Nervous system: WJEC A level Biology revision notes

Lesson 1 of 15

What are the parts of a nervous response?

These notes cover topic 3.8, The nervous system, in WJEC A level Biology Unit 3: Energy, and the Environment. Each lesson follows the WJEC specification, and the exam tips are based on BioWelsh analysis of WJEC past papers.

Animals must respond quickly to changes around them and inside them. A change that an organism can detect is a . The nervous system detects , passes information along cells called as electrical impulses, and brings about a response within milliseconds.

A is a cell, or group of cells, that detects one type of . act as : they convert the energy of the , such as light, heat or pressure, into impulses. Some detect external ; others detect internal , such as the in the that detect changes in the of the blood.

Examples of receptors and the stimuli they detect
Receptor	Where it is found	Stimulus detected
Rods and cones	Retina of the eye	Light
Pressure	Skin	Pressure and touch
Thermoreceptors	Skin and	Temperature
Stretch	Muscles	Stretching of the muscle
		of the blood
Chemoreceptors	Nose, tongue and carotid arteries	Chemicals, including carbon dioxide in the blood

Every nervous response follows the same pathway, shown in the diagram below.

Seven coloured boxes in a row joined by arrows: stimulus, a change inside or outside the body; receptor, detects the stimulus; sensory neurone; CNS, brain or spinal cord; motor neurone; effector, muscle or gland; response, the action that results. Beneath the row, a line is labelled sensory pathway towards the CNS on the left, coordinates in the middle and motor pathway away from the CNS on the right. A note at the foot says impulses carry information along neurones and receptors act as transducers, converting the energy of a stimulus into impulses. — The pathway of a nervous response, from stimulus to response. Original BioWelsh diagram.

A is detected by a .
A carries impulses from the to the (), which is the brain and spinal cord.
The coordinates a response, often through .
A carries impulses from the to an , which is a muscle or a gland.
The brings about the response: a muscle contracts or a gland secretes.

The is to every part of the body by , which together make up the . The diagram below shows how the branch from the spinal cord.

An outline of a human body seen from behind, with the brain in the head and the spinal cord running down the middle of the back. Many yellow nerves branch from the spinal cord into the arms, trunk and legs, and each nerve is labelled, for example brachial plexus, radial nerve, median nerve, sciatic nerve, femoral nerve and tibial nerve. — The human nervous system, showing the brain, spinal cord and main nerves. Photo: The Emirr, CC BY 3.0. Resized.

The brain and the spinal cord, running down the back, form the .
Pairs of leave the spinal cord between the , one on each side, all the way down.
The branch repeatedly to reach the skin, muscles and organs of the arms, trunk and legs.
A is a bundle of many , some sensory and some motor, wrapped in .

Lesson 2 of 15

What does a transverse section of the spinal cord show?

The spinal cord is a column of nervous tissue about 45 cm long, protected by the vertebral column. It carries impulses between the body and the brain, and it coordinates simple by itself. The diagram below is a transverse section (T.S.), a cut straight across the cord.

An oval section of spinal cord, dorsal side at the top and ventral side at the bottom, with a narrow notch at the bottom. Inside, a pale purple butterfly-shaped area is labelled grey matter, with its upper points labelled dorsal horn and lower points labelled ventral horn, and a small circle at its centre labelled central canal. The cream area around it is labelled white matter, and a thin blue outer layer is labelled meninges. On each side, a dorsal root curves from the upper part of the cord to an oval swelling labelled dorsal root ganglion, and a ventral root curves from the lower part of the cord; the two roots join to form a spinal nerve that extends outwards. — A transverse section of the spinal cord with its dorsal and ventral roots. Original BioWelsh diagram.

In the centre is a small , filled with .
Around it is an H-shaped or butterfly-shaped area of . It contains the of , , and non- fibres.
Outside the is the . It contains running up and down the cord. The fatty makes it look white.
The two points of the nearest the back are the dorsal horns; the two nearest the front are the ventral horns.
The whole cord is surrounded by protective membranes, the .

Along the cord, 31 pairs of leave it, one on each side. Near the cord, each divides into two roots. The enters the back of the cord and carries . It has a swelling, the , which contains the of the . The leaves the front of the cord and carries , whose are in the ventral horn of the . So each is a mixed , with sensory and together.

Dorsal and ventral roots compared
Feature	Dorsal root	Ventral root
Position	Back of the cord	Front of the cord
it carries	Sensory	Motor
Direction of impulses	Into the spinal cord	Out of the spinal cord
Where the are	In the	In the ventral horn of the

The specification asks you to examine a prepared slide of a T.S. of spinal cord. The photograph below shows a stained section through a real spinal cord, viewed under a light microscope.

