalon

The diencephalon (interbrain) is surrounded by the cerebrum and sits on top of the brainstem. It controls vital functions and visceral activities and is closely associated with those of the limbic system. The diencephalon has four divisions: epithalamus, thalamus, hypothalamus, and subthalamus (see Table 15.3 and Fig. 15.8). The epithalamus forms the roof of the third ventricle (a brain cavity) and composes the most superior portion of the diencephalon.

The thalamus is the largest component of the diencephalon, and it borders and surrounds the third ventricle. It is a major integrating center for afferent impulses to the cerebral cortex. Various sensations are perceived at this level, but cortical processing is required for interpretation. The thalamus also serves as a relay center for information from the basal ganglia and cerebellum to the appropriate motor area.

The hypothalamus forms the base of the diencephalon. The hypothalamus functions to (1) maintain a constant internal environment and (2) implement behavioral patterns. Integrative centers control ANS function, regulate body temperature and endocrine function, and adjust emotional expression. The hypothalamus exerts its influence through the endocrine system and neural pathways (Box 15.1). The subthalamus flanks the hypothalamus laterally. It serves as an important basal ganglia center for motor activities.

Box 15.1

Functions of the Hypothalamus

• • Visceral and somatic responses
• • Affectual responses
• • Hormone synthesis
• • Sympathetic and parasympathetic activity
• • Temperature regulation
• • Fluid balance
• • Appetite and feeding responses
• • Physical expression of emotions
• • Sexual behavior
• • Pleasure-punishment centers
• • Level of arousal or wakefulness

The spinal cord is the portion of the CNS that lies within the vertebral canal and is surrounded and protected by the vertebral column. The spinal cord has many functions, including connecting the brain and the body through a long nerve cable, somatic and autonomic reflexes, motor pattern control, and sensory and motor modulation. The spinal cord originates in the medulla oblongata and ends at the first or second lumbar vertebra level in adults (Fig. 15.11A). The end of the spinal cord, the conus medullaris, is cone-shaped. Spinal nerves continue from the end of the spinal cord and form a nerve bundle called the cauda equine. The filament anchor from the conus medullaris to the coccyx is the filum terminale. The coverings of the spinal cord are illustrated in Fig. 15.11C.

Illustration A shows the spinal cord and labels its different sections. From the top, the sections are as follows: subarachnoid space, cervical enlargement (of spinal cord), pedicles of vertebrae, spinal ganglion, lumbosacral enlargement (of spinal cord), conus medullaris, end of spinal cord at L 1 and L 2 vertebrae, cauda equina, and end of subarachnoid space-sacral vertebra 2. The vertebrae are labeled from the top as follows: C 1 to C 8, T 1 to T 12, L 1 to L 5, S 1 to S 5, and C o. Illustration B shows a section of spinal cord, C 5 to T 1. The spinal nerves connected to the spinal cord comprise branches, trunks, and roots. Illustration C shows the coverings of the spinal cord. From the inside to the outside, the structures are as follows: central canal, gray matter, white matter, rootlets, ventral root, dorsal root, dorsal root ganglion, spinal nerve, pia mater, arachnoid mater, dura mater, and sympathetic ganglion.

Grossly, the spinal cord is divided into vertebral sections (8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal) that correspond to paired nerves (see Fig. 15.11A). A cross section of the spinal cord (Fig. 15.12) is characterized by a butterfly-shaped inner core of gray matter (containing nerve cell bodies). The central canal is filled with CSF, lies in the center of this region, and extends through the spinal cord from its origin in the fourth ventricle. The gray matter of the spinal cord is divided into three regions and displays specific functional characteristics. These regions include the posterior horn, the lateral horn, and the anterior horn. The posterior horn, or dorsal horn, is composed primarily of interneurons and axons from sensory neurons whose cell bodies lie in the dorsal root ganglion. At the tip of the posterior horn is the substantia gelatinosa, a structure involved in pain transmission (see Chapter 16). The lateral horn contains cell bodies involved with the ANS. The anterior horn, or ventral horn, contains the nerve cell bodies for efferent pathways that leave the spinal cord by way of spinal nerves.

