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Coordination (Stimulus and response (Sequence of events: (stimulus –>…
Coordination
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Central nervous system
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CNS made up of brain and spinal cord
–> well protected by bones
–> brain inside skull, spinal cord inside spinal column
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Reflex Actions
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Response to painful stimulus: hand touches sharp object
–> hand is withdrawn => protects body from harm
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Stimulus detected by temperature/pain receptors in skin
–> generate impulses in sensory neurones
–> impulses enter CNS through part of spinal cord KA dorsal root
in spinal cord, sensory neurones connect by synapses with short relay neurones
–> these connect with motor neurones
motor neurones emerge from spinal cord through ventral root
–> send impulses back out to the muscles of the arm
–> muscles contract, pulling arm away from harmful stimulus
spinal cord
middle part consists mainly of nerve cell bodies, KA grey matter
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The Eye
Components
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choroid: dark layer containing pigment cells to stop light being reflected around inside the eye – underneath the sclera
retina:
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contains receptor cells KA rods and cones
–> react to light, producing impulses in sensory neurones
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rods: work well in dim light, cannot distinguish between colours
cones: only work in bright light, three types which respond to colours (RGB)
fovea: centre of retina where cones are particularly concentrated
–> cones give sharper image => can only see things clearly when looking directly at them when image is on the fovea
Forming an image
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Takes place at:
- air/cornea boundary
- the lens
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The Iris Reflex
Controls the amount of light entering the eye by changing the size of the pupil
–> too bright damages the rods and cones, too dim can't form an image
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Bright light:
Circular muscles contract, radial muscles relax
=> pupil is constricted
Dim light:
Radial muscles contract, circular muscles relax
=> pupil is dilated
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Action route:
light intensity –> retina –> coordination (optic nerve –> brain –> motor neurone to iris) –> iris muscles –> change in size of pupil
The blind spot
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Brain puts images from each eye together, cancelling out blind spot
Accommodation
Changes that take place in the eye which allow us to see objects at different distances are called accommodation
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Structure:
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Inside of eye filled with transparent water fluid which pushes outward
=> slight positive pressure within the eye
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Distant object
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Cornea refracts rays, lens does not need to refract much more
=> doesn't need to be very convex
=> ciliary muscles relax, pressure pushes outwards, lens flattens, suspensory ligaments stretched
Nearby object
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Lens has to be more convex
=> ciliary muscles contract, suspensory ligaments are slack, elastic lens bulges outwards
Synapses
Impulse arrives at a synapse
–> ends of fine branches of axon secret a chemical KA a neurotransmitter
–> chemical diffuses across the gap and attaches to membrane of second neurone
–> starts of impulses in the second cell
–> neurotransmitter broken down by an enzyme once finished
Synapses crossed by chemicals
=> easy for other chemicals to interfere with the working of the synapse – may imitate the neurotransmitter or block its action
<– how drugs work
Output of one cell may depend on multiple inputs
–> synapses important for integrating information in the CNS
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Structure of Neurones
Motor neurones
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Synapses: junctions with other neurones on any part of the cell body, dendrons or dendrites
Axon: extension from cell body – fibre that carries impulses to the effecter organ – end divides into many nerve endings
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myelin sheath: made of myelin – fatty material
–> insulates the axon, preventing short circuits
–> speeds up conduction of impulses
Sensory neurones
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cell body located on a side branch of the fibre, just outside the CNS
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