How a Signal Races From Your Fingertip to Your Brain
Compare neural and hormonal coordination and map the central, peripheral and visceral nervous system, describe the parts and types of a neuron, follow resting and action potentials along an axon, and compare synapses before touring the forebrain, midbrain and hindbrain.
How does the body coordinate so many actions at once?
Catching a ball needs your eyes, brain and arm muscles to act together within a fraction of a second.
This part covers the organisation of the neural system, neurons, nerve impulses, and synapses and the brain.
This part covers the organisation of the neural system, neurons, nerve impulses, and synapses and the brain.
How does neural coordination differ from endocrine coordination, and how is the human neural system divided?
Neural coordination is fast, short-lived and point-to-point, while endocrine coordination is slower and longer-lasting; the human neural system has a central part — brain and spinal cord — and a peripheral part of nerves with somatic and autonomic divisions, the autonomic being sympathetic and parasympathetic.
Divisions:
- Central neural system (CNS) — brain and spinal cord
- Peripheral neural system (PNS) — all nerves connected with the CNS
PNS fibres:
- Afferent — carry impulses from tissues and organs to the CNS
- Efferent — carry impulses from the CNS to tissues and organs
PNS divisions:
- Somatic — relays impulses from the CNS to skeletal muscles
- Autonomic — relays impulses to involuntary organs and smooth muscles; divided into sympathetic and parasympathetic
- Visceral nervous system — the nerves, fibres, ganglia and plexuses carrying impulses between the CNS and the viscera
An everyday example. Pulling your hand back from a hot tawa is neural control — instant and precise; growing taller over years is hormonal control.
The substance. Sympathetic and parasympathetic divisions usually act in opposition, for example speeding up and slowing down the heart.
Divisions:
- Central neural system (CNS) — brain and spinal cord
- Peripheral neural system (PNS) — all nerves connected with the CNS
PNS fibres:
- Afferent — carry impulses from tissues and organs to the CNS
- Efferent — carry impulses from the CNS to tissues and organs
PNS divisions:
- Somatic — relays impulses from the CNS to skeletal muscles
- Autonomic — relays impulses to involuntary organs and smooth muscles; divided into sympathetic and parasympathetic
- Visceral nervous system — the nerves, fibres, ganglia and plexuses carrying impulses between the CNS and the viscera
An everyday example. Pulling your hand back from a hot tawa is neural control — instant and precise; growing taller over years is hormonal control.
The substance. Sympathetic and parasympathetic divisions usually act in opposition, for example speeding up and slowing down the heart.
What are the parts of a neuron, and how do multipolar, bipolar and unipolar neurons differ?
A neuron has a cell body with Nissl's granules, short branched dendrites that carry impulses towards it, and a long axon that carries impulses away; neurons are multipolar, bipolar or unipolar according to how many processes leave the cell body.
Parts of a neuron:
- Cell body — cytoplasm with typical organelles and Nissl's granules
- Dendrites — short, repeatedly branched fibres that carry impulses towards the cell body
- Axon — a long fibre carrying impulses away from the cell body; its branches end in synaptic knobs filled with synaptic vesicles containing neurotransmitters
Myelinated and non-myelinated axons:
- Myelinated — wrapped by Schwann cells forming a myelin sheath; the gaps between sheaths are nodes of Ranvier; found in spinal and cranial nerves
Types by number of processes:
- Multipolar — one axon and two or more dendrites; in the cerebral cortex
- Bipolar — one axon and one dendrite; in the retina
- Unipolar — cell body with one axon only; usually in the embryonic stage
An everyday example. An electric wire with a plastic cover resembles a myelinated axon, with the myelin sheath acting like insulation.
The substance. Myelin speeds conduction, because the impulse jumps from node to node instead of travelling along every part of the axon.
Parts of a neuron:
- Cell body — cytoplasm with typical organelles and Nissl's granules
- Dendrites — short, repeatedly branched fibres that carry impulses towards the cell body
- Axon — a long fibre carrying impulses away from the cell body; its branches end in synaptic knobs filled with synaptic vesicles containing neurotransmitters
Myelinated and non-myelinated axons:
- Myelinated — wrapped by Schwann cells forming a myelin sheath; the gaps between sheaths are nodes of Ranvier; found in spinal and cranial nerves
Types by number of processes:
- Multipolar — one axon and two or more dendrites; in the cerebral cortex
- Bipolar — one axon and one dendrite; in the retina
- Unipolar — cell body with one axon only; usually in the embryonic stage
An everyday example. An electric wire with a plastic cover resembles a myelinated axon, with the myelin sheath acting like insulation.
