Peripheral Nervous System (PNS)

Overview

The Peripheral Nervous System (PNS) links the central nervous system (CNS) to the external environment and the rest of the body. It consists of neural structures outside the brain and spinal cord, enabling communication between the CNS and limbs, organs, and tissues.

• Links to and from the world outside our body

• Consists of neural structures outside the brain and spinal cord

Four Main Parts of the PNS

• Sensory Receptors: Detect changes (stimuli) in the environment.

• Transmission Lines: Nerves that carry sensory and motor signals.

• Motor Endings: Activate effectors (muscles and glands).

• Reflex Activity: Involuntary, subconscious movements.

Sensory Receptors

Classification of Receptors

Sensory receptors are specialized to respond to environmental changes (stimuli). They can be classified in three ways:

• Type of Stimulus

• Body Location

• Structural Complexity

Types of Stimulus Detected

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

• Thermoreceptors: Detect changes in temperature.

• Photoreceptors: Respond to light (e.g., receptors in the eyes).

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

• Nociceptors: Respond to pain, indicating possible tissue damage.

Body Location of Receptors

• Exteroceptors: Respond to stimuli from outside the body (e.g., skin, special sense organs).

• Interoceptors (Visceroceptors): Respond to stimuli from inside the body (e.g., chemical changes, tissue stretch, temperature changes in internal organs).

• Proprioceptors: Respond to stretch in muscles, tendons, joints, and connective tissues; inform the brain about body movements.

Structural Complexity

• Simple Receptors (General Senses): Modified dendritic endings of sensory neurons; detect tactile sensations, temperature, pain, and muscle sense.

• Nonencapsulated: Free nerve endings in epithelial and connective tissues; respond to temperature, pain, and light touch.

• Encapsulated: Dendritic endings enclosed in connective tissue capsules; mostly mechanoreceptors.

• Receptors for Special Senses: Vision, hearing, equilibrium, smell, and taste; housed in complex sense organs.

General Sensory Receptors: Structure and Function

Nonencapsulated Receptors

Encapsulated Receptors

Sensory Integration and Processing

Levels of Neural Integration

• Receptor Level: Sensory receptors detect stimuli and initiate graded potentials, which may generate action potentials.

• Circuit Level: Impulses are delivered to the appropriate region of the CNS via ascending pathways.

• Perceptual Level: Sensory information is interpreted in the cerebral cortex; perception depends on the location of the stimulus.

Sensation and Perception

• Sensation: Awareness of environmental changes.

• Perception: Conscious interpretation of those changes.

Receptor Adaptation

• Phasic Receptors: Fast-adapting; report changes in stimulus (e.g., pressure, touch, smell).

• Tonic Receptors: Slow-adapting or non-adapting; important for protective functions (e.g., nociceptors, most proprioceptors).

Pain and Its Modulation

Pain Receptors (Nociceptors)

• Warn of possible tissue damage

• Can be inhibited by:

  • Endogenous Opioids: Natural painkillers (e.g., endorphins) that change perception of pain.

  • Substance P: Neurotransmitter that can block excessive pain signals.

• Types of Pain:

  • Visceral: Originates from internal organs.

  • Somatic: Originates from skin, muscles, joints.

  • Referred: Pain perceived at a location other than the site of origin due to shared afferent pathways.

Nerves and Nerve Structure

Organization of Nerves

• Bundle of peripheral axons (fibers) enclosed by connective tissue

• Types:

  • Cranial Nerves: Originate from the brain.

  • Spinal Nerves: Originate from the spinal cord.

• Classification:

  • Sensory (Afferent): Carry impulses toward the CNS.

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

  • Mixed Nerves: Contain both sensory and motor fibers.

  • Somatic: Serve skin, skeletal muscles, joints.

  • Visceral: Serve internal organs.

Connective Tissue Coverings

• Endoneurium: Surrounds individual axons.

• Perineurium: Surrounds bundles of axons (fascicles).

• Epineurium: Surrounds the entire nerve.

Key Equations and Concepts

• Graded Potential: Local change in membrane potential; if threshold is reached, an action potential is generated.

• Action Potential: All-or-none electrical signal propagated along the axon.

Equation for Action Potential Generation:

Summary Table: Sensory Receptor Types

Additional info: These notes expand on the original slides by providing definitions, examples, and context for each receptor type and structural class, as well as the organization and function of nerves in the PNS.