How a Signal Races From Your Fingertip to Your Brain in a Split Second
Classify neurons and follow how a nerve impulse is generated and conducted, learn the organisation of the human nervous system and the parts of the brain and spinal cord, and see how impulses travel along myelinated fibres and across synapses.
How does your body react so quickly when you touch something hot?
Touch a hot tawa by mistake and your hand jerks away before you have even felt the pain. That speed comes from neurons — cells that carry electrical signals — linked into a nervous system that senses, decides and acts in fractions of a second.
This lesson covers neurons and the nerve impulse, the organisation of the nervous system with the brain and spinal cord, and how impulses travel along fibres and across synapses.
This lesson covers neurons and the nerve impulse, the organisation of the nervous system with the brain and spinal cord, and how impulses travel along fibres and across synapses.
What are the types of neurons, and how is a nerve impulse generated and conducted?
Neurons are classified by structure as multipolar, bipolar or unipolar, and a nerve impulse is a wave of depolarisation that begins when sodium ions rush into a resting neuron and travels along the axon as each patch of membrane excites the next.
Structure of a neuron. A cell body with the nucleus and Nissl's granules, short branched dendrites that carry impulses towards the cell body, and a long axon that carries impulses away to its synaptic knobs.
Types of neurons:
- Multipolar — one axon and two or more dendrites; in the cerebral cortex
- Bipolar — one axon and one dendrite; in the retina of the eye
- Unipolar — a cell body with one axon only; in the embryonic stage
Resting potential:
- At rest, the outside of the membrane is positive and the inside negative — the membrane is polarised
- The sodium-potassium pump moves 3 sodium ions out for every 2 potassium ions in, keeping sodium high outside and potassium high inside
Action potential:
- A stimulus makes the membrane at that point freely permeable to sodium ions, which rush in
- The inside becomes positive and the outside negative — depolarisation
- This reversed patch triggers the next one, so the impulse moves along the axon
- Potassium ions then move out, restoring the resting state — repolarisation
An everyday example. A numbing injection at the dentist blocks sodium channels in nearby nerves, so pain impulses cannot be generated.
The substance. An impulse is not current flowing down the axon like electricity in a wire — it is a chain of ion movements regenerated at every point of the membrane.
Structure of a neuron. A cell body with the nucleus and Nissl's granules, short branched dendrites that carry impulses towards the cell body, and a long axon that carries impulses away to its synaptic knobs.
Types of neurons:
- Multipolar — one axon and two or more dendrites; in the cerebral cortex
- Bipolar — one axon and one dendrite; in the retina of the eye
- Unipolar — a cell body with one axon only; in the embryonic stage
Resting potential:
- At rest, the outside of the membrane is positive and the inside negative — the membrane is polarised
- The sodium-potassium pump moves 3 sodium ions out for every 2 potassium ions in, keeping sodium high outside and potassium high inside
Action potential:
- A stimulus makes the membrane at that point freely permeable to sodium ions, which rush in
- The inside becomes positive and the outside negative — depolarisation
- This reversed patch triggers the next one, so the impulse moves along the axon
- Potassium ions then move out, restoring the resting state — repolarisation
An everyday example. A numbing injection at the dentist blocks sodium channels in nearby nerves, so pain impulses cannot be generated.
The substance. An impulse is not current flowing down the axon like electricity in a wire — it is a chain of ion movements regenerated at every point of the membrane.
How is the human nervous system organised, and what do the brain and spinal cord do?
The human nervous system has a central nervous system of brain and spinal cord that processes information and a peripheral nervous system of nerves that carries signals to and from it, and the brain is divided into forebrain, midbrain and hindbrain.
Organisation:
- Central nervous system (CNS) — the brain and spinal cord
- Peripheral nervous system (PNS) — all other nerves, with afferent fibres carrying impulses to the CNS and efferent fibres carrying them away
- The PNS has a somatic division, serving skeletal muscles, and an autonomic division, serving involuntary organs, split into sympathetic and parasympathetic parts
Protection. The brain lies inside the cranium, covered by three meninges — dura mater, arachnoid and pia mater.
Forebrain:
- Cerebrum — two hemispheres joined by the corpus callosum; the grey cerebral cortex holds motor, sensory and association areas for memory, thinking and speech
- Thalamus — a major coordinating centre for sensory and motor signals
- Hypothalamus — controls body temperature, hunger and thirst, and releases hormones
- Limbic system — with the hypothalamus, regulates emotions and motivation
Midbrain. Contains the corpora quadrigemina, involved in visual and auditory reflexes.
