Ch. 13 Peripheral Nervous System(1)

Chapter 13: Peripheral Nervous System

Overview of the Peripheral Nervous System (PNS)

  • 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.

Nervous System Flowchart

  • 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.

Sensory Receptors

  • 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.

Receptor Classification by Stimulus Type

  • 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.

Receptor Classification by Location

  • 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).

Structural Classification of Receptors

  • Receptors are categorized into:

    • Simple receptors: Most common, including encapsulated and unencapsulated varieties.

    • Complex receptors: Special sense organs (sight, smell, hearing, taste).

Types of Simple Receptors

  • 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.

From Sensation to Perception

  • Sensation: Awareness of changes in internal and external environments.

  • Perception: Conscious interpretation of sensory stimuli.

Organization of the Somatosensory System

  • 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.

Processing at the Receptor Level

  • 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 of Sensory Receptors

  • 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.

Processing at the Circuit Level

  • 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.

Processing at the Perceptual Level

  • 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.

Main Aspects of Sensory Perception

  • 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).

Structure of a Nerve

  • 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.

Classification of Nerves

  • General Types:

    • Sensory (afferent): Carries impulses to the CNS.

    • Motor (efferent): Carries impulses from the CNS.

    • Mixed Nerves: Carry both sensory and motor fibers.

Peripheral Nerves

  • 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.

Regeneration of Nerve Fibers

  • 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.

Levels of Motor Control

  • 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.

Reflexes

  • Definition: Quick, predictable response to stimuli.

  • Types: Can be intrinsic (inborn) or learned (acquired).

  • Involvement: May involve only peripheral nerves or higher brain centers.

Reflex Arc Components

  • 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.

Stretch and Deep Tendon Reflexes

  • Functionality: Vital for skeletal muscle operation through Golgi tendon organs and muscle spindles.

  • Importance: Maintains healthy muscle tone.