Nerve Cells Never Quite Touch, Yet Messages Leap Between Them in an Instant
Learn the parts of a neuron and its three types, what nerves and synapses are and how an impulse crosses the gap, how the nervous system is organised, and how the sympathetic and parasympathetic systems pull the body in opposite directions.
How does your body send messages from one part to another?
Touch a hot tawa and your hand jerks away before you have even thought about it. A message has travelled from your fingertip to your spinal cord and back to your arm muscles in a fraction of a second.
The nervous system carries these messages as electrical impulses along nerve cells, with chemicals passing them from one cell to the next. This part covers neurons, synapses, the organisation of the system and its two autonomic divisions.
The nervous system carries these messages as electrical impulses along nerve cells, with chemicals passing them from one cell to the next. This part covers neurons, synapses, the organisation of the system and its two autonomic divisions.
What is the structure of a neuron, and how do sensory, motor and relay neurons differ?
A neuron has a cell body, short branched dendrites that bring impulses in and a long axon that carries them away; sensory neurons carry impulses to the brain or spinal cord, motor neurons carry them out to muscles and glands, and relay neurons connect the two.
Parts of a neuron:
- Cell body (cyton) — contains the nucleus and most of the cytoplasm
- Dendrites — short branches that receive impulses and carry them towards the cell body
- Axon — a long fibre carrying impulses away from the cell body
- Myelin sheath — a fatty covering around many axons that insulates them and speeds up impulses
- Nodes of Ranvier — gaps in the myelin sheath
- Axon endings — fine branches with swollen synaptic knobs that pass the message on
Types of neuron:
- Sensory neurons — from receptors in sense organs to the brain or spinal cord
- Motor neurons — from the brain or spinal cord to effectors, muscles or glands
- Relay neurons — inside the brain and spinal cord, linking sensory and motor neurons
An everyday example. When a mosquito bites your arm, sensory neurons carry the sting to the spinal cord, relay neurons pass it on, and motor neurons move your other hand to slap it.
The boundary case. Mature neurons usually do not divide, which is why serious damage to the brain or spinal cord is often permanent.
Parts of a neuron:
- Cell body (cyton) — contains the nucleus and most of the cytoplasm
- Dendrites — short branches that receive impulses and carry them towards the cell body
- Axon — a long fibre carrying impulses away from the cell body
- Myelin sheath — a fatty covering around many axons that insulates them and speeds up impulses
- Nodes of Ranvier — gaps in the myelin sheath
- Axon endings — fine branches with swollen synaptic knobs that pass the message on
Types of neuron:
- Sensory neurons — from receptors in sense organs to the brain or spinal cord
- Motor neurons — from the brain or spinal cord to effectors, muscles or glands
- Relay neurons — inside the brain and spinal cord, linking sensory and motor neurons
An everyday example. When a mosquito bites your arm, sensory neurons carry the sting to the spinal cord, relay neurons pass it on, and motor neurons move your other hand to slap it.
The boundary case. Mature neurons usually do not divide, which is why serious damage to the brain or spinal cord is often permanent.
What are nerves and synapses, and how is a nerve impulse transmitted?
A nerve is a bundle of nerve fibres wrapped in connective tissue, and a synapse is the tiny gap between the axon ending of one neuron and the dendrite of the next, across which a chemical carries the impulse.
Types of nerves:
- Sensory nerves — contain only sensory fibres, such as the optic nerve from the eye
- Motor nerves — contain only motor fibres
- Mixed nerves — contain both, as most spinal nerves do
Transmission of an impulse:
- A stimulus at a receptor starts an electrical impulse
- The impulse travels along the dendrite, through the cell body and down the axon
- At the synaptic knob, a chemical messenger called a neurotransmitter is released into the gap
- It diffuses across and starts a new impulse in the next neuron
An everyday example. A thick electric cable in a building holds many separate wires inside one cover, just as a nerve holds many separate fibres.
The substance. Impulses cross a synapse in one direction only, because the neurotransmitter is released only from the axon side.
Types of nerves:
- Sensory nerves — contain only sensory fibres, such as the optic nerve from the eye
- Motor nerves — contain only motor fibres
- Mixed nerves — contain both, as most spinal nerves do
Transmission of an impulse:
- A stimulus at a receptor starts an electrical impulse
- The impulse travels along the dendrite, through the cell body and down the axon
- At the synaptic knob, a chemical messenger called a neurotransmitter is released into the gap
- It diffuses across and starts a new impulse in the next neuron
An everyday example. A thick electric cable in a building holds many separate wires inside one cover, just as a nerve holds many separate fibres.
