part two of the nervous system system system:

Overview of the Autonomic Nervous System (ANS) and Muscle Fiber Innervation

  • Introduction to the topic:
    • Background materials were discussed in a previous class (02/10).
    • Importance of reviewing background slides to understand terms and processes.

Autonomic Nervous System

  • Components of the ANS:
    • The ANS is part of the peripheral nervous system and controls involuntary functions.
    • Two primary divisions of the ANS:
    • Sympathetic Nervous System (SNS)
    • Parasympathetic Nervous System (PNS)

Sympathetic Nervous System (SNS)

  • Known as the 'fight or flight' response.
  • Key points:
    • Originates from the thoracic and upper lumbar regions of the spinal cord (thoracolumbar division).
    • Uses a two-neuron chain:
    • Preganglionic neurons release acetylcholine.
    • Ganglionic neurons primarily release norepinephrine.
    • Predominates in states of arousal, excitement, or physical exercise.

Parasympathetic Nervous System (PNS)

  • Known as the 'rest and digest' response.
  • Key points:
    • Originates from the brain stem and sacral spinal cord (cranial sacral division).
    • Uses a similar two-neuron chain:
    • Preganglionic neurons and ganglionic neurons both release acetylcholine.
    • Predominates when the body is at rest and engaged in digestion.

Neuromuscular Junctions and Motor Units

Muscle Fibers and Motor Neurons

  • Clarification of muscle fiber innervation:
    • Each muscle fiber is innervated by one alpha motor neuron (False statement was made regarding multiple innervation).
    • An alpha motor neuron can innervate multiple muscle fibers.

Motor Units

  • Definition: A motor unit consists of an alpha motor neuron and all muscle fibers it innervates.
  • Recruitment of motor units:
    • Motor units are usually recruited from smallest to largest during voluntary movements.
    • The size principle indicates that smaller motor units (Type I fibers) are engaged before larger units (Type II fibers).
    • Exceptions exist; fast ballistic movements recruit larger motor units immediately.

Brain Function in Voluntary Movement

Brain Regions Involved

  • Major brain regions concerning voluntary movement:
    • Cerebral Cortex: Responsible for conscious thought and control of voluntary movement (specifically the primary motor cortex).
    • Cerebellum: Involved in refining and initiating movements; crucial for fine-tuning and motor timing.
    • Brain Stem: Connects forebrain to spinal cord; regulates autonomic responses and posture.

Primary Motor Cortex

  • Defined as the area of the brain located in the frontal lobe that coordinates voluntary movements.
  • Finalizes movement plans and sends signals to alpha motor neurons for muscle activation.

Reflexes and Propriocepion

Proprioceptors and Reflex Actions

  • Proprioceptors are sensory receptors that provide feedback on body position and movement:

    • Muscle Spindles: Detect changes in muscle length. - Located within muscle fibers (intrafusal fibers).

    • Activate reflexive contraction of the muscle in response to stretching (e.g., knee-jerk reflex).

    • Golgi Tendon Organs: Located in tendons. - Monitor tension in muscles to prevent injury from excessive force. - Induce reflexive relaxation when tension is too high.

Reflex Pathways

  • Examples of the knee-jerk reflex illustrate how proprioceptors act in reflex pathways:
    • Muscle spindle detects stretching, sends signals to the spinal cord, synapses with alpha motor neurons, and contracts the muscle.
    • Golgi tendon organs inhibit muscle contraction when excessive tension is detected via an inhibitory interneuron pathway.

Central Governor Theory

  • Definition: A theory suggesting that the brain regulates fatigue to prevent catastrophic disruption of homeostasis during exercise.
  • Explains how perceived fatigue may stem from signals originating in the brain rather than purely from muscular or systemic stressors.

Afferent Information During Exercise

Types of Sensory Feedback

  • Proprioceptors: Provide feedback on muscle and joint mechanics, critical for maintaining proper movement.
  • Chemoreceptors: Respond to changes in chemical concentrations (e.g., hydrogen ions, carbon dioxide) to inform the state of the muscle environment.

Impact of Exercise on Brain Function

Exercise and Cognitive Function

  • Recent studies suggest that physical activity enhances learning and memory through:
    • Release of brain-derived neurotrophic factor (BDNF) and other synaptic growth factors.
    • Improved brain blood flow, potentially aiding cognitive functions and mood regulation.
  • Habitual physical activity can protect against cognitive decline in aging and reduce risk factors for neurodegenerative diseases like Alzheimer's.

Conclusion

  • Emphasis on understanding the autonomic nervous system, muscle innervation, proprioception, and how brain structures contribute to voluntary movement.
  • Importance of ongoing physical activity for healthy cognitive aging and preventing cognitive decline.