Chapter 13

NERVOUS SYSTEM OVERVIEW

Two principal divisions of the nervous system:

1. CNS consists of the brain and spinal cord. Within the CNS, various sorts of incoming sensory information are integrated and correlated. The CNS is connected to sensory receptors, muscles, and glands in peripheral parts of the body by the PNS

2. PNS consists of cranial nerves that arise from the brain and spinal nerves that emerge from the spinal cord. Portions of these nerves carry nerve impulses into the CNS while portions carry impulses out of the CNS. Input component of the PNS consists of nerve cells called (sensory or afferent) neurons. They conduct nerve impulses from sensory receptors in various parts of the body to the CNS. Output component consists of nerve cells called (motor or efferent) neurons. They originate within the CNS and conduct nerve impulses from the CNS to muscles and glands.

PNS subdivided into two parts: (based on body area response)

Somatic: body

Autonomic: smooth muscle, cardiac muscle, glands

Somatic consists of sensory neurons that convey information from cutaneous and special sense receptors primarily in the head, body wall, and extremities to the CNS and motor neurons from the CNS that conduct impulses to skeletal muscles only. Autonomic nervous system consists of sensory neurons that convey information from receptors primarily in the viscera (internal organs) to the CNS and motor neurons from the CNS that conduct impulses to smooth muscles, cardiac muscle, and glands.

Motor portion of the ANS consists of two branches:

Sympathetic

Parasympathetic

PERIPHERAL NERVOUS SYSTEM

The PNS provides links from and to the external environment. Its nerve fibers connect virtually every part of the body, enabling the CNS to receive information and to carry out its decisions.

The PNS includes: all neural structures outside the brain and spinal cord.

It includes sensory receptors, peripheral nerves and their associated ganglia, and efferent motor endings.

Sensory receptors are modified dendrites -- specialized structures that respond to changes in the environment known as stimuli. Activation of a sensory receptor triggers impulses along the afferent fibers coursing to the brain.

Classification of Sensory Receptors:

1. location in the body
2. type of stimulus detected
3. relative complexity of their structure

Location:

Three classes recognized:

1. Exteroceptors - sensitive to stimuli arising outside the body. Receptors located at or near the body surface. Include touch, pressure, pain, and temperature, and most special sense organs.

2. Interoceptors - respond to stimuli arising from within the body. Excited by a variety of stimuli, including chemical, stretching, and temperature. Activation may cause the feeling of pain, discomfort, hunger, thirst.

3. Proprioceptors – special category that responds to internal stimuli; however, their location is restricted to the musculoskeletal system. ccur in skeletal muscles, tendons, joints, and ligaments. Constantly advise the brain on body movements.

Stimulus Detected:

1. Mechanoreceptors - touch, pressure, vibrations, stretch.
2. Thermoreceptors - sensitive to temperature changes.
3. Photoreceptors - retina of the eye.
4. Chemoreceptors- respond to chemicals in solution, molecules smelled or tasted, changes in blood chemistry.
5. Nociceptors - respond to potentially damaging stimuli that result in pain. Virtually all receptors function as nociceptors at one time or another. (Excessive heat, cold, pressure and chemicals released at site of inflammation)

Structural Complexity:

1. Simple - majority of receptors - equivalent structurally to modified dendritic endings of sensory neurons. Found in the skin mucus membranes, muscles, and connective tissue. Monitor most types of general sensory information.

2. Complex - actually sense organs - localized collections of cells working together to accomplish a specific receptive process. Associated with special senses - vision, hearing, smell, taste.

Anatomy of General Sensory Receptors (Simple)

General sensory receptors are involved in: tactile sensations (touch, pressure, stretch), temperature monitoring, pain, and muscle sense

These receptors are either free dendritic endings or encapsulated dendritic endings.

1. Free - invade virtually all the body tissues, particularly abundant in epithelia and connective tissues.

The sensory fibers have a small diameter and their distal dendrites usually have small knob like swellings. Respond to pain and temperature, but some respond to tissue movements.

Some are associated with disc-shaped epidermal cells (merkel cells) that attach to the deeper layers of skin epidermis and function as light touch receptors.

Root hair plexuses are light touch receptors that detect bending of hairs.

2. Encapsulated - exhibit one or more terminal fibers enclosed in a C.T. capsule.

Virtually all are mechanoreceptors

Meissner' s corpuscles - small, egg-shaped receptors found just beneath the skin epidermis in the dermal papillae. (discriminative touch).

Pacinian corpuscles- scattered deep in the dermis and in subcutaneous tissue. Sense deep pressure.

Nerve Structure and Classification

Nerves vary in size. Every nerve consists of parallel bundles of peripheral axons enclosed by successive wrappings of connective tissue. Each axon is surrounded by a delicate layer of loose connective tissue called endoneurium, which also encloses the fiber's associated myelin and/or neurilemma sheath. Groups of fibers are bound into bundles or fascicles by a coarser connective tissue wrapping, the perineurium. All fascicles are enclosed by a tough fibrous sheath, the epineurium, to form the nerve. Blood vessels and lymphatics are also found within a nerve.

