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.