A roughly oval section of tissue stained dark purple, with a deep narrow notch at the bottom. In the middle is a paler pink butterfly-shaped area whose lower wings are broader than its upper wings, and a tiny pale gap at its centre. Thin strands of tissue and small cut nerve roots lie around the edge of the section. — A stained transverse section of a spinal cord seen with a light microscope. Photo: OpenStax, CC BY 4.0. Resized.

The section is roughly oval, with a deep notch at the bottom. This is the ventral side; the front of the cord is at the bottom of the image.
The paler pink, butterfly-shaped area in the middle is the . Its lower wings, the ventral horns, are broader than the upper, dorsal horns.
The darker purple tissue all round it is the , made mostly of cut across.
The small pale gap at the centre of the is the .

To measure structures on the slide, you use an , which is calibrated against a at each . To draw a , you draw only the outlines of the tissue areas, with no individual cells, keeping the proportions the same as in the specimen. If the actual widths of two regions are in the ratio 1 : 20, their lines in the plan must be in the same ratio.

Lesson 3 of 15

How does a reflex arc give a rapid, protective response?

A action is a rapid, automatic response to a . You do not decide to make it, so it is involuntary. protect the body: pulling your hand away from a hot object, blinking when something approaches your eye and narrowing the pupil in bright light all prevent damage. The pathway taken by the impulses is called a .

The diagram below shows a withdrawal passing through the spinal cord.

A transverse section of spinal cord with its roots on the right side of the image and a box of skin and an oval muscle on the left. A blue sensory neurone runs from a receptor in the skin along the spinal nerve and dorsal root to a cell body in the dorsal root ganglion, then into the dorsal horn of the grey matter. There it meets a green relay neurone at a synapse, marked with a dot. The relay neurone runs down through the grey matter and meets a red motor neurone at a second synapse. The motor neurone's cell body is in the ventral horn, and its axon leaves through the ventral root and runs out to the muscle, labelled effector. Arrows show the direction of the impulse. Text beneath gives the pathway stimulus, receptor, sensory neurone, relay neurone, motor neurone, effector, response, and says only the spinal cord is involved, so the response is rapid and involuntary, and that relay neurones also carry impulses up to the brain. — A withdrawal reflex arc passing through the spinal cord. Original BioWelsh diagram.

A in the skin detects the , for example heat from a hot object.
A carries impulses along the and into the spinal cord through the . Its is in the .
In the , the with a short .
The with a , whose is in the ventral horn.
The leaves through the and carries impulses to the , here a muscle in the arm, which contracts and pulls the hand away.

The is rapid because the pathway is short and contains very few , and each slows the impulse slightly. It is automatic because the pathway runs through the spinal cord and does not need the conscious parts of the brain to make a decision. do also send impulses up the spinal cord to the brain, so you become aware of the pain, but only after the response has begun.

The knee-jerk is even simpler. Tapping the below the kneecap stretches the quadriceps muscle at the front of the thigh. Stretch in the muscle send impulses along a that directly with a in the spinal cord. There is no , so there is only one , and the quadriceps contracts. It helps keep your balance when a muscle is suddenly stretched.

Two reflexes compared
Feature	Withdrawal reflex	Knee-jerk reflex
	Heat or pain in the skin	Stretch in the quadriceps
in the arc	Sensory, relay and motor	Sensory and motor only
in the arc	Two	One
	Muscle in the arm (biceps)	Quadriceps muscle
Benefit	Prevents burning or other damage	Maintains posture and balance

Lesson 4 of 15

How does the nerve net of Hydra compare with more complex nervous systems?

are simple aquatic animals such as jellyfish, sea anemones and the freshwater Hydra. Their body wall has just two layers of cells, and they have no brain. Instead, their form a spread through the body wall.

The photograph below shows a living Hydra attached to a water plant.

A thin, pale brown, tube-shaped animal attached by its base to a curved plant stem under water. A ring of very long, fine, thread-like tentacles spreads from the top of its body and hangs down, and a small side branch grows from its body near the stem. The background is blurred grey and green. — A living *Hydra* attached to the stem of a water plant. Photo: Marta Boroń, CC BY 2.0. Resized.

The body is a thin, pale tube about a centimetre long, attached at its base to the stem of a plant.
A ring of very long, thread-like tentacles hangs from the top of the body, around the mouth.
A small bud is growing out of the side of the body. This is how Hydra reproduces asexually.

