Peripheral and Autonomic Nervous System: Structure and Function

Peripheral Nervous System (PNS)

Overview of the Peripheral Nervous System

The peripheral nervous system connects the body and environment to the central nervous system (CNS). It transmits sensory stimuli, processes information, and sends signals to muscles and glands.

• Brain: Processes sensory information, develops command, and sends signals via the spinal cord to the PNS.

• Cranial nerves: Attach to brain structures and are named and numbered. There are 12 pairs, each with specific functions.

Cranial Nerves

Cranial nerves are classified by their origin and function. They include sensory, motor, and mixed nerves.

• Trochlear nerve: Moves the eye downward and inward; originates from the midbrain.

• Vestibulocochlear nerve: Responsible for hearing and equilibrium; originates from the pons and medulla.

• Optic nerve: Carries visual information; originates from the retina and forms the optic chiasma.

• Vagus nerve: Mixed nerve; controls contraction of muscles and visceral organs; originates from the medulla oblongata.

• Facial nerve: Controls facial expressions and taste; originates from the pons and medulla.

• Hypoglossal nerve: Controls tongue movement; originates from the medulla oblongata.

• Abducens nerve: Controls lateral movement of the eye; originates from the pons.

Spinal Nerves

Spinal nerves carry both sensory and motor information between the spinal cord and the body. There are 31 pairs, each with anterior and posterior roots.

• Anterior root: Carries motor signals from the spinal cord to muscles and glands.

• Posterior root: Carries sensory signals from receptors to the spinal cord.

• Ramus: Branches of spinal nerves that carry autonomic and somatic signals.

Plexuses of Spinal Nerves

Plexuses are networks formed by the anterior rami of spinal nerves. They innervate various body regions.

• Cervical plexus: Innervates the neck, skin, and muscles; includes the phrenic nerve (diaphragm).

• Brachial plexus: Innervates the shoulder, arm, and hand.

• Lumbar plexus: Innervates the lower limb, abdominal wall, and genitalia.

• Sacral plexus: Innervates the lower limb and pelvic muscles.

Major Nerves and Their Functions

• Femoral nerve: Innervates anterior thigh muscles.

• Obturator nerve: Innervates the adductor muscles of the thigh.

• Radial nerve: Innervates the posterior compartment of the arm.

• Musculocutaneous nerve: Innervates the biceps brachii and brachialis muscles.

Sensory Receptors and Their Classification

Types of Sensory Receptors

Sensory receptors detect changes in the internal and external environment. They are specialized cells that convert stimuli into electrical signals.

• Encapsulated nerve endings: Surrounded by supportive cells; detect touch and pressure.

• Free nerve endings: Detect pain and temperature.

• Mechanoreceptors: Respond to mechanical stimuli (touch, pressure, vibration).

• Thermoreceptors: Respond to changes in temperature.

• Electroreceptors: Respond to changes in electric field.

• Photoreceptors: Respond to light (vision).

• Baroreceptors: Respond to changes in pressure.

• Proprioceptors: Respond to changes in body position.

Classification by Location

• Exteroceptors: Respond to stimuli from the external environment (touch, vision, hearing).

• Interoceptors: Respond to stimuli from the internal environment (blood pressure, pH).

Classification by Function

• Effectors: Structures that respond to motor signals (muscles, glands).

• Receptors: Structures that detect sensory stimuli.

Reflexes and Reflex Arcs

Reflex Arc Structure

A reflex arc is a neural mechanism that controls an immediate response to a particular stimulus. It consists of a receptor, sensory neuron, integration center, motor neuron, and effector.

• Afferent pathway: Carries sensory information to the CNS.

• Efferent pathway: Carries motor commands from the CNS to effectors.

Types of Reflexes

• Monosynaptic reflex: Involves one synapse between the sensory and motor neuron (e.g., stretch reflex).

• Polysynaptic reflex: Involves multiple interneurons and synapses.

• Simple stretch reflex: Regulates muscle length (e.g., patellar reflex).

• Tendon reflex: Controls muscle contraction by causing relaxation.

• Crossed extensor reflex: A polysynaptic reflex; response occurs on the opposite side of the body from the stimulus.

Example: Crossed Extensor Reflex

When you step on a nail, the flexor reflex pulls the affected foot away, while the crossed extensor reflex straightens the other leg to support your body weight.

Table: Major Cranial Nerves and Their Functions

Cranial Nerve	Origin	Main Function
Olfactory (I)	Olfactory bulb	Smell
Optic (II)	Retina	Vision
Oculomotor (III)	Midbrain	Eye movement
Trochlear (IV)	Midbrain	Eye movement (downward/inward)
Trigeminal (V)	Pons	Facial sensation, chewing
Abducens (VI)	Pons	Eye movement (lateral)
Facial (VII)	Pons/Medulla	Facial expression, taste
Vestibulocochlear (VIII)	Pons/Medulla	Hearing, equilibrium
Glossopharyngeal (IX)	Medulla	Taste, swallowing
Vagus (X)	Medulla	Visceral organ control
Accessory (XI)	Medulla/Spinal cord	Neck muscles
Hypoglossal (XII)	Medulla	Tongue movement

Key Equations and Concepts

• Reflex arc pathway:

$\text{Stimulus} \rightarrow \text{Receptor} \rightarrow \text{Sensory Neuron} \rightarrow \text{Integration Center} \rightarrow \text{Motor Neuron} \rightarrow \text{Effector}$

• Types of Sensory Receptors:

$\text{Mechanoreceptor}, \text{Thermoreceptor}, \text{Electroreceptor}, \text{Photoreceptor}, \text{Baroreceptor}, \text{Proprioceptor}$

• Classification of Reflexes:

$\text{Monosynaptic Reflex} \quad \text{vs.} \quad \text{Polysynaptic Reflex}$

Additional info: Academic context and definitions have been expanded for clarity and completeness. Table entries and equations are inferred and formatted for study purposes.