Nervous Coordination

Lessons

Senior 6 · Biology

Nervous Coordination

5 lessons

Lesson 1 of 5

Control and coordination in mammals: the nervous system and neurones

25 min

A goalkeeper's half-second

A penalty kick takes less than half a second to reach the goal. In that time a goalkeeper must see the ball leave the striker's foot, judge its direction, and send signals to dozens of muscles in the right order. No hormone could do this; it needs the speed and precision of the nervous system. In Senior 3 you met the parts of the nervous system and the reflex arc. This unit explains, at A-level, what a nerve impulse actually is, how it travels, how it crosses from one neurone to the next, and why some responses are faster than others. This first lesson sets out the organisation of the system and the cells that make it.

Definition

Neurone (neuron): a nerve cell, specialised to carry electrical impulses.

Nerve: a bundle of the axons of many neurones, wrapped in connective tissue. A nerve is not a single cell.

Central nervous system (CNS): the brain and spinal cord. Peripheral nervous system (PNS): all the nerves and neurones outside the CNS.

Myelin sheath: a fatty insulating layer around many axons, formed by Schwann cells wrapped many times round the axon.

Node of Ranvier: a short gap between neighbouring Schwann cells where the axon membrane is not covered by myelin.

How the nervous system is organised

The autonomic nervous system controls the internal organs (heart, gut, glands, the smooth muscle of blood vessels) without conscious thought. Its two divisions usually have opposite effects on the same organ, so each organ can be speeded up or slowed down.

Feature	Sympathetic division	Parasympathetic division
General role	Prepares the body for activity and emergencies ("fight or flight")	Restores and conserves energy at rest ("rest and digest")
Heart rate	Increases	Decreases
Pupils	Dilate (widen)	Constrict (narrow)
Bronchioles	Dilate	Constrict
Gut movement and digestive secretions	Decrease	Increase
Transmitter released onto most effectors	Noradrenaline	Acetylcholine
Where its neurones leave the CNS	Thoracic and lumbar regions of the spinal cord	The brain (for example the vagus nerve) and the sacral spinal cord

Main regions of the brain

Region	Main functions
Cerebrum (two cerebral hemispheres with an outer cortex of grey matter)	Conscious thought, memory, language, interpreting sensory information, starting voluntary movements
Cerebellum	Coordinates the timing and force of muscle contractions; balance, posture and learned skills such as riding a bicycle
Medulla oblongata	Contains the centres that control breathing rate, heart rate and blood pressure; reflexes such as swallowing, coughing and sneezing
Hypothalamus	Controls body temperature, water balance, hunger and thirst; controls the pituitary gland, linking the nervous and endocrine systems
Spinal cord	Carries impulses between the brain and the body; coordinates spinal reflexes

Structure of a neurone

The features of a neurone suit its job of carrying impulses quickly over long distances:

Many dendrites receive impulses from other neurones; the cell body contains the nucleus, many ribosomes and mitochondria, and makes the proteins and neurotransmitters.
The axon is long (more than 1 m in a motor neurone running from the spinal cord to the foot) and carries impulses away from the cell body.
The myelin sheath insulates the axon so impulses jump from node to node, which is much faster (lesson 3).
Synaptic knobs at the axon terminals contain many mitochondria and vesicles of neurotransmitter, used to pass the signal across a synapse (lesson 4).

	Sensory neurone	Relay neurone	Motor neurone
Carries impulses	From a receptor to the CNS	Between sensory and motor neurones, within the CNS	From the CNS to an effector (muscle or gland)
Position of the cell body	Just outside the spinal cord, in the dorsal root ganglion, part-way along the fibre	In the CNS	In the CNS, at one end of the axon
Shape	A long dendron brings impulses to the cell body and a shorter axon takes them on into the CNS	Short, with many branches	Many dendrites, one long axon
Myelin	Usually myelinated	Usually unmyelinated	Usually myelinated

A myelinated axon cut across, seen with a transmission electron microscope. The pale axon in the centre contains a mitochondrion; it is wrapped in many dark, tightly packed layers of myelin, which are the membranes of a Schwann cell wound round and round. The scale bar is 500 nm. Photo: Roadnottaken, CC BY-SA 3.0, via Wikimedia Commons

Worked example 1: measuring from an electron micrograph

On a printed copy of the micrograph above, the 500 nm scale bar is 24 mm long, and the axon inside the myelin is 40 mm across at its widest. Calculate (a) the magnification and (b) the actual width of the axon in micrometres.

(a) Convert the bar to the same unit: $24\,\mathrm{mm} = 24\,000\,000\,\mathrm{nm}$ . Magnification $= \frac{\text{image size}}{\text{actual size}} = \frac{24\,000\,000}{500} = \times 48\,000$ .
(b) Actual width $= \frac{40\,\mathrm{mm}}{48\,000} = 8.3 \times 10^{-4}\,\mathrm{mm} = 0.83\,\mathrm{\mu m}$ (to 2 significant figures).

This is a small axon. The myelin around it is almost as thick as the axon itself, which shows how many times the Schwann cell membrane is wound round it.