The diagram below shows how the of Hydra are arranged.

A simple outline of a Hydra: a tube-shaped body with five tentacles at the top around the mouth and a basal disc at the bottom. Inside the body, purple dots joined by many short lines form a mesh with no central point. Labels read tentacle, mouth, nerve net: a network of nerve cells in the body wall, nerve cell with branches in several directions, and basal disc, attached to a surface. Text notes there is no brain and no separate sensory, relay and motor pathways, and that impulses spread out in all directions from a stimulated area. — The nerve net of *Hydra*. Original BioWelsh diagram.

The are short, branched cells joined to each other in a network throughout the body wall and tentacles.
There is no brain and no cord: no part of the net is in overall .
A to one area produces impulses that spread out in all directions through the net, so the response usually involves the whole animal.

Hydra responds slowly, with movements taking several seconds. Its are non- and very thin, and the impulse has to cross many because the are short. Many of these can pass impulses in either direction, so impulses spread out rather than travelling along a direct route.

The nervous systems of *Hydra* and a mammal compared
Feature	Hydra (nerve net)	Mammal
	None	Brain and spinal cord
Types of	One simple type	Sensory, relay and motor
Branching of	Short, highly branched	Most with a long
	Absent	Present on many
Direction of impulses	In all directions	In one direction only
Speed of conduction	Slow	Fast, up to about 120 m s⁻¹
Response	Whole body, simple	Localised, precise and complex

Lesson 5 of 15

What is the structure of a motor neurone?

are cells specialised to carry impulses. A carries impulses from the to an . You need to be able to draw and label one.

The diagram below shows a mammalian .

A motor neurone drawn horizontally. On the left, a purple cell body contains a nucleus and small dark dots labelled Nissl granules, with many branched dendrites radiating from it. A long axon runs to the right, covered by five yellow segments labelled myelin sheath, each with a small oval labelled nucleus of Schwann cell, and separated by narrow gaps labelled node of Ranvier. At the right end, the axon divides into four axon terminals ending in round synaptic knobs. An arrow above is labelled direction of the impulse, pointing from the cell body towards the terminals. — A mammalian motor neurone. Original BioWelsh diagram.

The contains the and many and much rough endoplasmic reticulum, seen as dark . These make proteins, including .
Many short branch from the . They receive impulses from other and carry them towards the .
One long carries impulses away from the . It can be more than a metre long, reaching from the spinal cord to a foot muscle.
wrap around the many times, forming a fatty . Each has its own .
Gaps about 1 mm apart between neighbouring , where the membrane is exposed, are the .
The ends in branched terminals with , which pass impulses to the muscle.

is mainly lipid, formed from many layers of the 's membrane. It is an electrical insulator: ions cannot flow across the membrane where it is covered. This is why the impulse travels faster along a .

The photograph below shows a stained smear of spinal cord under a light microscope, with one large in the centre.

A micrograph with a pale lilac background. In the centre is a large dark purple star-shaped cell with a darker area in its middle, and several thick branches radiating outwards, one running a long way to the left. Many small dark round dots are scattered all around it. — A motor neurone in a stained smear of spinal cord, seen with a light microscope. Photo: Berkshire Community College Bioscience Image Library, CC0.

The large, dark purple, star-shaped structure in the middle is the .
The darker area inside it is the .
Several thick processes radiate from the . These are and the start of the ; in a smear you cannot easily tell them apart.
The many small, dark dots all around are the of much smaller supporting cells.

Three types of neurone compared
Feature	Sensory neurone	Relay neurone	Motor neurone
Carries impulses	From a to the	Within the , between sensory and	From the to an
Position of	In a , part way along the	In the	In the , at one end
Length	Long	Short	Long

Lesson 6 of 15

How is the resting potential of a neurone maintained?

A that is not carrying an impulse still has a potential difference (a voltage) across its membrane. The inside is about 70 millivolts (mV) more negative than the outside, so the is about −70 mV. The membrane is said to be polarised.

The depends on how ions are distributed on the two sides of the membrane, and on the proteins that move them. The diagram below shows this.