The substance. Impulses cross a synapse in one direction only, because the neurotransmitter is released only from the axon side.
How is the nervous system organised into central, peripheral and autonomic divisions?
The central nervous system is the brain and spinal cord, the peripheral nervous system is the nerves that connect them to the rest of the body, and the autonomic nervous system is the part of the peripheral system that controls involuntary organs.
1. Central nervous system (CNS)
- Brain and spinal cord
- Receives information, processes it and sends out instructions
2. Peripheral nervous system (PNS)
- Cranial nerves — pairs, from the brain
- Spinal nerves — pairs, from the spinal cord
- Carries impulses between the CNS and all body parts
3. Autonomic nervous system (ANS)
- Controls organs that work without our will — heart, lungs, gut, blood vessels and glands
- Has two divisions: sympathetic and parasympathetic
An everyday example. After lunch you do not have to decide to digest your food; the autonomic system keeps your stomach and intestines working while you study.
The link. Autonomic does not mean independent. Its activity is regulated by centres in the brain such as the medulla and hypothalamus, described in Part 2.
1. Central nervous system (CNS)
- Brain and spinal cord
- Receives information, processes it and sends out instructions
2. Peripheral nervous system (PNS)
- Cranial nerves — pairs, from the brain
- Spinal nerves — pairs, from the spinal cord
- Carries impulses between the CNS and all body parts
3. Autonomic nervous system (ANS)
- Controls organs that work without our will — heart, lungs, gut, blood vessels and glands
- Has two divisions: sympathetic and parasympathetic
An everyday example. After lunch you do not have to decide to digest your food; the autonomic system keeps your stomach and intestines working while you study.
The link. Autonomic does not mean independent. Its activity is regulated by centres in the brain such as the medulla and hypothalamus, described in Part 2.
How do the sympathetic and parasympathetic systems affect the body differently?
The sympathetic system prepares the body for emergencies by speeding up the heart and breathing, while the parasympathetic system calms the body and supports rest and digestion; they usually have opposite effects on the same organ.
Effects compared — sympathetic first, parasympathetic second:
- Heartbeat — speeds up; slows down
- Pupils — dilate; constrict
- Air passages — widen; narrow
- Saliva — reduced; increased
- Digestion — slowed; stimulated
- Urinary bladder — relaxed; contracted
Origin. Sympathetic nerves arise from the chest and lower back regions of the spinal cord; parasympathetic nerves arise from the brain and the lowest part of the spinal cord.
Summary of roles. Sympathetic — fight or flight. Parasympathetic — rest and digest.
An everyday example. Waiting backstage to speak at the school annual day, your heart races and your mouth goes dry — sympathetic action. After a big festive lunch you feel relaxed and sleepy — parasympathetic action.
The substance. Most organs receive both kinds of nerve, and their activity at any moment depends on the balance between the two, not on one switching off completely.
Effects compared — sympathetic first, parasympathetic second:
- Heartbeat — speeds up; slows down
- Pupils — dilate; constrict
- Air passages — widen; narrow
- Saliva — reduced; increased
- Digestion — slowed; stimulated
- Urinary bladder — relaxed; contracted
Origin. Sympathetic nerves arise from the chest and lower back regions of the spinal cord; parasympathetic nerves arise from the brain and the lowest part of the spinal cord.
Summary of roles. Sympathetic — fight or flight. Parasympathetic — rest and digest.
An everyday example. Waiting backstage to speak at the school annual day, your heart races and your mouth goes dry — sympathetic action. After a big festive lunch you feel relaxed and sleepy — parasympathetic action.
The substance. Most organs receive both kinds of nerve, and their activity at any moment depends on the balance between the two, not on one switching off completely.
Exam tip
What earns full marks on neurons, synapses and the autonomic system?
Label diagrams with direction of impulse, define terms precisely, and compare the two autonomic divisions organ by organ.