An illustration of the central canal with the posterior (dorsal) side at the top and the anterior (ventral) side at the bottom identifies the ascending descending tracts in the spinal cord. The ascending tracts, from the top to the bottom, are as follows: fasciculus gracilis, fasciculus cuneatus, dorsal spinocerebellar tract, gray matter, ventral spinocerebellar tract, and spinothalamic tract. The descending tracts, from the top to the bottom, are as follows: fasciculus proprius, Lissauer’s tract, lateral corticospinal tract, rubrospinal tract, medial longitudinal fasciculus, medullary reticulospinal tract, lateral vestibulospinal tract, pontine reticulospinal tract, tectospinal tract, and anterior corticospinal tract.

Surrounding the gray matter is white matter, which forms ascending and descending pathways called spinal tracts. Spinal tracts are named to denote their beginning and ending points. For example, the spinothalamic tract (see Fig. 15.12) carries sensory nerve impulses from the spinal cord to the thalamus in the diencephalon. Numerous spinal tracts are grouped into columns according to their location within the white matter. These include the anterior columns, lateral columns, and posterior columns (see Fig. 15.12).

Neural circuits in the spinal cord, when activated, display specific sets of motor responses. Reflex arcs form basic units that respond to stimuli and provide protective circuitry for motor output. Structures needed for a reflex arc are a receptor, an afferent (sensory) neuron, an efferent (motor) neuron, and an effector muscle or gland. A simple reflex arc (e.g., knee-jerk reflex) may contain only two neurons (Fig. 15.13). Interneurons are usually present and provide a link between sensory and motor neurons. The motor effects of reflex arcs generally occur before the event is perceived in the brain's higher centers. Much internal environmental regulation is mediated by the ANS's reflex activity (e.g., cardiac muscle and smooth muscle contraction/relaxation and glandular responses).

An illustration shows the cross-section of the spinal cord. The following structures are labeled along structure: cell body in dorsal root ganglion, sensory neuron, stretch receptor, motor neuron, quadriceps muscle (effector), and patellar ligament.

Afferent pathways transmit sensory information from peripheral receptors toward the cerebrum. The pathways terminate in the cerebral or cerebellar cortex or both. Efferent pathways primarily relay information away from the cerebrum to the brainstem or spinal cord. Upper motor neurons (i.e., corticospinal and corticobulbar tracts) are completely contained within the CNS. Their primary roles are controlling fine motor movement and influencing/modifying spinal reflex arcs and circuits. Generally, upper motor neurons form synapses with interneurons, forming synapses with lower motor neurons that project into the periphery. Lower motor neurons directly influence muscles. Their cell bodies lie in the brainstem and spinal cord's gray matter, but their processes extend out of the CNS and into the PNS. Destruction of upper motor neurons usually results in initial paralysis followed within days or weeks by partial recovery, whereas destruction of the lower motor neuronsleads to paralysis unless peripheral nerve damage is followed by nerve regeneration and recovery (see Fig. 15.4). Differences in injury to upper and lower motor neurons are presented in Chapter 17.

Nerve impulses regulate muscle activity (i.e., stimulation and contraction). Motor neurons innervate one or more muscle cells, forming motor units, consisting of a neuron and the skeletal muscles it stimulates. The junction between the axon of the motor neuron and the plasma membrane of the muscle cell is called the neuromuscular (myoneural) junction (Fig. 15.14). (Injury to motor neurons is discussed in Chapter 17.)

Clinically relevant motor pathways are the lateral corticospinal (connects motor cortex with anterior horn cells in the spinal cord) and pyramidal tracts (connects the motor cortex with the medullary pyramids and descend to synapse with lower motor neurons of several cranial nerves in the brain stem or spinal cord); and the extrapyramidal reticulospinal, vestibulospinal, and rubrospinal tracts. The corticospinal and corticobulbar pathways are essentially the same tract and consist of a two-neuron chain (Fig. 15.15). The cell bodies (upper motor neurons) originate in and around the precentral gyrus; pass through the corona radiata (connects motor and sensory pathways) of the cerebrum, the internal capsule, middle three-fifths of the cerebral pedunculus, pons, and pyramid; and decussate (cross contralaterally) in the medulla oblongata and form the lateral corticospinal tract of the spinal cord (see Figs. 15.12 and 15.15A) and thus control the opposite side of the body. The corticobulbar tract axons synapse on motor cranial nuclei within the brainstem that control the face, head, and neck muscles. The lateral corticospinal tract axons leave the tract to go to specific interneurons or motor neurons in the anterior horn. The lateral corticospinal tract has the same somatotopic organization as the body (see Figs. 15.9 and 15.15A). These lower motor neurons project through nerves to specific muscles.