A horizontal membrane, drawn as two rows of circles, separates the outside of the axon at the top from the inside at the bottom. Plus signs line the outer surface and minus signs the inner surface. On the left, a purple protein labelled sodium-potassium pump has a red arrow going up labelled 3 Na plus out and a blue arrow going down labelled 2 K plus in, with ATP to ADP plus Pi written below. In the middle, a pair of blue channel proteins has a blue arrow going up, labelled K plus channels, many open, K plus diffuses out. On the right, a pair of red channel proteins is labelled Na plus channels, mostly closed, little Na plus diffuses in. Many Na plus ions are shown outside and many K plus ions inside, with a few large negative ions labelled A minus inside. Text says the outside has high Na plus and low K plus and is more positive, the inside has high K plus, low Na plus and large negative ions and is more negative, and the resting potential is about minus 70 mV, so the membrane is polarised. — How the resting potential of an axon is maintained. Original BioWelsh diagram.

The uses to actively transport 3 sodium ions (Na⁺) out of the for every 2 potassium ions (K⁺) it brings in. More positive charge leaves than enters.
This makes the concentration of Na⁺ high outside and the concentration of K⁺ high inside.
Many potassium ion channels are open, so K⁺ diffuses out of the down its , taking more positive charge out.
Most sodium ion channels are closed, so only a little Na⁺ can diffuse back in.
The cytoplasm also contains large negatively charged ions, such as proteins, which cannot leave.
The result is that the outside is positive compared with the inside.

Keeping the needs a steady supply of for the pump, which is why have many and use a lot of oxygen. (If and are unfamiliar, read the ATP lesson on why cells need ATP.)

Lesson 7 of 15

What happens during an action potential?

An impulse is a wave of electrical change that travels along a . At any one point on the membrane, the change is a brief reversal of the potential difference called an . It depends on ion channels: proteins that open or close when the potential difference across the membrane changes.

are studied by placing a very fine microelectrode inside an and a second electrode in the fluid outside, and displaying the potential difference on an , as shown below.

An axon lies in a blue dish of salt solution. A fine glass microelectrode, labelled microelectrode inside the axon, pierces it from above. A grey rod to its right, labelled reference electrode in the solution, dips into the solution. Wires from both electrodes lead to an oscilloscope, whose dark screen shows a green trace: a flat line, a steep rise to a sharp peak, a fall below the starting level and a gradual return. A note says the trace plots potential difference in mV against time in ms. — Recording the potential difference across an axon membrane. Original BioWelsh diagram.

The glass microelectrode pierces the membrane, so its tip is inside the .
The reference electrode sits in the salt solution outside the .
The displays the potential difference between the two electrodes as a trace, with time along the horizontal axis.

The graph below shows one .

Line graph of potential difference across the membrane in mV, from minus 90 to plus 50, against time in ms, from 0 to 6. The line is flat at minus 70, labelled resting, until 0.5 ms, rises slowly to the dashed threshold line at minus 55, then rises steeply, labelled depolarisation, to a peak of plus 40 at about 1.3 ms. It falls steeply, labelled repolarisation, to a minimum of minus 80 at about 2.4 ms, labelled hyperpolarisation, then rises slowly back to minus 70 by about 4.5 ms. — An action potential recorded from an axon, with its phases labelled. Original BioWelsh graph.

Resting: the potential difference is −70 mV.
A opens some sodium ion channels. Na⁺ diffuses in and the inside becomes less negative. If it reaches the of about −55 mV, many more sodium channels open.
: Na⁺ floods into the by , down its . The entering Na⁺ opens still more sodium channels, so the change speeds up. The inside becomes positive, reaching about +40 mV.
: at the peak, the sodium channels close and potassium ion channels open. K⁺ diffuses out, so the inside becomes negative again.
: the potassium channels are slow to close, so the potential overshoots to about −80 mV.
The potassium channels close and the and the resting permeability to K⁺ restore the of −70 mV.

What the ion channels do in each phase
Phase	Sodium ion channels	Potassium ion channels	Movement of ions
Resting	Closed	Some open (leak channels)	Pump: 3 Na⁺ out, 2 K⁺ in
	Open	Closed	Na⁺ diffuses in
	Closed	Open	K⁺ diffuses out
	Closed	Still open	K⁺ continues to diffuse out

Lesson 8 of 15

How does the strength of a stimulus affect the impulses?

An is only produced if the depolarises the membrane to the . Below , not enough Na⁺ enters and nothing happens. At or above , a full is always produced, and it is always the same size. This is the .

So how does the nervous system tell a weak from a strong one? The traces below show the impulses produced in a by of increasing strength.

