The neuron, nerves and the nerve impulse
A neuron is the unit of the nervous system. An impulse runs from dendrite to cell body to axon; chemicals carry it across the synapse to the next cell.
Parts of a neuron
| Part | What it is | Job |
|---|---|---|
| Dendrites | Short, branched projections of the cell body | Receive the stimulus; a chemical reaction at the tip starts an electrical impulse |
| Cell body | Holds a clear nucleus in cytoplasm, with the usual cell organelles | Passes the impulse on to the axon |
| Axon (nerve fibre) | One very long branch of the cell body | Carries the impulse away from the cell body |
| Myelin sheath (fatty sheath) | An insulating fatty cover on most axons, also called the medullary sheath. Your NIOS lesson calls it the neurilemma; strictly, the neurilemma is the thin outer layer around this sheath | Stops impulses in nearby axons from mixing |
| Nodes of Ranvier | Gaps where the fatty sheath is missing | Help produce the electrical activity that carries the impulse along |
| Axon endings (bulbs) | Swollen ends of the axon's branches | Store and release chemicals called neurotransmitters |
How an impulse travels and crosses a synapse
- 1A stimulus reaches the tip of a dendrite. It sets off a chemical reaction that creates an electrical impulse.
- 2The impulse passes through the cell body and along the axon.
- 3★At the axon endings, the impulse makes the bulbs release neurotransmitters into the synaptic cleft, the tiny gap at the synapse.
- 4The chemicals cross the gap and set off a new impulse of the same kind in a dendrite of the next neuron.
- 5In this way the impulse passes along a chain of neurons.
- 6Finally the impulse reaches an effector, a muscle cell or a gland cell, which acts.
★ key step
Three types of neurons
| Type | Carries the impulse | Where it is found |
|---|---|---|
| Sensory neuron | From a receptor (sense organ) to the brain or spinal cord | Between the sense organs and the CNS |
| Motor neuron | From the brain or spinal cord to an effector (muscle or gland) | Between the CNS and the muscles and glands |
| Association (connecting) neuron | Between sensory and motor neurons, linking them | Inside the brain and spinal cord |
Three kinds of nerves
| Kind | Contains | Carries impulses | Example |
|---|---|---|---|
| Sensory nerve | Sensory fibres | From the sense organs to the brain or spinal cord | Optic nerve, from the eye to the brain |
| Motor nerve | Motor fibres | From the brain or spinal cord to muscles or glands | A nerve from the brain to the muscles of the eye |
| Mixed nerve | Both sensory and motor fibres | Both ways | A spinal nerve |
How to draw a neuron
- Draw a rounded, star-shaped cell body with a round nucleus inside.
- Draw short, branching dendrites spreading out from the cell body.
- From one side, draw one long fibre: the axon.
- Shade the myelin sheath (fatty sheath) along the axon in segments, leaving small gaps: the nodes of Ranvier.
- At the far end, draw small branches with swollen axon endings (bulbs).
- Draw an arrow showing the direction of the impulse, from dendrite to axon ending. For a synapse, draw the bulb of one neuron close to a dendrite of the next, with a small gap.
A nerve is built like the underground cable that brings electricity or broadband to a home: many wires, each insulated, inside one cover. The insulation stops the messages in nearby fibres from mixing.
2 marks: “What is a synapse? How does a nerve impulse cross it?” The junction between the axon ending of one neuron and a dendrite of the next; its gap is the synaptic cleft. The impulse makes the axon ending release neurotransmitters, which cross the gap and start a new impulse.
3 marks: “Compare sensory, motor and association neurons: where does each one carry the impulse from, and where to?” Sensory: carries the impulse from a receptor to the brain or spinal cord. Motor: carries it from the brain or spinal cord to an effector, a muscle or gland. Association: lies inside the brain and spinal cord and links sensory and motor neurons.
1 marks: “Name the nerve that carries impulses from the eye to the brain.” The optic nerve, a sensory nerve.
Path of an impulse: Do Cats Always Nap? Dendrite, Cell body, Axon, Nerve ending.