The Peripheral Nervous System (PNS) and Reflex Activity

Overview and Chapter Structure

The Peripheral Nervous System (PNS) gathers input from sensory receptors and sends motor output to effectors, linking the external and internal environments to the Central Nervous System (CNS). This chapter explores sensory receptors, nerve structure and repair, motor endings, and reflex activity.

13.1 Sensory Receptors

Classification of Sensory Receptors

Sensory receptors are specialized to respond to environmental changes (stimuli). Their activation results in graded potentials that trigger nerve impulses, leading to sensation and perception in the brain. Receptors are classified by:

• Stimulus Type: Mechanoreceptors (touch, pressure, vibration, stretch), Thermoreceptors (temperature), Photoreceptors (light), Chemoreceptors (chemicals), Nociceptors (pain-causing stimuli).

• Location: Exteroceptors (external stimuli), Interoceptors (internal stimuli), Proprioceptors (muscle and joint stretch).

• Structure: Simple (general senses) or complex (special senses).

Structural Classification of Sensory Receptors

General sensory receptors are divided into nonencapsulated (free nerve endings) and encapsulated dendritic endings. These structures determine their function and location in the body.

Nonencapsulated (Free) Nerve Endings

• Abundant in epithelia and connective tissues.

• Respond to temperature, pain, and light touch.

• Include tactile (Merkel) discs and hair follicle receptors.

Encapsulated Dendritic Endings

• Mechanoreceptors encased in connective tissue capsules.

• Include tactile (Meissner's) corpuscles, lamellar (Pacinian) corpuscles, bulbous (Ruffini) endings, muscle spindles, tendon organs, and joint kinesthetic receptors.

Table: General Sensory Receptors Classified by Structure and Function

13.2 Sensory Processing

Levels of Neural Integration in Sensory Systems

The somatosensory system processes sensory input at three levels:

• Receptor Level: Sensory receptors generate signals when stimuli match their specificity and are within their receptive field. Transduction converts stimulus energy into graded potentials (generator or receptor potentials).

• Circuit Level: Sensory impulses are conducted by three-neuron pathways to the appropriate cortical regions. First-order neurons relay from receptors to the CNS, second-order neurons transmit to the thalamus, and third-order neurons reach the somatosensory cortex.

• Perceptual Level: Interpretation of sensory input occurs in the sensory cortex, including perceptual detection, magnitude estimation, spatial discrimination, feature abstraction, quality discrimination, and pattern recognition.

Adaptation of Sensory Receptors

Adaptation is a change in sensitivity in the presence of a constant stimulus. Fast-adapting receptors (e.g., pressure, touch, smell) send signals at the beginning or end of a stimulus, while slow-adapting receptors (e.g., nociceptors, most proprioceptors) adapt slowly or not at all.

Perception of Pain

Pain Mechanisms and Tolerance

Pain warns of actual or impending tissue damage. Stimuli include extreme pressure, temperature, and chemicals. Pain impulses travel on fibers releasing neurotransmitters like glutamate and substance P. Endogenous opioids (e.g., endorphins) can block some pain impulses. Pain tolerance varies among individuals and is influenced by genetics.

Visceral and Referred Pain

Visceral pain arises from internal organ receptors and is felt as vague aching or burning. Referred pain occurs when pain from one region is perceived as coming from another, due to shared nerve pathways (e.g., left arm pain during a heart attack).

13.3 Nerves

Structure of a Nerve

A nerve is a cordlike organ in the PNS, consisting of bundles of myelinated and nonmyelinated axons enclosed by connective tissue. The coverings include:

• Endoneurium: Encloses individual axons and their myelin sheaths.

• Perineurium: Bundles fibers into fascicles.

• Epineurium: Surrounds all fascicles to form the nerve.

Classification of Nerves

• Mixed nerves: Contain both sensory and motor fibers; impulses travel to and from CNS.

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

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

Most nerves are mixed, containing somatic and autonomic fibers.

Ganglia

Ganglia are clusters of neuron cell bodies associated with nerves in the PNS. Sensory ganglia (dorsal root ganglia) contain cell bodies of sensory neurons, while autonomic ganglia contain motor neurons.

Regeneration of Nerve Fibers

CNS vs. PNS Regeneration

Mature neurons are amitotic. Peripheral axons may regenerate if the soma is intact, but CNS axons rarely regenerate due to inhibitory proteins and scar tissue formation by astrocytes.

PNS Axon Regeneration Process

1. Axon fragments and myelin sheaths distal to injury degenerate (Wallerian degeneration).

2. Macrophages clean up debris; Schwann cells divide.

3. Axon filaments grow through a regeneration tube formed by Schwann cells.

4. Axon regenerates and new myelin sheath forms.

Summary Table: Steps of PNS Nerve Regeneration

Key Learning Outcomes

• Classify general sensory receptors by stimulus detected, body location, and structure.

• Outline the events that lead to sensation and perception.

• Describe receptor and generator potentials and sensory adaptation.

• Describe the general structure of a nerve and the process of nerve regeneration.

• Define ganglion and indicate the general body location of ganglia.