Peripheral Nervous System: Structure, Function, and Reflexes

Peripheral Nervous System (PNS) Structure and Organization

General Structure of Peripheral Nerves

The peripheral nerves are essential components of the PNS, responsible for transmitting signals between the central nervous system (CNS) and the rest of the body. Their structure is organized to protect and support nerve fibers.

• Nerve: The cordlike organ of the PNS, surrounded by the epineurium and composed of bundles called fascicles.

• Fascicle: A bundle of nerve fibers, each surrounded by the perineurium.

• Endoneurium: Each axon within a fascicle is further surrounded by the endoneurium.

• Nerve Classification:

  • Mixed nerves: Contain both sensory and motor fibers, transmitting signals to and from the CNS.

  • Sensory (afferent) nerves: Carry impulses toward the CNS.

  • Motor (efferent) nerves: Carry impulses away from the CNS.

Additional info: The layered structure of nerves provides both protection and efficient signal transmission.

Peripheral Nerve Damage vs. Central Nervous System Damage

Peripheral nerve damage is more likely to be repaired than damage within the CNS due to differences in cellular responses and regenerative capacity.

• Neurons: Generally do not divide; if the cell body is damaged, the neuron cannot be replaced.

• PNS Regeneration: If the soma remains intact, Schwann cells release growth factors and form a regeneration tube to guide axon regrowth.

• CNS Regeneration: CNS fibers do not naturally regenerate due to inhibitory factors and lack of supportive structures.

Wallerian degeneration occurs when axons are damaged, leading to breakdown of the distal axon and myelin sheath, followed by phagocytosis and regeneration in the PNS.

Additional info: Regeneration in the CNS is limited by the presence of inhibitory molecules and lack of growth-promoting cells.

Cranial Nerves: Order and Mnemonics

Order of Cranial Nerves

Cranial nerves are twelve pairs that emerge directly from the brain, each with specific functions. Mnemonics help students remember their order.

• Mnemonic 1: On occasion our trusty truck acts funny – very good vehicle anyhow, or

• Mnemonic 2: On old Olympus' towering top, a Finn and German viewed some hops.

Saying #1	Saying #2	Cranial Nerve
On	On	Olfactory (I)
Occasion	Old	Optic (II)
Our	Olympus'	Oculomotor (III)
Trusty	Towering	Trochlear (IV)
Truck	Top	Trigeminal (V)
Acts	A	Abducens (VI)
Funny	Finn	Facial (VII)
Very	Very	Vestibulocochlear (VIII)
Good	German	Glossopharyngeal (IX)
Vehicle	Viewed	Vagus (X)
Anyhow	Any	Accessory (XI)
Or	Hops	Hypoglossal (XII)

Additional info: Each cranial nerve has a unique function, ranging from sensory (e.g., olfactory) to motor (e.g., oculomotor).

Spinal Nerves and the Spinal Cord

Structure of Spinal Nerves

Spinal nerves connect the spinal cord to the rest of the body, transmitting sensory and motor information.

• Dorsal (posterior) root: Contains sensory (afferent) fibers; cell bodies reside in dorsal root ganglia.

• Ventral (anterior) root: Contains motor (efferent) fibers; cell bodies are located in the spinal cord.

Additional info: The dorsal and ventral roots merge to form a mixed spinal nerve.

Plexuses of the PNS

Major Plexuses and Peripheral Nerves

Plexuses are networks of intersecting nerves that provide motor and sensory innervation to specific regions.

• Nerve Plexus: A network of nerves, primarily serving limbs.

• Major Plexuses: Cervical, brachial, lumbar, sacral.

• Function: Each muscle receives nerve supply from multiple spinal nerves, providing redundancy.

Additional info: Damage to one spinal nerve may not result in complete loss of function due to overlapping innervation.

Reflex Arcs and Reflex Types

Reflex Arc Structure

A reflex arc is the basic functional unit of the nervous system, allowing for rapid, predictable responses to stimuli.

• Receptor: Site of stimulus.

• Sensory neuron: Transmits afferent impulse to the CNS.

• Integration center: Interneuron in the spinal cord.

• Motor neuron: Conducts efferent impulses from the integration center to an effector.

• Effector: Muscle or gland that responds to the efferent impulse.

Types of Reflexes: Ipsilateral vs. Contralateral

Reflexes can be classified based on the side of the body affected relative to the stimulus.

• Ipsilateral reflex arc: Stimulus and motor activity occur on the same side of the body (e.g., knee-jerk/patellar reflex).

• Contralateral reflex arc: Motor activity occurs on the opposite side of the body (e.g., crossed extensor reflex).

Example: In the crossed extensor reflex, stepping on a sharp object causes withdrawal of the affected foot (ipsilateral) and extension of the opposite leg (contralateral) to maintain balance.

Reflex Classification

• Innate (intrinsic) reflexes: Present at birth.

• Learned (acquired) reflexes: Developed through experience.

Additional info: Reflexes are essential for survival, allowing for quick responses to potentially harmful stimuli.