The substance. Myelin speeds conduction, because the impulse jumps from node to node instead of travelling along every part of the axon.
How is a nerve impulse generated and conducted using resting potential, the sodium-potassium pump and action potential?
A resting axon stays positive outside and negative inside because of ion gradients kept by the sodium-potassium pump; a stimulus lets sodium rush in, reversing the charge to create an action potential, and this reversal spreads along the axon as the nerve impulse.
Resting state:
- The axon membrane is more permeable to **K, nearly impermeable to Na, and impermeable to negatively charged proteins inside
- The axoplasm has high K and low Na; the fluid outside has low K and high Na
- The sodium-potassium pump moves 3 Na out for every 2 K in
- The outer surface is positive and the inner surface negative — the membrane is polarised, and this difference is the resting potential
Action potential:
- A stimulus makes the membrane at that site freely permeable to Na**
- Na rushes in, so the inside becomes positive and the outside negative — depolarisation
- This potential difference is the action potential, which is the nerve impulse
Conduction:
- Current flows from the depolarised site to the next site along the axon, depolarising it in turn
- Behind the impulse, Na permeability falls and K diffuses out, restoring the resting potential — repolarisation
An everyday example. A crowd doing a wave in a cricket stadium passes a disturbance along the stands without anyone leaving their seat — as an impulse travels while ions cross only locally.
The substance. The pump does not create the impulse — it maintains the ion gradients that make impulses possible.
Resting state:
- The axon membrane is more permeable to **K, nearly impermeable to Na, and impermeable to negatively charged proteins inside
- The axoplasm has high K and low Na; the fluid outside has low K and high Na
- The sodium-potassium pump moves 3 Na out for every 2 K in
- The outer surface is positive and the inner surface negative — the membrane is polarised, and this difference is the resting potential
Action potential:
- A stimulus makes the membrane at that site freely permeable to Na**
- Na rushes in, so the inside becomes positive and the outside negative — depolarisation
- This potential difference is the action potential, which is the nerve impulse
Conduction:
- Current flows from the depolarised site to the next site along the axon, depolarising it in turn
- Behind the impulse, Na permeability falls and K diffuses out, restoring the resting potential — repolarisation
An everyday example. A crowd doing a wave in a cricket stadium passes a disturbance along the stands without anyone leaving their seat — as an impulse travels while ions cross only locally.
The substance. The pump does not create the impulse — it maintains the ion gradients that make impulses possible.
How do electrical and chemical synapses differ, and what do the forebrain, midbrain and hindbrain do?
At electrical synapses current passes directly between closely joined neurons, while at chemical synapses neurotransmitters cross a fluid-filled cleft; the forebrain handles thinking, sensation and hormone control, the midbrain handles visual and auditory reflexes, and the hindbrain controls balance, breathing and heart reflexes.
Synapses:
- Electrical — membranes in very close contact; current flows directly; faster; rare in our system
- Chemical — membranes separated by a synaptic cleft; neurotransmitters such as acetylcholine cross it and bind receptors, producing an excitatory or inhibitory potential
Forebrain:
- Cerebrum — two hemispheres joined by the corpus callosum
- Thalamus — the major coordinating centre for sensory and motor signalling
- Hypothalamus — controls body temperature, eating and drinking, and releases hypothalamic hormones
Midbrain:
- Corpora quadrigemina — four round swellings concerned with vision and hearing
Hindbrain:
- Pons — fibre tracts linking different regions of the brain
- Cerebellum — coordinates posture, balance and precise movements
- Medulla oblongata — controls respiration, cardiovascular reflexes and gastric secretions
An everyday example. Riding a bicycle without wobbling depends on the cerebellum constantly adjusting balance.
The substance. Most human synapses are chemical, which is why drugs that alter neurotransmitters can change mood and movement.
Synapses:
- Electrical — membranes in very close contact; current flows directly; faster; rare in our system
- Chemical — membranes separated by a synaptic cleft; neurotransmitters such as acetylcholine cross it and bind receptors, producing an excitatory or inhibitory potential
Forebrain:
- Cerebrum — two hemispheres joined by the corpus callosum
- Thalamus — the major coordinating centre for sensory and motor signalling
- Hypothalamus — controls body temperature, eating and drinking, and releases hypothalamic hormones
Midbrain:
- Corpora quadrigemina — four round swellings concerned with vision and hearing
Hindbrain:
- Pons — fibre tracts linking different regions of the brain
- Cerebellum — coordinates posture, balance and precise movements
- Medulla oblongata — controls respiration, cardiovascular reflexes and gastric secretions
An everyday example. Riding a bicycle without wobbling depends on the cerebellum constantly adjusting balance.
