THE PERIPHERAL NERVOUS SYSTEM

Components:

1) sensory receptors

2) nerves and ganglia

3) efferent motor endings

I) Sensory Receptors: specialized structures that respond to stimuli or changes in the environment by sending nerve impulses along afferent neurons to the CNS. Sensory receptors are classified according to...

1) The type of stimulus they detect

a) Mechanoreceptors - detect mechanical forces such as touch, pressure (including blood pressure), vibrations, stretch, and itch.

b) Thermoreceptors - detect temperature changes.

c) Photoreceptors - detect light energy.

d) Chemoreceptors - detect chemicals in solution such as smell, taste,and changes in blood chemistry.

e) Nociceptors - also called pain receptors they detect damaging stimuli that result in pain.

2) By location

a) Exteroceptors - sensitive to stimuli outside the body (located at or near the body surface) includes receptors of the skin and special sense organs.

b) Interoceptors - also called visceroceptors, are sensitive to stimuli arising within the body (within the internal visceral organs and within  blood vessels).

c) Proprioceptors - sensitive to internal stimuli in skeletal muscles, tendons, joints, ligaments, and connective tissue covering bones and muscles.

3) Structural Complexity

a) Simple receptors -  The majority of all receptors , they are modified nerve endings of sensory neurons, and include those found in the skin, mucous membranes, muscles, and connective tissue. They can be encapsulated, or unencapsulated and are given special names and classified in table 13.1 page 476-477.

b) Complex receptors - These include the special senses of vision, hearing, smell, and taste.  Complex receptors (the eye, ear, nose, and the tongue) will be studied later in chapter 16 : The Special Senses.

3) Sensory Reception or Function

a) Pain

---Specialized pain receptors are called nociceptors ("all" receptors can function as nociceptors  if over stimulated--overstimulation of any receptor is painful)---

----Nociceptors are found in the skin, joint capsules, periostea of bones, walls of blood vessels (not found in the CNS)---

--Pain is classified clinically as either visceral pain (pain in the organs present in the thorax and abdominal cavity), or somatic pain (pain in the skin, muscles, joints) ( can be classified as superficial or deep).----

----Types of pain include: fast pain also called prickling pain. This results in an injection or  a cut. It is localized pain carried by myelineated fibers to primary sensory cortex (superficial somatic pain).---- slow pain (a burning, aching, dull pain that lasts a long time-chronic pain). This type of pain is carried by unmyelenated neurons, and we can only the general area involved (can be deep somatic pain or visceral pains) ( involves NMDA receptors)---and referred pain which involves the perception of pain coming from parts of the body not stimulated ( due to two parts of the body being stimulated by the same spinal nerve). Examples include cardiac pain being perceived as from pain in the upper chest and left arm,  and gall bladder pain being perceived from pain in the shoulder. From cutaneous areas (superficial), visceral pain can be perceived as being somatic in origin (page 523)--

b) Temperature

---Pain receptors (thermoreceptors) consist of free nerve endings, and are found in skeletal muscles, the liver, the skin, and the hypothalamus.--

---Two types: cold receptors (more numerous) and warm receptors (less numerous----

---When stimulated, the room feels too hot or too cold (quickly adapt to a stable temperature) (very active when temperatures are changing---

---relayed along the same pathway as pain to the reticular formation, the thalamus, and to a lesser extent toward the primary sensory cortex----

c) Mechanoreception

---Called mechanoreception, they are sensitive to the distortion of the cell membrane such as stretching, compression, twisting.....)---

----Three classes of mechanoreceptors:

1) Tactile (touch receptors)- found in the skin and sensitive to touch, pressure, and vibrations, and consist of the following:

-------Free nerve endings- (same structure as pain and temperature receptors): found between epidermal cells, and detect touch and pressure---

-------The root hair plexus- consist of free nerve endings and monitor the distortion or movement of hair---

------Merkel's Discs - dendrites contact large epidermal cells (Merkel's cells) in the lower epidermis and detect fine touch and pressure -----

------Meissner's Corpuscles - abundant in the eyelids, lips, fingertips, nipples, and external genitalia (encapsulated types includes Krause's end bulbs), sensitive to fine touch and pressure---