The PNS is divided into sensory (afferent) and motor (efferent) divisions.

Nerves are classified according to the direction in which they transmit impulses:

1. mixed nerves - contain both sensory and motor fibers and transmit impulses in both directions.
2. sensory (afferent) - carry impulses toward the CNS.
3. motor (efferent) - carry impulses away from the CNS.

Most nerves are mixed.

Fibers can be classified further according to the region they innervate:

somatic afferent - sensory impulses from the body surface
somatic efferent - motor impulses to the skeletal muscles
visceral afferent - sensory impulses from the internal organs
visceral efferent - motor impulses to the internal organs

For convenience, the peripheral nerves are classified as cranial nerves or spinal nerves depending on whether they arise from the brain or spinal cord.

Ganglia are collections of neuron cell bodies associated with nerves in the PNS. Afferent nerve fiber ganglia contain only cell bodies of sensory neurons (dorsal root ganglia). Efferent nerve fiber ganglia contain cell bodies of autonomic motor neurons, as well as a special variety of integrative neurons.

Regeneration of Nerve Fibers

Damage to nervous tissue is serious because mature neurons do not divide. If the damage is severe or close to the cell body, the entire neuron may die. Other neurons normally stimulated by the damaged axon may die as well. In certain cases, cut or compressed axons in peripheral nerves can regenerate.

Almost immediately after a peripheral axon has been severed or crushed, the separated ends seal off and then swell. Within a few minutes, the axon and its sheath distal to the site of injury, begin to disintegrate.

This process spreads distally from the site of injury. Generally, the entire axon distal to the injury is degraded within a week.

After removal of the debris, the surviving Schwann cells proliferate and migrate into the site of in jury forming cellular cords that guide the regenerating axon "sprouts" across the gap and attempt to make their original contacts. The Schwann cells protect, support, and remyelinate the regenerating axons and release growth factors to encourage growth.

Within two days of injury, the cell body swells, and its chromatophilic substance breaks apart and disperses within the cell. This signals that the neuron is preparing to synthesize proteins to support regeneration.

Axons regenerate at the rate of 1 to 5 mm a day. The greater the distance to regenerate, the less chance of recovery because the axonal sprouts tend to escape into surrounding areas and form a tissue mass called a neuroma. Post-trauma axon regrowth is never exactly the same as what existed before injury.

Much of the functional recovery involves retraining the nervous system to respond appropriately so that stimulus and response are coordinated.

Fibers within the CNS never regenerate distances more than 1 mm under any circumstances. Damage to the brain or spinal cord are viewed as irreversible.

Motor Endings

Somatic motor fiber terminals that innervate voluntary muscles form neuromuscular junctions with their effector cells. The motor endings of autonomic motor neurons form much simpler junctions with smooth and cardiac muscle and visceral glands. The axons branch repeatedly, each branch forming synapses with its effector cells. The axonal ending serving smooth muscle or glands (not cardiac) exhibit a series of varicosities, that make it look like a string of beads.

Cranial Nerves

Twelve pairs of cranial nerves are associated with the brain and pass through various foramina of the skull.

The first two pair originate from the forebrain; the rest originate from the brain stem. The cranial nerves serve only head and neck structures, except the vagus nerve which serves abdominal and thoracic structures.

The names usually reveal the most important structures they serve or their primary function. Most are mixed nerves; however, three pair (olfactory, optic, and vestibulocochlear) axe associated with special sense organs and are considered purely sensory.

Spinal Nerves

Thirty-one pairs of spinal nerves, arise from the fusion of the ventral and dorsal roots of the spinal cord, and supply all parts of the body except the head and some areas of the neck. All spinal nerves are mixed nerves (motor & sensory).

The nerves are named according to their point of issue from the spinal cord:

8 pairs of cervical nerves (C1 –C8)

12 pairs of thoracic nerves (TI1-T12)

5 pairs of lumbar nerves (L1-L5)

5 pairs of sacral nerves (S1 -S5)

1 pair of tiny coccygeal nerves.

The first 7 pairs of nerves leave the canal superior to the vertebrae for which they are named. C8 emerges inferior to the seventh cervical vertebrae between C7 and T1. Below the cervical level, each spinal nerve leaves the column inferior to the same numbered vertebra.

Each spinal nerve is connected to the spinal cord and formed from the fusion of the dorsal and ventral roots. Each root actually consists of a series of rootlets that attach along the whole length of the spinal cord segment. The ventral roots contain motor (efferent) fibers arising from anterior horn motor neurons that extend to and innervate the skeletal muscles. Dorsal roots contain sensory (afferent) fibers, arising from sensory neurons in the dorsal root ganglia, that conduct impulses from peripherally located receptors to the spinal cord.

The spinal roots pass laterally from the cord and unite distal to the dorsal root ganglion, forming a spinal nerve before emerging from the column. Almost immediately after emerging from its foramen, each nerve divides into a large ventral ramus, a smaller dorsal ramus and a tiny meningeal branch. Each ramus is mixed. The spinal nerve rami supply the entire somatic region of the body from the neck down. The dorsal rami serve the skin and musculature of the posterior body trunk. The ventral rami of spinal nerves (T2-T12) pass anteriorly as the intercostal nerves. The ventral rami of all other spinal nerves form complex networks of nerves called plexuses, which serve the motor and sensory needs of the rest of the body trunk and the limbs.