The substance. Most human synapses are chemical, which is why drugs that alter neurotransmitters can change mood and movement.
Exam tip
What earns full marks on the neural system?
Draw an axon segment showing charges at rest and during depolarisation.
- Divisions: CNS and PNS; afferent to CNS, efferent from CNS; somatic and autonomic; sympathetic and parasympathetic
- Neuron: cell body with Nissl's granules, dendrites, axon, synaptic knobs; myelin and nodes of Ranvier
- Impulse: outside positive at rest; Na influx depolarises; pump moves 3 Na out, 2 K in
- Brain: cerebrum, thalamus, hypothalamus; corpora quadrigemina; pons, cerebellum, medulla
The trap. Saying the inside of a resting axon is positive. At rest the outside is positive; the charges reverse only during an action potential.
- Divisions: CNS and PNS; afferent to CNS, efferent from CNS; somatic and autonomic; sympathetic and parasympathetic
- Neuron: cell body with Nissl's granules, dendrites, axon, synaptic knobs; myelin and nodes of Ranvier
- Impulse: outside positive at rest; Na influx depolarises; pump moves 3 Na out, 2 K in
- Brain: cerebrum, thalamus, hypothalamus; corpora quadrigemina; pons, cerebellum, medulla
The trap. Saying the inside of a resting axon is positive. At rest the outside is positive; the charges reverse only during an action potential.
Did you know
Why does a stubbed toe hurt twice — a sharp jolt and then a dull ache?
Stub your toe and you often notice two feelings: a quick, sharp jolt, and a moment later a deeper, throbbing ache.
The difference lies in the nerve fibres carrying the signals. Some pain fibres are myelinated and conduct impulses fast, so the sharp pain arrives first. Others are thin and non-myelinated, and their slower impulses bring the dull ache a little later.
The difference lies in the nerve fibres carrying the signals. Some pain fibres are myelinated and conduct impulses fast, so the sharp pain arrives first. Others are thin and non-myelinated, and their slower impulses bring the dull ache a little later.
Exam relevance
How are neurons, nerve impulses and the brain tested in NEET?
Neural Control and Coordination is part of the Human Physiology unit of NEET Biology, and it is rich in sequence and function-matching questions.
What gets asked. Afferent versus efferent fibres, types of neurons with their locations, the ionic basis of resting and action potentials, electrical versus chemical synapses, and matching brain parts with functions such as the corpora quadrigemina, hypothalamus and medulla.
Question types. Statement-based questions, match-the-column lists and assertion-reason questions.
The trap that costs marks. Assigning balance to the medulla — balance is the cerebellum; the medulla controls breathing and heart reflexes.
What gets asked. Afferent versus efferent fibres, types of neurons with their locations, the ionic basis of resting and action potentials, electrical versus chemical synapses, and matching brain parts with functions such as the corpora quadrigemina, hypothalamus and medulla.
Question types. Statement-based questions, match-the-column lists and assertion-reason questions.
The trap that costs marks. Assigning balance to the medulla — balance is the cerebellum; the medulla controls breathing and heart reflexes.
Key takeaways
What must you be able to do from this part?
- Organisation: neural versus endocrine coordination; CNS and PNS; afferent and efferent fibres; somatic, autonomic, sympathetic and parasympathetic divisions
- Neuron: cell body, dendrites, axon, synaptic knobs; myelin with nodes of Ranvier; multipolar, bipolar and unipolar types
- Impulse: resting potential outside positive; Na influx gives an action potential; K outflow repolarises; pump moves 3 Na out and 2 K in
- Synapses and brain: electrical and chemical synapses; cerebrum, thalamus, hypothalamus; corpora quadrigemina; pons, cerebellum, medulla
Explain why an impulse crosses a chemical synapse in only one direction, then name the brain part you rely on to walk along a narrow wall.
- Neuron: cell body, dendrites, axon, synaptic knobs; myelin with nodes of Ranvier; multipolar, bipolar and unipolar types
- Impulse: resting potential outside positive; Na influx gives an action potential; K outflow repolarises; pump moves 3 Na out and 2 K in
- Synapses and brain: electrical and chemical synapses; cerebrum, thalamus, hypothalamus; corpora quadrigemina; pons, cerebellum, medulla
Explain why an impulse crosses a chemical synapse in only one direction, then name the brain part you rely on to walk along a narrow wall.