Homeostasis and response

• Homeostasis is about the regulations in your body to maintain a stable internal environment in response to changes
• negative feedback counteracts changes

  a receptor detects a stimulus level is too high

  the coordination centre receives and processes the information and organises a response

  effector produces a response restoring the optimum level, decreasing it

  a receptor detects a stimulus level is too low and the cycle repeats
• the nervous system detects and reacts to stimuli


  a single celled organism can just respond to its environment but the cells of multicellular organisms have to communicate first
• the nervous system is made up of different parts

  the central nervous system

  sensory neurons

  motor neurons

  effectors
• Central nervous system (CNS)

  in vertebrates this consists of the spinal cord and brain only

  in mammals the CNS is connected to the body by sensory and motor neurons
• Sensory neurons


  the neurons that carry information as electrical impulses from the receptor to the CNS
• motor neurons


  the neurons that carry electrical impulses from the CNS to the effector
• effector


  all the muscles and glands that respond to nervous impulses
• receptors are the cells that detect stimuli


  there are many different types of receptors such as taste receptors on the tongue

  receptors can form part of a larger complex organ- the retina of the eye
• effectors respond to nervous impulses and bring about a change


  muscles and glands are known as effectors- they respond in different ways

  muscles contract in response

  glands secrete hormones
• The CNS is a coordination centre 


  stimulus-receptor-sensory neuron-CNS-motor neuron-effector-response
• synapses connect neurons


  the nerve signal is transferred by chemicals which diffuse across the gap

  the chemicals set off a new electrical signal in the next neuron
• reflexes are rapid, automatic responses to certain stimuli that don't involve the conscious part of the brain


  this can reduce the chance of injury

  if a bright light shines in an eye, the pupils automatically get smaller to reduce the light entering the eye
• the passage of information in a reflex is called a reflex arc


  the reflex arc goes through the CNS
• reflex arc


  when a stimulus is detected by receptors impulses are sent along a sensory neuron to the CNS

  when the impulses reach a synapse between a sensory neuron and a relay neuron they trigger chemicals to be released which cause impulses to be sent along the relay neuron

  this happens in the synapse between they relay + motor neuron

  they travel along to the effector which causes a response
• bee sting reflex arc

  bee stings finger- stimulation of pain receptors

  impulses travel along a sensory neuron

  impulses are passes along a relay neuron via a synapse

  impulses travel along a motor neuron via a synapse

  when impulse reaches muscles it contracts, moving finger away from bee
• relay neurons connect sensory neurons to motor neurons
• reaction time can be measured using a computer


  computers can give a more precise reaction time because they remove the possibility of human error from the measurment
• the brain is part of the CNS and made up of billions of interconnected neurons


  the brain is in charge of all complex behaviours

  it controls and coordinates everything you do
• the brain is made up of different parts

  cerebral cortex

  cerebellum

  medulla

  spinal cord
• cerebral cortex


  responsible for consciousness, intelligence, memory, language
• cerebellum 


  responsible for muscle coordination
• medulla


  controls unconscious activities like breathing and the heartbeat
• scientists use a range of methods to study the brain


  studying patients with brain damage-the effect this has tells us what that part of the brain did

  electrically stimulating the brain-observing what it does reveals what those parts of the brain do

  MRI scans- detailed picture of the brain to find what parts are active when people are doing things
• parts of the EYE

  sclera ciliary muscles + suspensory ligaments

  cornea optic nerve

  iris retina

  lens pupil
• sclera


  tough, supporting wall of the eye
• cornea


  transparent outer layer at the front of the eye

  it refracts light into the eye
• iris


  contains muscles that control the diameter of the pupil and therefore how much light enters the eye
• pupil


  the hole in the middle of the eye
• lens


  refocuses light onto the retina
• retina


  contains receptor cells sensitive to light intensity and colour
• ciliary muscles and suspensory ligaments


  controls the shape of the lens
• optic nerve


  carries impulses from the receptors on the retina to the brain
• when light receptors in the eye detect bright light, a reflex is triggered that makes the pupil smaller. the circular muscles in the iris contract and the radial muscles relax, reducing the amount of light that can enter. the opposite happens in dim light
• to look at near objects

  ciliary muscles CONTRACT

  suspensory ligaments RELAX

  lens becomes fat and this increases the amount of light it bends
• to look at far objects

  ciliary muscles RELAX

  suspensory ligaments CONTRACT

  lens goes thin so smaller amount of light refracted
• long sighted people HYPEROPIA


  unable to focus on near objects

  the lens is the wrong shape/eyeball too short meaning the light is focused behind the retina

  glasses with a CONVEX lens
• short sighted people MYOPIA

  unable to focus on distant objects

  lens is the wrong shape/eyeball too long meaning light is focused in front of the retina

  glasses with a CONCAVE lens
• other options to wearing glasses

  contact lenses- sit on surface of eye

  laser eye surgery- vaporise tissue changing shape of cornea

  replacement lens surgery- natural lens removed replaced with artificial lens
• body temperature must be kept constant


  the body has to balance the amount of energy gained and lost to keep the core body temperature constant

  there is a thermoregulatory centre in the brain which contains receptors that are sensitive to the temperature of the blood and receives impulses from temperature receptors in the skin