Worked example 2: who controls the resting heart?

Illustrative data: a heart with all its nerves cut beats at about 100 beats per minute, set by its own pacemaker. Stimulating its parasympathetic (vagus) nerve slows it to 60; stimulating its sympathetic nerve raises it to 140. A healthy person's resting heart rate is about 70. What do these data suggest about the nervous control of the heart at rest?

The resting rate, about 70, is below the heart's own rate of 100.
Only the parasympathetic nerve can slow the heart below 100, so at rest the vagus nerve must be sending impulses all the time.
During exercise or fear, heart rate can rise above 100 only through sympathetic stimulation (and the hormone adrenaline).

Conclusion: the two autonomic divisions act antagonistically on the same pacemaker, and at rest the parasympathetic effect dominates. This dual control lets the heart rate be adjusted quickly in both directions.

Warning

Common mistakes

"A nerve is a long cell." A neurone is one cell; a nerve is a bundle of many axons.
"Sensory neurones carry impulses from the brain." Sensory neurones carry impulses towards the CNS; motor neurones carry them away from it.
"The sympathetic system speeds everything up." It slows digestion and narrows the blood vessels of the gut while it speeds up the heart and breathing.

CrashCourse: "The Nervous System, Part 1: Crash Course Anatomy & Physiology #8". Watch for the divisions of the nervous system and the parts of a neurone.

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Practice: the nervous system and neurones

Distinguish between a neurone and a nerve.
Name the part of the brain that (a) contains the centre controlling breathing rate, (b) coordinates balance and fine movements, (c) controls body temperature.
State two effects of the sympathetic nervous system and the parasympathetic effect on the same two organs.
Give three ways in which the structure of a motor neurone differs from that of a sensory neurone.
Explain the function of (a) the myelin sheath, (b) the many mitochondria in a synaptic knob.
On a micrograph at ×12 000, a neurone cell body is 300 mm wide. Calculate its actual width in µm.
Explain why the autonomic nervous system needs two divisions with opposite effects.
A man's spinal cord is damaged in the neck. Suggest why he may be unable to move his legs although the muscles and the nerves in his legs are undamaged.

Show answersHide answers

A neurone is a single nerve cell; a nerve is a bundle of the axons of many neurones, wrapped in connective tissue.
(a) Medulla oblongata. (b) Cerebellum. (c) Hypothalamus.
For example: heart rate is increased by the sympathetic and decreased by the parasympathetic; pupils are dilated by the sympathetic and constricted by the parasympathetic (also bronchioles, gut activity).
Any three: the motor neurone's cell body is at the end of the fibre, in the CNS, while the sensory neurone's is part-way along it, in the dorsal root ganglion; the motor neurone has many dendrites on its cell body, the sensory neurone a long dendron; the motor neurone carries impulses from the CNS to effectors, the sensory neurone from receptors to the CNS.
(a) It insulates the axon, so impulses jump between the nodes of Ranvier and travel faster. (b) They release ATP for making and releasing neurotransmitter, and for pumping ions.
$\frac{300\,\mathrm{mm}}{12\,000} = 0.025\,\mathrm{mm} = 25\,\mathrm{\mu m}$ .
So that each organ can be both speeded up and slowed down quickly, and its activity matched to the body's needs, such as raising heart rate in exercise and lowering it at rest.
Impulses from the brain that start voluntary movement travel down the spinal cord to the motor neurones serving the legs. Damage in the neck cuts this pathway, so no impulses reach those motor neurones from the brain.

Key points

CNS = brain and spinal cord; PNS = sensory and motor neurones; motor neurones are somatic or autonomic; the autonomic system has antagonistic sympathetic and parasympathetic divisions.
Cerebrum: conscious thought and voluntary action; cerebellum: coordination and balance; medulla: breathing and heart rate; hypothalamus: temperature, water balance and the pituitary.
A neurone has dendrites, a cell body and an axon; many axons are myelinated by Schwann cells, with nodes of Ranvier between them.
Sensory neurones carry impulses to the CNS, relay neurones within it, and motor neurones from it to effectors.

Sources and credits

7.3.2: Neurons and Glial Cells, 7.3.4: The Central Nervous System and 7.3.5: The Peripheral Nervous System, from Biology 2e by OpenStax (Mary Ann Clark, Matthew Douglas, Jung Choi), via LibreTexts Biology, licensed under CC BY 4.0. Used for the structure of neurones, the regions of the brain and the autonomic nervous system. Adapted by Ganzaa.
Photo: Myelinated neuron, Roadnottaken, licensed under CC BY-SA 3.0, via Wikimedia Commons.
Video: "The Nervous System, Part 1: Crash Course Anatomy & Physiology #8", CrashCourse, embedded from YouTube.

Figures 1 and 2, the tables, the worked examples (with illustrative data) and the practice questions were written by Ganzaa. This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.

1A goalkeeper's half-second

2How the nervous system is organised

3Main regions of the brain

4Structure of a neurone

A goalkeeper's half-second

How the nervous system is organised

Main regions of the brain

Structure of a neurone