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Body movement and the brain

Making a Voluntary Movement

Help Link : Animation: Synaptic vesicle fusion and neurotransmitter release at the neuromuscular junction.

In order to contract, voluntary muscles must be stimulated by the somatic nervous system. In this respect, they differ from cardiac muscles and smooth muscles, which can contract on their own. The nerves of the autonomic nervous system that innervate these muscles serve to modulate the strength and frequency of their contractions, rather than to trigger them.

Skeletal muscles are composed of muscle fibres that measure from .01 to .10 millimetres in diameter and up to several centimetres in length. Muscle fibres are terminated at their tips by collagen filaments that form tendons and attach the muscles to the bones.


When the brain decides to move part of the body and gives the command to the motor neurons to execute this movement, it is the muscles at the end of the chain of command that ultimately contract to move the body part concerned.

To transmit this command, the axons of these motor neurons, emerging from the spinal cord, form a nerve that extends to the muscles. Where the tip of each axon comes into proximity with a muscle fibre, it forms a synapse with that fibre. This special form of synapse between a motor neuron axon and a muscle fibre is called a neuromuscular junction.

The arrival of a nerve impulse at the neuromuscular junction causes thousands of tiny vesicles (pouches) filled with a neurotransmitter called acetylcholine to be released from the axon tip into the synapse.

On the opposite side of the synapse, this acetylcholine then binds to the surface of the muscle fibre at special sites where there are large numbers of acetylcholine receptors.

Just like in a synapse between two neurons, when this neurotransmitter binds to a receptor, it triggers a new nerve impulse on the muscle fibre membrane. Because of the special way that muscle fibres are structured, this nerve impulse propagates rapidly throughout the fibre and makes it contract.



Axons Play Unexpected Role in Processing Information

There are many molecules that are produced by plants but can bind to certain receptors in the human body, simply because these molecules happens to be similar in structure to neurotransmitters that are produced by the body and bind to these same receptors naturally. One such molecule is nicotine, which is produced by the tobacco plant and can bind to the body's nicotinic acetylcholine receptors, thereby producing the same effect as its own acetylcholine. (It is believed that nicotine's function in the tobacco plant is to protect it from certain insects.)

The effect of nicotine on these receptors explains why smokers develop a dependency on cigarettes. Nicotine molecules are so small that they can make their way across the blood-brain barrier and bind to the nicotinic receptors in the brain. When people smoke regularly, they are regularly exposing their receptors to so much nicotine that these receptors eventually become desensitized. When smokers quit smoking, it is this desensitization that causes them to feel a craving for nicotine.


Acetylcholine is a small molecule that acts as a chemical messenger to propagate nerve impulses across the neuromuscular junction between a nerve and a muscle.


When the nerve impulse from a motor neuron arrives at the tip of its axon, acetylcholine molecules stored there in vesicles are released into the synaptic gap. Some of these molecules then bind to nicotinic receptors: large proteins embedded in the membrane of the muscle fibre.

The reason that these receptors are called nicotinic is that nicotine can bind to them just like acetylcholine (see sidebar). Nicotinic receptors were the first kind to be studied in detail, because they are present in high concentrations in the electrical organ that sting rays use to paralyze their prey.

These first studies, which date back to the early 1970s, revealed the special structure of the nicotinic receptor. It is simultaneously the site to which the acetylcholine binds and the channel that is opened by this binding to let sodium enter the muscle fibre. And it is this sodium that regenerates the nerve impulse in the muscle fibre and makes it contract.

Research has shown that each nicotinic receptor actually consists of five subunits: two alphas, one beta, one delta and one gamma. Each of these subunits consists of proteins that are manufactured by various genes and then assembled to form a barrel-shaped structure with a channel down the middle. Research has also shown that one molecule of acetylcholine must bind to each of the two alpha subunits for this central channel to open.

Nicotinic receptors at neuromuscular junctions are not the only place in the nervous system where acetylcholine plays a role. It is also active at many other locations, where it can bind to another, very different type of receptor.

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