-----Pacinian Corpuscles - found in the skin of fingers (dermis), breasts, external genitalia, joint capsules, mesenteries, pancreas, and urinary bladder - sensitive to deep pressure and pulsating high frequency vibrations---

-----Ruffini Corpuscles - found in the dermis and sensitive to pressure and distortion of the skin-----

2) Baroreceptors (pressure receptors)

------free nerve endings that monitor "changes" in pressure---

------found in distensible organs such as blood vessels, respiratory organs, digestive organs, and organs of the urinary system----

-----Blood pressure is monitored in the aorta (aortic sinus), and the carotid artery (carotid sinus)----

3) Proprioceptors (position receptors)

-----monitor "position" of joints, "tension" in tendons and ligaments, state of muscular contraction---

-----called tendon organs and muscle spindles - subconsciously processed- we are unaware of their stimulation, they continually send information to the CNS---

d) Chemoreceptors (chemical receptors)

----respond to lipid and water soluble substances--

----found in the respiratory center in the medulla oblongata (sensitive to changes in pH  and CO2 in the CSF)----

-----found in carotid bodies (near the origin of the internal carotid arteries) and in the aortic bodies (between the major branches of the aortic arch) ---

---Glossopharyngeal and vagus nerves carry impulses to the respiratory center (the carotid and aortic bodies aid in control of respiration and cardiovascular activity---

II) Nerves and Ganglia

A) Nerves:

a) Structure (page 481)

-----consist of parallel bundles of myelenated or unmyelenated axons---

-----endoneurium encloses each axon---

-----contains fascicles which are bundles of axons---

----perineurium covers a fascicle----

----epineurium covers groups of fasciculi---

----also contain an artery and a vein----

b) Classification

----can be motor nerves consisting of motor neurons only (efferent), sensory nerves consisting of sensory neurons only (afferent), or mixed nerves consisting of sensory and motor neurons----

-----can be classified as somatic or visceral (autonomic)----

---Nerves are classified  using both of the above characteristics as somatic afferent, somatic efferent, visceral afferent, or visceral efferent.----

B) Ganglia

----collections of neuron cell bodies associated with sensory neurons (afferent - dorsal root ganglia), and efferent neurons---

C) Regeneration of Nerve Fibers

---mature neurons do not divide---

----if neuron dies then other neurons stimulated by its axon also die--

----death of a neuron occurs if damage occurs to (or close to) the cell body---

----cut or compressed axons can regenerate if cell body remains in tact---

----separated ends seal themselves off and swell up---

---A process called Wallerian Degeneration occurs in which degeneration of the myelin sheath and axon occurs due to no nutrients from the cell body. This spreads distally from the injury site, fragmenting the axon---

----macrophages and schwann cells then phagocytize debris from the axon and myelin sheath---

-----CAMs (cell surface adhesion molecules -NGf and IGF) encourage axonal growth in which  a regeneration tube is formed first from schwann cells and form axon sprouts guiding them across the injured site with the regeneration tubes. Remyelination of axons occur occur with schwann cells. Regeneration occurs at a  rate of 1.5 mm/day (page 482)----

D) Motor Nerve Endings

----activate effectors by releasing neurotransmitters ( found in the skeletal muscle, cardiac muscle and glands---

E) Cranial Nerves and functions (pages 483-491)

F) Spinal Nerves

----all mixed nerves---

----formed from combination of dorsal and ventral roots from spinal cord---

---spinal nerve is actually only 1-2 cm long in that it divides into three sections after leaving the intravertebral foramen:

a) the dorsal ramus

b) the ventral ramus (largest) (containing the rami communicantes)

c) the meningeal branch (meninges and blood vessels of CNS)

----All ventral rami (except for T2-T12) join together to form nerve plexi (occur in the brachial, lumbar, and sacral regions and serve the limbs) They consist of the cervical plexus, brachial plexus, lumbar plexus, and sacral plexus---

-----Each branch of a plexus contain s fibers from several different spinal nerves (fibers from different ventral rami crisscross and are re distributed)--

----Each muscle receives innervation from more than one spinal nerve----

Reflex Activity

Five components;

1) receptor 2) sensory neuron 3) integration center (CNS) 4) motor neuron 5) the effector (muscle or gland)

Types of Reflexes (also called spinal or somatic reflexes and can be ipsilateral or contralateral)

The Stretch Reflex

a) purpose is to maintain healthy muscle tone (resistance to active or passive stretch at rest)---

b) depends on muscle spindles whose structure consists of....

