Unit 14 - Coordination and response

Question 1

The Nervous System

Answer 1

A stimulus is a change in the environment that can be detected by an organism. The nervous system is an organ system that allows humans and other animals to detect stimuli and respond to them.

Question 2

Features of the nervous system

Answer 2

The nervous system contains many nerve cells called neurones. A neurone is a specialised cell that transmits electrical nerve impulses - travel along these cells as electrical signals.

Consists of:
- CNS - the brain and spinal cord, form the central nervous system, responsible for coordinating all reactions and nervous communication around the body.
- PNS - nerves in the other parts, which form the peripheral nervous system, responsible for transmitting the impulses from the CNS to all parts of the body.

Question 3

Neurones

Answer 3

The cells that make up the majority of the nervous system. They carry electrical impulses around your body at incredible speeds. There are three main types of neurone:

• sensory neurones
• relay neurones
• motor neurones.

Question 4

Sensory Neurones

Answer 4

Nerve cell that carries nerve impulses from a receptor to the spinal cord when a stimulus is detected. Sensory neurones can be long because they may need to transmit nerve impulses from receptors in distant parts of the body towards the spinal cord.

Question 5

Relay Neurones

Answer 5

Nerve cell in the spinal cord that carries nerve impulses from a sensory neurone to motor neurones, coordinating a response to a stimulus - connects one neurone to the next.
Found in the brain. Makes links between sensory neurones and motor neurones in the spinal cord.

Question 6

Motor Neurones

Answer 6

Nerve cell that carries nerve impulses from the spinal cord to an effector such as muscles or glands.

Question 7

The synapse

Answer 7

Junction between two neurones. It allows an impulse to travel from an axon terminal of one neurone to a dendrite in another neurone. This means that, although neurones are not physically connected to each other, impulses may travel the length of the body.

Question 8

How synapses work

Answer 8

Axon terminals contain many tiny, bubble-like structures called vesicles .
- Sub-cellular structure involved in transport, storage and the breakdown of waste material.

These contain substances called neurotransmitters.
- A substance that allows the transmission of information across a synaptic gap.

Dendrites have receptor molecules that can bind with these neurotransmitters.

Question 9

When a nerve impulse arrives at the axon terminal of the left-hand neurone:

Answer 9

1. Vesicles release neurotransmitters into the synaptic cleft (also called the synaptic gap).
2. The neurotransmitters travel across the gap by diffusion .
3. The neurotransmitters bind with receptor molecules on the dendrite of the right-hand neurone.
4. A nerve impulse is triggered in this neurone.

Note that the way the synapse works means that impulses can travel in one direction only.

Question 10

Reflex Action

Answer 10

A means of automatically and rapidly integrating and coordinating stimuli with the responses of effectors (muscles and glands).

Reflex actions are:
- automatic (you do not have to think about what to do)
- rapid (they happen very quickly)
- innate (you do not need to learn how to do them).

Reflexes are rapid, automatic responses that do not involve choice or thought.

A means of automatically and rapidly integrating and coordinating stimuli with the responses of effectors (muscles and glands).

Question 11

Reflex Arcs

Answer 11

Pathway through the body that brings about a reflex action.

Beginning with a stimulus (change in the environment that can be detected), a reflex arc works like this:
receptor → sensory neurone → relay neurone → motor neurone → effector

Question 12

Receptors

Answer 12

In the nervous system, a cell that detects a stimulus. In the hormonal system, a protein on the outside of a cell that can bind to a hormone, causing a response inside the cell.

The human body contains many different types of receptor, such as:
- Touch receptors in the skin
- Sound receptors in the ear
- Light receptors in the eye.
- Chemical (taste) receptors in the nose, tongue and mouth
- Temperature receptors in the skin and brain.

A ‘receptor’ is something that ‘receives’ something. In this case, a stimulus.

Question 13

Effector

Answer 13

In the nervous system, a muscle or gland that produces a response to a stimulus.

Question 14

Hormone

Answer 14

A substance, produced by a gland and carried by the blood, which alters the activity of one or more specific target organs.

Question 15

Explaining sense organs

Answer 15

A group of receptor cells that respond to a specific stimulus.

Question 16

The Eye

Answer 16

Sense organ that is sensitive to light. The eye consists of:
- Iris
- Pupil
- Cornea
- Lens
- Retina
- Blind Spot
- Optic Nerve

Light entering the eye travels in this direction:
cornea → pupil → lens → optic nerve

Question 17

Iris

Answer 17

A coloured ring of muscles that controls the size of the pupil in the eye.
The iris responds to changes in light intensity, but is not itself sensitive to the light.

Question 18

Pupil

Answer 18

The hole in the iris at the front of the eye that allows light to enter.

Question 19

Cornea

Answer 19

A tough, colourless and transparent outer layer covering the iris and pupil of the eye. Most refraction of light happens through the cornea.

Question 20

Lens

Answer 20

Transparent rounded structure that refracts light, focusing it onto the retina.

Question 21

Retina

Answer 21

Layer containing receptor cells that responds to light levels and to light of different colours.

The retina is at the back of the eye, and receives the focused image.

Question 22

Blind Spot

Answer 22

Part of the retina where the optic nerve enters the eye. As it does not contain receptor cells, it is not sensitive to light.
Where the optic nerve joins the retina, there are no cones and rods, so no light can be detected.

Question 23

Optic Nerve

Answer 23

Bundle of neurones that carries nerve impulses and signals from the retina (eye) to the brain.

Question 24

The Pupil Reflex

Answer 24

Reflex action in the eye that controls the amount of light entering the pupil, controlled by the radial and circular muscles of the iris working antagonistically.

It prevents damage to the eye.

Light enters the eye through the pupil and hits the retina at the back of the eye. The retina can be damaged if too much light enters the eye.

The pupil reflex is sometimes called the iris reflex. This is because the muscles of the iris control the size of the pupil in response to light. As the light intensity increases, the diameter of the pupil decreases. This means that:
- In dim light, the pupil becomes larger, letting more light into the eye
- In bright light, the pupil becomes smaller, letting less light into the eye.

This response is involuntary (you do not have to think about it) and happens rapidly.

Question 25

Antagonistic Pairs

Answer 25

Muscles often work in pairs called antagonistic pairs: as one muscle contracts, the other muscle relaxes. The radial muscles and circular muscles in the iris are an example of an antagonistic pair:
- In dim light, the pupil dilates, letting more light into the eye. To make this happen, the radial muscles contract and the circular muscles relax.
- In bright light, the pupil constricts, letting less light into the eye. To make this happen, the radial muscles relax and the circular muscles contract.

Question 26

Radial Muscle

Answer 26

Muscles in the iris of the eye that control the size of the pupil, working in an antagonistic pair with the circular muscles.

Question 27

Circular Muscles

Answer 27

Muscles in the iris of the eye that control the size of the pupil.