Chapter 7 (Part IV): The Peripheral Nervous System (PNS) & the Autonomic Division

The Peripheral Nervous System (PNS) and Autonomic Division

Overview of the Nervous System

The nervous system is divided into the Central Nervous System (CNS) and the Peripheral Nervous System (PNS). The PNS consists of nerves and ganglia outside the CNS and is further divided into sensory and motor divisions. The motor division includes the somatic and autonomic divisions, with the autonomic division split into sympathetic and parasympathetic branches.

Sensory Receptors

Sensory receptors are specialized cells that detect changes in the environment (stimuli) and initiate nerve impulses. Sensation is the awareness of a stimulus, while perception is the interpretation of its meaning in the brain.

• Mechanoreceptors: Respond to touch, pressure, vibration, stretch, and itch.

• Thermoreceptors: Detect temperature changes.

• Photoreceptors: Stimulated by light (e.g., retina).

• Chemoreceptors: Respond to chemicals (e.g., smell, taste, blood chemistry).

• Nociceptors: Sensitive to pain-causing stimuli.

• Proprioceptors: Detect body position and movement.

Nerve Structure and Regeneration

Nerve Anatomy

Peripheral nerves are composed of bundles of axons (nerve fascicles), blood vessels, and connective tissues:

• Endoneurium: Surrounds individual axons and their myelin sheaths.

• Perineurium: Bundles groups of axons into fascicles.

• Epineurium: Encloses the entire nerve.

Regeneration of Nerve Fibers

Unlike CNS neurons, PNS neurons can regenerate if the soma is intact. The process involves macrophages, Schwann cells, and axons.

• Macrophages: Remove debris and stimulate Schwann cell division.

• Schwann cells: Form a regeneration tube and secrete growth factors.

• Axons: Regenerate and reconnect to their targets.

Cranial Nerves

Overview and Classification

There are 12 pairs of cranial nerves, each identified by Roman numerals and names. They innervate the head and neck, except for the vagus nerve, which extends to thoracic and abdominal organs.

• I Olfactory: Sensory; smell

• II Optic: Sensory; vision

• III Oculomotor: Motor; eye movement, pupil contraction

• IV Trochlear: Motor; vertical eye movement

• V Trigeminal: Both; facial sensation, mastication

• VI Abducens: Motor; lateral eye movement

• VII Facial: Both; taste, facial expression, salivation

• VIII Vestibulocochlear: Sensory; hearing, equilibrium

• IX Glossopharyngeal: Both; taste, swallowing, gag reflex

• X Vagus: Both; parasympathetic functions, sensory from aorta, swallowing

• XI Accessory: Motor; neck and shoulder movement

• XII Hypoglossal: Motor; tongue movement, swallowing

Spinal Nerves

Classification and Function

The spinal cord gives rise to 31 pairs of spinal nerves, classified by region:

• 8 cervical pairs (C1–C8)

• 12 thoracic pairs (T1–T12)

• 5 lumbar pairs (L1–L5)

• 5 sacral pairs (S1–S5)

• 1 coccygeal pair

Each nerve pair innervates specific regions and functions of the body.

The Autonomic Nervous System (ANS)

Structure and Function

The ANS is part of the motor division of the PNS and regulates involuntary functions. It is divided into the sympathetic and parasympathetic divisions, both aiming to maintain homeostasis but through different mechanisms.

• Sympathetic Division: Prepares the body for intense situations (fight, flight, freeze), moving away from homeostasis.

• Parasympathetic Division: Returns the body to homeostasis, promoting rest and digest functions (SLUDDD: Salivation, Lacrimation, Urination, Digestion, Defecation, Decrease of heart rate).

Sympathetic vs. Parasympathetic Division

Both divisions have pre-synaptic and post-synaptic neurons, and their neurotransmitter effects depend on the receptor and target cell.

• Sympathetic: Short pre-synaptic, long post-synaptic neurons; increases heart rate, blood pressure, bronchodilation, pupil dilation.

• Parasympathetic: Long pre-synaptic, short post-synaptic neurons; decreases heart rate, promotes SLUDDD functions, bronchoconstriction, pupil constriction.

Homeostasis is the ultimate goal, with each division inhibiting the other to maintain balance.

Summary Table: Sympathetic vs. Parasympathetic Functions

Division	Main Functions	Neuron Structure	Response
Sympathetic	Increase heart rate, blood pressure, bronchodilation, pupil dilation, glucose release	Short pre-synaptic, long post-synaptic	Fight, flight, freeze
Parasympathetic	SLUDDD, decrease heart rate, bronchoconstriction, pupil constriction	Long pre-synaptic, short post-synaptic	Rest and digest

Key Equations and Concepts

• Action Potential: $\Delta V = V_{final} - V_{initial}$

• Homeostasis: Maintenance of internal conditions despite external changes.

Additional info: Academic context was added to clarify the structure and function of the nervous system, cranial and spinal nerves, and the autonomic division. The included images directly reinforce nerve regeneration, cranial nerve locations, spinal nerve classification, and autonomic responses.