Definition: PNS consists of all neural structures outside the brain and spinal cord.
Components:
Sensory receptors
Peripheral nerves
Associated ganglia
Motor endings
Function: Links the body to the external environment.
Central Nervous System (CNS) vs. Peripheral Nervous System (PNS)
Divisions of PNS:
Sensory division
Motor division (further divided into autonomic and somatic nerve systems)
Autonomic nervous system is subdivided into sympathetic and parasympathetic divisions.
Function: Specialized structures that respond to stimuli.
Process:
Activation leads to depolarizations, triggering impulses sent to the CNS.
Sensation and perception occur in the brain.
Classification:
Based on type of stimulus detected, body location, and structural complexity.
Types of Receptors:
Mechanoreceptors: Touch, pressure, vibration, stretch.
Thermoreceptors: Temperature changes.
Photoreceptors: Light energy (found in the retina).
Chemoreceptors: Chemicals (smell, taste).
Nociceptors: Pain-causing stimuli.
Types by Location:
Exteroceptors: Respond to stimuli outside the body (e.g., touch, pain).
Interoceptors: Respond to internal stimuli (e.g., from organs and blood vessels).
Proprioreceptors: Respond to stretch within the body (e.g., muscles, tendons).
Receptors are categorized into:
Simple receptors: Most common, including encapsulated and unencapsulated varieties.
Complex receptors: Special sense organs (sight, smell, hearing, taste).
Examples:
Free nerve endings: Detect pain and temperature.
Merkel cells: Light touch receptors.
Meissner's corpuscles: Light touch.
Hair follicle receptors: Light touch.
Pacinian corpuscles: Deep pressure and vibration.
Muscle spindles and Golgi tendon organs: Stretch receptors.
Sensation: Awareness of changes in internal and external environments.
Perception: Conscious interpretation of sensory stimuli.
Input from exteroceptors, interoceptors, and proprioceptors.
Neural Integration Levels:
Receptor level: Sensory receptors collect data.
Circuit level: Ascending pathways transmit signals.
Perceptual level: Neuronal circuits in cerebral cortex interpret data.
Criteria for effective receptor activation:
Specificity for stimulus energy.
Stimulus must stimulate the receptor’s receptive field.
Conversion of stimulus energy into a graded potential.
Reaching a threshold in sensory neurons to activate.
Adaptation: Receptors become less responsive to unchanging stimuli.
Frequently declines in response frequency.
Categories of Adaptation:
Quick adaptation: Pressure, touch, smell receptors.
Slow adaptation: Merkel’s discs, Ruffini’s corpuscles, interoceptors.
Pain receptors and proprioceptors show no adaptation.
Neuronal Chain: Three neurons conduct sensory impulses to the brain.
First-order neurons: Impulses from skin to spinal cord/brain stem.
Second-order neurons: From spinal cord or medulla to thalamus/cerebellum.
Third-order neurons: From thalamus to somatosensory cortex.
Thalamus Function: Projects fibers to the somatosensory cortex and sensory areas for processing.
One modality first, followed by integration.
Creates a conscious representation of the stimulus.
Perceptual Detection: Awareness that a stimulus occurred, summation necessary.
Magnitude Estimation: Measures intensity of the stimulus.
Spatial Discrimination: Identifies site or pattern of the stimulus.
Feature Abstraction: Determining texture/shape.
Quality Discrimination: Differentiating submodalities (e.g., sweet versus sour).
Pattern Recognition: Identifying familiar patterns (faces, melodies).
Nerve: A cord-like organ of the PNS with peripheral axons wrapped in connective tissue.
Connective Tissue Coverings:
Endoneurium: Loose connective tissue around axons.
Perineurium: Bundles fibers into fascicles.
Epineurium: Tough sheath around the nerve.
General Types:
Sensory (afferent): Carries impulses to the CNS.
Motor (efferent): Carries impulses from the CNS.
Mixed Nerves: Carry both sensory and motor fibers.
Composition: Mixed nerves carry both somatic and autonomic impulses.
Types include somatic and visceral afferent/efferent.
Origin: Nerves arise from the brain or spinal cord.
Significance: Damage is serious as mature neurons are amitotic.
Repair Possibility: If the soma is intact, regeneration can occur.
Mechanism: Involves Schwann cells (form regeneration tube) and macrophages (remove debris).
CNS Limitations: CNS fibers rarely regenerate due to oligodendrocytes inhibiting growth.
Three Levels:
Segmental level: Lowest hierarchy, spinal cord circuits, controlled by CPGs.
Projection level: Controls reflexes and patterns through cortical and brain stem areas.
Precommand level: Regulates motor activity; involves cerebellum and basal nuclei.
Definition: Quick, predictable response to stimuli.
Types: Can be intrinsic (inborn) or learned (acquired).
Involvement: May involve only peripheral nerves or higher brain centers.
Five Components:
Receptor: Site of stimulus.
Sensory neuron: Transmits signal to CNS.
Integration center: Monosynaptic or polysynaptic CNS region.
Motor neuron: Conducts signal to effector.
Effector: Muscle or gland that responds.
Functionality: Vital for skeletal muscle operation through Golgi tendon organs and muscle spindles.
Importance: Maintains healthy muscle tone.