Three graphs side by side of potential difference in mV against time from 0 to 10 ms, each with a dashed threshold line, and a blue bar underneath each showing the size of the stimulus. Left, stimulus below threshold, the shortest bar: only a small bump that stays below threshold, labelled no action potential. Middle, medium stimulus, a taller bar: two action potentials rising to plus 40, labelled 2 action potentials in 10 ms. Right, strong stimulus, the tallest bar: four action potentials of the same height, labelled 4 action potentials in 10 ms. A line at the foot reads every action potential reaches the same peak; a stronger stimulus only makes them more frequent. — How the strength of a stimulus changes the action potentials in a sensory neurone. Original BioWelsh diagram.

Each column shows the same given a of a different size, shown by the blue bar.
The smallest causes a small that does not reach , so there is no .
A produces two in 10 ms.
The strongest produces four in 10 ms, each exactly the same height as before.

The strength of a is therefore coded by the of the impulses, not their size. A strong also tends to stimulate more and so more , and the brain interprets both as a stronger sensation.

After an , there is a short , lasting a few milliseconds. During the , about 1 ms, the sodium channels cannot reopen, so no new can be generated however strong the . During the that follows, while the membrane is hyperpolarised, a new can be generated, but only by a stronger than normal. The has three effects. It keeps separate from each other. It limits how many can pass in a second. And it makes the impulse travel in one direction only, because the membrane behind it cannot be depolarised again.

Lesson 9 of 15

How do you analyse oscilloscope traces?

Exam questions often give an trace and ask you to read values from it or calculate from it. First check the axes: the potential difference is usually in mV and the time in ms (1 ms = 0.001 s).

: the steady value before the , for example −70 mV.
Peak: the highest point, for example +40 mV.
Amplitude of the : peak minus , for example +40 − (−70) = 110 mV.
Duration: the time from the start of to the return to .
: the number of in one second.

Some questions ask for the answer in standard form. For example, if an travels 52 µm (52 × 10⁻⁶ m) along an at 100 m s⁻¹, the time taken is 52 × 10⁻⁶ ÷ 100 = 5.2 × 10⁻⁷ s.

Lesson 10 of 15

How does an impulse travel along an axon?

An at one point on the triggers the next point, so the impulse moves along. In a non- this happens by local circuits, shown below.

A long horizontal band representing an axon, with plus and minus signs on the outside and inside of its membranes. On the left, the region labelled refractory region has plus outside and minus inside. In the middle, a pink region labelled action potential, depolarised: inside positive, has minus outside and plus inside. On the right, a long region labelled resting region, about to be depolarised, has plus outside and minus inside. Red curved arrows inside the axon point from the action potential to the right, labelled Na plus diffuse along the axon. A grey dashed arrow points left, labelled cannot depolarise here. An arrow at the top shows the direction of the impulse from left to right. — How local currents move an impulse along a non-myelinated axon. Original BioWelsh diagram.

At the site of the , the inside of the is positive because Na⁺ has entered.
Some of these Na⁺ ions diffuse sideways along the inside of the towards the next, resting region, which is still negative inside. These movements of charge are .
They make the next region less negative, depolarising it to , so its sodium channels open and a new is generated there.
Behind the , the membrane is in its , so it cannot be depolarised again. The impulse can only move forwards.

In this way, a wave of passes along the whole length of the membrane, one region after another. Each new is the same size as the last, so the impulse does not fade however long the is.

Lesson 11 of 15

What affects the speed of conduction of an impulse?

The speed of conduction varies a great deal between animals and between , from under 1 m s⁻¹ in a Hydra to about 120 m s⁻¹ in the fastest mammalian . Three factors are important: myelination, the diameter of the and temperature.

The most important factor is the . The diagram below shows how an impulse travels along a .

A horizontal axon covered above and below by long yellow blocks of myelin, with narrow gaps between them. One gap is labelled node of Ranvier. A minus sign above the second gap marks the node where the action potential is, labelled action potential at this node. A red curved arrow inside the axon runs from this node to the next node on the right, labelled local current jumps to the next node, and that node is labelled next node: depolarised to threshold. A grey dashed arrow runs back to the previous node, labelled previous node is refractory. The myelin is labelled myelin sheath: insulates. Text at the foot says depolarisation happens only at the nodes, so the action potential jumps from node to node: saltatory conduction. — Saltatory conduction along a myelinated axon. Original BioWelsh diagram.

