Peripheral Nervous System (PNS)

Overview

The Peripheral Nervous System (PNS) consists of all neural structures outside the brain and spinal cord. It connects the central nervous system (CNS) to limbs and organs, facilitating communication between the body and the CNS.

• Components: Sensory receptors, peripheral nerves, ganglia, and motor endings

• Main Functions: Sensory input, integration, and motor output

Sensory Receptors

Definition and Function

Sensory receptors are specialized structures that detect changes in the environment (stimuli) and convert them into nerve impulses. They are classified by the type of stimulus detected, location, and structural complexity.

• Transduction: Conversion of stimulus energy into graded potentials, which may trigger action potentials if threshold is reached.

Classification by Stimulus Detected

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

• Thermoreceptors: Detect changes in temperature

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

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

• Nociceptors: Detect pain, typically from damaging stimuli

Classification by Location

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

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

• Proprioceptors: Detect stretch in muscles, tendons, joints, and ligaments

Classification by Structure

• Free Nerve Endings (Nonencapsulated): Detect pain, temperature, and some pressure

• Encapsulated Nerve Endings: Dendrites enclosed in connective tissue capsules, generally mechanoreceptors

Encapsulated Receptors

• Tactile (Meissner's) Corpuscles: Light touch, located in dermal papillae

• Lamellar (Pacinian) Corpuscles: Deep pressure and vibration, found in dermis and subcutaneous tissue

• Bulbous Corpuscles (Ruffini endings): Deep, continuous pressure, found in dermis, subcutaneous tissue, and joint capsules

• Muscle Spindles: Detect muscle stretch, located in skeletal muscle perimysium

• Tendon Organs: Detect tendon stretch, found in tendons

• Joint Kinesthetic Receptors: Monitor joint position and movement, found in articular capsules

Sensory Integration

Levels of Sensory Integration

Sensory information is processed at three main levels:

• Receptor Level: Sensory receptors transduce stimuli into graded potentials

• Circuit Level: Impulses are relayed to appropriate CNS regions via ascending pathways

• Perception Level: Sensory input is interpreted in the cerebral cortex

Receptor Level

• Transduction: Conversion of stimulus energy to graded potential energy

• Threshold: Action potential generated if threshold is reached

• Frequency Coding: Frequency of nerve impulses encodes stimulus intensity

Circuit Level

• First-order neurons: From receptor to spinal cord/brainstem

• Second-order neurons: To cerebellum or thalamus

• Third-order neurons: From thalamus to somatosensory cortex

Perception Level

• Perceptual Detection: Awareness that a stimulus has occurred

• Magnitude Estimation: Ability to detect intensity

• Spatial Discrimination: Identifying site or pattern of stimulation

• Feature Abstraction: Identifying more complex aspects of sensation

• Quality Discrimination: Differentiating submodalities (e.g., sweet vs. sour)

• Pattern Recognition: Recognizing familiar or significant patterns

Nerves and Nerve Structure

Connective Tissue Coverings

• Endoneurium: Surrounds individual nerve fibers

• Perineurium: Surrounds fascicles (bundles of fibers)

• Epineurium: Surrounds the entire nerve

Types of Nerves

• Sensory (Afferent) Nerves: Carry impulses to CNS

• Motor (Efferent) Nerves: Carry impulses from CNS

• Mixed Nerves: Contain both sensory and motor fibers (most common)

• Anatomical Classes: Cranial nerves and spinal nerves

Regeneration of Nerves

• PNS Axons: Can regenerate if cell body is intact and Schwann cells are functional

• CNS Axons: Generally do not regenerate due to inhibitory factors and lack of supportive environment

Cranial Nerves

Overview

Cranial nerves emerge from the brain and primarily serve the head and neck. There are 12 pairs, each with specific sensory, motor, or mixed functions.

• I – Olfactory: Sensory (smell)

• II – Optic: Sensory (vision)

• III – Oculomotor: Motor (eye movement)

• IV – Trochlear: Motor (eye movement)

• V – Trigeminal: Mixed (facial sensation, mastication)

• VI – Abducens: Motor (eye movement)

• VII – Facial: Mixed (facial expression, taste)

• VIII – Vestibulocochlear: Sensory (hearing, balance)

• IX – Glossopharyngeal: Mixed (taste, swallowing)

• X – Vagus: Mixed (viscera, heart, lungs)

• XI – Accessory: Motor (neck muscles)

• XII – Hypoglossal: Motor (tongue movement)

Spinal Nerves and Plexuses

Spinal Nerves

• 31 pairs: C1–C8 (cervical), T1–T12 (thoracic), L1–L5 (lumbar), S1–S5 (sacral), C0 (coccygeal)

• Ventral roots: Motor (efferent) fibers

• Dorsal roots: Sensory (afferent) fibers

• Plexus: Network of diverging and converging nerve fibers

Plexuses

• Cervical Plexus (C1–C4): Sensory from skin, motor to diaphragm (phrenic nerve)

• Brachial Plexus (C5–T1): Axillary, musculocutaneous, median, ulnar, and radial nerves supply upper limb

• Lumbar Plexus (L1–L4): Femoral and obturator nerves supply anterior and medial thigh

• Sacral Plexus (L4–S4): Pudendal and sciatic nerves supply lower limb and genitals

Motor Integration

Levels of Motor Control

• Segmental Level: Spinal cord circuits generate basic movements and reflexes

• Projection Level: Upper motor neurons in cortex and brainstem send commands to spinal cord

• Precommand Level: Cerebellum and basal nuclei plan and coordinate complex movements

Reflexes and Reflex Arcs

Types of Reflexes

• Intrinsic Reflex: Rapid, predictable, unlearned response to a stimulus

• Acquired Reflex: Learned through practice or repetition

Reflex Arc Components

• Receptor: Site of stimulus

• Sensory Neuron: Transmits afferent impulses to CNS

• Integration Center: Synapse(s) in CNS

• Motor Neuron: Conducts efferent impulses to effector

• Effector: Muscle or gland that responds