5.2 The Human Nervous System

What does the central nervous system (CNS) consist of?

• Brain

• Spinal cord

Describe the function of the nervous system.

Allows humans to:

• react to their surroundings

• coordinate their behaviour

Describe how the nervous system brings about a response.

• Information from receptors passes along neurones (as electrical signals) → CNS

• CNS coordinates response of effectors:

  • muscles contracting

  • glands secreting hormones

• stimulus → receptor → coordinator → effector → response

Describe and explain how the various structures in a reflex arc relate to their function.

• Stimuli detected by receptors

• Electrical impulses pass along relay neurones → spinal cord

• Neurotransmitters diffuse across synapse → relay neurone

• Electrical impulses passed → motor neurones — cause effectors to bring about a response

What are reflex actions?

• Rapid and automatic responses to stimuli

• Actions that do not involve conscious parts of brain

Explain why reflex actions are important.

Prevent people from getting hurt

Give 2 examples of reflex actions.

• Pupils getting smaller — avoid damage from bright lights

• Moving hand from hot surface — prevents damage

Define a synapse.

Gap between two neurones

What is reaction time?

How long it takes to respond to a stimulus

Give examples of factors that could be investigated in this experiment. (possible independent variables)

• Practice

• Hand used

• Chemicals — e.g. caffeine

• Background noise

• Exercise

State the dependent and control variables for required practical 7.

• Dependent — distance that ruler drops (reaction time)

• Control:

  • same starting distance between thumb and index

  • measuring from top of thumb each time

  • room conditions — e.g. lighting, temperature

Describe the effects that each of the possible independent variables could have on reaction time.

• Practice – decreases RT

• Chemicals (caffeine) – speeds up reflex actions; decreases RT

• Background noise – distraction so increases RT

• Exercise:

  • light exercise – increases blood flow -> decreases RT

  • heavy exercise – could become fatigued -> increases RT

• Hand used:

  • dominant – decreases RT (used to using it)

  • non-dominant – increases RT

Describe a method to investigate the effect of a factor on reaction time.

• Person 1 lays dominant hand across table w/ arm overhanging edge

• Person 2 holds metre ruler vertically w/ 0 cm mark between person 1’s thumb and index finger

• Person 2 drops ruler and person 1 catches it

• Record number on ruler that is level with top of person 1’s thumb

• Repeat steps 1 - 4

• Use conversion table to convert ruler measurements → reaction time

Describe the structure and function of the brain.

• Controls complex behaviour

• Has billions of interconnected neurones

• Has different regions that carry out different functions

Label each part of the brain.

• A — Cerebellum

• B — Pituitary gland

• C — Cerebral cortex

• D — Medulla

• E — Hypothalamus

Describe the function of the cerebral cortex.

Controls:

• memory

• intelligence

• language

• consciousness

Describe the function of the cerebellum.

Controls:

• fine movement of muscles

• balance

What is the function of the medulla?

Controls unconscious actions — e.g. breathing, heart rate

Explain why investigating brain function and treating brain damage and disease is difficult.

• Brain is:

  • easily damaged

  • complex

  • delicate

• Drugs given to treat disease can’t always reach brain —- membranes that surround it

State 3 ways that neuroscientists can map out regions of the brain.

• Studying patients w/ brain damage

• Electrically stimulating parts of brain

• MRI scanning techniques

Describe and evaluate how studying patients with brain damage can be useful for understanding more about the brain.

• Scientists could determine the function of a part of brain by observing changes in the patient’s behaviour or personality after damage

  • provides evidence of what happens when function is lost

  • multiple areas damaged → scientists not able to pinpoint what function specific parts have

Describe and evaluate how electrically stimulating different parts of the brain can be useful for understanding more about the brain.

• Putting electrodes in someone’s brain + observing what happens based on mental or physical changes

  • allows scientists to know functions of parts of brain

  • patient can give feedback

  • very invasive

  • requires surgery

    • infection

    • brain damage

Describe and evaluate how using MRI scanning techniques can be useful for understanding more about the brain.

• Using MRI scanner to create image of brain

• Shows which parts of brain are:

  • affected by a tumour

  • active during tasks

    • not invasive

    • very safe

    • provides detailed images of brain in action

    • patient has to stay still

    • very expensive equipment

Describe the functions of the eye.

• Accommodation to focus on near or distant objects

• Adaptation to dim light

Label each part of the eye.

