🏥 Chapter 10 - Nervous System and The Eye

• stimulus

  • any change in the environment which causes an organism to react
• response

  • reaction to a stimulus
• sensitivity

  • ability to respond to a stimulus
• role of nervous system

  • regulates how we react to the surroundings
  • contains highly-specialized sense organs that detect stimuli
• involuntary actions

  • actions that are not controlled consciously (i.e. automatic and not controlled by nervous system)
  • e.g. heartbeat, peristalsis,breathing
• voluntary actions

  • actions performed by nervous system
• central nervous system (CNS)

  • consists of brain and spinal cord
  • receives information from receptors

  
• peripheral nervous system (PNS)

  • consists of cranial nerves (from brain), spinal nerves (from spinal cord) and sense organs

  
• sense organs

  • they receive stimuli from receptors found in them
  • inform the CNS for any changes in surroundings by producing nerve impulses (electrical signals)
  • these impulses are transmitted via nerves (cranial & spinal)
  • nerve impulses are transmitted in a fraction of a second
• example of nerve impulse

  1. when a person touches your hand, you feel it almost immediately.

  2. as a response to this stimulus, the CNS will send nerve impulses to muscles

  
  

  3. muscles will then effect and action (e.g. muscles in arm contract and hand is jerked away)

  
  

  • muscles are also effectors
• how nerve impulses are transmitted

  f

  A) receptors

  B) CNS

  C) effectors

  
• neurons

  1. Sensory Neuron (receptor neuron)

  • transmit nerve impulses from the receptor to the CNS

  2. Relay Neuron (intermediate neuron)

  • transmit nerve impulses from the sensory to motor neuron
  • found in CNS

  3. Motor neuron (effector neuron)

  • transmits nerve impulses from CNS to effectors

  
• structure of motor neurones

  • cell body : irregular shape, has protoplasm and organelles
  • nerve fibre: strand of cytoplasm extending from the cell body

  
• structure of sensory neurone


  • has a cell body of circular shape
  • long nerve fibre extending towards receptor
  • short nerve fibre extending towards relay neurone

  
• synapse


  • junction between two neurones
  • Nerve impulses are transmitted across a synapse by chemicals

  released by the neurones.

  
• sensation

  • when stimuli stimulate thermoreceptors in skin , leading to production of nerve impulses
  • sensation is the brains' interpretation of a nerve impulse

  A) receptor

  B) sensory

  C) relay

  D) brain

  
• voluntary actions

  • deliberate action
  • nerve impulse is formed in the brain
  • opposite reflex arc of sensation

  A) brain

  B) relay

  C) motor

  D) effector

  
• reflex action

  • an immediate response to a specific stimulus without any conscious control (definition)
  • rapid and automatic
  • cannot be prevented consciously
  • involuntary action
• reflex centres

  • cranial reflexes ( controls upper body) : controlled by the brain subconsciously

  e.g. blinking

  
  

  • spinal reflexes (controls lower body) : controlled by spinal cord

  e.g. withdrawal of hand when touching hot surface
• process when you touch a hot object

  1. heat from the object stimulates nerve endings, and nerve impulses are produced
  2. sensory neurones transmit this nerve impulse to spinal cord
  3. in spinal cord, nerve impulses are transmitted across a synapse to the relay neurone, and across another synapse to the motor neurone
  4. motor neurone transmits nerve impulses from spinal cord to effector

  
• reflex arc

  • shortest pathway a nerve impulse can travel from receptor to effector in a reflex action (definition)

  
• differences between endocrine control and nervous control

  fg

  A) hormones

  B) nerve impulses

  C) blood

  D) neurones

  
• external structure of human eye (I)

  1. Sclera : the white of the eye. is tough , and continuous with the cornea and protects the eyeball from damage
  2. Conjunctiva : thin transparent membrane covering the sclera. it secretes mucus to keep the eye ball moist
  3. Eyelashes : shields eyes from dust particles
  4. tear gland : secretes tears that wash away dust particles, keep cornea moist for atmospheric oxygen to dissolve and diffuse into cornea, and lubricates cornea to reduce friction when eyelids move

  
• external structure of human eye (II)

  1. eyelids : protect cornea from damage. can be partly closed (squinting), to regulate amount of light entering the eyelid and damaging retina. blinking spreads tears over cornea and conjunctiva , and wipes dust particles away
  2. iris : sheet of muscles that control pupil size and amount of light entering eye
  3. pupil : hole in the centre of the iris that allows light to enter the eye

