Principles of Human Physiology - The Nervous System: Autonomic and Motor Systems

The Nervous System: Autonomic and Motor Systems

Chapter Outline

  • Overview of the Somatic Nervous System
  • Two branches of the Autonomic Nervous System

Learning Outcomes

  • Describe the anatomy of the somatic nervous system and the two branches of the autonomic nervous system.
  • Describe the chemical messengers and receptor types associated with the peripheral nervous system.

11.2 The Somatic Nervous System

Anatomy of the Somatic Nervous System

  • Motor Neurons:

    • Originate in the ventral horn of the spinal cord.
    • Innervate skeletal muscle.
    • Key neurotransmitter: acetylcholine (ACh).
    • Receptors involved are nicotinic cholinergic.

  • Spinal Cord Overview:

    • Contains the ventral horn, which is essential for motor control.
The Neuromuscular Junction
  • Anatomy of the Neuromuscular Junction:
    • Structure includes the axon terminal (terminal bouton) of the motor neuron.
    • All somatic motor neurons release ACh.
    • Presence of synaptic vesicles that contain ACh.
    • The motor end plate is part of skeletal muscle that interacts with motor neurons.
Excitation Mechanism at the Neuromuscular Junction

  • Mechanism of Activation:

    1. Action Potential travels down the motor neuron.
    2. Voltage-gated calcium channels open in response to action potential.
    3. Calcium ions (Ca2+) trigger the release of ACh into the synaptic cleft from synaptic vesicles.

  • Structure of the neuromuscular junction:

    • Terminal Bouton: The end of a motor neuron that releases neurotransmitters.
    • Motor End Plate: Contains nicotinic receptors that respond to ACh.
Activities of the Neuromuscular Junction
  • Activation Process:
    • The activation of somatic motor neurons relies on the summation of EPSPs (excitatory postsynaptic potentials) and IPSPs (inhibitory postsynaptic potentials).
    • An action potential in the motor neuron initiates the release of ACh at the neuromuscular junction, leading to end-plate potential (EPP) which acts as a graded potential.
Communication at the Neuromuscular Junction

  • Presynaptic Cell:

    1. Action potential arrives at the axon terminal.
    2. Voltage-gated calcium channels open.
    3. Calcium influx triggers the release of ACh.

  • Synaptic Cleft:

    1. ACh binds to nicotinic receptors on the motor end plate, opening monovalent cation channels (Na+ and K+).
    2. This leads to a net positive charge movement, resulting in depolarization (EPP).

  • Postsynaptic Cell:

    1. EPP causes an action potential (AP) in the muscle cell.
    2. The action potential spreads through the muscle cell leading to contraction.
    3. Acetylcholinesterase (AChase) degrades ACh in the synaptic cleft, leading to cessation of the signal.
    4. Choline is recycled back to the axon terminal for the synthesis of ACh.
Comparison of Autonomic and Somatic Nervous Systems
  • Table 11.4 Properties of the Autonomic and Somatic Nervous Systems:
    • Property:
    • Somatic System:
      • Origin: Ventral horns of the spinal cord
      • Neurons in pathway: One motor neuron
      • Effector organs: Skeletal muscle
      • Neurotransmitters at neuroeffector junction: Acetylcholine
      • Receptor type at effector organ: Nicotinic cholinergic
      • Effects on effector organ: Excitation
      • Control: Primarily voluntary
    • Autonomic Nervous System - Parasympathetic:
      • Origin: Brainstem or lateral horns of the sacral spinal cord
      • Neurons in pathway: Two (preganglionic and postganglionic)
      • Effector Organs: Cardiac muscle, smooth muscle, glands
      • Neurotransmitters at neuroeffector junction: Acetylcholine
      • Receptor type at effector organ: Muscarinic cholinergic
      • Effects on effector organ: Either excitation or inhibition
      • Control: Primarily involuntary
    • Autonomic Nervous System - Sympathetic:
      • Origin: Lateral horns of the thoracic and lumbar spinal cord
      • Neurons in pathway: Two (preganglionic and postganglionic)
      • Effector Organs: Cardiac muscle, smooth muscle, glands, adipose tissue
      • Neurotransmitters at neuroeffector junction: Norepinephrine
      • Receptor type at effector organ: Adrenergic (all classes)
      • Effects on effector organ: Either excitation or inhibition
      • Control: Primarily involuntary
Conclusion
  • Clear understanding of the events at the neuromuscular junction is essential for grasping how the somatic nervous system operates. The interplay between motor neurons, acetylcholine, and muscle cell action potentials showcases a complex mechanism pivotal to voluntary muscle control.