Biology: 4.5.2.3/4 - The human nervous system (the eye) (control of body temperature)

The eye

Sense organ with receptors sensitive to light intensity and colour which has many structures allowing the shape to change

Retina

• layer of light sensitive cells at the back of the eye

• light hits retina → impulses sent to the brain

Optic nerve

• nerve connecting eye and brain carrying impulses

Sclera

• white outer layer that supports structures inside the eye

• strong to prevent damage

Cornea

• see-through layer at the front of the eye

• lets light through

• focuses light onto retina

Iris

muscles surrounding the pupil

• in bright light - makes pupil smaller to avoid damage

  → circular muscles contract

  → radial muscles relax

• in dim light - enlarges pupil to allow more light in

  → circular muscles relax

  → radial muscles contract

Ciliary muscles & suspensory ligaments

• control the shape of lens

Focus on a near object

• ciliary muscles contract

• suspensory ligaments loosen

• lens becomes thicker and more curved

• light refracts more

Focus on a distant object

• ciliary muscles relax

• suspensory ligaments tighten

• lens becomes thinner

• light refracts less

Myopia

Short-sightedness where lens is too curved. So a concave lens is used.

Hyperopia

Long-sightedness where lens is too flat. So a convex lens is used.

Laser eye surgery

• reduces thickness of cornea so less light is refracted for myopia

• changes cornea curvature so light is refracted strongly for hyperopia

Replacement lens

• natural lens is replaced with an artificial on

• may damage retina

• cataracts can develop

Thermoregulatory centre

• found in the brain

• controls body temperature

• contains receptors sensitive to blood temperature

• receives impulses from temp receptors in skin

More than 37.5°C

• sweat is produced from sweat glands

• vasodilation where blood vessels dilate

• both cause transfer of energy from skin to environment

Less than 37.5°C

• sweating stops

• shivering - skeletal muscles contract

• hairs stand on end for insulation

• vasoconstriction where blood vessels constrict