B10 The Human Nervous System

Question 1

What is the function of the nervous system?

Answer 1

The nervous system enables humans to react to their surroundings and to coordinate their behaviour

Question 2

What does the human nervous system consists of?

Answer 2

The human nervous system consists of:
- Central nervous system (CNS) – the brain and spinal cord
- Peripheral nervous system (PNS) – all of the nerves in the body

Question 3

How does a stimulus lead to a response being carried out by the body?

Answer 3

• Stimulus is converted into an electrical impulse by the receptors
• The electrical impulse passes along sensory neurones to the Central Nervous System (CNS)
• The CNS coordinates an appropriate response and an electrical impulse is sent along motor neurones to the effector, which carries out the response
  OR
• Information from receptors passes along cells (neurones) as electrical impulses to the central nervous system (CNS)
• The CNS is the brain and spinal cord
• The CNS coordinates the response of effectors which may be muscles contracting or glands secreting hormones

Question 4

What is a nerve?

Answer 4

It is a bundle of neurones

Question 5

What is the pathway through the nervous system?

Answer 5

Stimulus - Receptor - Coordinator - Effector - Response

Question 6

What is a reflex action?

Answer 6

• 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

Question 7

Why are reflex actions important?

Answer 7

They aid survival by preventing harm to the body

Question 8

Describe how a reflex action occurs via a reflex arc

Answer 8

1. The stimulus is detected by a receptor
2. A sensory neurone sends electrical impulses to the spinal cord (the coordinator)
3. An electrical impulse is passed to a relay neurone in the spinal cord
4. At a synapse between a sensory neurone and a relay neurone, a chemical diffuses across the gap and stimulates a new impulse which passes along the relay neurone
5. The same process as above occurs at a synapse between a relay and a motor neurone
6. A motor neurone carries an impulse to a muscle in the leg (the effector)
7. At the effector, an appropriate response is carried out. (eg The muscle will contract and pull the foot up and away from the sharp object)

Question 9

What are the three types of neurones in a reflex arc?

Answer 9

• Sensory: carry impulses from sense organs to the CNS (brain or spinal cord)
• Relay are found inside the CNS and connect sensory and motor neurones
• Motor carry impulses from the CNS to effectors (muscles or glands)

Question 10

What are the main differences between relay, motor and sensory neurons?

Answer 10

• Sensory neurones are long and have a cell body branching off the middle of the axon
• Relay neurones are short and have a small cell body at one end with many dendrites branching off it
• Motor neurones are long and have a large cell body at one end with long dendrites branching off it

Question 11

What are synapses?

Answer 11

• Neurones never touch each other, they are separated by junctions (gaps) 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)
• These chemicals then arrive at receptors sites on the next neurone. Here they attach to the surface of the next neurone and set up a new electrical impulse

Question 12

Exam Tip

Answer 12

Exam Tip
Exam questions about neurotransmitters are a good opportunity for examiners to introduce unfamiliar examples and contexts.
Remember the following:
- Neurotransmitters move by diffusion – remember this requires a concentration gradient and is a passive process
- Receptors that are complementary in shape to neurotransmitters are located on the postsynaptic neurone
Drugs (such as heroin, ecstasy and cocaine) can bind to
neurotransmitter receptors, triggering impulses in different regions of the brain
Frequent drug-use can lead to overstimulation of neurones, leading to loss of function or overstimulation of different regions of the brain and body

Question 13

What is the difference between a reflex pathway and a conscious pathway?

Answer 13

Within a reflex pathway, the coordination centre is a relay neurone found in the spinal cord/ unconscious part of the brain. In a conscious pathway, the coordination centre is in the conscious part of the brain

Question 14

What is the brain and what is it made up of?

Answer 14

• 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 15

Talk about the different regions of the brain

Answer 15

• The 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
• The cerebellum: this is underneath the cerebral cortex and is responsible for balance, muscle coordination and movement
• The medulla: this region controls unconscious activities such as heart rate and breathing

Question 16

What are some of the specialised parts of the brain?

Answer 16

• Hypothalamus: involved in controlling body temperature
• Pituitary glands: produce many different chemical (hormones) which play a big part in coordinating and controlling body systems

Question 17

Why is our understanding of the brain limited?

Answer 17

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 18

How have neuroscientists been able to map the regions of the brain to particular functions?

Answer 18

• By studying patients with brain damage,
• electrically stimulating different parts of the brain and - using MRI scanning techniques

Question 19

How can patients with brain damage help scientists learn about the brain?

Answer 19

Patients with brain damage can be studied to see what effect it has on them physically or on their personality or capabilities.
The most famous example of this is Phineas Gage in 1848, a railroad construction worker who survived a large iron rod being driven completely through his head – his frontal left lobe was completely destroyed and his personality and temperament changed drastically. Modern scans revealed that the accident damaged part of his cerebral cortex, including the area which controls social inhibitions. Modern scientists explained what happened nearly 200 yrs ago

Question 20

How can electrical stimulation help scientists learn about the brain?

Answer 20

Tiny electrodes can be pushed into different parts of the brain, tiny jolts of electricity stimulate these regions and the effects can be observed
For example, if a region in the medulla responsible for movement is stimulated, the movement caused can be observed. People can experience fear, hunger, thirst, anger

Question 21

Why is brain surgery usually done on conscious patients?

Answer 21

Because there are no sensory nerve ending in the brain

Question 22

How can MRI help scientists learn about the brain?

