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The human nervous system

5.2.1 Structure and function

Central nervous system (CNS)
  • Brain & spinal cord
  • Coordinates response of effectors eg muscles contracting / glands secreting hormones

Stimulus → receptor → coordinator (CNS) → effector → response

Describe the function of receptors. (2)

  • Detect changes in surroundings
  • Convert stimulus to impulse
 Describe the function of effectors. (2)

  • Convert impulse to an action
  • Eg muscle / gland

Reflex action

  • Automatic & rapid
  • Over short period of time
  • Don’t involve conscious part of brain
  • Involves nerve cells & impulses
  • Affects only one part of body

Structure in a reflex action

Type of effector Response effector makes
muscle contraction
gland Produce chemical / enzyme

Describe the stages that happened in a reflex action. (6)

Stimulus → receptor → sensory neurone (in peripheral) → synapse → relay neurone (in CNS) → synapse → motor neurone (in peripheral) → effector → response

  1. Stimulus detected by receptor
  2. Receptor sends electrical impulses along sensory neurons and reaches synapse
  3. Chemical releases from sensory neurons
  4. Chemical diffuses across synapse and attaches to relay neurone to stimulate electrical impulse
  5. Impulse passes along relay neurone and reaches synapse
  6. Chemical release from relay neurone
  7. Chemical diffuses across synapse and attaches to motor neurone to stimulate electrical impulse
  8. Impulse passes along motor neurone to an effector
  9. Effector causes a response

How can you tell that it’s not a conscious action? (1)

  • Not connected to brain
  • Coordinated only by spinal cord

Reflex actions like this are useful. Explain why. (2)

  • Rapid response protects body from damage or pain

Reflex action times investigation

  1. Student A sits with his elbow resting on the edge of a table.
  2. Student B holds a ruler with the bottom of the ruler level with the thumb of Student A.
  3. Student B drops the ruler.
  4. Student A catches the ruler and records the distance.
  5. Steps 1 to 4 are then repeated.

Give two variables the students controlled in their investigation. (2)

  • Drop ruler from same height
  • Use same weight of ruler

What improvement could the students make to the method so the results are more valid? (1)

  • Carry out more repeats

Give a conclusion about the effect of caffeine on reflex actions & reaction time. (1)

Caffeine speeds up reflex actions & decreases reaction time

5.2.2 The brain (biology only)

Functions

Cerebral cortex – consciousness, intelligence, memory & language

Cerebellum – coordination of muscular activity & balance

Medulla – unconscious activities eg heartbeat & breathing

Methods of finding out how much the brain is damaged

Neuroscientists map regions of brain to particular functions by…

  1. Studying patients with brain damage
  2. Electrode stimulaton – electrically stimulating different parts of brain
  3. MRI scan

Name the technique a doctor would use to find out if the man’s brain has been damaged. (1)

  • MRI scan

Difficulties of brain investigation

Explain why it is difficult to treat brain damage. (1)

  • High risk of causing more damage to brain
  • It’s complex & delicate
  • Difficult to treat brain disorders

5.2.3 The eye (biology only)

Eye – a sense organ containing receptors sensitive to light intensity & colour

Functions

Retina contains light receptors, rods & cones
Optic nerve carries sensory neurons from retina to brain
Sclera tough outer coat
Cornea Focus light onto retina
Iris controls amount of light entering eye
Ciliary muscles controls shape of lens
Suspensory ligaments attach lens to ciliary muscles

Pupil reflex in response to light

Accommodation

process of changing the shape of the lens to focus on near or distant objects

To focus on near object To focus on a distant object
  • ciliary muscles contract
  • suspensory ligaments loosen
  • lens is then thicker & refracts light rays strongly
  • ciliary muscles relax
  • suspensory ligaments are pulled tight
  • lens is then pulled thin & only slightly refracts light rays

Eye defects

Occur when light doesn’t focus on retina

Types Myopia (short sightedness) Hyperopia (long sightedness)
  • Lens too curved
  • Distant objects appear blurry
  • Lens too flat
  • Cannot refract light enough

Treatments

 

1. Spectacle lenses
  • Refract light rays so they focus on retina
Myopia Hyperopia

Concave lens bends light so light focuses on the retina

Convex lens bring rays together
2. Contact lenses Hard or soft last for different lengths of time
3. Laser surgery Myopia Hyperopia
  • Reduce thickness of cornea
  • So it refracts less light
  • Change its curvature
  • So it refracts light more strongly
4.Replacement lens
  • Treat hyperopia by replacing it with artificial lens
  • Could damage retina

5.2.4 Control of body temperature (biology only)

Describe what happens in the body to keep the body temperature constant. (6)

  • Body temp monitored by thermoregulatory centre (TC)
  • TC contains thermoreceptor
  • Thermoreceptor in skin send nervous impulses to TC, giving info about skin temp

If core body temp too high

  • Blood vessels supplying blood to skin dilate
  • So more blood flows near skin surface
  • More heat loss
  • Sweat glands release more sweat to cool body

If core body temp too low

  • Blood vessels supplying blood to skin constrict
  • Less blood flows near skin surface
  • Less heat loss
  • Muscles may shiver to release heat energy

Why it is important to control body temperature. (1)

  • Enzymes work best
  • So chemical reactions are fastest
  • Prevent damage to cells