Plexus

A feature of all spinal nerves except T1-T12 is that their ventral rami branch and join one another lateral to the column, forming complicated nerve plexuses. Such networks serve the limbs. Only the ventral rami form plexuses. Each muscle in a limb receives its nerve supply from more than one spinal nerve, an advantage in that damage to one spinal segment or root cannot completely paralyze any limb muscle.

Cervical Plexus

Buried deep in the neck. Supply the muscles of the shoulder and neck.

Its single most important nerve is the phrenic nerve, which supplies both motor and sensory innervation to the diaphragm. The primary danger of a broken neck is that the phrenic nerve may be severed. Irritation of the phrenic nerve causes spasms of the diaphragm, or hiccups. Severing of the phrenic nerve and/or C3-C5 results in paralysis of the diaphragm.

Brachial Plexus

Located partly in the neck and partly in the axilla. Provides virtually all the nerves that innervate the upper limb.

Five nerves are especially important:

1. axillary - supplies the deltoid and teres minor and the skin and joint capsule of the shoulder

2. musculocutaneous – supplies motor fibers to the arm muscles that flex the forearm. It provides cutaneous sensation to the lateral forearm.

3. median - gives off branches in the forearm to the skin and most of the flexor muscles. It activates muscles to pronate the forearm, flex the wrist and fingers and oppose the thumb.

4. ulnar - produces wrist and finger flexion and adduction and abduction of the medial fingers.

5. radial - the largest nerve of the brachial plexus. It innervates all the extensor muscles of the upper limb.

Lumbosacral Plexus

The sacral and lumbar plexuses overlap substantially. They mainly serve the pelvic region of the trunk and the lower limbs. Lumbar plexus arises from the first four lumbar spinal nerves. The major branches descend to innervate the anterior and medial thigh. Sacral plexus arises from the spinal nerves L4-S4. About half the nerves serve the buttock and the lower limbs, the rest innervate pelvic structures.

The major branch of the sacral plexus is the sciatic nerve, the thickest and longest nerve in the body.

It is actually2 nerves wrapped in a common sheath. The sciatic nerve leaves the pelvis via the greater sciatic notch and runs deep to the gluteus maximus and enters the posterior thigh where it gives off motor branches to the hamstring muscles. Injury to the sciatic nerve results in a number of lower limb impairments. Sciatica is characterized by stabbing pain radiating over the course of the sciatic nerve. When the sciatic nerve is completely transected, the leg becomes nearly useless. Recovery is usually slow and incomplete.

Dermatomes

The skin of the body can be mapped to the various nerves that provide sensory innervation. The area of skin innervated by the branches of a single nerve is called a dermatome.

Reflex Activity

Many of the body's control systems belong to the general category of stimulus-response sequences known as reflexes. A reflex is a rapid, predictable motor response to a stimulus, it is unlearned, unpremeditated, and involuntary. In many cases the final response to the reflex activity is apparent, while in others the reflex activities go unnoticed (inborn).

There are learned or acquired reflex responses that result from practice or repetition.

These are largely automatic but only because of conscious effort expended to learn the skill.

Most reflex actions are subject to modification by learning and conscious effort.

Components of a Reflex Arc

1. Receptor: site of the stimulus action.

2. Sensory neurons: transmits the afferent impulses to CNS.

3. Integration center: simple: may be a single synapse between the sensory neuron and a motor neuron. complex: involves multiple synapses with chains of interneurons. The center is always within the CNS.

4. Motor neuron: conducts efferent impulses from the center to an effector organ.

5. Effector: muscle fiber or gland cell that responds in a characteristic way.

Reflexes are classified functionally as somatic reflexes if they activate skeletal muscle, and as autonomic reflexes when visceral effectors are activated.

Spinal Reflexes - Somatic reflexes mediated by the spinal cord.

Flexor reflex - Initiated by a painful stimulus and causes automatic withdrawal of the threatened part from the stimulus.

Crossed Extensor Reflex - Reflex when someone grabs your arm or when you step on a piece of glass barefooted, (combined withdrawal reflex with a contralateral extensor reflex).

Superficial Reflex - Elicited by gentle cutaneous stimulation. (Plantar reflex)

Stretch and Deep Tendon Reflexes - Important to normal skeletal muscle function, posture, and locomotion.

Stretch Reflex – initiated by the muscle spindle (a special sensory receptor in muscle) when the muscle is stretched, such as occurs when we carry a heavy weight. The muscle spindle senses the stretch and causes the stretched muscle to contract (to support the weight) and also causes reciprocal inhibition (the relaxation of the antagonistic muscles).

Deep Tendon Reflex – initiated by the Golgi tendon organ (a special sensory receptor in tendons). When muscle tension increases, the Golgi tendon organ is stimulated and it causes the contracting muscle to relax and also reciprocal activation (the contraction of antagonistic muscles).