Chapter 13: The Peripheral Nervous System and Reflex Activity – Study Guide

Chapter 13: The Peripheral Nervous System and Reflex Activity

Overview

This chapter focuses on the structure and function of the peripheral nervous system (PNS), including sensory receptors, nerves, ganglia, and reflexes. Understanding these components is essential for grasping how the body detects and responds to stimuli.

Sensory Receptors and Sensation

Classification of Sensory Receptors

• By Stimulus Type:

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

  • Thermoreceptors: Detect temperature changes.

  • Photoreceptors: Respond to light (e.g., retina of the eye).

  • Chemoreceptors: Detect chemicals in solution (e.g., smell, taste, changes in blood chemistry).

  • Nociceptors: Respond to potentially damaging stimuli that result in pain.

• By Location:

  • Exteroceptors: Sensitive to stimuli outside the body (e.g., skin, special senses).

  • Interoceptors (Visceroceptors): Respond to stimuli within the body (e.g., internal organs, blood vessels).

  • Proprioceptors: Respond to stretch in skeletal muscles, tendons, joints, and ligaments; inform the brain of body movements.

• By Structural Complexity:

  • Simple receptors: General senses (touch, pressure, pain, temperature, proprioception).

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

General vs. Special Senses

• General senses: Touch, pressure, pain, temperature, proprioception.

• Special senses: Vision, hearing, equilibrium, smell, taste.

Structure of a Nerve

Connective Tissue Coverings

• Endoneurium: Surrounds individual axons.

• Perineurium: Bundles groups of axons into fascicles.

• Epineurium: Encloses all fascicles to form the nerve.

Classification of Nerves

• 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; most common type.

Ganglia

• Definition: Collections of neuron cell bodies in the PNS.

• Types:

  • Dorsal root ganglia: Sensory, associated with afferent nerve fibers.

  • Autonomic ganglia: Motor, associated with efferent nerve fibers of the autonomic nervous system.

Cranial Nerves

Overview

• There are 12 pairs of cranial nerves, each with specific sensory, motor, or mixed functions.

• They are numbered I–XII and named according to their structure or function (e.g., Olfactory for smell, Optic for vision).

Spinal Nerves and Plexuses

Spinal Nerves

• 31 pairs of mixed nerves arising from the spinal cord.

• Each spinal nerve connects to the spinal cord via dorsal (sensory) and ventral (motor) roots.

Nerve Plexuses

• Cervical plexus: Serves the head, neck, and diaphragm (e.g., phrenic nerve).

• Brachial plexus: Serves the upper limb (e.g., median, ulnar, radial nerves).

• Lumbar plexus: Serves the anterior thigh (e.g., femoral nerve).

• Sacral plexus: Serves the buttock, lower limb, pelvic structures (e.g., sciatic nerve).

Reflex Activity

Reflex Arc Components

• Receptor: Site of stimulus action.

• Sensory neuron: Transmits afferent impulses to the CNS.

• Integration center: May be a single synapse (monosynaptic) or multiple synapses (polysynaptic) in the CNS.

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

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

Types of Reflexes

• Somatic reflexes: Activate skeletal muscle.

• Autonomic (visceral) reflexes: Activate smooth muscle, cardiac muscle, or glands.

General Reflex Arc Diagram

• Label the five components: receptor, sensory neuron, integration center, motor neuron, effector.

Table: Comparison of Sensory Receptor Types

Type	Stimulus Detected	Location	Example
Mechanoreceptor	Mechanical force	Skin, ear, muscles	Touch receptor
Thermoreceptor	Temperature	Skin, hypothalamus	Cold/warm receptors
Photoreceptor	Light	Eye (retina)	Rods and cones
Chemoreceptor	Chemicals	Nose, tongue, blood vessels	Olfactory cells
Nociceptor	Pain	Throughout body	Free nerve endings

Summary

• Be able to classify sensory receptors by stimulus, location, and structure.

• Understand the structure and function of nerves and ganglia.

• Identify the major cranial and spinal nerves and their functions.

• Describe the organization and function of nerve plexuses.

• Explain the components and types of reflex arcs.