Motor Neurons, Units, Pools and Twitch
The basic unit of communication in the nervous system is the nerve cell (neuron).
Each nerve cell consists of the CELL BODY or SOMA, which includes the nucleus, a major branching fibre (AXON) and numerous smaller branching fibres DENTRITES.
MOTOR NEURON is nerve responsible for sending the signal to the muscle to move. It makes up ONE part of the MOTOR UNIT.
MOTOR NEURON
A motor unit is comprised of the following 3 items:
- The motor cell body called the SOMA
- The AXON (pathway)
- The muscle fibres it stimulates (junction)
From the beginning to the end:
Electrical impulses are collected from other neurons in the DENTRITES and centralized to follow down the axon to the muscle.
Travelling down the axon, electrical impulse moves very rapidly through the myelinated section (Myelin is an insulating layer, or sheath that forms around nerves) only to slow down and concentrate between each section at the Nodes of Ranvier before moving once again down the axon.
The main functions of the myelin sheath are:
- It acts as an electrical insulator for the neuron - it prevents electrical impulses travelling through the sheath.
- The sheath prevents the movement of ions into or out of the neuron/ it prevents depolarization.
Once the electrical impulse reaches the ends of the axon, the terminal knobs, it causes vesicles filled with the neurotransmitter Acetylcholine (Ach) to move closer to the outer membrane of the knob.
Attaching to the membrane, the vesicle is ready to release the Ach molecules into the synaptic cleft, from the nervous system, into the space across to the muscular system.
MOTOR UNIT
- A motor unit consists of the motor neuron and the grouping of muscle fibres innervated (stimulated) by the neuron.
- Small Motor units can exercise greater precision of movement compared to larger motor units.
- Larger Motor units can be activated to facilitate more forceful actions.
- When a motor unit is activated, all of its fibres contract.
- Groups of motor units are innervated to coordinate contraction of a whole muscle and generate appropriate movement; all of the motor units within a muscle are called a MOTOR POOL.
There are often multiple sizes of motor unit within a motor pool as a means of modulating the precision and force produced by a single muscle
- These multiple motor units of different sizes within a motor pool allow for very fine control of force either spatially or temporally.
