5 - Homeostasis and Response

• What is homeostasis?


  - the regulation of the internal conditions of a cell or organism to maintain optimal conditions for function in response to internal and external changes
• Why is homeostasis important?

  - cells and enzymes in the body can only survive within narrow physical and chemical limits (they require a constant temperature and pH as well as a constant supply of dissolved food and water)

  - the body requires control systems that constantly monitor and adjust the composition of the blood and tissues
• What does homeostasis control?

  - blood glucose concentration

  - body temperature

  - water levels
• What are the general features of an automatic control system?

  Involves the nervous system or hormones;

  
  

  - receptor cells which detect stimuli (changes in the environment)

  
  

  - coordination centres (such as the brain, spinal cord and pancreas) that receive and process information from receptors

  
  

  - effectors, muscles or glands, which bring about responses => restore optimal levels
• What 2 parts does the nervous system control?

  - central nervous system (CNS) (brain and spinal cord) => coordinates the response of effectors which may be muscles contracting or glands secreting hormones

  
  

  - neurones which carry electrical impulses to the CNS from receptors
• What is the flow of information in the nervous system?

  stimulus => receptor => coordinator => effector => response
• What happens when you touch a hot object?

  1. STIMULUS (heat) detected by RECEPTOR (in the skin)

  
  

  2. electrical impulses pass from the receptor along a SENSORY NEURONE to the CNS

  
  

  3. junction (SYNAPSE), chemical is released + diffuses to a RELAY NEURONE in the CNS => triggers an electrical impulse

  
  

  4. electrical impulse passes across relay neurone to another synapse (chemical released again & triggers electrical impulse in MOTOR NEURONE)

  
  

  5. motor neurone passes electrical impulse to an EFFECTOR (muscle in this case, which contracts to pull the hand away from the heat) => RESPONSE
• What is a key feature of the reflex arc?

  - rapid and automatic

  (no decision-making by the conscious part of the brain)
• How can we investigate the reaction time of a person?

  1. Person A sits on a stool with good upright posture, and places the forearm of their dominant arm across the table (with their hand overhanging the table edge).

  
  

  2. Person B holds a ruler vertically (the 0cm mark should be between A's thumb + index finger).

  
  

  3. B tells A to prepare to catch the ruler.

  
  

  4. Person B drops the ruler at a random time.

  
  

  5. Person A tries to catch the ruler as quickly as possible when the ruler drops.

  
  

  6. Person A now records the measurement on the ruler that is level with the top of A's thumb.

  
  

  7. Person A has a short rest and then the test is repeated 5 times, and a mean is calculated.

  
  

  8. Using a conversion table from the internet, convert the measurements to a reaction time.

  
  

  9. The two people switch places and the experiment is carried out again.
• What is the IV of the reaction time experiment?

  
  

  What is the DV of the reaction time experiment?

  
  

  What is are the CVs of the reaction time experiment?


  - person having their reaction time tested

  
  

  - reaction time

  
  

  - starting distance bwteen thumb and index finger

  - lighting and background noise
• With the reaction time practical, what other independent variables could we have?

  - effect of which hand (dominant or non-dominant)

  - effect of practise

  - effect of caffeine
• Why does the brain play a critical role in the CNS?

  - the brain controls complex behaviour, eg. language
• What features of the brain enable it to perform its functions effectively?

  - made up of billions of interconnected neurones

  - different regions that carry out different functions
• What is the highly folded, outer part of the brain called?

  the cerebral cortex

  
• What are the 3 functions of the cerebral cortex?

  - language

  - memory

  - consciousness
• What is the function of the cerebellum?

  - controls our balance and co-ordinates our movements

  
• What is the function of the medulla?

  - controls our heart rate and our breathing rate

  
• Why is studying the brain difficult? Why is it also difficult to treat brain damage or brain diseases?

  - the brain is protected by the skull => very hard to access

  
  

  - structures of the brain are extremely complex => difficult to work out exactly which parts of the brain carry out specific functions

  
  

  - brain is extremely delicate and easy to damage
• What are 3 ways that neuroscientists can use to map the regions of the brain to particular functions?

  - by studying patients with brain damage and locating where the damage has taken place => linking that region of the brain to its function

  
  

  - electrically stimulating different parts of the brain and observing the effects on the person's behaviour => narrow down specific regions to their functions

  
  

  - using MRI scanning techniques to look at which parts of the brain are most active during different activities
• What is the 'eye' in the human body?

  - a sense organ containing receptor sensitive to light intensity and colour
• What is the cornea?

  - the transparent layer forming the front of the eye where light rays pass through

  
• What is the function of the cornea?

  - start the focusing of the light rays
• What is the pupil?

  - the hole in the centre of the iris where light enters
• What is the iris?

  - the coloured part of the eye
• How does the iris cause changes in the size of the pupil? (Exam Q)

  muscle contraction
• What is the function of the lens?

  - to focus the light rays onto the retina

  - can change its shape, allowing us to focus on distant or near objects
• What is the retina?

  - the back of the eye which contains receptor cells that allow us to detect light intensity and light colour
• What happens after the receptor cells in the retina detect light intensity and colour?

  - they send electrical impulses down the optic nerve to the brain

  - brain then sends electrical impulses to specific muscles in the iris
• What is the sclera, and what is its function?

  - the white part of the eye which forms a tough, outer, protective layer

  
• How does your eye adapt to dim light?

  - entered a dark room (light intensity is low)

  - drop in light intensity is sensed by light receptors in the retina => send electrical impulses to the brain via the optic nerve

  
  

  - brain then sends electrical impulses to specific musclees in the iris => these muscles contract => pupil becomes larger => now allows more light to enter the eye
• Why is your pupil decreasing in size in conditions with bright light a reflex action?

  does not involve the conscious part of the brain
• What is 'accomodation'?

  - the ability to change the shape of the lens to focus on near/distant objects
• How does the lens change shape?

  - by contraction/relaxation, the ciliary muscle can change the thickness of the lens
• What happens when the ciliary muscle contracts?

  the suspensory ligaments loosen

  
  

  - lens becomes thicker => refracts light rays more strongly
• What happens when the ciliary muscle relaxes?

  the suspensory ligaments are pulled tight

  
  

  - lens becomes thinner => only slightly refracts light rays
• How does the eye focus on distant objects?

  - the light from distant objects needs to be focused only a relatively small amount

  
  

  - the ciliary muscle relaxes => the suspensory ligaments are pulled tight => lens is thinner (now only a small amount of refraction of the light rays)
• How does the eye focus on near objects?

  - light from near objects needs to be focused a large amount

  
  

  - ciliary muscle contracts => suspensory ligaments loosen => lens is thicker (refracts the light rays more strongly)
• What is the term for short sightedness

  myopia (cannot focus on distant objects)
• What is the term from long sightedness?


  hyperopia (cannot focus on near objects)
• What are causes for hyperopia?

  - eyeball is too short => light is focused on a point behind the retina

  
  

  - (especially in the elderly) lens becomes less elastic => lens cannot become thick enough to focus on near objects