----3-10 intrafusal muscle fibers enclosed in a capsule of connective tissue (extrafusal fibers are contractile units of a muscle, the intrafusal fibers only have myosin and actin myofilaments on their ends so is the only part that can contract----

-----the center of intrafusal fibers serve as receptor surfaces for the spindle, they are "wrapped" with two types of afferent nerve fibers: Type Ia (stimulated by rate and amount of stretch and innervates the middle section), and Type II (stimulated by degree of stretch  and innervated at the ends)----

----the ends of the intrafusal fibers are innervated by gamma efferent fibers (alpha efferent fibers stimulate extrafusal muscle fibers in order to contract---

c) Stimulation of the muscle spindle result from either external forces that lengthen the muscle, or activation of the gamma motor neurons producing and "internal stretch".

d) The Patellar Reflex is an example of a stretch reflex and involves the following steps:

1) Stimulation of a muscle spindle occurs

2) The impulse travels to the spinal cord via afferent fibers to the alpha motor neurons where the impulse takes two paths

3) An impulse is carried to the alpha motor neurons causing the stretched muscle to contract and interneurons in the spinal cord carry the impulse to the alpha fibers of an antagonistic muscle (reciprocal inhibition) which inhibits the contraction of the antagonistic muscle

Other Reflexes

1) Deep Tendon Reflexes: reciprocal activation (stimulation: golgi tendon organ) An example is running which involves rapid switching between flexion and extension.

2) The Flexor Reflex: initiated by a painful stimulus also called the withdrawal reflex.

3) The Crossed Extensor Reflex: important in maintaining balance (when someone grabs your right arm, you extend your left arm)

4) Superficial Reflexes:

- - - The Plantar Reflex: Tests nerves L4 to S2 by rubbing blunt instrument downward along the sole of the foot (plantar surface) results in a downward flexion of the toes (normal response) , or an abnormal response called Babinski's sign in which the "great toe" dorsoflexes while the other toes fan laterally.  Infants up to one year old exhibit Babinski's sign because all of their neurons are incompletely myelenated----

- - -  Abdominal Reflexes: Tests nerves T8 to T12. Stroking the skin of the lateral abdomen above, to the side, or below the umbilicus will cause the umbilicus to move toward the stimulated site- - -

THE AUTONOMIC NERVOUS SYSTEM

I) The Sympathetic Division

a) Epinephrine and/or norepinephrine released by postganglionic axon at the target organ (andrenergic)

b) Acetylcholine released at the sympathetic ganglion ( involves alpha and beta andrenergic receptors (page 524)

***epinephrine (adrenalin) is produced by the adrenal gland

***norepinephrine is a neurotransmitter

II) The Parasympathetic Division

a) Acetylcholine is released by the postganglionic neuron at the target organ (cholinergic)

b) Cholinergic receptors bind Ach (acetylcholine) and are named for drugs that bind receptors and mimic Ach effects:   nicotinic (always stimulatory and muscarinic (a mushroom poison) can be either inhibitory (heart's pacemaker by the vagus nerve), or stimulatory (smooth muscle of the gastrointestinal tract.  Muscarinic receptors are found in the PSN and the SNS.

NERVE INFORMATION

1) Clawhand: Occurs from damage to the ulnar (funny bone) nerve and causes loss of sensations and paralysis to the small finger and the ring finger. Individuals have trouble making a fist.  The two fingers hyperextend at the knuckles and flexed at the interphalangeal joints (clawhand appearance). Atrophy of the muscles have occurred.

2) Sciatic Nerve: The largest branch of the sacral plexus.                            

3) Radial Nerve:  The largest branch of the brachial plexus. Trauma to this nerve will result in "Saturday night paralysis" or wrist drop in which one can't extend the hand at the wrist.

4) Hypoactive Reflexes: Absence of stretch reflexes due to peripheral nerve damage or ventral horn injury.  Absent in those with chronic diabetes, coma, neurosyphilis.

5) Hyperactive Reflexes: Present in polio and stroke patients (due to nerve damage to the corticospinal tract or the pyramidal system).  The large neurons in the precentral gyrus are called pyramidal cells and involve "skilled" voluntary movements of the skeletal muscles.