ch 13. Peripheral Nervous System: Structure, Function, and Special Senses

Peripheral Nervous System (PNS) Overview

Main Functions of the PNS

The Peripheral Nervous System (PNS) includes all neural structures outside the brain and spinal cord. Its primary role is to provide communication between the Central Nervous System (CNS) and the rest of the body.

• Sensory Input: Collects information from sensory receptors and transmits it to the CNS.

• Integration: Processes and interprets sensory input (mainly in the CNS, but some integration occurs in the PNS).

• Motor Output: Sends commands from the CNS to effector organs (muscles and glands).

PNS Organization

Divisions of the PNS

• Sensory (Afferent) Division: Carries information toward the CNS from sensory receptors.

• Motor (Efferent) Division: Carries information away from the CNS to effector organs.

The motor division is further subdivided into:

• Somatic Nervous System: Controls voluntary movements of skeletal muscles.

• Autonomic Nervous System (ANS): Controls involuntary functions (smooth muscle, cardiac muscle, glands).

The ANS is divided into:

• Sympathetic Division: Prepares the body for 'fight or flight' responses.

• Parasympathetic Division: Promotes 'rest and digest' activities.

Sensation vs. Perception

Definitions and Differences

• Sensation: Awareness of changes in the internal or external environment, detected by sensory receptors.

• Perception: The conscious interpretation of sensory input, influenced by memory and previous experiences.

Types of Sensory Receptors

Classification Criteria

Sensory receptors are classified by:

• Stimulus Type

• Location

• Structure

Classification by Stimulus Type

• Mechanoreceptors: Detect touch, pressure, vibration, and stretch.

• Thermoreceptors: Detect changes in temperature.

• Photoreceptors: Respond to light (primarily in the eye).

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

• Nociceptors: Detect pain from potentially damaging stimuli.

Classification by Location

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

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

• Proprioceptors: Detect body position and movement (e.g., in muscles, tendons, joints).

Classification by Structure

• Complex Receptors: Associated with special senses (vision, hearing, equilibrium, taste, smell).

• Simple Receptors: Found in general senses; can be nonencapsulated (free nerve endings) or encapsulated (enclosed in connective tissue).

Receptor Processing and Sensory Pathways

Stimulus Processing

• A stimulus must excite a receptor and be converted into a graded potential through transduction.

• Graded potentials can be:

  • Generator Potentials: In general senses.

  • Receptor Potentials: In special senses.

Sensory Pathways

1. First-order neuron: Sensory neuron that transmits impulses from the receptor to the CNS.

2. Second-order neuron: Transmits impulses within the CNS (spinal cord or brainstem) to the thalamus or cerebellum.

3. Third-order neuron: Transmits impulses from the thalamus to the somatosensory cortex.

Levels of Processing

• Receptor Level: Sensory receptors detect stimuli.

• Circuit Level: Processing in ascending pathways.

• Perceptual Level: Processing in the cerebral cortex, leading to conscious perception.

Sensory Adaptation

Adaptation Types

• Occurs when a stimulus remains constant over time.

• Phasic Receptors: Fast-adapting (e.g., pressure, touch).

• Tonic Receptors: Slow-adapting (e.g., pain, proprioception).

• Nociceptors: Do not adapt; continue to signal pain.

Nerve Structure and Classification

Nerve Anatomy

• A nerve is a bundle of neuron fibers (axons) in the PNS.

• Connective tissue layers:

  • Endoneurium: Surrounds individual axons.

  • Perineurium: Surrounds bundles of axons (fascicles).

  • Epineurium: Surrounds the entire nerve.

Classification of Nerves

• Mixed Nerves: Contain both sensory and motor fibers.

• Sensory Nerves: Carry impulses toward the CNS.

• Motor Nerves: Carry impulses away from the CNS.

Cranial and Spinal Nerves

Cranial Nerves

• 12 pairs, connect directly to the brain.

• Can be sensory, motor, or mixed in function.

Spinal Nerves

• 31 pairs, connect to the spinal cord.

• Regions: Cervical, Thoracic, Lumbar, Sacral, Coccygeal.

• All are mixed nerves (contain both sensory and motor fibers).

Reflexes and Reflex Arcs

Reflexes

• A reflex is a rapid, involuntary, and predictable response to a stimulus.

• Types:

  • Inborn (Intrinsic): Present at birth (e.g., knee-jerk reflex).

  • Learned (Acquired): Developed through experience (e.g., driving skills).

Reflex Arc Components

1. Receptor: Detects the stimulus.

2. Sensory Neuron: Transmits afferent impulses to the CNS.

3. Integration Center: Processes information (may be a single synapse or multiple synapses).

4. Motor Neuron: Conducts efferent impulses from the integration center to an effector.

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

Types of Reflexes

• Somatic Reflexes: Activate skeletal muscles.

• Autonomic Reflexes: Activate smooth muscle, cardiac muscle, or glands.

Special Senses

Overview

• Vision

• Hearing

• Equilibrium (Balance)

• Taste

• Smell

Vision

• Most sensory receptors are located in the eye.

• Photoreceptors:

  • Rods: Sensitive to dim light; important for night vision.

  • Cones: Detect color; important for sharp, detailed vision.

• Visual pathway: Photoreceptor → Bipolar cell → Ganglion cell → Optic nerve → Brain.

Hearing

• Ear divided into external, middle, and inner ear.

• Hearing receptors are located in the cochlea (specifically, the organ of Corti).

Balance (Equilibrium)

• Semicircular Canals: Detect angular (rotational) movement.

• Vestibule: Detects gravity and head position.

Chemical Senses: Taste and Smell

• Smell (Olfaction): Detected by the olfactory epithelium in the nasal cavity.

• Taste (Gustation): Detected by taste buds on the tongue.

• Five basic tastes: Sweet, Sour, Salty, Bitter, Umami.

Summary Table: Sensory Receptor Classification

Classification	Types	Examples
By Stimulus Type	Mechanoreceptors, Thermoreceptors, Photoreceptors, Chemoreceptors, Nociceptors	Touch, Temperature, Light, Chemicals, Pain
By Location	Exteroceptors, Interoceptors, Proprioceptors	Skin, Organs, Muscles/Joints
By Structure	Complex (special senses), Simple (general senses)	Eye, Ear, Free nerve endings

Key Concepts and Big Ideas

• The PNS connects the CNS to the rest of the body.

• Sensory (afferent) pathways carry information toward the CNS; motor (efferent) pathways carry information away from the CNS.

• Receptors detect specific types of stimuli and initiate neural signals.

• Reflexes are automatic, rapid responses to stimuli, essential for protection and homeostasis.

• Nerves are bundles of axons, protected by connective tissue layers.