CH 13

Chapter 13: The Peripheral Nervous System and Reflex Activity

I. Sensory Receptors and Sensation

The peripheral nervous system (PNS) is responsible for transmitting sensory information from the body to the central nervous system (CNS). Sensory receptors detect changes in the environment and initiate neural signals.

• Definition: Sensory receptors are specialized neural structures that respond to specific stimuli.

• Classification by Stimulus Type:

  • Mechanoreceptors: Respond to mechanical forces such as touch, pressure, vibration, and stretch.

  • Thermoreceptors: Detect changes in temperature.

  • Photoreceptors: Respond to light energy (e.g., in the retina).

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

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

• Classification 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).

  • Proprioceptors: Respond to internal stimuli in muscles, tendons, joints, and ligaments; provide information about body position and movement.

• Classification by Structure:

  • Simple receptors: General senses; may be free nerve endings or encapsulated dendritic endings.

  • Encapsulated dendritic endings: Surrounded by connective tissue capsules; detect touch, pressure, vibration.

• Levels of Sensory Integration:

  • Receptor level: Sensory receptors transduce stimuli into graded potentials.

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

  • Perceptual level: Processing in cortical sensory areas; interpretation of sensory input.

• Adaptation: Many sensory receptors exhibit adaptation, a decrease in response to a constant stimulus.

• Visceral Pain: Visceral pain may be referred to areas distant from the affected organ due to shared neural pathways.

II. The Cranial Nerves

The cranial nerves are twelve pairs of nerves that emerge from the brain and primarily innervate the head and neck. Each has specific sensory, motor, or mixed functions.

• General Features:

  • Numbered I to XII; most are mixed nerves, some are purely sensory or motor.

  • Transmit sensory input and motor output between the brain and peripheral structures.

• Major Cranial Nerves and Functions:

  Cranial Nerve	Type	Main Function
  Olfactory (I)	Sensory	Smell
  Optic (II)	Sensory	Vision
  Oculomotor (III)	Motor	Eye movement, pupil constriction
  Trochlear (IV)	Motor	Eye movement (superior oblique muscle)
  Trigeminal (V)	Mixed	Sensory to face, motor to mastication muscles
  Abducens (VI)	Motor	Eye movement (lateral rectus muscle)
  Facial (VII)	Mixed	Facial expression, taste (anterior tongue)
  Vestibulocochlear (VIII)	Sensory	Hearing, balance
  Glossopharyngeal (IX)	Mixed	Taste (posterior tongue), swallowing
  Vagus (X)	Mixed	Parasympathetic control of heart, lungs, digestive tract
  Accessory (XI)	Motor	Head and neck movement
  Hypoglossal (XII)	Motor	Tongue movement

• Clinical Relevance: Damage to cranial nerves can result in loss of specific sensory or motor functions (e.g., facial paralysis, loss of taste, impaired vision).

III. The Spinal Nerves

Spinal nerves are mixed nerves that connect the spinal cord to the body, transmitting sensory and motor information. There are 31 pairs, each associated with a specific region of the spinal cord.

• Structure:

  • Each spinal nerve is formed by the union of dorsal (sensory) and ventral (motor) roots.

  • Spinal nerves branch into dorsal and ventral rami, which innervate different body regions.

• Plexuses: Networks of nerves formed by ventral rami; major plexuses include cervical, brachial, lumbar, and sacral.

• Dermatomes: Regions of skin innervated by specific spinal nerves; useful for diagnosing nerve injuries.

• Key Functions:

  • Transmit sensory input from the body to the CNS.

  • Transmit motor output from the CNS to muscles and glands.

IV. Reflex Activity

Reflexes are rapid, involuntary responses to stimuli, mediated by neural pathways called reflex arcs. They help maintain homeostasis and protect the body from harm.

• Reflex Arc Components:

  1. Receptor: Detects stimulus.

  2. Sensory neuron: Transmits afferent impulses to CNS.

  3. Integration center: Processes information (may be monosynaptic or polysynaptic).

  4. Motor neuron: Transmits efferent impulses to effector.

  5. Effector: Muscle or gland that responds.

• Types of Reflexes:

  • Somatic reflexes: Involve skeletal muscles.

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

• Examples:

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

  • Withdrawal (flexor) reflex: Rapidly withdraws a limb from a painful stimulus.

  • Crossed extensor reflex: Maintains balance during withdrawal reflexes.

• Clinical Application: Reflex testing is used to assess the integrity of the nervous system.

Additional info:

• Reflexes can be monosynaptic (single synapse, e.g., stretch reflex) or polysynaptic (multiple synapses, e.g., withdrawal reflex).

• Damage to specific nerves or regions can be diagnosed by loss of reflexes or sensory/motor deficits in corresponding dermatomes.