Co-ordination & Control

• Co-ordination in animals is achieved through the nervous system and the hormonal system
• a change in environment is called a stimulus
• a stimulus produces a response
• in animals each type of stimulus is recognised by a receptor in the body
• when a receptor is stimulated it causes an effector (muscle or hormone producing gland) to produce a response
• a co-ordinator is needed to link between the receptor and the effector, usually brain or spinal cord - central nervous system
• 1. stimulus, change inside or outside body causing a change in behaviour
  2. receptors, sense cells or organs which detect stimuli
  3. co-ordinators, organs which receive info from receptors (brain or spinal cord)
  4. effector, controlled by co-ordinators e.g. muscles and glands
  5. response, behaviour resulting from original stimulus
• each eye consists of an eyeball which sits in an orbit in the skull, the sides and back of the eyeball are thick and tough and is know as the 'white of the eye'
• the front of the eye is transparent and is called the cornea
• the cornea is covered by a membrane called the conjunctiva
• the conjunctiva is moistened by tears, this is lubricating fluid produced by a gland under the eyelid, tears contain the enzyme lysozyme which kills bacteria
• the pupil is the centre of the eye, it is the aperture of the eye, in bright light the pupil gets smaller to stop too much light getting into the eye and in the dark the pupil gets bigger to let more light in, controlled by the iris
• the pupil is surrounded by the iris, the coloured part of the eye
• the eyeball is held in place by muscles that can move the eye up and down and from side to side
• the optic nerve runs from the back of the eyes to the brain, this nerve transmits impulses to the brain as to what is being seen
• conjunctiva, a protective transparent covering that stops dust and dirt damaging the cornea and keeps cornea moist
• cornea, transparent part of the sclera that allows light to enter
• pupil, the opening in the middle of the iris through which light passes
• iris, consists of radial and circular muscles that contract and relax to control the size of the pupil
• lens, bends (refracts) light and focuses the light rays on the retina
• retina, contains the receptor cells that are sensitive to light, the central part of the retina (fovea) contains a lot of light receptors
• optic nerve, contains sensory neurones that link the retina to the brain, carries electrical impulses from the retina to the brain
• the blind spot in the part of the retina where the optic nerve leaves (no image at this point and an image cannot be seen if it is focused on this part of the retina)
• aqueous humour, maintains the correct pressure at the front part of the eye and it nourishes the cornea which has no blood vessels
• vitreous humour, a jelly-like fluid which presses on the inner wall of the eyeball keeping it spherical
• too much light can damage the cells in the retina
• the muscles in the iris can contract or relax to change the size of the pupil, these muscles are called radial and circular muscles
• dim light produces a large pupil (dilates) to get as much light as possible into the eye and to the retina
• bright light reduces the size of the pupil (constricts) to reduce the amount of light entering the eye and falling on the retina
• in dim light, circular muscles are relaxed and radial muscles are contracted, so the pupil dilates
• in bright light, circular muscles are contracted and radial muscles are relaxed, so pupil constricts
• when looking at an object light is reflected from the object and enters the eye
• rays from the object pass through the cornea and lens they are bent inwards so that they can produce an image on the retina
• the lens focuses the light rays so that they meet exactly on the retina, this is called refraction, the refraction of the light produces an image on the retina which is upside down
• the cornea and lens are involved in focusing the light rays onto the retina
• light rays from distant objects reparallel and don't need much refraction whereas rays from a near object need a lot of refraction
• the lens of the eye can change shape in order to accommodate for near or far objects, this is called accommodation
• the ciliary muscle (body) is a ring of muscle that surrounds the lens
• the lens is attached to the ciliary muscle by suspensory ligaments, which are like small threads
• the ciliary muscle can contract and relax and this will either make the suspensory ligaments tighter or slacker