  
• internal structure of the eye (I)

  diagram

  A) sclera

  B) choroid

  C) retina

  D) optic nerve

  E) lens

  F) iris

  G) cornea

  H) suspensory ligament

  I) ciliary body

  J) vitreous chamber

  K) aqueous chamber

  
• internal structure of eye (II)

  1. retina : innermost layer (closest to vitreous chamber) of the eyeball. is light sensitive and contains photoreceptors (rod and cones) that connect the nerve endings to optic nerve
  2. lens : transparent biconvex structure that changes its shape or thickness to focus light into retina
  3. blind spot : region where optic nerve leaves the eye, and since it has no rods or cones it is not photosensitive and you will not see an object if it falls in blind spot
  4. optic nerve : transmits nerve impulses to the brain when the photoreceptors in retina are stimulated
• internal structure of the eye (III)

  1. fovea : small yellow depression in retina directly behind the lens, and is where images are focused. has greatest concentration of cons but no rods, providing a detailed colour vision in bright light
  2. vitreous chamber : transparent, jelly-like substance that keeps eyeball firm and helps refract (bend) light into retina
  3. choroid : middle layer of the eyeball (between sclera and retina) which is pigmented black to prevent internal refraction of light, and contains blood vessels to bring oxygen and nutrients to eyeball as well as to remove waste products
• internal structure of the eye (IV)

  1. ciliary body : thickened region at the front end of choroid, and controls ciliary muscles which control curvature and thickness of lens
  2. suspensory ligament : connective tissue attaches lens to ciliary body
  3. cornea : transparent layer continuous with sclera and refracts light into the eye, and has greatest refraction of light into the eye
  4. aqueous chamber : spaced filled between the lens and the cornea. keeps front of eyeball firm and helps to refract light into the pupil
• pupil reflex

  • in bright light, pupils become smaller to reduce amount of light entering the eye
  • in dim light, pupils become larger to increase amount of light entering the eye

  
• how does iris control amount of light entering the eye

  • controlled by two involuntary muscles (circular and radial)
  • these muscles are antagonistic

  when circular muscles contract , radial muscles relax :

  • pupil constricts , reducing amount of light entering the eye

  when circular muscles relax, radial muscles contract:

  • pupil dilates , increasing amount of light entering the eye
• reflex arc of pupil reflex

  this

  A) stimulus

  B) receptors

  C) sensory

  D) brain

  E) motor

  F) effector

  G) iris

  H) light

  
• how do we see

  1. Light rays are refracted through cornea and and aqueous humour onto the lens
  2. Lens causes further refraction and light rays are brought to a focus on the retina
  3. image on the retina stimulates either rods or cones
  4. image formed is vertically and laterally inverted, and is diminished (smaller than actual object)

  
• focusing (accommodation)

  • the adjustment of the lens on the eye so that clear images of objects at different distances are formed on the retina
• focusing on distant object (>7 m)

  1. ciliary muscles relax, pulling on suspensory ligaments
  2. suspensory ligaments become taut, pulling on the edge of the lens
  3. lens becomes thinner and less convex
  4. light rays from the distant object are sharply focused on the fovea of the retina
  5. photoreceptors are stimulated
  6. nerve impulses are produced and are transmitted to the brain via the optic nerve. the brain interprets the nerve impulses, and an image of the distant object that we can see is formed

  
• focusing on a near object

  1. ciliary muscles contract, relaxing its pull on the suspensory ligament
  2. suspensory ligament slackens, relaxing its pull on the edges of the lens
  3. the lens becomes thicker and more convex
  4. light rays from the near object are sharply focused onto the fovea of the retina
  5. photoreceptors are stimulated
  6. nerve impulses are produced, and are transmitted to the brain via the optic nerve. the brain interprets the nerve impulses, and an image of the distant object that we can see it formed

  
• detailed answer for pupil reflex

  1. when entering, bright/dim light, light intensity increase/decreases.
  2. photoreceptors in the retina are stimulated, and nerve impulses are produced
  3. nerve impulses are transmitted via sensory neurone in optic nerve, to the brain, where it crosses a synapse to the relay neurone
  4. in the brain, impulses travel across another synapse to the motor neurone.
  5. motor neurone then transmits nerve impulses to iris muscles
  6. circular muscles contract/relax, radial muscles relax/contract