5.2 Human nervous system

Question 1

Human nervous system

Answer 1

The nervous system protects organisms from harm by responding to changes in the environment. It does this by coordinating communication between different parts of organisms.
Central nervous system (CNS) – the brain and spinal cord
Peripheral nervous system (PNS) – all of the nerves in the body
Information is sent through the nervous system as electrical impulses.

Question 2

Reflex actions

Answer 2

An involuntary (or reflex) response does not involve the conscious part of the brain as the coordinator of the reaction.
Awareness of a response having happened occurs after the response has been carried out.
Responses are therefore automatic and rapid – this helps to minimise damage to the body.
Some examples of reflexes are: Blinking, sneezing.

Question 3

Reflex arc

Answer 3

A stimulus can be any change in the environment to which the body needs to respond.
The stimulus is detected by a receptor.

Receptors are found all over the body.
They detect the change in the environment and initiate a signalling process within the body.
The signal is picked up by a neurone (nerve cell).

The coordinator formulates a response (the spinal cord or the CNS).

An effector is a muscle or gland that brings about an action in response to the change in the internal or external environment.

The response can be any action that helps the organism to avoid the harmful situation.

Question 4

Practical 7 (reaction time)

Answer 4

IV - dominant or non-dominant hand
DV - distance the ruler falls (reaction time)
CV - same ruler, environment, influence of drugs
Test - ruler drop test.

Question 5

Brain

Answer 5

The brain is a very complex organ that controls all conscious and unconscious thoughts in order to keep an organism alive.
The brain alongside the spinal cord is part of our central nervous system.
The brain is made of billions of interconnected neurones and is responsible for controlling complex behaviours.
Within the brain are different regions that carry out different functions.

Question 6

Parts of the brain

Answer 6

Cerebral cortex - This is the outer layer of the brain which is divided into two hemispheres. It’s highly folded and is responsible for higher-order processes such as intelligence, memory, consciousness and personality.
Cerebellum - This is underneath the cerebral cortex and is responsible for balance, muscle coordination and movement.
Medulla - This part is responsible for unconscious activities (e.g. breathing and heartbeat).

Question 7

Investigating the brain

Answer 7

The brain is an incredibly complex and delicate organ – this makes it extremely difficult for neuroscientists to study it to find out how it works.
Our understanding is limited because the brain is so complex and different regions can’t be studied in isolation.
Our limited understanding means that treating brain damage and disease is very difficult; in addition, any potential treatment carries risks of further damage occurring which can lead to increased problems.
Accidental damage could lead to speech or motor issues, or changes to personality which are permanent.

Question 8

Brain damage

Answer 8

The brain is delicate, complex, and not well understood.
Therefore, the treatment of brain damage and brain disease is difficult.
By studying patients with brain damage, where part of their brain doesn’t function, neuroscientists have been able to link particular regions of the brain to particular functions.

Question 9

MRI scanners

Answer 9

MRI stands for Magnetic Resonance Imaging.
MRI scanners are very important diagnostic tools used to study the brain and other regions of the body using magnetic fields and the effect these have on protons in the water molecules of the body
Functional MRIs can produce images of different regions of the brain that are active during different activities (the scanners can detect changes in blood flow – more active regions of the brain have increased blood flow)
MRI scanners have allowed us to learn which areas of the brain are active during different activities, such as moving, speaking and listening.

Question 10

Electrical stimulation

Answer 10

Electrical stimulation has also allowed us to treat certain disorders of the brain.
Tiny electrodes can be pushed into different parts of the brain, tiny jolts of electricity stimulate these regions and the effects can be observed.
Because the nervous system communicates using electrical impulses, electrical stimulation is used to help treat conditions such as Parkinson’s disease (causes tremors).

Question 11

Spinal cord

Answer 11

The spinal cord is the other component of the CNS. It is also important in coordinating the response of effectors to changes in the environment.

Question 12

Sensory neurons

Answer 12

The sensory neurone carries the signal in the form of an electrical impulse to the central nervous system (CNS).
Sensory neurones are long and have a cell body branching off the middle of the axon.

Question 13

Relay neuron

Answer 13

Relay neurones are found inside the CNS and connect sensory and motor neurones.
Relay neurones are short and have a small cell body at one end with many dendrites branching off it.

Question 14

Motor neurons

Answer 14

Motor neurones carry impulses from the CNS to effectors (muscles or glands).
Motor neurones are long and have a large cell body at one end with long dendrites branching off it.

Question 15

Nerve cells

Answer 15

Neurones (nerve cells) carry electrical impulses between receptors, the central nervous system (CNS) and effectors.

Question 16

Reflex to being stabbed with a pin

Answer 16

The pin (the stimulus) is detected by a (pain/pressure/touch) receptor in the skin.
A sensory neurone sends electrical impulses to the spinal cord (the coordinator).
An electrical impulse is passed to a relay neurone in the spinal cord.
A relay neurone synapses with a motor neurone.
A motor neurone carries an impulse to a muscle in the leg (the effector).
The muscle will contract and pull the foot up and away from the sharp object (the response) when stimulated by the motor neurone.

Question 17

Synapses

Answer 17

Neurones never touch each other, they are separated by junctions called synapses.
Synaptic junctions are incredibly small - around 10nm in size - and electrical impulses cannot cross them.
In a reflex arc, there are synapses between the sensory and relay neurones, and the relay and motor neurones
Chemicals called neurotransmitters (such as dopamine and serotonin) are released into the synaptic cleft and diffuse across it (down a concentration gradient).