The insulates the , so ions cannot cross the membrane where it is covered.
can happen only at the , where the membrane is exposed and the ion channels are concentrated.
flow from one node to the next, so the jumps from node to node. This is , from the Latin for 'to jump'.
Because far less of the membrane has to be depolarised, the impulse travels much faster than along a non- of the same diameter.

also saves energy. After an impulse, the must restore the ion gradients using . In a ions cross only at the nodes, so less is needed. If the is damaged, as can happen in people exposed to some chemicals and in multiple sclerosis, the whole membrane must be depolarised. Conduction slows, more pumps must work, and the respires faster and uses more oxygen. Symptoms include slower reactions, weakness, loss of feeling and loss of muscle .

The second factor is the diameter of the . The graph below shows how conduction speed changes with diameter.

Line graph of conduction speed in metres per second, from 0 to 120, against axon diameter in micrometres, from 0 to 20. A solid blue line for myelinated axons rises steeply and straight from about 3 at 0.5 micrometres to 60 at 10 micrometres and 120 at 20 micrometres. A dashed red line for non-myelinated axons rises slowly and curves, from about 2 at 0.5 micrometres to about 9 at 10 micrometres and about 13 at 20 micrometres. — Conduction speed against axon diameter in myelinated and non-myelinated axons. Original BioWelsh graph.

In both types of , speed increases as the diameter increases, because a wider offers less to the flow of ions along it.
At every diameter, conduct much faster than non- .
For example, at a diameter of 10 µm the conducts at about 60 m s⁻¹, several times faster than the non- .

Some invertebrates, such as the squid, have no but have giant up to 1 mm across, which carry impulses for rapid escape. The distance between nodes also matters: longer internodes fewer jumps and faster conduction, up to a point; if the nodes are too far apart, cannot depolarise the next node. The third factor is temperature. Up to about 40 °C, higher temperature increases the rate of of ions, so impulses travel faster. This is one reason why mammals and birds, which keep a high body temperature, conduct faster than animals whose temperature follows the surroundings.

Lesson 12 of 15

What is the structure and role of a synapse?

A is the junction between two , or between a and an cell. The two cells do not touch: they are separated by a gap about 20 nm wide, the . Impulses cannot jump this gap electrically, so a chemical called a carries the signal across. that use as their are called .

The diagram below shows the structure of a .

A presynaptic axon with a short myelin sheath enters from the left and widens into a blue synaptic knob. Inside the knob are two oval mitochondria and many small circles labelled synaptic vesicle containing acetylcholine. One vesicle is fusing with the flat right-hand edge, labelled presynaptic membrane, which also carries small gold blocks labelled calcium ion channel. A narrow white gap, labelled synaptic cleft about 20 nm, separates it from a green postsynaptic neurone on the right, labelled postsynaptic neurone, dendrite. Small red receptor shapes, labelled receptor proteins, line the postsynaptic membrane. An arrow at the top shows the direction of the impulse from left to right. — The structure of a cholinergic synapse. Original BioWelsh diagram.

The of the presynaptic ends in a swelling, the .
The knob contains many , which supply for making and moving the .
It also contains many , small membrane sacs each filled with .
The presynaptic membrane contains calcium ion channels.
Across the , the postsynaptic membrane has proteins that bind . Each is also a sodium ion channel.
The cleft contains the , which breaks down .

are far too small to see with a light microscope. The cleft is about 20 nm wide, well below the resolution of a light microscope of about 200 nm, so are studied with an electron microscope.

They make impulses travel in one direction only, because the are only in the presynaptic and the only on the postsynaptic membrane.
They allow : several small inputs can add together to trigger an .
They filter out weak, low-level that do not release enough to reach .
They allow one to connect with many others, so information can be spread out or brought together.
They protect against overstimulation: if impulses arrive too often, the runs out and the is fatigued.

Lesson 13 of 15

What happens during synaptic transmission?

Synaptic transmission is the process by which an impulse in the presynaptic produces an impulse in the postsynaptic . The diagram below numbers the steps at a .

The same synapse with six numbered circles placed where each step happens: 1 in the presynaptic axon, 2 above a calcium channel with a gold arrow labelled Ca2 plus going into the knob, 3 beside a vesicle fusing with the presynaptic membrane, 4 in the cleft, 5 in the postsynaptic neurone with a red arrow labelled Na plus going in, and 6 in the cleft. A key beneath reads: 1 an action potential arrives and depolarises the synaptic knob; 2 calcium ion channels open and Ca2 plus diffuse into the knob; 3 vesicles move to the presynaptic membrane, fuse with it and release acetylcholine by exocytosis; 4 acetylcholine diffuses across the cleft and binds to receptors on the postsynaptic membrane; 5 sodium ion channels open, Na plus diffuse in and the postsynaptic membrane is depolarised; 6 acetylcholinesterase hydrolyses acetylcholine and the products are taken back into the knob and reused. — The steps of synaptic transmission at a cholinergic synapse. Original BioWelsh diagram.