The extrapyramidal tracts are involved in precise motor movements. The reticulospinal tract arises in the reticular formation of the medulla or pons (see Fig. 15.12) and modulates motor movement by inhibiting and exciting spinal activity. The vestibulospinal tract arises from a vestibular nucleus in the pons and causes the extensor muscles of the body to rapidly contract, most dramatically witnessed when a person starts to fall backward. The rubrospinal tract originates in the red nucleus, decussates, and terminates in the cervical spinal cord. It is important for muscle movement and fine muscle control in the upper extremities.

The three clinically important spinal afferent pathways are the posterior column, anterior spinothalamic tract, and lateral spinothalamic tract (see Figs. 15.12 and 15.15B). The posterior (dorsal) column (fasciculus gracilis and fasciculus cuneatus) carries fine-touch sensation, two-point discrimination, and proprioceptive information (i.e., epicritic information). The posterior column is formed by a three-neuron chain. The first neuron of the chain is the primary afferent neuron. It also is the sensory neuron of the reflex arc. After entering the spinal cord, it sends its axon ipsilaterally (on the same side) up the spinal cord to a specific part of the posterior column and synapses in the three posterior column nuclei in the medulla oblongata. For example, a basketball player who is above 6 feet tall has primary afferent neurons that could be 6 feet long, running from the great toe up to the medulla oblongata. The axon of the second-order neuron crosses contralaterally in the medial lemniscus and ascends in the medulla and pons to synapse with a specific nucleus of the thalamus. The third-order neuron, originating in the thalamus, continues the tract into the internal capsule, corona radiata, and postcentral gyrus (Brodmann areas 3, 1, 2) (see Fig. 15.8, and 15.15B).

The anterior and lateral spinothalamic tracts are responsible for vague touch sensation and pain and temperature perception, respectively (see Figs. 15.12 and 15.15; see Chapter 16). These modalities are referred to as protopathic. These tracts also form a three-neuron chain. However, their primary afferent neurons synapse in the posterior horn of the spinal cord is not just at the level they enter the intervertebral foramen but in several spinal segments above and below their point of entry. This is an example of divergence. The axons of the second-order neurons in the posterior horn cross to the contralateral side in the spinal cord in the lateral column, ascend to the same thalamic nucleus as the posterior column pathway, and continue with the posterior column pathway to the postcentral gyrus.

The brain derives its arterial supply from the internal carotid arteries and the vertebral arteries (Fig. 15.19). The internal carotid arteries supply a proportionately greater amount of blood flow. They originate at the common carotid arteries, enter the cranium through the base of the skull, and pass through the cavernous sinus. After forming some small branches, these arteries divide into the anterior and middle cerebral arteries (Fig. 15.20). The vertebral arteries originate at the subclavian arteries and pass through the transverse foramina of the cervical vertebrae, entering the cranium through the foramen magnum. They join at the junction of the pons and medulla to form the basilar artery (see Fig. 15.20). The basilar artery divides at the level of the midbrain to form paired posterior cerebral arteries.

An illustration shows the right lateral view of the human skull and neck, highlighting the major arteries within the structure. The following arteries are identified from the top to the bottom along the anterior side: maxillary artery, facial artery, external carotid artery, superior thyroid artery, internal carotid artery, common carotid artery, inferior thyroid artery, thyrocervical trunk, and subclavian artery. The following arteries are identified from the top to the bottom along the posterior side: superficial temporal artery, occipital artery, posterior auricular artery, vertebral artery, deep cervical artery, transverse cervical artery, and suprascapular artery.

Illustration A shows the inferior view of the human brain and identifies the following structures from the top to the bottom: olfactory bulb, anterior cerebral artery, optic nerve [3] (cut), middle cerebral artery, posterior cerebral artery, internal carotid artery, vertebral artery, brainstem, and cerebellum. Illustration B shows and labels the following arteries at the base of the brain, from the top to the bottom: circle of Willis, anterior communicating artery, anterior cerebral artery, middle cerebral artery, posterior communicating artery, posterior cerebral artery, basilar artery, and vertebral artery.

