Chapter summary

The peripheral nervous system consists of sensory receptors, nerves conducting impulses to and from the CNS, their associated ganglia, and motor endings.

Nerves and ganglia

Nerves

1.

2. Nerves are surrounded by a series of connective tissue layers. The epineurium is the outermost layer, consisting of a dense network of collagen fibers. The perineurium is the next innermost layer, and it partitions the nerve into a series of fascicles, each containing a bundle of axons. The innermost layer is the endoneurium that sur­rounds each individual axon.

Classification of nerves

1. Nerves carrying impulses toward the CNS are called sensory or afferent nerves, whereas those carrying impulses away from the CNS are motor or efferent nerves.

2. Peripheral nerves can function within the auto­nomic (visceral) nervous system or somatic nervous system.

Spinal nerves

1. Nerves leaving the CNS are called either spinal nerves or cranial nerves. There is a pair of spinal nerves exiting at each spinal segment.

2. Each spinal nerve has a dorsal and ventral root that enters and exits the spinal cord, respectively. Near the spinal cord, the two roots merge, forming a spinal nerve, which is a mixed nerve containing both afferent and efferent fibers.

3. Each dorsal root has a swelling called the dorsal root ganglion, situated near the spinal cord, which contains the cell bodies of the neurons running through the dorsal root.

Degeneration and regeneration of nerves

1. Neurons do die. It is believed that neurotrophic factors are responsible for keeping neurons alive. The presence of these factors suppresses a latent biochemical pathway present in all cells that causes the cells to commit suicide.

2. Cells can die by a process called apoptosis, or programmed cell death, which involves four steps.

3. In apoptosis, the loss of neurotrophic factors such as nerve growth factor decreases activity of the MAP kinase and phosphatidylinositol 3-kinase pathways, resulting in an increase in reactive oxygen species. This leads to an increase in c-jun N-terminal kinases and phosphorylation of c-jun protein. Then, there is an increased expres­sion of genes, including c-jun, cyclin Dl, and c-fos, and a decrease in RNA and protein synthe­sis. There is a decrease in Bcl-2 family proteins. When damaged, Bcl-2 family proteins release Apaf-I, which activates caspases (cysteine aspar­tate specific proteases). Caspases are a family of over a dozen proteins that cleave cellular pro­teins at aspartate residues.

Ganglia

Ganglia are collections of neuron cell bodies located in the peripheral nervous system.

Sensory receptors

1. Sensations are the awareness of a stimulus, whereas perception involves the interpretation of the sensation, and occurs in the CNS.

2. All senses involve three steps: (1) a physical stim­ulus, (2) transformation of the stimulus into a nerve impulse, and (3) a response to the sensa­tion in the form of a perception or conscious experience of sensation.

3. All sensory systems give four types of informa­tion about the stimulus, including modality, location, intensity, and timing, which collectively yield sensation.

Classes of sensory receptors

1. There are five classes of sensory receptors: mechanical, chemical, nociceptors (nocere = to injure), thermal, and electromagnetic.

2. Mechanoreceptors can detect touch, propriocep­tive sensation (muscle stretch or contraction), joint position, hearing, and sense of balance. Chemo­receptors function in the sense of itches, taste, and smell. Nociceptors detect pain. Thermoreceptors

can sense either hot or cold while photoreceptors sense electromagnetic energy.

3. These receptors can be further classified by location as exteroceptors, interoceptors, or proprioceptors.

4. Exteroceptors are sensitive to stimuli outside the body. Located near the surface of the body, they can detect touch, pressure, pain, and tempera­ture, as well as special senses such as smell, taste, vision, and auditory.

5. Interoceptors monitor the visceral organs and their function. They monitor chemical and tem­perature changes, as well as stretching within the viscera.

6. Proprioceptors are restricted to those receptors in muscles and joints that provide information concerning the position of the bones and muscles.