Hindbrain:
- Cerebellum — coordinates movement and balance
- Pons — links different regions of the brain
- Medulla oblongata — controls breathing, heartbeat and gastric secretions
Spinal cord. Runs from the medulla inside the vertebral column, carries impulses to and from the brain, and is the centre for many reflex actions.
An everyday example. A cyclist staying balanced while weaving through busy traffic relies on the cerebellum to coordinate every small movement.
The substance. A reflex does not wait for the brain — the knee-jerk reflex is completed through the spinal cord before the brain even registers the tap.
Organisation:
- Central nervous system (CNS) — the brain and spinal cord
- Peripheral nervous system (PNS) — all other nerves, with afferent fibres carrying impulses to the CNS and efferent fibres carrying them away
- The PNS has a somatic division, serving skeletal muscles, and an autonomic division, serving involuntary organs, split into sympathetic and parasympathetic parts
Protection. The brain lies inside the cranium, covered by three meninges — dura mater, arachnoid and pia mater.
Forebrain:
- Cerebrum — two hemispheres joined by the corpus callosum; the grey cerebral cortex holds motor, sensory and association areas for memory, thinking and speech
- Thalamus — a major coordinating centre for sensory and motor signals
- Hypothalamus — controls body temperature, hunger and thirst, and releases hormones
- Limbic system — with the hypothalamus, regulates emotions and motivation
Midbrain. Contains the corpora quadrigemina, involved in visual and auditory reflexes.
Hindbrain:
- Cerebellum — coordinates movement and balance
- Pons — links different regions of the brain
- Medulla oblongata — controls breathing, heartbeat and gastric secretions
Spinal cord. Runs from the medulla inside the vertebral column, carries impulses to and from the brain, and is the centre for many reflex actions.
An everyday example. A cyclist staying balanced while weaving through busy traffic relies on the cerebellum to coordinate every small movement.
The substance. A reflex does not wait for the brain — the knee-jerk reflex is completed through the spinal cord before the brain even registers the tap.
How do nerve impulses travel along myelinated and non-myelinated fibres and across a synapse?
In non-myelinated fibres the impulse moves continuously along the membrane, in myelinated fibres it jumps from one node of Ranvier to the next, which is much faster, and at a synapse it is passed on electrically or, more often, by chemical neurotransmitters.
Non-myelinated fibres:
- The axon is enclosed by a Schwann cell that does not form a myelin sheath
- Depolarisation spreads to each neighbouring patch in turn, so conduction is slower
- Found in the autonomic and somatic neural systems
Myelinated fibres:
- Schwann cells wrap a fatty myelin sheath around the axon, leaving gaps called nodes of Ranvier
- Myelin insulates the axon, so ion exchange happens only at the nodes
- The impulse jumps from node to node — saltatory conduction — which is faster and uses less energy
- Found in spinal and cranial nerves
Electrical synapse. The two membranes are very close, and current passes directly through channels, so transmission is very fast; such synapses are rare in humans.
Chemical synapse:
- The membranes are separated by a fluid-filled synaptic cleft
- An arriving impulse makes synaptic vesicles release a neurotransmitter, such as acetylcholine
- The neurotransmitter crosses the cleft and binds receptors on the post-synaptic membrane, opening ion channels
- The result can be a new impulse (excitatory) or a reduced chance of one (inhibitory)
An everyday example. Cobra venom can block acetylcholine receptors where nerves meet muscles, which is why a bite can paralyse the breathing muscles.
The substance. Transmission at a chemical synapse is one-way — the neurotransmitter is released only from the pre-synaptic knob, so impulses cannot travel backwards.
Non-myelinated fibres:
- The axon is enclosed by a Schwann cell that does not form a myelin sheath
- Depolarisation spreads to each neighbouring patch in turn, so conduction is slower
- Found in the autonomic and somatic neural systems
Myelinated fibres:
- Schwann cells wrap a fatty myelin sheath around the axon, leaving gaps called nodes of Ranvier
- Myelin insulates the axon, so ion exchange happens only at the nodes
- The impulse jumps from node to node — saltatory conduction — which is faster and uses less energy
- Found in spinal and cranial nerves
Electrical synapse. The two membranes are very close, and current passes directly through channels, so transmission is very fast; such synapses are rare in humans.