- Label a neuron: dendrites, cell body, nucleus, axon, myelin sheath, node of Ranvier, axon endings
- Show the direction of the impulse with an arrow
- Define synapse as a gap, not a join, and name the neurotransmitter as the carrier
- Classify nerves as sensory, motor and mixed with one example each
- Give the numbers pairs cranial and pairs spinal nerves
- Compare sympathetic and parasympathetic on the same organs
The trap. Writing that dendrites carry impulses away from the cell body. Dendrites bring impulses in; the axon takes them out.
- Label a neuron: dendrites, cell body, nucleus, axon, myelin sheath, node of Ranvier, axon endings
- Show the direction of the impulse with an arrow
- Define synapse as a gap, not a join, and name the neurotransmitter as the carrier
- Classify nerves as sensory, motor and mixed with one example each
- Give the numbers pairs cranial and pairs spinal nerves
- Compare sympathetic and parasympathetic on the same organs
The trap. Writing that dendrites carry impulses away from the cell body. Dendrites bring impulses in; the axon takes them out.
Did you know
Why does your leg tingle with pins and needles after sitting cross-legged too long?
Sit cross-legged on the floor through a long puja or a family function and your foot may go numb, then prickle painfully as you stand up.
Pressing on a leg squeezes a nerve and the small blood vessels that feed it. Starved of oxygen, the nerve fibres stop sending normal messages, so the foot feels numb.
When the pressure is released, blood returns and the fibres restart unevenly, firing bursts of impulses that the brain reads as tingling. Within a minute or two the signals settle and normal feeling returns.
Pressing on a leg squeezes a nerve and the small blood vessels that feed it. Starved of oxygen, the nerve fibres stop sending normal messages, so the foot feels numb.
When the pressure is released, blood returns and the fibres restart unevenly, firing bursts of impulses that the brain reads as tingling. Within a minute or two the signals settle and normal feeling returns.
Exam relevance
How does the neuron and nerve impulse lead into NEET Biology?
This is foundation work for Class 11 Neural Control and Coordination in NEET Biology.
What gets built on. The chapter explains the resting potential of a neuron membrane, depolarisation as sodium ions rush in, and how the impulse moves along myelinated and non-myelinated fibres. It describes electrical and chemical synapses, the synaptic cleft and neurotransmitters, and divides the peripheral system into somatic and autonomic parts.
Question types. Statement questions on ion movement during an impulse, labelled neuron diagrams.
The trap that costs marks. Assuming every synapse is chemical. Electrical synapses also exist, where the impulse passes directly and faster; chemical synapses use a neurotransmitter across a cleft.
What gets built on. The chapter explains the resting potential of a neuron membrane, depolarisation as sodium ions rush in, and how the impulse moves along myelinated and non-myelinated fibres. It describes electrical and chemical synapses, the synaptic cleft and neurotransmitters, and divides the peripheral system into somatic and autonomic parts.
Question types. Statement questions on ion movement during an impulse, labelled neuron diagrams.
The trap that costs marks. Assuming every synapse is chemical. Electrical synapses also exist, where the impulse passes directly and faster; chemical synapses use a neurotransmitter across a cleft.
Key takeaways
What must you be able to do from this part?
- Neuron: dendrites bring impulses in, cell body holds the nucleus, axon carries impulses out, myelin speeds them up
- Sensory neurons to the CNS, motor neurons to effectors, relay neurons link them
- Nerve: a bundle of fibres; sensory, motor or mixed
- Synapse: a gap crossed by a neurotransmitter, one way only
- CNS: brain and spinal cord; PNS: pairs cranial and pairs spinal nerves
- ANS controls involuntary organs through two divisions
- Sympathetic: faster heart, dilated pupils, less saliva, slower digestion
- Parasympathetic: slower heart, constricted pupils, more saliva, active digestion
Think of the last time you were nervous before an exam, and see if you can name every organ your sympathetic system changed.
- Sensory neurons to the CNS, motor neurons to effectors, relay neurons link them
- Nerve: a bundle of fibres; sensory, motor or mixed
- Synapse: a gap crossed by a neurotransmitter, one way only
- CNS: brain and spinal cord; PNS: pairs cranial and pairs spinal nerves
- ANS controls involuntary organs through two divisions
- Sympathetic: faster heart, dilated pupils, less saliva, slower digestion
- Parasympathetic: slower heart, constricted pupils, more saliva, active digestion
Think of the last time you were nervous before an exam, and see if you can name every organ your sympathetic system changed.