General senses

Mechanoreceptors

1. Mechanoreceptors detect distortions in their cell membranes such as bending and stretching.

2. There are three classes of mechanoreceptors: (1) tactile receptors are responsible for the sensa­tions of touch, pressure, and vibration; (2) baro­receptors detect changes in pressure in the walls of blood vessels, as well as the digestive, repro­ductive, and urinary tracts; and (3) propriocep­tors detect changes in the position of joints and muscles.

3. The two principal tactile receptors located in the superficial skin layers are MerkeTs discs and Meissner's corpuscles. The two tactile receptors found in the deep subcutaneous layers are the Pacinian corpuscle and the Ruffini ending.

4. There are three groups of proprioceptors: (1) muscle spindles detect the length of skeletal muscles; (2) Golgi tendon organs, located at the junction between skeletal muscle and its tendon, detect stretch of the tendons; and (3) receptors in joint capsules monitor the position of the body, and joint capsules are innervated with free nerve endings that detect pressure, tension, and move­ment of the joint.

Nociceptors

1. Pain is mediated by nociceptors. These receptors respond to stimuli that can damage tissue.

2. Some nociceptors respond directly to stimuli, while others respond indirectly since they respond to chemicals released by damaged tissue.

Thermoreceptors

1. Thermoreceptors alter their firing rate as a result of changes in temperature.

2. Thermoreceptors maintain a low, tonic firing rate (2-5 spikes∕s) at normal body temperature.

3. There are separate cold and warm receptors located in the dermis of the skin, skeletal muscles, liver, and hypothalamus.

Chemoreceptors

1. Chemoreceptors are responsible for detecting changes in concentrations of specific chemicals or compounds.

2. These receptors are also responsible for the special senses of taste, smell, or olfaction. They are responsible for sensing irritating substances on the skin, nutrients within the GI tract or brain, or carbon dioxide or oxygen levels in our blood.

Detection of sensory stimuli

1. When sensory receptors are stimulated, they transform the stimulus into an electrical signal called a receptor potential. The amplitude and duration of the receptor potential are related to the magnitude and length of time of the stimu­lus. If the receptor potential is large enough to reach threshold, it is termed a generator poten­tial, and causes an action potential to form in the sensory neuron.

2. The process of converting the stimulus into a receptor potential is called stimulus trans­duction.

Reflexes

1. Reflexes are automatic, neural responses to spe­cific stimuli. Reflexes work to preserve homeo­stasis by making rapid adjustments that do not require conscious activity.

2. The neural path controlling a reflex is called a reflex arch. The reflex arch begins with a sensory receptor and ends with the effector.

3. There are four steps to a reflex arch: (a) stimulus activates receptor, (b) information processing, (c) activation of the motor neuron, and (d) response of the peripheral effector.

Classification of reflexes

1. Reflexes can be classified based on their develop­ment, site of information processing, or resulting motor response.

2. Development of reflex. Animals are born with some reflexes, termed innate reflexes, such as suckling, chewing, a withdraw reflex from painful stimuli, and tracking objects with the eyes.

3. Site of information processing. If the sensory information is processed in the spinal cord, the reflex is called a spinal reflex (e.g., withdrawal reflex). The processing of information in the brain results in cranial reflexes.

4. Resulting motor response. Reflexes that involve the contraction of skeletal muscle are termed somatic reflexes, while those that involve smooth muscle, cardiac muscle, or glands are called vis­ceral reflexes.

Spinal reflexes

Monosynaptic reflex

1. The stretch reflex, also called the myotatic reflex, is the simplest reflex in the body, and it is an example of a proprioceptive message involved in maintaining posture and muscle tone.

2. The stretch reflex is a monosynaptic reflex pro­viding autonomic control of skeletal muscle. The sensory mechanism involves the muscle spindles, which are small, encapsulated sensory receptors located within skeletal muscle that provide information about the changes in length of the muscle.

