Chapter 10 [the nervous system and the eye] .pdf

Sense organs Sense organs receive stimuli through...

Sense organs Sense organs receive stimuli through Key terms receptors found in them Stimulus: a change in the Role of the nervous systems They produce electrical signals or messages called nerve impulses to inform the CNS of environment that evokes a speci c functional reaction in an coordinated and regulates the activities of the body and changes in the environment organ or tissue how it responds to stimuli Sense organs → CNS → effector (re ex Response: an organism’s reaction When our body responds to stimuli, the response can be action) to a stimulus involuntary or voluntary Sense organs range from simple structures Sensitivity: the ability to detect → involuntary actions: automatic activities that are not (photoreceptors) to highly-specialised and respond to changes in the controlled consciously structures (eye) environment → activities that are controlled consciously The eye is the sense organ that we use for sight Consists of: Nerves, brain, spinal cord, The human nervous nervous tissues, neurone cells system Central nervous system (CNS) Peripheral nervous system (PNS) consists of cranial nerves (from the brain) and spinal nerves (from consists of brain and spinal cord the spinal cord) Neurones nerve cells in Synapse nervous tissues A synapse is the junction between two neurone Sensory neurone or between a neurone and an effector An area where impulses are transmitted from transmit nerve impulses from the an axon to a dendron receptors to the CNS Transmission across a synapse is by chemical means through neurotransmitters Motor neurone Relay neurone transmit nerve impulses from transmits nerve impulses from the the CNS to the effectors sensory neurone to the motor neurone Found within the CNS  Re ex action a re ex action is an immediate response to a speci c Knee-jerk stimulus without conscious control (involuntary action) stimulus: the tendon under the kneecap The spinal cord and brain (CNS) are re ex centres Reflex arc is tapped and stretches upper thigh → spinal re ex: re exes controlled by the spinal cord Relay 1. Nervous impulses are produced and neurone (knee-jerk, hang with drawl to hot object) transmitted along the sensory → cranial re exes: re exes controlled by the brain without neurone to the spinal cord a person’s consciousness, occurring in the head region (pupil 2. The impulse is transmitted from re ex, blinking, salivation) sensory neurone to motor neurone Re ex arc (NO RELAY NEURONE) re ex arc is the shortest pathway which nerve impulses 3. The motor neurone transmit the travel from the receptor to the effector in a re ex action Touching a hot object impulse to the effector, which in this Consists of a receptor → sensory neurone → relay neurone case is the thigh muscle 1. Heat from the object stimulates the in re ex centre (CNS) → motor neurone → effector 4. (Response) the muscle contracts and receptors in your skin causes the lower leg to jerk upwards 2. Nerve impulses are produced and transmitted Automatic Internal along the sensory neurone to the spinal cord activities that are regulatory Involuntary actions 3. In the spinal cord, the nerve impulses are not controlled eg. heartbeat, processes breathing, transmitted rst across a synapse to the consciously relay neurone, and then across another temperature synapse to the motor neurone regulation 4. Upon receiving the nerve impulses from the relay neurone, the motor neurone transmits nerve impulses to the effector Information processing (Sensation, re ex action (involuntary action), voluntary action) 5. The effector muscle, in this case the bicep muscle, contracts and causes your hand to withdraw suddenly Sensation Voluntary action Pathway of nerve impulses Receptor → sensory neurone → actions that are controlled consciously relay neurone in the spinal cord/ Nerve impulses are produced in the brain brain → brain Transmitted by a relay neurone from the brain down the spinal cord pathway of nerve impulses: Brain → relay neurone in spinal cord/brain → motor neurone → effector  External structure of the eye Two main functions of Eyelash Tear gland shields the eye from secretes tears to: external structures dust particles wash away dust particles refraction of light rays and control of amount keep the cornea moist for atmospheric of light entering eye oxygen to dissolve so that it can cornea, pupil, iris, eyelid diffuse into the cornea Protects the eye from damage lubricate the conjunctiva, helping to eyelids, conjunctiva, tear glands, eyelashes reduce friction when the eyelids move Sclera Conjunctiva tough, white outer covering a mucus membrane covering of the eyeball which is the sclera, secretes mucus continuous of the cornea to keep the front of the eyeball moist Eyelid Iris a circular sheet of muscles Pupil