Three major paired arteries perfuse the cerebellum and brainstem: the posterior inferior cerebellar artery, the anterior inferior cerebellar artery, and the superior cerebellar arteries. They originate from the basilar artery. The basilar artery also gives rise to small pontine arteries. The large arteries on the surface of the brain and their branches are called superficial arteries (conducting arteries). Small branches that project into the brain are termed projecting arteries (nutrient arteries).

The circle of Willis (see Fig. 15.20B) provides an alternative route for blood flow when one of the contributing arteries is obstructed (collateral blood flow). The circle of Willis is formed by the posterior cerebral arteries, posterior communicating arteries, internal carotid arteries, anterior cerebral arteries, and anterior communicating arteries. The anterior cerebral, middle cerebral, and posterior cerebral arteries leave the circle of Willis and extend to various brain structures, serving their associated brain territories. The border zone is the area between the major arterial territories. (Table 15.5 and Fig. 15.21 describe the structures served, the functional relationships, and the pathologic considerations related to occlusion of cerebral arteries.)

Cerebral venous drainage does not parallel its arterial supply, whereas the venous drainage of the brainstem and cerebellum does parallel the arterial supply of these structures. The cerebral veins are classified as superficial and deep veins. The veins drain into venous plexuses and dural sinuses (formed between the dural layers) and eventually join the internal jugular veins at the skull base (Fig. 15.22). Adequacy of venous outflow can significantly affect intracranial pressure. For example, when individuals with a head injury turn or let their heads fall to the side, the action partially occludes venous return, and intracranial pressure can increase because of decreased flow through the jugular veins.

An illustration shows the right lateral view of the human skull and neck, highlighting the major veins within the structure. The following veins are identified from the top to the bottom along the anterior side: maxillary vein, facial vein, lingual vein, superior thyroid vein, internal jugular vein, external jugular vein, inferior thyroid vein, right brachiocephalic vein, and right subclavian vein. The following veins are identified from the top to the bottom along the posterior side: superior sagittal sinus, superficial temporal vein, straight sinus, right transverse sinus, sigmoid sinus, occipital vein, and suprascapular vein.

The blood-brain barrier (BBB) describes cellular structures that selectively inhibit certain potentially harmful substances in the blood from entering the interstitial spaces of the brain or CSF, thus allowing neurons to function normally. Endothelial cells in brain capillaries with their intracellular tight junctions are the site of the BBB (Fig. 15.23). Supporting cells include astrocytes, pericytes, and microglia.³ Some substances, including glucose, lipid-soluble molecules, electrolytes, and chemicals, can cross into and out of the brain facilitated by transport molecules. This has substantial implications for drug therapy because certain antibiotics and chemotherapeutic drugs show a greater propensity than others to cross this barrier. Breakdown of the BBB can contribute to brain invasion by toxic molecules or pathogens promoting neuroinflammation and neurodegeneration. The epithelium of the choroid plexus and the arachnoid membrane also provide barrier functions.⁴

Illustration A is a lateral view of a brain capillary. The following structures are labeled, from the inside: tight junctions, endothelium, pericyte under the basal lamina, perivascular cell (macrophage), pia mater (larger vessels only), basal lamina, and astrocyte end-foot. Illustration B is a cross-sectional view of a brain capillary. The following structures are labeled, from the inside: endothelium, basal lamina, pericyte, perivascular cell (macrophage), and astrocyte end-foot.

The spinal cord derives its blood supply from branches of the vertebral arteries and from branches from various regions of the aorta. The anterior spinal artery and the paired posterior spinal arteries branch from the vertebral artery at the base of the cranium and descend alongside the spinal cord. Arterial branches from vessels exterior to the spinal cord follow the spinal nerve through the intervertebral foramina, pass through the dura, and divide into the anterior and posterior radicular arteries (Fig. 15.24).

Illustration A is a posterior view of the cervical cord, identifying the following arteries from the top to the bottom: basilar artery, anterior spinal artery, posterior inferior cerebellar artery, vertebral artery, posterior spinal artery, and posterior radicular artery. Illustration B is a cross-section view of the cervical cord, identifying the following arteries from the inside: anterior central artery, internal spinal arteries, posterior central artery, anterior spinal artery, posterior spinal artery, anterior radicular artery, posterior radicular artery, postcentral branch, and neural branch.

