Coordination & Response

3. The human nervous system

The human nervous system is built from two parts:

• The central nervous system (CNS): the brain and spinal cord. This is where the decisions are made.
• The peripheral nervous system (PNS): every nerve outside the CNS. These connect the CNS to the receptors and effectors all over the body.

A nerve is a bundle of long thin cells called neurones. Each neurone carries electrical impulses along its length.

Three types of neurone

The standard pathway through the nervous system, in summary form:

receptor → sensory neurone → CNS (relay neurones) → motor neurone → effector

4. Synapses: passing the signal between neurones

Neurones are not actually connected to each other. A tiny gap, called a synapse, sits between the end of one neurone and the start of the next. The electrical impulse cannot jump across this gap; instead, the message has to be turned into a chemical signal for the crossing.

How a synapse works

1. An electrical impulse travels along the first neurone and arrives at the end of it (called the presynaptic terminal).
2. The arrival of the impulse triggers the release of neurotransmitter molecules stored in tiny sacs in the end of the neurone. The neurotransmitters spill out into the synaptic gap.
3. The neurotransmitters diffuse across the gap to the next neurone.
4. They bind to receptor proteins on the membrane of the next neurone.
5. This triggers a new electrical impulse in the next neurone, which then carries the signal onwards.
6. The neurotransmitter is quickly broken down or reabsorbed so the signal does not last forever.

Why synapses are useful

• One-way only. Neurotransmitter is released only on one side of the synapse and detected only on the other, so signals can only pass in one direction. This stops signals bouncing back and getting tangled.
• Signal modulation. Synapses can amplify, dampen or block a signal. Many drugs (and some natural processes) work by changing what happens at a synapse.

5. Reflex actions and the reflex arc

A reflex action is a very fast, automatic, involuntary response to a stimulus. You do not have to think about it; by the time you become aware of the stimulus, your body has already responded. Examples include pulling your hand away from a sharp object, blinking when something flies near your eye, coughing when food goes the wrong way, and the pupil narrowing in bright light.

Reflex actions are useful because they are fast and automatic, which protects the body from damage in dangerous situations.

The reflex arc

The neural pathway followed by a reflex action is called a reflex arc. It bypasses the conscious part of the brain to save precious milliseconds. The arc usually goes through the spinal cord rather than the brain.

Tracing the reflex when a person steps on a sharp pin:

1. Stimulus: the sharp point of the pin pressing on the foot
2. Receptor: pain receptors in the skin of the foot detect the pressure and damage
3. Sensory neurone: an electrical impulse travels from the receptor up the leg into the spinal cord
4. Relay neurone: inside the spinal cord, the sensory neurone synapses with a short relay neurone, which immediately synapses with a motor neurone. This is the arc: the signal arcs across the spinal cord without going up to the brain
5. Motor neurone: an impulse travels back down the leg to the muscles
6. Effector: the muscles in the leg contract
7. Response: the foot is pulled up and away from the pin

The whole thing takes a fraction of a second. A separate, slower signal also goes up to the brain so that you become aware of the pain shortly after the foot has moved.

The reflex arc: stimulus → receptor → sensory neurone → relay neurone (in spinal cord) → motor neurone → effector → response

6. The human eye

The eye is a sense organ that detects the stimulus of light. It contains light-sensitive receptor cells, plus a sophisticated lens system that focuses light onto those cells.

The main structures of the eye

Accommodation: focusing on near and far objects

The eye uses two parts to focus light: the cornea does most of the bending, and the lens does the fine-tuning. The lens can change shape, which lets the eye focus on objects at different distances. This is called accommodation.

Looking at a near object (e.g. reading a book):

1. The ciliary muscle contracts, so the ring of muscle becomes smaller in diameter
2. This slackens the suspensory ligaments
3. The lens, no longer pulled tight, becomes fatter and more rounded
4. A fatter lens refracts (bends) the light more, focusing the light from the close object onto the retina

Looking at a far object (e.g. a tree on a distant hill):

1. The ciliary muscle relaxes, so the ring of muscle becomes wider in diameter
2. This pulls the suspensory ligaments tight
3. The taut ligaments pull on the lens, making it thinner and flatter
4. A thinner lens refracts the light less, which is enough for the parallel rays of light from the distant object to focus on the retina

A common mistake: people sometimes write that "the suspensory ligaments contract". They don't, because they are ligaments, not muscles. The right words are tighten or slacken.

The pupil reflex: responding to light intensity

The amount of light reaching the retina is controlled by the size of the pupil. The iris changes the pupil size automatically through a reflex, protecting the retina from too much light and helping with vision in dim light.

The iris contains two sets of muscles, working as antagonistic pairs:

• Circular muscles run around the pupil. When they contract, the pupil constricts (narrows).
• Radial muscles run from the centre outwards like spokes on a wheel. When they contract, the pupil dilates (widens).

In bright light:

1. Photoreceptors detect high light intensity
2. The circular muscles contract, the radial muscles relax
3. The pupil constricts (becomes smaller)
4. Less light enters the eye, protecting the retina from damage

In dim light:

1. Photoreceptors detect low light intensity
2. The circular muscles relax, the radial muscles contract
3. The pupil dilates (becomes larger)
4. More light enters the eye, improving vision

The pupil reflex is automatic; you do not consciously control it. It is a classic example of a reflex action coordinated by the brain (specifically, by the brain stem) rather than by the spinal cord.

7. The skin in temperature regulation

Keeping the body's core temperature close to 37 °C is essential because the body's enzymes only work properly within a narrow temperature range. Even a 2 °C swing is potentially fatal. The skin plays a major role in adjusting heat loss to keep core temperature steady.

