Principles of Human Physiology: The Nervous System
Principles of Human Physiology: The Nervous System
Chapter Outline
- Overview of the Somatic Nervous System.
- Focus on voluntary movements and functions.
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 from the ventral horn of the spinal cord.
- Function: Innervate skeletal muscles.
- Neurotransmitter: Acetylcholine (ACh).
- Receptor Type: Nicotinic cholinergic receptors.
The Neuromuscular Junction
Anatomy of the Neuromuscular Junction
- Key Structures:
- Axon Terminal (Terminal Bouton): All somatic motor neurons release ACh.
- Synaptic Vesicle: Contains neurotransmitter ACh.
- Motor End Plate: Specialized region of the skeletal muscle cell membrane for ACh reception.
- Skeletal Muscle Cell Membrane: Site of action potential generation.
Activities of the Neuromuscular Junction
- Activation of Somatic Motor Neuron: Depends on summation of excitatory postsynaptic potentials (EPSPs) and inhibitory postsynaptic potentials (IPSPs).
- Action Potential Triggering: In the motor neuron, release of ACh occurs at the neuromuscular junction, leading to the end-plate potential (EPP), a graded potential.
Communication at the Neuromuscular Junction
Presynaptic Cell Events
- Action Potential:
- Propagates down the axon terminal.
- Voltage-Gated Calcium Channels Open:
- Allow Ca$^{2+}$ influx into the axon terminal.
- Calcium Triggers ACh Release:
- Release of ACh from the synaptic vesicles.
Synaptic Cleft Dynamics
- ACh Binds to Nicotinic Receptors:
- Found on the motor end plate leading to the opening of monovalent cation channels for Na$^{+}$ and K$^{+}$ ions, which results in a high electrochemical gradient.
- Net Charge Movement:
- Positive charge influx causes depolarization, leading to the end-plate potential (EPP).
Postsynaptic Cell Response
- Action Potential Generation in Muscle Cell:
- EPP causes action potential (AP) in the muscle cell.
- AP spreads through the muscle cell membrane causing muscle contraction.
- Role of Acetylcholinesterase (AChase):
- Breaks down ACh in the synaptic cleft after its action is completed.
- Choline Recycling:
- Choline, a byproduct, is taken back into the presynaptic terminal for reuse.
Comparative Properties of the Autonomic and Somatic Nervous Systems
Important Properties
| Property | Autonomic: Parasympathetic | Autonomic: Sympathetic | Somatic |
|---|
| Origin | Brainstem or lateral horns of the sacral spinal cord | Lateral horns of thoracic and lumbar spinal cord | Ventral horns of the spinal cord |
| Neurons in Pathway | Two (preganglionic and postganglionic) | Two (preganglionic and postganglionic) | One (motor neuron) |
| Effector Organs | Cardiac muscle, smooth muscle, glands | Cardiac muscle, smooth muscle, glands, adipose tissue | Skeletal muscle |
| Neurotransmitters at Neuroeffector Junction | Acetylcholine | Norepinephrine | Acetylcholine |
| Receptor Type at Effector Organ | Muscarinic cholinergic | Adrenergic (all classes) | Nicotinic cholinergic |
| Effects on Effector Organ | Either excitation or inhibition | Either excitation or inhibition | Excitation |
| Control | Primarily involuntary | Primarily involuntary | Primarily voluntary |
Summary of Key Concepts
- The somatic nervous system consists primarily of motor neurons influencing voluntary skeletal muscle movements through the neurotransmitter acetylcholine via nicotinic receptors.
- The neuromuscular junction plays a crucial role in the communication between motor neurons and muscle fibers, featuring key structures such as the axon terminal, synaptic vesicles, and the motor end plate.
- The physiological pathways and mechanisms of the autonomic and somatic nervous systems differ significantly in terms of control, number of neurons involved, and the nature of the responses in target organs.