The radicular arteries eventually connect to the spinal arteries. Branches from the radicular and spinal arteries form plexuses whose branches penetrate the spinal cord, supplying the deeper tissues. Venous drainage parallels the arterial supply closely and drains into venous sinuses located between the dura and periosteum of the vertebrae.

The sympathetic nervous system mobilizes energy stores in times of need (e.g., in the “fight or flight” or stress response) (see Chapter 11 and Fig. 11.2). The sympathetic division is innervated by cell bodies located from the first thoracic (T1) through the second lumbar (L2) regions of the spinal cord and therefore is called the thoracolumbar division (see Fig. 15.26). The preganglionic axons of the sympathetic division form synapses shortly after leaving the spinal cord in the sympathetic (paravertebral) ganglia (see Fig. 15.27A). These preganglionic axons travel in several different ways: (1) directly synapsing with postganglionic neurons in the sympathetic chain ganglion at their level; (2) by traveling up the sympathetic chain ganglia; (3) by traveling down the sympathetic chain ganglion before forming synapses with a higher or lower postganglionic neuron; or (4) by traveling through the sympathetic chain ganglion to synapse with collateral ganglia (i.e., the splanchnic nerves, which lead to collateral ganglia on the front of the aorta). The collateral ganglia are named according to the branches of the aorta nearest them, namely the celiac, superior mesenteric, and inferior mesenteric ganglia (see Fig. 15.26). Postganglionic neurons leave the collateral ganglia and innervate the viscera below the diaphragm.

Preganglionic sympathetic neurons that innervate the adrenal medulla also travel in the splanchnic nerves and do not synapse before reaching the gland (see Figs. 15.26 and 15.27B). The secretory cells in the adrenal medulla are considered modified postganglionic neurons. Because preganglionic sympathetic fibers are all myelinated, travel to the adrenal medulla is quick, and innervation causes the rapid release of epinephrine and norepinephrine, which are mediators of the fight-or-flight response (see Chapter 11).

The parasympathetic nervous system conserves and restores energy when a person is at rest. The nerve cell bodies of this division are located in the cranial nerve nuclei and the sacral region of the spinal cord and therefore constitute the craniosacral division. Unlike the sympathetic branch, the preganglionic fibers in the parasympathetic division are longer and travel close to the organs they innervate before forming synapses with the relatively short postganglionic neurons (see Fig. 15.26). Parasympathetic nerves arising from nuclei in the brainstem travel to the viscera of the head, thorax, and abdomen within cranial nerves—including the oculomotor (III), facial (VII), glossopharyngeal (IX), and vagus (X) nerves.

Preganglionic parasympathetic nerves that originate from the sacral region of the spinal cord run either separately or together with some spinal nerves. The preganglionic axons unite to form the pelvic splanchnic nerve, which innervates the viscera of the pelvic cavity. These preganglionic axons synapse with postganglionic neurons in terminal ganglia located close to the organs they innervate.

Similar to the spinal nerves, the autonomic nerves also form plexuses. These are networks of nerves that have both parasympathetic and sympathetic neurons. They are usually located near the organs they innervate. Examples include the celiac (solar) plexus (contains the celiac ganglia and innervates many abdominal and pelvic organs), the cardiac plexus (innervates the heart), and Meissner and Auerbach plexuses (innervate the gastrointestinal tract).

Sympathetic preganglionic fibers and parasympathetic preganglionic and postganglionic fibers release acetylcholine—the same neurotransmitter released by somatic efferent neurons (see Fig. 15.27). These fibers are characterized by cholinergic transmission. Most postganglionic sympathetic fibers release norepinephrine (noradrenaline) and thus are considered to function by adrenergic transmission. A few postganglionic sympathetic fibers, such as those that innervate the sweat glands, release acetylcholine.