3. Anatomy of the muscle spindle. There are two types of muscle fibers within skeletal muscle: extrafusal muscle fibers that are found outside of the muscle spindle, making up the bulk of skel­etal muscle, and intrafusal muscle Abersthat are located within the connective tissue capsule that surrounds the muscle spindle and run parallel to the extrafusal fibers.

4. There are two types of sensory fibers: primary sensory endings and secondary sensory endings.

5. Gamma motor neurons innervate each intrafusal fiber, whereas α-motor neurons innervate the extrafusal fibers.

6. When a muscle is stretched, the intrafusal fibers are also stretched, thus increasing activity in the sensory endings.

7. The components of the stretch reflex include (a) stretch of the muscle spindle, (b) activation of the sensory neurons in the muscle spindle, (c) trans­mission of the sensory signal to the α-motor neurons located in the dorsal horn in the spinal cord, and (d) stimulation of muscle contraction induced by the α-motor neurons.

8. Stimulation of the gamma motor neurons causes the intrafusal fibers to contract at either end. Since they are attached to the ends of the capsule, this causes the sensory endings to increase their firing rate because they sense stretch.

9. Gamma motor neuron loop. Stimulation of a gamma motor neuron causes contraction of intrafusal fibers, which causes the muscle spindle to detect stretch. The sensory endings in the muscle spindle detect the stretch, resulting in an increased firing rate in the sensory neuron. This information is transmitted back to the CNS where the sensory neurons synapse directly on the α-motor neurons going to the same muscle. Increased firing rate in the α-motor neuron results in contraction of the muscle in order to reduce the stretch in the muscle spindle.

Polysynaptic reflexes

1. Tendon reflex. The tendon reflex, also called the inverse myotatic reflex or reverse myotatic reflex, functions to prevent tearing of tendons. Golgi tendon receptors located within the tendons of the muscle increase their firing rate in response to increased tension in the tendon. This signal then causes inhibition of contraction of the muscle from which the signal initiated while causing reciprocal activation of antagonist muscles.

2. Withdrawal reflex. The withdrawal reflex, also called the flexor reflex, allows for the immediate withdrawal of a body part in response to painful stimuli. A painful stimuli causes a sensory signal to be transmitted to the spinal cord where it causes excitation of α-motor neurons going to flexors in that region while simultaneously inhib­iting the extensors in the same region.

3. Crossed-extensor reflex. The crossed-extensor reflex is a polysynaptic reflex in which a signal is sent to the contralateral side of the spinal cord to initiate an extensor reflex at the same time the withdrawal, or flexor, reflex is occurring on the ipsilateral side. The crossed-extensor reflex immediately allows the animal to support its weight on the contralateral side as it shifts its weight off the ipsilateral side.

Use of reflexes in diagnostics

1. Reflexes are routinely examined when assessing the nervous system. They provide a diagnostic

tool at the site of a spinal cord, brain, spinal nerve, or cranial nerve injury.

2. Table 10.3 gives examples of reflexes used in diagnostics.

Autonomic nervous system

Overview

1. In contrast to conscious activities controlled by the somatic nervous system, most physiological and endocrine activities of the body require no conscious activity, but are instead controlled automatically. These life-sustaining activities are controlled by the autonomic nervous system that coordinates and integrates the visceral functions of the body.

2. The autonomic nervous system controls visceral effectors, including smooth muscle, cardiac muscle, glandular tissue, and visceral reflexes.

3. Efferent fibers of the autonomic nervous system generally originate in the hypothalamus and travel to either autonomic nerve nuclei in the brain or preganglionic neurons located in the anterior horn of the spinal cord. The pregangli­onic neurons then leave the spinal cord and synapse on postganglionic neurons located in autonomic ganglia located in the periphery.

Divisions of the autonomic nervous system

1. The autonomic nervous system is subdivided into the sympathetic and parasympathetic subdivision.

2. The sympathetic division, sometimes called the flight-or-fight division, generally causes excita­tion and results in catabolism. This division is activated during periods of stress and exertion.

