Nervous System Structure & Coordination

The Standard Coordination Pathway:

1. Stimulus: A change in the environment (e.g., a rapidly approaching car).
2. Receptor: Specialized cells that detect the stimulus (e.g., light-sensitive cells in the retina).
3. Sensory neurone: Carries the electrical impulse from the receptor to the CNS.
4. Coordinator (CNS): Receives and processes the information, then sends a new impulse down the next neurone.
5. Motor neurone: Carries the electrical impulse from the CNS to the effector.
6. Effector: A muscle or gland that brings about the response.
7. Response: The specific action taken. Muscles will always contract to move, while glands will always secrete substances (like adrenaline).

Adaptations of Neurones

You can snap a piece of chalk easily, but try snapping a diamond; structure always determines function. Neurones are highly adapted to carry electrical impulses quickly over long distances.

Some neurones have very long axons, which can be over 1 metre in length. This allows impulses to travel long distances efficiently by minimizing the number of slow synapses needed in a single pathway. The axon is also surrounded by a fatty insulation layer called the myelin sheath. This insulation prevents electrical "short circuits" or signal loss, greatly speeding up the transmission of the impulse.

Neurones also possess branched extensions from their cell body called dendrites. These provide a massive surface area to receive impulses from many other neurones, forming complex coordination networks.

You can identify neurone types by their structure:

• A sensory neurone usually has its cell body located on a side branch in the middle of the axon.
• A motor neurone has its cell body at one end of the cell, heavily surrounded by dendrites.
• A relay neurone is found entirely within the CNS and directly connects sensory neurones to motor neurones.

Synaptic Transmission

Why does a sprint relay race always slow down during the baton pass? The exact same delay happens when signals jump between nerves. Neurones never actually touch each other; instead, they are separated by a tiny gap (approximately $10$ to $20\text{ nm}$ wide) called a synapse.

Because electrical impulses cannot "jump" this gap, the signal must be converted into a chemical messenger. When an electrical impulse reaches the end of the first (pre-synaptic) neurone, it triggers the release of chemicals called neurotransmitters from storage vesicles.

These neurotransmitters move across the synaptic gap by diffusion. Once they reach the other side, they bind to specific receptors on the second (post-synaptic) neurone membrane, which triggers a brand new electrical impulse. This diffusion of chemicals is the slowest step in the entire nervous pathway.

Impulses can only travel in one direction across a synapse. This unidirectional flow is guaranteed because neurotransmitter vesicles are only found in the first neurone, and receptor molecules are only located on the second neurone.

Reflex Actions

It is a surprising fact that your body can react to a painful burn before your conscious brain even registers the heat. A reflex action is a rapid, automatic response to a stimulus that does not involve the conscious part of the brain.

Because reflexes bypass the conscious brain, they save valuable time. Their primary biological function is to protect the body from harm and damage.

The nerve pathway of a reflex is called a reflex arc and involves three neurones in a strict sequence: Sensory neurone $\rightarrow$ Relay neurone $\rightarrow$ Motor neurone. In this arc, the relay neurone handles the coordination within the spinal cord (or the unconscious part of the brain) without waiting for conscious thought.