The action of catecholamines (adrenalin, noradrenaline, dopamine) varies with the type of neuroreceptor stimulated. Catecholamines are also released by the adrenal medulla gland, which physiologically and biochemically resembles the sympathetic nervous system. Two types of adrenergic receptors exist, α and β. Cells of the effector organs may have one or both types of adrenergic receptors. α-Adrenergic receptors have been further subdivided according to the action produced. α₁-Adrenergic activity is associated mostly with excitation or stimulation; α₂-adrenergic activity is associated with relaxation or inhibition. Most of the α-adrenergic receptors on effector organs belong to the α₁ class. β-Adrenergic receptors are classified as β₁-adrenergic receptors (which facilitate increased heart rate and contractility and cause the release of renin from the kidney) and β₂-adrenergic receptors (which facilitate all remaining effects attributed to β receptors).⁶ Norepinephrine stimulates all α₁ and β₁ receptors and only certain β₂ receptors. The primary response from norepinephrine, however, is stimulation of α₁-adrenergic receptors that cause vasoconstriction. Epinephrine strongly stimulates all four types of receptors and induces general vasodilation because of the predominance of β receptors in muscle vasculatures. (Table 15.7 summarizes the effects of neuroreceptors on their effector organs.)

Many body organs are innervated by both the sympathetic and parasympathetic nervous systems. The two divisions often cause opposite responses; for example, sympathetic stimulation of the stomach causes decreased peristalsis, whereas parasympathetic stimulation of the intestine increases peristalsis. In general, sympathetic stimulation promotes responses for the protection of the individual. For example, sympathetic activity increases blood glucose levels and temperature and increases heart rate and blood pressure. In emergency situations, a generalized and widespread discharge of the sympathetic system occurs and is known as the “fight or flight” reflex or acute stress response (see Chapter 11). This is accomplished by an increased firing frequency of sympathetic fibers and by activation of sympathetic fibers normally silent and at rest (fibers to the sweat glands, pilomotor muscles, and the adrenal medulla, as well as vasodilator fibers to muscle). Regulation of vasomotor tone is considered the single most important function of the sympathetic nervous system. (Fig. 15.28 illustrates some of the most important functions of the sympathetic nervous system.)

Flowchart A shows the functions of the sympathetic activation (vasomotor tone). • Adrenal medulla activation; release of epinephrine and norepinephrine; and increased strength of contraction of heart. • Contraction of arteriolar smooth muscles; vasoconstriction; increased peripheral resistance; and increased blood pressure. Flowchart B shows the functions of sympathetic activation (strenuous muscular exercise). • Peripheral vasoconstriction; and increased venous return leading to increased cardiac output or shifting of cardiac output to muscles. • Stimulation of beta-receptors of muscle vasculature; vasodilation; and increased blood flow to muscles. • Stimulation of beta-receptors of bronchiole vasculature; and increased bronchodilation; increased oxygenation. • Metabolic effects; and increased release of epinephrine. Increased release of epinephrine leads to the following three responses. • Glycogenolysis in the liver; and increased blood sugar. • Glycolysis in muscle; and increased lactic acid. • Breakdown of adipose tissue; and release of free fatty acids.

Increased parasympathetic activity promotes rest and tranquility and is characterized by reduced heart rate and enhanced visceral functions concerned with digestion. Stimulation of the vagus nerve (cranial nerve X) in the gastrointestinal tract increases peristalsis and secretion, as well as the relaxation of sphincters. Activation of parasympathetic fibers in the head, provided by cranial nerves III, VII, and IX, causes pupil constriction, tear secretion, and increased salivary secretion. Stimulation of the sacral division of the parasympathetic system contracts the urinary bladder and facilitates the process of penile erection.

The parasympathetic system lacks the generalized and widespread response of the sympathetic system. Specific parasympathetic fibers are activated to regulate particular functions. Although the actions of the parasympathetic and sympathetic systems are usually antagonistic, there are exceptions. Peripheral vascular resistance, for example, is increased dramatically by sympathetic activation but is not altered appreciably by activity of the parasympathetic system. Most blood vessels involved in the control of blood pressure are innervated by sympathetic nerves. To decrease blood pressure, therefore, it is more important to block or paralyze the continuous (tonic) discharge of the sympathetic system than to promote parasympathetic activity.

The CNS mechanisms involved in the aging process are extremely complex, and many questions have yet to be answered. Geriatric Considerations: Aging & the Nervous System summarizes the structural, cellular, cerebrovascular, and functional changes that occur in the nervous system with aging.

The ANS is involved in the involuntary control of organ systems. The ANS is divided into sympathetic and parasympathetic divisions.