An arrives at the and depolarises the presynaptic membrane.
This opens calcium ion channels, and calcium ions (Ca²⁺) diffuse into the .
The Ca²⁺ ions cause the to move to the presynaptic membrane and fuse with it, releasing into the cleft by .
diffuses across the cleft and binds to the proteins on the postsynaptic membrane.
The sodium ion channels in the open, Na⁺ diffuses into the postsynaptic and the membrane is depolarised. If the reaches , an is generated.
into choline and ethanoic acid. These diffuse back into the and are made into again, using , ready for the next impulse.

Breaking down is essential. It releases the , so the sodium channels close and the postsynaptic membrane repolarises. Without it, the postsynaptic membrane would stay depolarised and no separate impulses could pass.

One impulse often releases too little to reach . The small depolarisations can add together in two ways, shown in the graph below. This is .

One graph of the postsynaptic membrane potential difference in mV against time from 0 to 26 ms, with a dashed threshold line. Blue arrows below mark impulses arriving. At 1 ms, one impulse from neurone A gives a small rise that stays below threshold and dies away, labelled one impulse from neurone A: below threshold. At 9 and about 10 ms, two impulses from neurone B arrive in quick succession; the second rise adds to the first, reaches threshold and a full action potential to plus 40 follows, labelled temporal summation. At 18 ms, impulses from neurones C and D arrive together; the larger rise reaches threshold straight away and a full action potential follows, labelled spatial summation. — Temporal and spatial summation at a synapse, shown on one trace. Original BioWelsh diagram.

At A, one impulse arrives at one . The small stays below and dies away.
At B, two impulses arrive at one in quick . The second adds to the first and reaches . This is .
At C + D, impulses arrive at the same time at two from different . Together they release enough to reach . This is .
In both cases, once is reached, a full-sized is produced.

Some are inhibitory. Their opens channels that make the inside of the postsynaptic membrane more negative (hyperpolarised), so it is harder to reach . The balance of excitatory and inhibitory inputs decides whether a fires.

Lesson 14 of 15

How do drugs and other chemicals affect synaptic transmission?

Many drugs, poisons and medicines act at or on membranes. To work out what a chemical does, identify which step it affects and then follow the consequences through. The diagram below marks four sites of action.

The same synapse with four lettered circles and leader lines. A points to the presynaptic axon, B to a calcium channel on the presynaptic membrane, C to a receptor on the postsynaptic membrane and D to the synaptic cleft. A key beneath reads: A, local anaesthetics such as lidocaine block Na plus channels, so no action potential reaches the knob; B, calcium channel blockers, for example a cone shell toxin, stop Ca2 plus entering, so no acetylcholine is released; C, nicotine has a similar shape to acetylcholine, binds to its receptors and opens the Na plus channels; D, organophosphates inhibit acetylcholinesterase, so acetylcholine stays bound and keeps depolarising the membrane. — Four sites where chemicals affect transmission at a synapse. Original BioWelsh diagram.

A: such as lidocaine block the sodium ion channels of . Na⁺ cannot enter, is not reached and no is generated, so impulses from the pain do not reach the brain.
B: some , such as one made by a marine cone snail, block the calcium ion channels in the presynaptic membrane. Ca²⁺ cannot enter, the do not fuse with the membrane and no is released.
C: nicotine has a similar shape to and binds to some of its , opening the sodium ion channels and stimulating the postsynaptic .
D: inhibit , so is not broken down.

are used as insecticides, and some have been used as gases. With inhibited, stays in the cleft and keeps binding to the . The postsynaptic membrane stays depolarised and continue to be generated. At this causes uncontrolled muscle contraction and spasms, then paralysis, and it can be fatal if the breathing muscles are affected.

How chemicals can change synaptic transmission
Type of effect	How it works	Example
Increase transmission	Mimics the and binds to its	Nicotine at
Increase transmission	Stops the being broken down
Increase transmission	Stops the being taken back into the presynaptic	Cocaine with the
Decrease transmission	Blocks the without opening the channels	Curare at
Decrease transmission	Prevents release of the	Calcium channel blockers; botulinum
Block impulses in the	Blocks the sodium ion channels	Lidocaine and other

are drugs that act on the brain and change mood, perception or behaviour. Many work at . Cocaine, for example, blocks the proteins that take back into the presynaptic , so stays in the cleft and keeps stimulating the postsynaptic in the brain's reward pathways. The brain responds to repeated use by reducing the number of , which is to tolerance and addiction.