Answer 22

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 like listening to music or recalling a memory
(the scanners can detect changes in blood flow – more active regions of the brain have increased blood flow)

Question 23

Exam Tip

Answer 23

Exam Tip
In the exam you may be asked to evaluate the benefits and risks of procedures carried out on the brain and nervous system. The benefits of procedures being carried out usually involve improving the quality of
someone’s life (as the procedure is used to treat a disorder of some kind) but there are risks of more permanent damage, some of these will be
because we still don’t fully understand how the brain and nervous system works!

Question 24

What is the eye?

Answer 24

• The eye is a sense organ containing receptors sensitive to light intensity and colour
• Receptors are groups of specialised cells that can generate an electrical impulse in a sensory neurone
• The eye contains two types of receptor cell: rod cells which are sensitive to light intensity and cone cells which are sensitive to different wavelengths of visible light (colour)
• The eye is an organ made from several different types of tissue. All of the structures function together to allow light to hit the retina, which sends signals to the brain

Question 25

What is the structure of the eye?

Answer 25

The purpose the eye is to receive light and focus it onto the retina at the back of the eye
- The retina is a layer at the back of the eye, where the light sensitive cells are located (rod - detect light intensity- and cone cells - detect colour)
- Th sclera is the white outer layer of the eye. It is relatively tough and strong so the eyeball is not easily damaged.
- Cornea: this is a transparent lens at the front of the eyeball which refracts (bends) light as it enters into the eye. Its curved surface is very important for changing the direction of the light rays coming into the eye, making sure that they enter the eye and are focused on the retina
- Iris: It controls how much light enters the pupil. The iris is made of muscles which contract or relax to change the size of the pupil and so control the amount of light reaching the retina. In dim light the pupil is enlarged so as much light as possible enters the eye. In bright light the iris makes the pupil very small so that too much bright light does not damage the delicate light sensitive cells
- Pupil: hole that allows light to enter the eye
- Lens: transparent disk that can change shape to focus light on the retina. The lens is kept in place by suspensory ligaments and ciliary muscles. The lens fine tunes the focusing of the light rays, changing their direction to produce a clear image on the retina.
- Optic nerve: when the light hits the retina the light-sensitive cells are stimulated. they send impulses to the brain along sensory neurons in the optic nerve. When the brain receives them, it interprets it as visual image

Question 26

What is a blind spot?

Answer 26

It is point where the optic nerve leaves the eye. It has no retina so it is a blind spot. You are unaware of the blind spot because your brain “fills the gap”

Question 27

What is the pupil reflex?

Answer 27

Very bright light can damage the receptor cells of the retina, the pupil reflex protects the eye by altering the diameter of the pupil.
It’s a reflex action so it happens automatically
The reflex action is controlled by two groups of muscle: - radial muscle and
- circular muscles
In dim light, the pupil dilates (widens) in order to allow as much light into the eye as possible. To achieve this the circular muscles relax, the radial muscles contract and the pupil dilates
In bright light, the pupil constricts (narrows) in order to prevent too much light entering the eye and damaging the retina. The circular muscles contract and the radial muscles relax.

Question 28

What is accomodation?

Answer 28

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.
The changes are brought about by the contraction or relaxation of the ciliary muscles.

Question 29

What happens when we focus on a near object?

Answer 29

To focus on a near object:
- The ciliary muscles contract
- The suspensory ligaments loosen
- The lens is then thicker and refracts light rays more strongly
See picture pg 156 biology book

Question 30

What happens to focus on a distant object?

Answer 30

To focus on a distant object:
The ciliary muscles relax
The suspensory ligaments are pulled tight
The lens is then pulled thin and only slightly refracts light rays

Question 31

Exam Tip
Accommodation

Answer 31

Exam Tip
Accommodation is something you can work out in an exam if you have forgotten – staring at your hand right in front of your eye will make your eyes feel tight after a few seconds. This is because the ciliary muscles are
contracted. Staring at an object far away feels relaxing and comfortable because the ciliary muscles are relaxed.

Question 32

What is myopia?

Answer 32

short-sightednes. If you have myopia you can see close objects in focus, but distant objects look blurred. The light is focused in front of the retina, so the images that actually land on the retina are out of focused and blurry. This might be caused by a lens that is too curved or a particularly long eyeball.
Treatment: most common treatment is wearing glasses with a concave lens that spreads out the light from distant objects before it reaches the eye

Question 33

What is hyperopia?

Answer 33

long-sightednes. Closer objects appear blurred. This might be the result of a lens which is too flat and thin, or a particularly short eyeball. As a a result the lens cannot refract the rays of light strong enough. The light is focused behind the retina, so the image which lands on the retina is out of focus.
To correct this, glasses with convex lenses are used. They bring the light rays together more before they reach the eye itself.

Question 34

Exam tip on ray diagrams

Answer 34

Exam Tip
You should expect to see ray diagrams, showing myopia and hyperopia of
the eye and be able to demonstrate how spectacle lenses can correct them.

Question 35

New technologies are now available which can treat both of these defects rather than using spectacle lenses

Answer 35

Treatments include:
- 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
- Laser surgery: lasers can be used to change the shape of the cornea (changing how it refracts light onto the retina) although like all surgical procedures there is 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
- Lens replacement 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) but this procedure is more invasive than laser surgery and carries a risk of damage occurring to the retina leading to complete sight loss

Question 36

What are the two types of contact lenses:

Answer 36

• Hard - rigid material, last a long time, must be kept sterile
• Soft - flexible material, last for a shorter time, more comfortable