Question 18

How does synapse work

Answer 18

The electrical impulse travels along the first axon.
When an electrical impulse arrives at the end of the axon on the presynaptic neurone, chemical messengers called neurotransmitters are released from vesicles.
The neurotransmitters diffuse across the synaptic gap and bind with receptor molecules on the membrane of the second neurone (known as the postsynaptic membrane).
This stimulates the second neurone to generate an electrical impulse that travels down the second axon.
The neurotransmitters are then destroyed or recycled to prevent continued stimulation of the second neurone which would cause repeated impulses to be sent.
Synapses ensure that impulses only travel in one direction, avoiding confusion within the nervous system if impulses were travelling in both directions.
As this is the only part of the nervous system where messages are chemical as opposed to electrical, it is the only place where drugs can act to affect the nervous system - eg this is where heroin works.

Question 19

Adaptations of the nervous system

Answer 19

Neurones have a cell body (where the nucleus and main organelles are found) and cytoplasmic extensions from this body called axons and dendrites.
Some human neurones have axons over a metre in length (but only 1 - 4 micrometres wide).
This is far more efficient than having multiple neurones to convey information from the CNS to effectors – less time is wasted transferring electrical impulses from one cell to another.
The axon is insulated by a fatty myelin sheath with small uninsulated sections along it (called nodes) which the impulse jumps along.

Question 20

Retina scanning

Answer 20

Your retina is full of receptor cells, which are sensitive to both the brightness (light intensity) and the colour of light.
Retina scanning looks at the pattern of blood vessels in your retina to identify you.

Question 21

Eye

Answer 21

The eye is a sense organ containing receptor cells which are sensitive to light intensity and colour.
The purpose of the eye is to receive light and focus it onto the retina at the back of the eye.

Question 22

Iris

Answer 22

The iris controls pupil diameter and the quantity of light reaching the retina.

Question 23

Retina

Answer 23

Controls the light receptor cells that detect light intensity and colour of light.

Question 24

Optic nerve

Answer 24

Sensory neurone that carries electrical impulses from the eye to the brain.

Question 25

Sclera

Answer 25

White outer layer which supports the structures inside the eye. It is strong to prevent damage to the eye.

Question 26

Cornea

Answer 26

Transparent covering of the front of the eye that refracts light.

Question 27

Ciliary muscles

Answer 27

Ring of muscles around the lens which relaxes and contracts to change the shape of the lens.

Question 28

Suspensory ligaments

Answer 28

Work with the ciliary muscles to change the shape of the lens.

Question 29

Lens

Answer 29

Transparent disc that changes shape to focus light onto the retina.

Question 30

Vitreous humour

Answer 30

Fluid behind the lens that helps maintain the shape of the eye and lens. It also keeps the retina against the wall of the eye.

Question 31

Aqueous humour

Answer 31

Fluid in front of the lens that helps maintain the shape of the eye and lens.

Question 32

Fovea

Answer 32

A region of the retina with the highest density of cones (colour detecting cells) where the eye sees particularly good detail.

Question 33

Adaption of the eye

Answer 33

Dim light - Photoreceptors detect change in environment (dark). Radial muscles contract. Circular muscles relax. In dim light, the pupil dilates (widens) to allow as much light into the eye as possible. Bright light - Photoreceptors detect change in environment (bright). Radial muscles relax. Circular muscles contract. In bright light, the pupil constricts (narrows) to prevent too much light from entering the eye and damaging the retina

Question 34

Accommodation of the eye

Answer 34

Accommodation is the process of changing the shape of the lens to focus on near or distant objects. The lens is elastic and its shape can be changed when the suspensory ligaments attached to it become tight or loose. Changing the shape of the lens alters how much light is refracted . This is important in making sure that light is focused on the retina of the eye rather than in front or behind it. The contraction or relaxation of the ciliary muscles brings about the changes.

Question 35

Focusing on objects

Answer 35

Close objects - The ciliary muscles contract. The suspensory ligaments loosen. The lens is then thicker and refracts light rays more strongly. Distant objects - The ciliary muscles relax. The suspensory ligaments are pulled tight. The lens is then pulled thin and only slightly refracts light rays.

Question 36

Defects of the eye

Answer 36

Two common defects of the eyes are myopia (short-sightedness). hyperopia (long-sightedness). In both defects rays of light do not focus on the retina Generally, these defects are treated with spectacle lenses (glasses). Which refract the light rays so that they focus on the retina.

Question 37

Myopia

Answer 37

Myopia is short sightedness The lens is too thick and curved and the eyeball is too elongated so the distance between the retina and lens is too great. This causes the image to be in focus before reaching the retina. A concave lens can be used to correct light rays so they focus on the retina.

Question 38

Hyperopia

Answer 38

Hyperopia is long sightedness The eyeball is too short and the distance between the retina and the lens is too short. This causes the image to be in focus behind the retina. A convex lens can be used to refract light rays so they focus on the retina.

Question 39

Contact lenses

Answer 39

Hard and soft contact lenses: These sit on the surface of the eye and are almost invisible, making them ideal for activities like sports. Soft lenses are more comfortable but carry a higher infection risk.

Question 40

Laser eye surgery

Answer 40

Lasers can be used to change the shape of the cornea (changing how it refracts light onto the retina). All surgical procedures have a risk of unexpected damage occurring during the procedure which could lead to worse vision or an infection. For myopia: the cornea is slimmed down, reducing the refractive power. For hyperopia: the cornea shape is changed so the refractive power is increased.

Question 41

Lens replacement surgery

Answer 41

This surgery completely replaces the lens of the eye with a plastic artificial lens (rather than changing the shape of the cornea during laser eye surgery). The procedure is more invasive than laser surgery and carries a risk of damage occurring to the retina leading to complete sight loss.