Peripheral and Autonomic Nervous System: Study Guide

Peripheral and Autonomic Nervous System

Cranial Nerves

The cranial nerves are twelve pairs of nerves that emerge directly from the brain and brainstem, each with specific sensory, motor, or mixed functions. Damage to these nerves can result in characteristic deficits depending on the nerve affected.

• Olfactory (I): Sense of smell. Damage: Loss of smell (anosmia).

• Optic (II): Vision. Damage: Blindness in part or all of the visual field.

• Oculomotor (III): Eye movement, pupil constriction. Damage: Drooping eyelid, dilated pupil, double vision.

• Trochlear (IV): Eye movement (superior oblique muscle). Damage: Difficulty looking down and in.

• Trigeminal (V): Facial sensation, chewing. Damage: Loss of facial sensation, impaired chewing.

• Abducens (VI): Eye movement (lateral rectus muscle). Damage: Inability to move eye laterally.

• Facial (VII): Facial expression, taste (anterior 2/3 tongue). Damage: Facial paralysis, loss of taste.

• Vestibulocochlear (VIII): Hearing and balance. Damage: Deafness, dizziness, loss of balance.

• Glossopharyngeal (IX): Taste (posterior 1/3 tongue), swallowing. Damage: Loss of taste, difficulty swallowing.

• Vagus (X): Parasympathetic control of heart, lungs, digestive tract. Damage: Hoarseness, loss of voice, impaired swallowing, digestive issues.

• Accessory (XI): Head, neck, shoulder movement. Damage: Impaired movement of head/shoulders.

• Hypoglossal (XII): Tongue movement. Damage: Difficulty with speech and swallowing.

Example: Damage to the facial nerve (VII) can cause Bell's palsy, resulting in facial muscle weakness on one side.

Dermatomes and Referred Pain

A dermatome is an area of skin supplied by sensory fibers from a single spinal nerve root. Understanding dermatomes is important for diagnosing nerve injuries and understanding patterns of referred pain.

• Dermatome: Each spinal nerve (except C1) innervates a specific skin region.

• Referred Pain: Pain perceived at a location other than the site of the painful stimulus, often due to convergence of nerve fibers in the spinal cord.

Example: Pain from a heart attack is often felt in the left arm or jaw due to referred pain along the same spinal segments.

Reflex Arc and Flexion-Crossed Extension Reflex

A reflex arc is the neural pathway that mediates a reflex action, typically involving a sensory neuron, interneuron(s), and a motor neuron.

• Steps of the Flexion-Crossed Extension Reflex:

  1. Stimulus: Painful stimulus (e.g., stepping on a sharp object).

  2. Activation of Sensory Neuron: Transmits signal to spinal cord.

  3. Integration Center: Interneurons in spinal cord process information.

  4. Motor Output (Flexion): Motor neurons stimulate flexor muscles to withdraw the affected limb.

  5. Crossed Extension: Interneurons stimulate extensor muscles on the opposite limb to support body weight.

Example: When you step on a tack, your foot withdraws (flexion) while the opposite leg extends to maintain balance.

Somatic Motor vs. Autonomic Nervous System

The somatic motor system and the autonomic nervous system (ANS) are two divisions of the peripheral nervous system with distinct functions.

Feature	Somatic Motor System	Autonomic Nervous System
Effector Organs	Skeletal muscles	Cardiac muscle, smooth muscle, glands
Control	Voluntary	Involuntary
Neural Pathway	One neuron from CNS to muscle	Two-neuron chain (preganglionic and postganglionic)
Neurotransmitter	Acetylcholine	Acetylcholine, norepinephrine

Sympathetic vs. Parasympathetic Nervous System

The ANS is divided into the sympathetic and parasympathetic divisions, which often have opposing effects on target organs.

Feature	Sympathetic	Parasympathetic
Origin	Thoracolumbar (T1-L2)	Craniosacral (brainstem, S2-S4)
Function	"Fight or flight" (prepares body for action)	"Rest and digest" (conserves energy)
Neurotransmitters	Acetylcholine (preganglionic), norepinephrine (postganglionic)	Acetylcholine (both pre- and postganglionic)
Effect on Heart	Increases rate and force	Decreases rate
Effect on Digestive Tract	Decreases activity	Increases activity
Pupil	Dilates	Constricts

Main Effects of Sympathetic and Parasympathetic Divisions at Target Organs

The following table summarizes the main effects of each division at major organs (see Figure 14.7 and 14.10 in standard A&P textbooks):

Organ	Sympathetic Effect	Parasympathetic Effect
Heart	Increases heart rate and force	Decreases heart rate
Bronchioles (lungs)	Dilates airways	Constriction of airways
Digestive Tract	Decreases motility and secretion	Increases motility and secretion
Pupil	Dilates (mydriasis)	Constricts (miosis)
Bladder	Relaxes bladder wall, contracts sphincter (inhibits urination)	Contracts bladder wall, relaxes sphincter (promotes urination)
Salivary Glands	Decreases secretion	Increases secretion
Blood Vessels	Constriction (increases blood pressure)	Little or no effect

Example: During exercise, the sympathetic system increases heart rate and dilates bronchioles to supply more oxygen to muscles.

Additional info: For more detailed effects, refer to standard A&P textbook figures (e.g., Marieb & Hoehn, Figure 14.7 and 14.10).