Lesson 15 of 15

How do the ideas in the nervous system fit together?

How the ideas in this topic connect

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Nervous response: , , , , ,
- Spinal cord: grey and ; sensory, motor
  - : few , spinal cord only, so rapid and involuntary
- in Hydra: no , non-, impulses in all directions
- : , , , from , nodes
  - −70 mV: pump 3 Na⁺ out, 2 K⁺ in; K⁺ diffuses out
    - : Na⁺ in (), K⁺ out (),
      - Speed: , diameter, temperature
      - : Ca²⁺ in, released, binds, Na⁺ in;
        Chemicals: anaesthetics, calcium blockers, nicotine, ,

A is one cell; a is a bundle of .
enter by the ; their are in the .
A is rapid because of few and automatic because it does not involve the conscious brain.
Say 'sodium ions' or 'Na⁺', never 'sodium'.
Ions move into the , not 'into the membrane'.
are all the same size; strength changes their .
In , happens only at the .
inhibit ; they do not increase release.
When a chemical blocks one step, explain what follows from that step, not the whole process.

What are the parts of a nervous response?

What does a transverse section of the spinal cord show?

How does a reflex arc give a rapid, protective response?

How does the nerve net of Hydra compare with more complex nervous systems?

What is the structure of a motor neurone?

How is the resting potential of a neurone maintained?

What happens during an action potential?

How does the strength of a stimulus affect the impulses?

How do you analyse oscilloscope traces?

How does an impulse travel along an axon?

What affects the speed of conduction of an impulse?

What is the structure and role of a synapse?

What happens during synaptic transmission?

How do drugs and other chemicals affect synaptic transmission?

How do the ideas in the nervous system fit together?

Worksheets

Worksheet 1: 3.8 The nervous system (139 marks)

Check your understanding.

Receptors are described as transducers. What does this mean?

The nervous system

What are the parts of a nervous response?

What does a transverse section of the spinal cord show?BWRBioWelsh recommendedWe recommend revising this because it has appeared regularly in past papers. Asked in 2 of 7 WJEC exam series (2017 to 2025), 12 marks in total. This is a revision priority, not a prediction of your exam.

How does a reflex arc give a rapid, protective response?BWRBioWelsh recommendedWe recommend revising this because it has appeared regularly in past papers. Asked in 3 of 7 WJEC exam series (2017 to 2025), 19 marks in total. This is a revision priority, not a prediction of your exam.

How does the nerve net of Hydra compare with more complex nervous systems?

What is the structure of a motor neurone?

How is the resting potential of a neurone maintained?

What happens during an action potential?BWRBioWelsh recommendedWe recommend revising this because it has appeared regularly in past papers. Asked in 6 of 7 WJEC exam series (2017 to 2025), 28 marks in total. This is a revision priority, not a prediction of your exam.

How does the strength of a stimulus affect the impulses?

How do you analyse oscilloscope traces?BWRBioWelsh recommendedWe recommend revising this because it has appeared regularly in past papers. Asked in 3 of 7 WJEC exam series (2017 to 2025), 6 marks in total. This is a revision priority, not a prediction of your exam.

How does an impulse travel along an axon?

What affects the speed of conduction of an impulse?BWRBioWelsh recommendedWe recommend revising this because it has appeared regularly in past papers. Asked in 3 of 7 WJEC exam series (2017 to 2025), 12 marks in total. This is a revision priority, not a prediction of your exam.

What is the structure and role of a synapse?

What happens during synaptic transmission?BWRBioWelsh recommendedWe recommend revising this because it has appeared regularly in past papers. Asked in 5 of 7 WJEC exam series (2017 to 2025), 22 marks in total. This is a revision priority, not a prediction of your exam.

How do drugs and other chemicals affect synaptic transmission?BWRBioWelsh recommendedWe recommend revising this because it has appeared regularly in past papers. Asked in 4 of 7 WJEC exam series (2017 to 2025), 22 marks in total. This is a revision priority, not a prediction of your exam.

How do the ideas in the nervous system fit together?

Worksheets

Worksheet 1: 3.8 The nervous system (139 marks)

Check your understanding.

What does a transverse section of the spinal cord show?

How does a reflex arc give a rapid, protective response?

What happens during an action potential?

How do you analyse oscilloscope traces?

What affects the speed of conduction of an impulse?

What happens during synaptic transmission?

How do drugs and other chemicals affect synaptic transmission?