Peripheral Nervous System and Reflex Activity: Chapter 13 Study Guide

Peripheral Nervous System Overview

Composition of the Peripheral Nervous System (PNS)

The Peripheral Nervous System (PNS) consists of all neural structures outside the brain and spinal cord. It connects the central nervous system (CNS) to limbs and organs.

• Nerves: Bundles of axons (nerve fibers) outside the CNS.

• Ganglia: Clusters of neuron cell bodies located outside the CNS.

• Receptors: Specialized structures that detect changes in the environment.

Sensory Receptors

Classification of Sensory Receptors

Sensory receptors are classified based on three main criteria:

• By Stimulus Type:

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

  • Thermoreceptors: Detect temperature changes.

  • Photoreceptors: Respond to light (found in the retina).

  • Chemoreceptors: Detect chemicals (taste, smell, blood chemistry).

  • Nociceptors: Respond to pain-causing stimuli.

• By Location:

  • Exteroceptors: Sensitive to stimuli outside the body (touch, pressure, pain, temperature).

  • Interoceptors (Visceroceptors): Respond to stimuli within the body (internal organs).

  • Proprioceptors: Detect stretch or position in muscles, tendons, joints.

• By Structural Complexity:

  • Simple receptors: Modified dendritic endings of sensory neurons (general senses).

  • Complex receptors: Sense organs (vision, hearing, equilibrium, smell, taste).

Nerve Structure

Connective Tissue Layers of a Nerve

Nerves are organized into bundles and protected by connective tissue layers:

• Endoneurium: Surrounds individual axons.

• Perineurium: Encloses bundles of axons (fascicles).

• Epineurium: Encloses the entire nerve.

Additional info: A labeled diagram is helpful to visualize these layers.

Nerve Classification

• Direction of Transmission:

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

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

  • Mixed nerves: Contain both sensory and motor fibers.

• Origin:

  • Cranial nerves: Originate from the brain.

  • Spinal nerves: Originate from the spinal cord.

Nerve Regeneration

Regeneration in the PNS vs. CNS

Damaged axons in the PNS can regenerate if the cell body is intact, while regeneration in the CNS is limited due to inhibitory factors and lack of supportive cells.

• PNS: Schwann cells help guide axon regrowth.

• CNS: Oligodendrocytes do not support regeneration; scar tissue inhibits regrowth.

Sensory Processing

Levels of Sensory Integration

Sensory information is processed at three levels:

• Receptor Level: Sensory receptors detect stimuli.

• Circuit Level: Processing in ascending pathways to the CNS.

• Perceptual Level: Interpretation in the cerebral cortex.

Pain Perception

Visceral Pain and Referred Pain

Visceral pain arises from internal organs and is often poorly localized. Referred pain occurs when pain from one area is perceived as coming from another area, due to shared nerve pathways.

• Example: Pain from a heart attack may be felt in the left arm.

Spinal Nerves and Plexuses

Spinal Nerve Structure

Spinal nerves are formed by the union of dorsal (sensory) and ventral (motor) roots. They branch into dorsal and ventral rami.

• Dorsal root: Contains sensory fibers.

• Ventral root: Contains motor fibers.

• Rami: Branches that serve different body regions.

Nerve Plexuses

Nerve plexuses are networks of intersecting nerves. Major plexuses include:

• Cervical plexus: Serves the neck and diaphragm.

• Brachial plexus: Serves the upper limb.

• Lumbar plexus: Serves the anterior thigh.

• Sacral plexus: Serves the posterior thigh, leg, and foot.

Brachial Plexus Nerves

• Axillary nerve

• Musculocutaneous nerve

• Radial nerve

• Median nerve

• Ulnar nerve

Each nerve has specific motor and sensory functions in the upper limb.

Lumbar and Sacral Plexus Nerves

• Femoral nerve: Innervates anterior thigh muscles.

• Obturator nerve: Innervates medial thigh muscles.

• Gluteal nerves: Innervate gluteal muscles.

• Pudendal nerve: Innervates perineum.

• Sciatic nerve: Largest nerve, innervates posterior thigh, leg, and foot.

Dermatomes

Definition and Clinical Importance

A dermatome is an area of skin supplied by sensory fibers from a single spinal nerve. Dermatomes are important for diagnosing nerve injuries.

Reflexes

Reflex Arc Components

A reflex arc is the neural pathway involved in a reflex action. It consists of:

• Receptor

• Sensory neuron

• Integration center

• Motor neuron

• Effector

Types of Reflexes

• Stretch reflex: Maintains muscle tone and posture (e.g., knee jerk).

• Tendon reflex: Prevents muscle damage from excessive tension.

• Flexor (withdrawal) reflex: Protects from injury by withdrawing from painful stimuli.

• Plantar reflex: Tests integrity of spinal cord and brain (Babinski sign).

• Abdominal reflex: Tests function of spinal nerves in the abdominal region.

Reflex Testing

Reflex tests are used to assess the integrity of the nervous system. Abnormal responses may indicate neurological disorders.

Summary Table: Major Nerves and Their Functions

Nerve	Plexus	Main Function
Axillary	Brachial	Deltoid and teres minor muscles; shoulder sensation
Musculocutaneous	Brachial	Anterior arm muscles; lateral forearm sensation
Radial	Brachial	Posterior arm and forearm muscles; posterior limb sensation
Median	Brachial	Anterior forearm muscles; lateral palm sensation
Ulnar	Brachial	Hand muscles; medial hand sensation
Femoral	Lumbar	Anterior thigh muscles; anterior thigh sensation
Obturator	Lumbar	Medial thigh muscles; medial thigh sensation
Sciatic	Sacral	Posterior thigh, leg, and foot muscles; lower limb sensation