3. The parasympathetic division is responsible for rest, digestion, and anabolism (i.e., building phase of metabolism).

Role of the sympathetic division

1. The sympathetic division allows the body to respond to emergency situations resulting from sudden changes in the internal or external envi­ronment. It mediates an increase in alertness, heart rate, blood pressure, metabolism, respira­tion rate, sweating, piloerection, and mobiliza­tion of energy within the body.

2. Simultaneously, it decreases activity of the diges­tive, urinary, and immune systems. It causes an increase in blood flow to the skeletal mus­cles while decreasing blood flow to the visceral organs.

Role of the parasympathetic division

1. The parasympathetic nervous system stimu­lates restful activities while inhibiting stress responses. Therefore, it is active during non­stressful conditions.

2. The parasympathetic nervous system promotes activities such as digestion while simultaneously conserving energy, and decreasing blood pres­sure, heart rate, and respiration rate. Metabolic rate is decreased by the parasympathetic nervous system.

Anatomy of the autonomic nervous system

Sympathetic division

1. The sympathetic division, also called the thoraco­lumbar division, exits the CNS from the thoracic and lumbar vertebrae.

2. After leaving the spinal cord through the ventral root, the preganglionic sympathetic fibers enter the spinal nerve along with somatic motor fibers. Shortly thereafter, the sympathetic preganglionic fibers separate from the spinal nerve and pass through the white rami to enter the sympathetic chains (paravertebral chain) lying on either side of the spinal cord. Therefore, the preganglionic fibers are short and the postganglionic fibers are long.

3. The unmyelinated postganglionic fibers exit the sympathetic chain via the gray rami.

Parasympathetic division

The parasympathetic division, or craniosacral divi­sion, originates from brain stem nuclei and S₂-S₄ of the sacrum. The preganglionic fibers synapse on postganglionic fibers either on or near the target organ. Therefore, the preganglionic fibers are long, and the postganglionic fibers short.

Physiology of the autonomic nervous system

Neurotransmitters and receptors

1. The major neurotransmitters in the autonomic nervous system are ACh and norepinephrine. Postganglionic neurons of the parasympathetic nervous system release ACh and are thus called cholinergic neurons while those of the sympa­thetic nervous system generally release norepi­nephrine and are called adrenergic fibers.

2. Both sympathetic and parasympathetic pregan­glionic fibers generally release ACh that acts at nicotinic receptors to induce fast EPSPs in post- synaptic cells.

Cholinergic fibers

1. Acetylcholine can bind to two types of receptors called nicotinic and muscarinic.

2. Nicotinic receptors are found at skeletal muscle end plates and in autonomic ganglionic neurons in both the parasympathetic and sympathetic nervous systems. Therefore, somatic neurons and all preganglionic neurons release ACh that acts at nicotinic receptors.

3. All cholinergic postganglionic fibers act at mus­carinic receptors. These include all parasympa­thetic fibers as well as those sympathetic fibers innervating sweat glands.

Adrenergic fibers

1. Postganglionic sympathetic fibers, except those innervating sweat glands, release norepineph­rine. The sympathetic postganglionic fibers in the adrenal medulla release both norepinephrine and epinephrine.

2. There are two major classes of adrenergic recep­tors: alpha (α) and beta (β). Norepinephrine and epinephrine act at both types of receptors.

3. When the dose of norepinephrine or epineph­rine is relatively low, they cause vasodilation in skeletal muscle. Such a response is seen when an animal is alarmed or experiencing a flight- or-fight response in which there needs to be increased blood flow to skeletal muscle to sup­port its increased metabolic activity. This effect is mediated by the neurotransmitters acting at β₂-adrenergic receptors. When these neurotrans­mitters are administered at high doses, they cause vasoconstriction mediated by α ∣ -a d renergi c receptors.

