Structure and Function of Motor Units in Skeletal Muscle
Definition
Motor unit
A motor unit comprises a single motor neuron and all the skeletal muscle fibers it innervates.
- The motor neuron sends electrical signals (action potentials) that simultaneously activate all its muscle fibers.
- Neuromuscular junctions (NMJs) are specialized synapses where the neurotransmitter acetylcholine (ACh)initiates muscle fiber contraction.
Components of a Motor Unit
- Motor Neuron
- Cell body in the spinal cord or brainstem, axon extends to the target muscle.
- Transmits the nerve impulses needed for muscle contraction.
- Muscle Fibers
- Cylindrical cells that respond to stimulation by contracting.
- A single motor unit may contain few or hundreds of fibers, depending on precision vs. strength tasks.
- Neuromuscular Junction (NMJ)
- Synaptic Terminal: End of the motor neuron’s axon storing ACh in vesicles.
- Synaptic Cleft: Gap between neuron and muscle fiber.
- Motor End Plate: Region of muscle me
How Motor Units Work
- Nerve Impulse Transmission
- The action potential travels along the motor neuron to the synaptic terminal.
- Neurotransmitter Release
- ACh is released into the synaptic cleft upon arrival of the action potential.
- Muscle Fiber Activation