protects cornea from mechanical damage, prevents entry of light which may damage the retina (circular muscles and radial a hole in the centre of the iris (squinting, prevents entry of excessive light) and muscles) controls the size that allows light to enter the spreads tears over eyes to wipe off dust by blinking of pupil and therefore eye, its size is controlled by the controls the amount of muscles in iris light entering the eye Cornea dome-shaped transparent layer, refracts light rays into the eye (covers the iris and the pupil)  Internal structure of the eye Retina Choroid Ciliary body innermost layer of the eye wall containing light-sensitive cells known black pigmented middle layer of the contains ciliary muscles eyeball, which prevents the internal as photoreceptors (consists of ‘rods’ which control the re ection of light, contains blood vessels and ‘cones’), connected to nerve curvature and thicken of that carry oxygen and nutrients to bres from the ‘rods’ and ‘cones’ lens eyeballs and remove metabolic waste products from the eyeballs Suspensory ligaments Vitreous chamber space behind the lens, lled with attaches the edge of the lens to the vitreous humour ciliary body Vitreous humour transparent jelly-like substance, keeps eyeball rm and refracts Aqueous humour light onto the retina a transparent watery uid, keeps Fovea the front of the eyeball rm and a small yellow depression where refracts light into the pupil images are focused, contains Aqueous chamber cones but not rods, enables a space between lens and the cornea, person to have detailed colour lled with aqueous humour vision in bright light Optic nerve Lens transparent, circular and Blind spot transmit nerve impulses to the brain biconvex structure, No retina when photoreceptors shape or thickness can region where the optic are stimulated be changed in order to nerve leaves the eye, refract light onto the does not contain retina photoreceptors, therefore not sensitive to light  Pupil re ex (bright and dim) in order for a person to see clearly, only the right amount of light should enter the eye In bright light: The amount of light entering the eye is determined by the size In dim light: of the pupil which is controlled by the iris muscles The circular muscles of the iris contract The circular muscle of the iris contract The pupil re ex is a re ex action while the radial muscles of the iris relax while the radial muscles of the iris relax It is a response to changes in the light intensity The pupil dilates (increase in size) The pupil constricts (decrease in size) It protects the eye from excessive light exposure, which could This increases the amount of light This reduces the amount of light damage the retina entering the eye entering the eye Pathway of nerve impulses in the pupil re ex Stimulus (change in light intensity) → receptor (retina) → sensory neurons in optic nerve → brain → motor neurone → effector (iris) how does the iris control the amount of light entering the eye? the iris is controlled by two sets on involuntary muscles, the circular and radial muscles The circular and radial muscles are antagonistic muscles The human eye Focusing on a distant object The light rays re ecting off the distant object (7m or Focusing on nearby objects more) are almost parallel to each other Rays are refracted through the cornea and aqueous Focusing (far and near) light rays re ecting off the near object are divergent focusing or accommodation is the adjustment of humour into the pupil Rays are refracted through the cornea and aqueous the lens of the eye so that clear images of objects Minimal refraction is required since the rays are almost humour into the pupil at different distances are formed on the retina parallel thus the lens became thinner More refraction is required since the rays are diverging It’s necessary so that objects at different 1. Ciliary muscles relax, pulling on suspensory ligaments thus the lens become thicker distances can be seen clearly 2. Suspensory ligaments become taut, pulling on the edge 1. Ciliary muscles contract, relaxing their pull on In focusing, the curvature or thickeners of the of the lens suspensory ligaments lens is adjusted, allowing light rays to be focused 3. Lens become thinner and less convex 2. Suspensory ligaments slacken, relaxing their pull in on the retina 4. Light rays from the distant object is sharply focused on the lens the retina, stimulating the photoreceptors 3. Lens becomes thicker and more convex 5. Nerve impulses produced are transmitted to the brain via 4. Light rays are sharply focused on the retina, the optic nerve stimulating the photoreceptors 6. The brain interprets the impulses and the person sees 5. Nerve impulses generated are transmitted to the the distant object brain via the optic nerve 6. The brain interprets the impulses and the person sees the near object