Other neurotransmitters

1. Adenosine triphosphate (ATP) is frequently coreleased with norepinephrine at many post­ganglionic sympathetic neurons. Adenosine, which is produced by the hydrolysis of ATP, can act both pre- and postsynaptically at purine (P₂) receptors. Adenosine reduces the release of nor­epinephrine and ATP from nerve terminals, par­ticularly after intense sympathetic activity.

2. Many neuropeptides are also coreleased with norepinephrine and ACh from autonomic neurons. Cholinergic preganglionic fibers may contain enkephalins, neurotensin, somatostatin, or substance P Cholinergic postganglionic fibers can also contain calcitonin gene-related peptide and VIP Corelease of VIP may enhance the effect of ACh since VIP causes vasodilation. For example, when ACh causes salivary gland secre­tion, VIP enhances blood flow to support the secretory response.

Interactions of the sympathetic and paraysmpathetic divisions

Both divisions of the autonomic nervous system innervate most organs, with the two divisions having opposite effects. The effect on the organ is dependent on the relative activity of each division.

Central nervous system control of the autonomic nervous system

1. The two divisions of the autonomic nervous system are highly coordinated at the level of the CNS.

2. The hypothalamus serves a role in regulating five autonomic functions:

a. It regulates blood pressure and electrolyte composition by controlling fluid and salt intake, thus maintaining blood osmolality and volume.

b. It regulates body temperature by controlling the set point for body temperature and acti­vating either heat loss or heat production pathways.

c. It regulates energy metabolism by control­ling food intake, digestion, and metabolic rate.

d. In controls an animal's response to stress by regulating adrenal function, blood flow to muscles, and immunological responses.

e. It regulates reproductive functions including mating, pregnancy, and lactation.

Visceral reflexes

Visceral reflexes control an array of autonomic responses.

Ocular reflexes

The diameter of the pupil and the shape of the lens are controlled by the autonomic nervous system. Sympathetic fibers originating from the superior cervical ganglia innervate the muscles controlling dilation of the pupil, while parasympathetic fibers innervate circular muscles constricting the pupil. When excited, the autonomic nervous system inhib­its pupillary constriction, while it simulates pupil- Iodilator muscles.

Cardiovascular reflexes

The sympathetic nervous system can increase heart rate, strength of cardiac contraction, and periph­eral resistance, while the parasympathetic nervous system can decrease heart rate and peripheral resis­tance, although its effect on peripheral resistance is less than that of the sympathetic nervous system.

Glandular reflexes

1. The parasympathetic nervous system stimu­lates gastrointestinal glands such as the nasal, lacrimal, and gastric glands. Sympathetic input causes a viscous secretion high in amylase, while parasympathetic input causes a watery secretion of higher volume.

2. In general, the sympathetic nervous system decreases glandular secretions, while the para­sympathetic nervous system increases glandular secretions.

Gastrointestinal reflexes

The parasympathetic nervous system stimulates gastric acid secretion, whereas the sympathetic nervous system inhibits such function. The enteric nervous system controls peristalsis.

Urogenital reflexes

Bladder emptying is generally under autonomic control, although there can be some voluntary control. When the bladder is extended, there is a visceral sensory reflex in which parasympathetic postganglionic neurons in the pelvic ganglion plexus promote contraction of the bladder. The sympathetic nervous system promotes bladder smooth muscle relaxation.

Review questions and answers are available ¾ online.

References

Getty, R. 1964. Atlas for Applied Veterinary Anatomy. Iowa State Press, Ames, Iowa.

Kandel, E.R., J.H. Schwartz, and T.M. Jessell. 2000. Princi­ples of Neural Science, 4th edition. McGraw-Hill, New York.

Riegel, RJ. and S.E. Hakola. 1996. Illustrated Atlas of Clinical Equine Anatomy and Common Disorders of the Horse, Vol. 1, Musculoskeletal System & Lameness Disorders. Equistar Publication, Limited, Marysville, Ohio.