Chemical synapse:
- The membranes are separated by a fluid-filled synaptic cleft
- An arriving impulse makes synaptic vesicles release a neurotransmitter, such as acetylcholine
- The neurotransmitter crosses the cleft and binds receptors on the post-synaptic membrane, opening ion channels
- The result can be a new impulse (excitatory) or a reduced chance of one (inhibitory)
An everyday example. Cobra venom can block acetylcholine receptors where nerves meet muscles, which is why a bite can paralyse the breathing muscles.
The substance. Transmission at a chemical synapse is one-way — the neurotransmitter is released only from the pre-synaptic knob, so impulses cannot travel backwards.
Exam tip
What earns full marks on neural control and coordination?
Draw the neuron and the synapse with every part labelled — dendrites, cell body, axon, myelin sheath, node of Ranvier, synaptic knob, vesicles, cleft and receptors.
- Neurons: multipolar (cortex), bipolar (retina), unipolar (embryo)
- Resting: outside positive, inside negative; action potential: sodium ions rush in
- Brain: forebrain, midbrain and hindbrain, each with named parts
- Myelinated fibres: saltatory conduction between nodes of Ranvier
The trap. Writing that the cerebellum controls breathing. Breathing and heartbeat are controlled by the medulla oblongata; the cerebellum controls balance and coordination.
- Neurons: multipolar (cortex), bipolar (retina), unipolar (embryo)
- Resting: outside positive, inside negative; action potential: sodium ions rush in
- Brain: forebrain, midbrain and hindbrain, each with named parts
- Myelinated fibres: saltatory conduction between nodes of Ranvier
The trap. Writing that the cerebellum controls breathing. Breathing and heartbeat are controlled by the medulla oblongata; the cerebellum controls balance and coordination.
Did you know
Why does hitting your funny bone feel so strange?
The so-called funny bone is not a bone at all. It is the ulnar nerve, which passes close to the skin behind the elbow with very little padding over it.
A knock presses directly on the nerve, triggering a burst of impulses in many fibres at once. The brain reads these signals as tingling and pain running down the forearm to the little finger — the area the nerve normally supplies.
The odd feeling is the brain doing its job: it assumes the signals came from the skin the nerve serves, not from the elbow where the nerve was struck.
A knock presses directly on the nerve, triggering a burst of impulses in many fibres at once. The brain reads these signals as tingling and pain running down the forearm to the little finger — the area the nerve normally supplies.
The odd feeling is the brain doing its job: it assumes the signals came from the skin the nerve serves, not from the elbow where the nerve was struck.
Exam relevance
How does NEET test neurons, the brain and synaptic transmission?
Neural Control and Coordination is a recurring NEET chapter, and many of its questions test mechanisms step by step.
What gets asked. Ion movements during resting and action potentials, types of neurons and where they occur, functions of brain regions such as the hypothalamus, cerebellum and medulla, and the sequence of events at a chemical synapse.
Question types. Mostly statement-based and match-the-column questions, with some assertion-reason questions on saltatory conduction.
Why it matters later. The hypothalamus returns in Chemical Coordination and Integration, and the nerve-muscle junction links to Locomotion and Movement.
The trap that costs marks. Reversing the charges of a resting neuron — at rest the inside is negative and the outside positive, and this flips during depolarisation.
What gets asked. Ion movements during resting and action potentials, types of neurons and where they occur, functions of brain regions such as the hypothalamus, cerebellum and medulla, and the sequence of events at a chemical synapse.
Question types. Mostly statement-based and match-the-column questions, with some assertion-reason questions on saltatory conduction.
Why it matters later. The hypothalamus returns in Chemical Coordination and Integration, and the nerve-muscle junction links to Locomotion and Movement.
The trap that costs marks. Reversing the charges of a resting neuron — at rest the inside is negative and the outside positive, and this flips during depolarisation.
Key takeaways
What must you be able to do from this lesson?
- Neurons and impulses: multipolar, bipolar and unipolar neurons; a resting potential kept by the sodium-potassium pump; depolarisation as sodium ions rush in
- Nervous system: CNS and PNS; forebrain, midbrain and hindbrain with their functions; the spinal cord and reflexes
- Conduction: continuous in non-myelinated fibres, saltatory in myelinated fibres, and chemical or electrical transmission at synapses
Why can an impulse cross a chemical synapse in only one direction?
- Nervous system: CNS and PNS; forebrain, midbrain and hindbrain with their functions; the spinal cord and reflexes
- Conduction: continuous in non-myelinated fibres, saltatory in myelinated fibres, and chemical or electrical transmission at synapses
Why can an impulse cross a chemical synapse in only one direction?