• A — Cornea

• B — Lens

• C — Optic nerve

• D — Retina

• E — Sclera

• F — Ciliary muscle

• G — Suspensory ligament

What is the eye?

• Sense organ

• Contains receptors sensitive to:

  • light intensity

  • colour

Define accommodation.

Process of changing shape of lens to focus on near or distant objects

Describe what the eye does to focus on a near object.

• Ciliary muscles contract

• Suspensory ligaments loosen

• Lens is thicker + refracts light rays strongly

Describe what the eye does to focus on a distant object.

• Ciliary muscles relax

• Suspensory ligaments are pulled tight

• Lens is pulled thin + only slightly refracts light rays

Describe the structure and function of the retina.

• Layer of light sensitive cells at the back of eye

• Light rays hit retina → cells are stimulated → produce impulses (sent to brain along optic nerve)

Describe the structure and function of the optic nerve.

• Nerve that leaves the eye and leads to brain

• Sends impulses from retina → brain — can form an image

Describe the structure and function of the sclera.

• Tough, white outer layer of eye

• Supports structures inside eye

• Strong — prevents damage to eye

Describe the structure and function of the cornea.

• See-through layer at front of eye

• Allows light through + curved surface bends and focuses light onto retina

Describe the structure and function of the iris.

• Muscles that surround pupil

• Contract and relax to alter size of pupil

Describe what the eye does in bright light.

• Circular muscles contract

• Radial muscles relax

• Makes pupil smaller — avoids damage to retina

Describe what the eye does in dim light.

• Circular muscles relax

• Radial muscles contract

• Makes pupil larger — lets more light enter → creates better image

Describe the function of ciliary muscles and suspensory ligaments.

• Hold the lens in place

• Control the lens’ shape

What are two common defects of the eye?

• Myopia

• Hyperopia

What is myopia also known as?

Short sightedness

What is hyperopia also known as?

Long sightedness

What happens to the light entering your eye if you have myopia or hyperopia?

Rays of light do not focus on the retina:

• Myopia — in front

• Hyperopia — behind

People with ___________ see close objects in clear focus, but light from distant objects is focused __ _______ __ the retina, so the images on the retina are out of ______ and ________.

• myopia

• in front of

• focus

• blurry

What can having myopia be a result of? (2)

• Lens that is too curved

• Long eyeball

How is myopia commonly treated?

Glasses with a concave lens.

How do concave lenses work for myopia?

They spread out the light from distant objects so it is in perfect focus on the retina.

People with ____________ focus clearly on distant objects but close objects appear _________. The lens cannot ________ the rays of light __________ enough. The light is focused _________ the retina, so the images are out of ________.

• hyperopia

• blurred

• refract

• strongly

• behind

• focus

What can having hyperopia be a result of? (2)

• Lens is too flat and thin

• Short eyeball

How is hyperopia commonly treated?

Glasses with a convex lens.

How do convex lenses work for hyperopia?

They bring the light rays together so that they can be focused on the retina.

What are the 4 ways to treat hyperopia or myopia?

• Glasses

• Contact lenses

• Laser eye surgery

• Replacement lenses

_________ _________ are lenses that are placed on the surface of the eye. They work like conventional glasses, but cannot be _______.

• Contact lenses

• seen

What are the advantages of hard contact lenses?

• Makes it easier to play sports and general activities

• Lasts a long time

What are the disadvantages of hard contact lenses?

• Risk of eye infections

• Can be uncomfortable

What are the advantages of soft contact lenses?

• More comfortable to wear

• Makes it easier to play sports and general activities

What are the disadvantages of soft contact lenses?

• Risk of eye infections

• Doesn’t last very long

Some soft lenses last for a __________, but have to be removed and kept in _______ solution overnight. Others are __________ — worn for a day and thrown away.

• month

• sterile

• disposable

How does laser eye surgery treat myopia?

• Reducing the thickness of the cornea so it refracts the light less strongly.

• This focuses distant light on the retina.

How does laser eye surgery treat hyperopia?

• Lasers are used to change the curve of the cornea.

• This makes it refract light from close objects more effectively.

What is the main advantage of laser eye surgery?

It is a permanent solution that eliminates the need for glasses.

What are the disadvantages of laser eye surgery?

• Expensive

• Surgery carries a risk of infection

How do replacement lenses work?

• A permanent contact lens is implanted into the eye.

or

• The faulty lens is replaced with an artificial one.

What is the main advantage of replacement lenses?

It is a permanent solution that eliminates the need for glasses.

