Why do animals need control and coordination?
Every action, from blinking to running, needs many body parts to work together at the right moment. This teamwork is coordination. In animals two systems do it: the fast nervous system (electric messages in nerves) and the slower endocrine system (hormones in blood).
Special nerve endings called receptors pick up changes around us (stimuli): light in the eyes, sound in the ears, smell in the nose (olfactory receptors), taste on the tongue (gustatory receptors), touch and heat in the skin.
Neuron: structure and how a message travels
A neuron is the working unit of the nervous system. It has three parts:
- Dendrites: short branches that collect the message.
- Cell body: contains the nucleus and processes the message.
- Axon: a long fibre that carries the message away as an electric impulse to the nerve ending.
The message always travels in one direction: dendrite → cell body → axon → nerve ending.
Synapse
Two neurons do not touch. The tiny gap between the end of one and the start of the next is the synapse. At the nerve ending the electric impulse makes the cell release chemicals. These cross the gap and start a fresh impulse in the next neuron. The same happens where a nerve meets a muscle or gland cell.
Reflex action and the reflex arc
A reflex action is a sudden, automatic response to a stimulus, done without thinking. Examples: pulling back from a flame, blinking at bright light, sneezing, coughing, knee jerk.
The path the message takes is the reflex arc: receptor (skin) → sensory neuron → relay neuron in the spinal cord → motor neuron → effector (muscle). The spinal cord makes the decision, so the reply is very fast. The message also goes on to the brain, which becomes aware afterwards.
Why do reflex arcs exist? Thinking in the brain takes time; in danger that delay could cause harm. Reflex arcs evolved in animals with simple nervous systems and are still useful to us.
Human nervous system: central and peripheral
The central nervous system (CNS) is the brain plus the spinal cord. The peripheral nervous system is all the nerves joining the CNS to the body: cranial nerves from the brain and spinal nerves from the spinal cord.
Actions we choose (writing, kicking a ball) are voluntary and controlled by the fore-brain. Actions like heartbeat, breathing and salivation are involuntary and controlled by the mid-brain and hind-brain.
Human brain: fore-brain, mid-brain and hind-brain
- Fore-brain (cerebrum): the largest part and the thinking part. It has areas that receive sensations (sight, hearing, smell, touch), areas that link and store information (memory, learning) and areas that send orders to voluntary muscles. The feeling of hunger also comes from a centre here.
- Mid-brain: connects fore- and hind-brain and handles some involuntary actions such as eye and head reflexes.
- Hind-brain: cerebellum keeps posture and balance and makes movements precise (walking in a straight line, picking up a pencil); medulla controls heartbeat, blood pressure, breathing, salivation and vomiting; pons helps control breathing.
How are the brain and spinal cord protected?
Nervous tissue is soft and delicate. The brain sits inside the bony skull (cranium) and is surrounded by a fluid-filled balloon that acts as a shock absorber. The spinal cord is protected by the vertebral column (backbone), a chain of ring-like bones.
How does nervous tissue cause action?
When a motor nerve impulse reaches a muscle, muscle cells change their shape. Muscle cells contain special proteins that change their arrangement in response to the electric impulse, so the cell becomes shorter. Many cells shortening together pull a bone and create movement. Muscles that we move at will are voluntary muscles; the heart and gut muscles are involuntary.
Board exam tip: the reflex arc diagram/flow, neuron diagram and brain-part functions are asked almost every year (2 to 5 marks).
Key formulas and definitions
- Neuron: dendrite → cell body → axon → nerve ending (one-way flow)
- Synapse: electric impulse → chemical across the gap → new electric impulse
- Reflex arc: receptor → sensory neuron → relay neuron (spinal cord) → motor neuron → effector
- CNS = brain + spinal cord; PNS = cranial nerves + spinal nerves
- Fore-brain: thinking, senses, voluntary actions, hunger
- Mid-brain: involuntary reflexes of eye and head
- Hind-brain: cerebellum (balance), medulla (heartbeat, breathing, vomiting), pons (breathing)
- Protection: skull + fluid cushion (brain); vertebral column (spinal cord)
Worked examples
1. Name the part of a neuron where information is first picked up, and the part along which it travels as an impulse.
Information is picked up by the dendrites. It then travels along the axon as an electric impulse.
2. You step on a sharp pin and lift your foot at once. Trace the path of the message.
Pain receptors in the skin of the foot → sensory neuron → spinal cord, where a relay neuron passes the signal → motor neuron → leg muscles contract and lift the foot. This is a reflex arc; the brain learns about it a moment later.
3. A person walks unsteadily and cannot pick up small objects neatly after a head injury. Which part of the brain is likely hurt?
The cerebellum, part of the hind-brain. It keeps balance and makes voluntary movements precise.
4. Why can a message pass only one way across a synapse?
Only the nerve ending (end of the axon) releases the chemicals; the dendrite side only receives them. So the signal moves from axon of one neuron to dendrite of the next, never backwards.
5. Is thinking about what to eat for lunch a reflex? Explain.
No. It needs memory, choice and decision in the fore-brain and takes time. A reflex is automatic, fast and handled by the spinal cord or lower brain.
Common mistakes
- Saying the brain controls a reflex action. The spinal cord decides; the brain is informed later.
- Writing the neuron path backwards (axon to dendrite inside one neuron). It is dendrite → cell body → axon.
- Thinking neurons touch each other. There is always a gap, the synapse, crossed by chemicals.
- Mixing up cerebellum and cerebrum. Cerebrum (fore-brain) thinks; cerebellum (hind-brain) balances.