Temperature receptors in the skin and in the hypothalamus (a region at the base of the brain) detect changes in body temperature. The hypothalamus then coordinates the response, which is carried out by structures in the skin.

When the body is too hot

Three cooling mechanisms in the skin:

• Vasodilation of skin arterioles. The small arterioles that supply the surface capillaries widen (because the muscles in their walls relax). This sends more blood close to the skin's surface, where it loses heat to the surrounding air by radiation. The skin looks flushed red.
• Sweating. Sweat glands secrete more sweat onto the skin's surface. Water in the sweat evaporates, taking thermal energy from the body with it as latent heat. The skin cools as it evaporates.
• Hairs lie flat. Hair erector muscles relax, so the hairs lie flat against the skin. Less air is trapped close to the skin, so the insulating layer is thinner and heat can escape more easily.

When the body is too cold

Three warming (or heat-conserving) mechanisms:

• Vasoconstriction of skin arterioles. The arterioles narrow (their muscles contract), sending less blood through the surface capillaries. Less heat is lost by radiation. The skin looks pale.
• Less sweating. Sweat glands secrete less sweat, so less heat is lost by evaporation.
• Shivering. Skeletal muscles contract and relax rapidly. The contractions need ATP, which is supplied by respiration; respiration releases heat as a by-product, warming the body.
• Hairs stand on end. Hair erector muscles contract, pulling the hairs upright. This traps a layer of air next to the skin. Air is a poor conductor of heat, so the trapped air acts as insulation, reducing heat loss. (We have less hair than most mammals, so this gives "goosebumps" but does not insulate much in humans.)

Note for exams: the Edexcel mark scheme expects students to mention sweating, vasodilation and vasoconstriction by name. Hair erection (goosebumps) is mentioned in the textbook but is not strictly required.

8. Major human hormones

A hormone is a chemical messenger produced by an endocrine gland and carried in the blood to target cells in another part of the body. Endocrine glands have no ducts; they secrete their hormones directly into the bloodstream.

The major endocrine glands and their key hormones are summarised below. The detailed mechanisms (such as how insulin lowers blood glucose, or how ADH controls water reabsorption) are covered in topic 11 Homeostasis and topic 9 Excretion respectively.

How adrenaline works (the "fight-or-flight" response)

The adrenal glands release adrenaline in response to fear, excitement, or sudden physical demand. Adrenaline causes a coordinated set of changes that prepare the body to either fight a threat or run from it:

• Heart rate increases. Blood is pumped to the muscles faster.
• Breathing rate and depth increase. More oxygen enters the blood; more CO₂ leaves.
• Pupils dilate. More light enters the eye, improving vision in low-light or distant scenes.
• Blood is redirected away from non-essential organs (such as the gut) and towards muscles. The arterioles supplying muscles dilate; those supplying the gut constrict.
• Glucose is released into the blood. Adrenaline triggers the liver to break down stored glycogen back into glucose, raising blood sugar so muscle cells have more fuel for respiration.

All of these effects begin within seconds of the adrenaline reaching its target cells and fade within minutes once the danger has passed.

9. Plant responses: tropisms

Plants cannot move from place to place, but they can change their orientation by growing one way rather than another. A directional growth response in a plant is called a tropism.

A tropism is named after the stimulus it responds to:

• Phototropism is a response to light. Shoots show positive phototropism (grow towards the light); roots show negative phototropism (grow away from light).
• Geotropism (also called gravitropism) is a response to gravity. Roots show positive geotropism (grow downwards, in the direction of gravity, anchoring the plant and reaching water); shoots show negative geotropism (grow upwards, against gravity, lifting the leaves towards the sky).

The plant's behaviour makes biological sense: shoots find light for photosynthesis; roots find soil for water and minerals.

Auxin: the plant hormone behind tropisms

Plants control directional growth using plant hormones, the most important of which is auxin.

How auxin causes phototropism in a shoot:

1. Auxin is made in the tip of the shoot and diffuses down to the region just behind the tip.
2. In normal even lighting, auxin spreads equally through the shoot and the cells on all sides elongate at the same rate, so the shoot grows straight up.
3. When light shines on the shoot from one side, auxin moves to the shaded side of the shoot, away from the light.
4. The cells on the shaded side receive more auxin and so elongate more than the cells on the lit side.
5. The unequal elongation makes the shoot bend towards the light.

Important: auxin causes existing cells to elongate, not to divide. The number of cells does not change; they just get longer on one side than the other.

How auxin causes geotropism in a root:

• Auxin in roots collects on the lower side of a horizontal root (due to gravity).
• In roots, high auxin concentration slows cell elongation (unlike in shoots where it speeds it up).
• The cells on the upper side therefore elongate more than the cells on the lower side, and the root bends downwards.

So auxin works in opposite directions in shoots and roots, even though the chemical itself is the same. This nicely explains why shoots curl up towards the sun while roots curl down into the soil.

Practical uses of plant hormones

The discovery of how auxin works has led to several useful agricultural applications:

• Selective weedkillers (herbicides) that contain synthetic auxin-like chemicals make broad-leaved weeds grow uncontrollably until they die. Cereal crops (which have narrow leaves) are mostly unaffected, so farmers can spray a whole field and kill only the weeds.
• Rooting powders for cuttings contain auxin. Dipping the cut end of a stem cutting in rooting powder encourages new roots to grow, which makes propagating plants from cuttings much more reliable.
• Fruit ripening hormones (especially ethene) are used commercially to ripen fruit on demand, so that bananas and tomatoes can be shipped green and ripened just before sale.