BIO 225 ch13 part 3 notes

Chapter 13: Peripheral Nervous System

Overview of Peripheral Nervous System (PNS)

  • The PNS consists of all neural structures outside of the brain and spinal cord.
      - Includes nerves and receptors.
      - Focus on motor innervation of skeletal muscles.

Structure of Chapter 13

  1. Receptors: Understanding how receptors work.

  2. Transmission Lines: Overview of sensory and motor nerves.

  3. Specific Nerves: Examination of nerves derived from spinal nerves.

  4. Motor End Plates: Innervation of skeletal muscle.

Focus on Motor Innervation

  • Somatic Component: Involvement of somatic motor neurons to innervate skeletal muscles at the neuromuscular junction.
      - Chapter 14 will focus on the autonomic component of the PNS.

Neuromuscular Junction

  • Definition: The area where somatic motor neurons meet skeletal muscle fibers.
      - Axon terminals filled with synaptic vesicles containing acetylcholine (ACh).

  • Nerve Impulse:
      - Action potential travels down the somatic motor neuron, causing a change in voltage.
      - Voltage gated calcium channels open, allowing calcium ions to enter the axon terminal.
      - Calcium facilitates the binding of synaptic vesicles to the neuron's plasma membrane, resulting in the release of ACh into the synaptic cleft.

  • Receptor Interaction:
      - ACh binds to chemically gated receptors on the muscle cell.
      - This initiates the opening of chemically gated sodium channels, leading to:
        - Sodium influx into muscle cells
        - Muscle cell depolarization due to increased positive charge.

Visceral Muscle Innervation

  • Definition of Visceral Muscle: Includes smooth muscle and cardiac muscle; innervated by the autonomic nervous system (ANS).

  • Key Differences between Somatic and Autonomic Innervation:
      - Varicosities, rather than neuromuscular junctions, are responsible for neurotransmitter release in the ANS.
      - NT used in ANS includes acetylcholine and catecholamines (epinephrine, norepinephrine).

Levels of Motor Control

  • Three Levels of Motor Control:
      1. Segmental Level (specific segment of spinal cord).
      2. Projection Level (descending pathways from the brain).
      3. Pre Command Level (higher brain centers that modulate motor control).

Segmental Level

  • Focuses on lower motor neurons in ventral horn of spinal cord for innervating specific muscles.
      - Central Pattern Generators (CPGs): Networks in the spinal cord responsible for rhythmic movements (e.g., walking, cycling).

Projection Level

  • Includes upper motor neurons (pyramidal cells) from the primary motor cortex (precentral gyrus) projecting down to spinal cord.
      - Pyramidal Tracts: Bundles of axons descending through CNS to synapse with lower motor neurons.
      - Data flow: Upper motor neurons (precentral gyrus) → Lower motor neurons (ventral horn of spinal cord).

Pre Command Level

  • Involves structures such as the cerebellum and basal nuclei, important for the fine-tuning of motor movements and coordination.

Reflex Activity

  • Definition: Most reflexes occur in the spinal cord without brain involvement, though the brain is made aware of them.

  • Types of Reflexes:
      - Inborn Reflexes: Reflexes that are innate (e.g., knee jerk).
      - Learned Reflexes: Reflexes that require learning (e.g., stopping at a red light).

Reflex Arc Components

  1. Sensory Receptor: Detects external stimulus.

  2. Sensory Neuron: Relays information to the CNS.

  3. Integration Center: Usually an interneuron within the spinal cord that processes the stimulus and decides the response.

  4. Motor Neuron: Transmits command from CNS to effector.

  5. Effector: Muscle or gland responding to the motor neuron command.

Types of Reflexes Based on Effectors

  • Somatic Reflexes: Involves skeletal muscle.

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

Specific Types of Somatic Reflexes

  • Includes stretch reflexes, flexor reflexes, and superficial reflexes, all aiding in muscle function and response behaviors.

Proprioceptors and Muscle Spindles

  • Muscle Spindle: A sensory receptor located in skeletal muscle detecting changes in muscle length and rate of stretch.
      - Extrafusal Muscle Fibers: Regular skeletal muscle cells, responsible for contraction.
      - Intrafusal Muscle Fibers: Modified fibers within connective tissue sheaths of muscle spindles.
        - Anulospiral Endings: Detect rate and degree of stretch.
        - Flower Spray Endings: Detect only degree of stretch.

  • The interaction of sensory and motor fibers plays a critical role in muscle response to stimuli.

Alpha-Gamma Coactivation

  • Definition: A neural mechanism ensuring that both extrafusal and intrafusal muscle fibers contract simultaneously, preserving muscle spindle tension and responsiveness, even when extrafusal fibers are activated by alpha motor neurons.

  • Importance: Prevents muscle spindles from becoming lax and ceasing to send action potentials, thus maintaining constant feedback to the CNS regarding muscle stretch.

Conclusion

  • The materials discussed are integral to understanding how muscle movements and reflexes are coordinated and managed by the nervous system.

  • Future detailed study will enrich understanding of complex autonomic reflexes in Chapter 14.