What are the disadvantages of replacement lenses?

• Damage to the retina

• Cataracts developing if the natural lens remains in place

• Infections

Body temperature is monitored and controlled by the __________________ ________ in the ________.

• thermoregulatory centre

• brain

What does the thermoregulatory centre have?

Receptors that are sensitive to the temperature of the blood.

The skin contains ______________ ____________ and sends __________ ___________ to the _________________ _________.

• temperature receptors

• nervous impulses

• thermoregulatory centre

What happens when body temperature is too high?

• Your enzymes stop working effectively and begin to denature.

• The reactions of respiration stop working, causing death.

What does your body do when the core temperature is too high?

• Blood vessels dilate (vasodilation).

• Sweat is produced from the sweat glands.

• Both these mechanisms cause a transfer of energy from the skin to the environment.

How does vasodilation cool you down?

More blood flows closer to the surface of the skin, resulting in increased energy transfer from the body.

How does sweating cool you down?

Sweat evaporates from skin surface resulting in increased energy transfer away from the body.

What happens when body temperature is too low?

• The rate of the enzyme-controlled reactions in your cells falls too low.

• Cells begin to die.

What does your body do when the core temperature is too low?

• Blood vessels constrict (vasoconstriction)

• Sweating stops

• Skeletal muscles contract (shivering)

How does vasoconstriction warm you up?

It reduces the energy transferred by radiation through the surface of the skin.

How does stopping sweat production warm you up?

Less water from sweat evaporates, so less energy is transferred to the environment.

How does shivering warm you up?

• These muscle contractions need lots of respiration — an exothermic process.

• The energy transferred from these exothermic reactions raises your body temperature until shivering stops.

Figure 2 shows some structures involved in the coordination of a reflex action.

Describe how the structures shown in Figure 2 help to coordinate a reflex action. [6 marks]

• A receptor detects a stimulus

• The receptor generates impulses

• Neurones conduct impulses

• Neurone A conducts impulses to spinal cord

  • neurone A = sensory neurone

• There is a synapse between neurones

• A neurotransmitter diffuses across the synapse

• The neurotransmitter stimulates impulses in neurone B

  • neurone B = relay neurone

  • neurone C = motor neurone

• An effector carries out response

• The muscles contract or a gland secretes chemicals

• An insect flies near a person's eye.

• The person blinks.

• This is a reflex action.

Figure 2 shows the coordination system for this reflex action.

Complete Figure 2.

Choose answers from the box below. [2 marks]

brain

cornea

iris

muscles

retina

• A — retina

• B — brain

• C — muscles

• Humans keep their internal conditions almost constant.

• Body temperature is kept within a narrow range.

• When the core body temperature is too low, this is detected by the thermoregulatory centre in the brain.

Describe how the body responds when a decrease in core body temperature is detected. [6 marks]

• The blood vessels supplying skin constrict (vasoconstriction)

• There is less blood flow to the skin

• So less energy is lost to the surroundings

• The muscles contract and relax rapidly (shivering)

• So they are respiring more which produces heat

• Sweating stops so less water is evaporating from the skin (which means less heat loss)

• In most MRI scanners the person being scanned needs to stay completely still.

• A functional MRI (fMRI) scanner allows a person to move while the scanner makes images of the person's brain activity.

Suggest how the fMRI scanner could help to find out more about the brain damage a person has. [3 marks]

• They can ask people to do different tasks while taking the scan

• To see which part of brain is active/inactive

• To compare with a person without brain damage

• To see exactly where the damage is

Many human actions are reflexes.

Which two of the following are examples of reflex actions? [2 marks]

• Releasing saliva when food enters the mouth

• Withdrawing the hand from a sharp object

• The diagram shows how a nerve impulse passing along a relay neurone causes an impulse to be sent along another type of neurone, neurone X.

• Scientists investigated the effect of two toxins on the way in which information passes across synapses.

• The table below shows the results.

Toxin	Effect at the synapse
Curare	Decreases the effect of the chemical on neurone X
Strychnine	Increases the amount of the chemical made in the relay neurone

Describe the effect of each of the toxins on the response by muscles. [2 marks]

• Curare — no contraction

• Strychnine — more contraction

• During hibernation an echidna wakes up several times.

• Each time the echidna wakes up it becomes active and its body temperature increases to over 30 °C.

Explain why the echidna has a higher body temperature when it is active. [2 marks]

• Movement requires energy from respiration

• Respiration releases heat which increases body temperature