The Nervous System – Neurons and Reflex Action

Introduction: Why Do We Need Control and Coordination?

Imagine you are walking barefoot and suddenly step on a sharp pebble. Before your mind even registers the pain, your foot is already off the ground. Or imagine a sunflower slowly turning its face as the sun moves across the sky. In both cases, a living body is sensing something in its surroundings and responding in a useful way. This sensing-and-responding is what biologists call control and coordination.

Animals do it mainly through a nervous system made of specialised cells called neurons, aided by chemical messengers called hormones. Plants, which have no nerves or muscles, achieve the same result through growth hormones and slow directional bending. In this part we focus on the animal nervous system — the neuron, the reflex arc and the way information travels inside us.

6.1 Animals — Nervous System

Every living body must receive information about its environment and send out the right response. In animals, this job is carried out by the nervous system. The inputs come from specialised cells called receptors present in our sense organs — eyes (photoreceptors), ears (phonoreceptors), tongue (gustatory), nose (olfactory) and skin (thermo- and mechanoreceptors). The outputs are delivered to effectors, which are usually muscles (producing movement) or glands (producing secretions).

6.1.1 Reflex Actions — The Body's Emergency Response

When your finger touches a hot object, you pull it away instantly. You do not stop to think, "this is hot, I should move". The arm has already moved before your brain consciously noticed the heat. Such a sudden, involuntary response to a harmful stimulus is called a reflex action.

Why does the body bypass the brain? Because thinking takes time. In the extra fraction of a second needed to "decide", a burn would get worse. Evolution has wired a short-cut pathway through the spinal cord so the response can be issued immediately. Thinking happens a moment later — that is when you feel the pain.

6.1.2 The Neuron — Structural and Functional Unit

The nervous system is built from a single specialised cell type — the neuron. A typical neuron has three parts:

• Dendrites — many short branching fibres that receive information from other neurons or from receptors.
• Cell body (cyton) — contains the nucleus and most cell organelles. Here the incoming signals are added up.
• Axon — a single long fibre that carries the impulse away from the cell body towards the next neuron or an effector.

The message, in the form of an electrical impulse, enters at the dendrite, travels through the cell body, and leaves through the axon. At the axon's tip the electrical signal is converted into a chemical signal that crosses a tiny gap called the synapse.

How Does a Signal Cross the Synapse?

The electrical impulse cannot jump across an empty gap. Instead, when the impulse reaches the axon terminal, it triggers the release of tiny packets of chemical molecules — neurotransmitters. These molecules diffuse across the synaptic gap and bind to receptor sites on the dendrite of the next neuron, setting up a fresh electrical impulse there.

Types of Neurons

CNS and PNS — Two Halves of One System

For convenience, the nervous system is split into two divisions:

• Central Nervous System (CNS) — the brain and the spinal cord. This is the processing centre where decisions are made and reflexes are switched.
• Peripheral Nervous System (PNS) — all the nerves that branch out of the CNS. These include the cranial nerves (12 pairs from the brain) and the spinal nerves (31 pairs from the spinal cord).

The Reflex Arc — A Ready-made Circuit

A reflex action needs a pathway that bypasses the slow, thinking brain. This built-in short-cut is called the reflex arc.

Nervous Tissue as an Organised Network

Many neurons are bundled together by connective tissue to form a nerve. Nerves are the "cables" of the body. Information in the nervous system is always electrical inside the neuron and chemical at the synapse. This combination gives two advantages: speed (electrical signals) and selectivity (each synapse can strengthen, weaken or block the signal).