Detecting change
Animals survive by reacting to changes around them and inside them: the shadow of a predator, the smell of food, a drop in temperature. A change that an animal can detect is a stimulus. Every response follows the same chain: a receptor detects the stimulus, the nervous system acts as the coordinator, and an effector, usually a muscle or a gland, produces the response. Nervous systems are fast: they carry signals in milliseconds, where hormones in the blood take seconds to hours.
Neurons
The nervous system is built from nerve cells, or neurons. A typical neuron has:
- Dendrites, branched extensions that receive signals from receptors or other neurons.
- A cell body with the nucleus, which combines the incoming signals.
- An axon, a long fibre that carries impulses away from the cell body, often wrapped in a fatty myelin sheath.
- Axon terminals, which pass the signal to the next cell at a synapse.
Sensory neurons carry signals from receptors towards the central nervous system;motor neurons carry them out to muscles and glands; and relay neurons(interneurons) connect neurons to each other. Neurons are supported by other cells called glial cells, which include the cells that make myelin.
How an impulse works
At rest, the inside of a neuron is about 70 mV more negative than the outside: the resting potential. Pumps in the membrane keep more sodium ions (Na⁺) outside and more potassium ions (K⁺) inside. A stimulus makes a small change in voltage, a graded potential, which fades as it spreads. If the voltage at the start of the axon reaches the threshold, about −55 mV, channels open and Na⁺ rushes in, making the inside briefly positive (about +30 mV). Then K⁺ flows out and the voltage falls back, briefly dipping below the resting level. This whole event is an action potential, and it lasts only about 2 milliseconds.
Action potentials are all-or-none: either the threshold is reached and a full-sized impulse fires, or nothing is sent. A stronger stimulus does not make bigger impulses; it makes them more frequent. Each impulse triggers the next patch of membrane, so it travels along the axon without fading. Just behind it, the membrane cannot fire again for a moment (the refractory period), so the impulse only travels forwards. Myelin insulates the axon, so the impulse jumps from one gap to the next (the nodes of Ranvier). This raises the speed from about 1 m/s to as much as 100 m/s. For a closer look at the ion channels, seeNervous System and Neuron Firing.
Synapses
Neurons do not touch. Between the axon terminal of one neuron and the next cell is a tiny gap, about 20 nanometres wide: the synapse. When an impulse arrives, small vesicles release a chemicalneurotransmitter into the gap. It diffuses across and binds to receptors on the next neuron, changing its voltage. The neurotransmitter is then broken down or taken back up, so the signal is brief. Synapses make signals travel one way, and they let the nervous system combine, strengthen or block signals; changes at synapses are the basis of learning and memory. Many drugs, toxins and medicines act at synapses, for example by blocking receptors.
Brain and spinal cord
In vertebrates, the brain and spinal cord form the central nervous system (CNS). Nerves (bundles of axons) connect it to the rest of the body and form the peripheral nervous system. The brain processes information from the senses, stores memories and controls behaviour. The spinal cord, protected by the vertebrae, carries signals between the brain and the body, and handles many reflexes on its own.
Reflex arcs
A reflex is a fast, automatic response that does not need conscious thought. The pathway is areflex arc. If you touch something hot, heat receptors in the skin start impulses in a sensory neuron, which passes them to a relay neuron in the spinal cord, which passes them to a motor neuron, which makes the biceps contract and pulls your hand away, in about 50 milliseconds. The signal also travels up to the brain, so you feel the pain, but only afterwards. The knee-jerk reflex is even simpler: the sensory neuron connects straight to the motor neuron, with only one synapse. Reflexes protect the body, and help keep you balanced and upright.
Receptors and senses
Receptors are cells or structures that detect a particular kind of change and turn it into nerve impulses.Photoreceptors detect light; mechanoreceptors detect touch, pressure, vibration, sound and stretch; chemoreceptors detect chemicals (taste and smell); andthermoreceptors detect temperature. An animal can only respond to stimuli its receptors can detect, and only when the stimulus is strong enough. Many animals sense things humans cannot: bees see ultraviolet light, bats and dolphins hear ultrasound, and sharks detect the weak electric fields of their prey.
Simple and complex nervous systems
| Animal | Nervous system | Main senses in the model |
|---|---|---|
| Hydra | Nerve net, no brain | Touch; light-sensitive cells |
| Planarian | Head ganglia and two nerve cords | Eyespots; touch |
| Earthworm | Small brain and a nerve cord with a ganglion in each segment | Light-sensitive skin; touch; vibration |
| Insect | Brain and a ventral nerve cord with ganglia | Compound eyes; air-movement hairs; touch |
| Cat | Brain and spinal cord | Eyes, ears, touch and whiskers, smell |
Sea anemones, jellyfish and hydra have a nerve net with no centre, which suits animals that can meet food or danger from any side. Animals that move head-first have their senses and a cluster of neurons at the front, a trend called cephalisation. In flatworms this is a pair of ganglia; in earthworms and insects, a brain connected to a nerve cord with a ganglion in each segment, which can run some behaviours locally and very fast. In vertebrates a large brain integrates many senses and allows learning and flexible behaviour. More complex is not “better”: each system suits the animal’s way of life.
Common misconceptions
- “Nerve impulses are electricity flowing along a wire.” An impulse is a wave of changing voltage caused by ions crossing the membrane. It is regenerated at each point and travels far more slowly than current in a wire.
- “A stronger stimulus makes a bigger impulse.” Impulses are all-or-none; strength is coded by how often they fire.
- “The brain controls every reflex.” Many reflexes are handled by the spinal cord; the brain is informed afterwards.
- “Animals without a brain cannot respond.” A hydra’s nerve net detects and responds to touch and light.
- “Neurons touch each other.” Most are separated by a synaptic gap that chemicals cross.
What the model simplifies
The moving glow shows where the membrane voltage is changing. In a real neuron nothing travels along the inside of the axon: ions cross the membrane at each point in turn. Everything is slowed down about a thousand times, and the speeds, sizes and number of ions are not to scale. The voltage is recorded at one point, the stimulus acts on one dendrite, and the refractory period and graded potentials are simplified. Real neurons receive thousands of synapses, some exciting and some inhibiting. The reflex timings are typical approximate values for a person. The animal drawings are schematic, the thresholds are illustrative, and real animals’ responses depend on their state, experience and surroundings.
Key takeaways
- Responses follow the chain stimulus → receptor → coordinator → effector → response.
- Neurons receive signals with dendrites, combine them in the cell body and send impulses along the axon.
- An impulse fires only if the threshold is reached, and is always the same size (all-or-none); myelin makes it faster.
- At a synapse, neurotransmitter carries the signal across a gap to receptors on the next neuron.
- Reflex arcs through the spinal cord give fast, automatic responses; the brain is informed afterwards.
- Nervous systems range from a hydra’s nerve net to the centralised brain and spinal cord of vertebrates.