Recptors

• What is a stimulus?


  - a detectable change in the environment
• What is the function of receptors?


  - convert stimuli into nerve impulses

  
  

  
  

  -transducers : transfer energy associated with a stimulus into an electrical change in a neurone.
• How do receptors work ?

  > specific stimulus causes membrane potential in the receptor to change (i.e. depolarise)

  
  

  > producing a generator potential

  
  

  
  

  > more intense the stimulus, the larger the generator potential

  
  

  
  

  >If this causes the membrane potential to exceed a certain threshold value it will cause an action potential to be produced

  
  

  
  

  > This can then be transmitted to a sensory neurone via synaptic transmission.
• What are pacinian corpuscles?

  >really modified neurones - consist of concentric layers (lamellae) of connective tissue surrounding the unmyelinated end of a myelinated sensory neurone

  
  

  - viscous gel between layers
• What are pacinian corpuscles known as?

  -mechanoreceptors as are sensitive to changes in mechanical pressure

  
  

  
  

  > allow you to detect firm pressure applied to the skin, textured surfaces etc
• What type of channels do pacinian corpuscles contain?

  - stretch-mediated Na+ channels

  
  

  
  

  - found in membrane of sensory nerve ending

  
  

  -change shape when under pressure so only

  Na+ pass through to initiate generator potential
• What shape is the pacinian corpuscle at rest ?

  - round and Na+ channels are the wrong shape to let the ions pass through.
• What happens when pacinian corpuscle are under pressure?

  - membrane stretched + Na+ channels OPEN + Na+ DIFFUSES into neurone (axon)

  
  

  
  

  - leads to generator potential which if large enough leads to action potential (impulse)

  
  

  
  

  - greater pressure greater the generator potential
• What happens after pressure has been registered in pacinian corpuscle ?

  - no further action potentials sent until pressure released

  
  

  
  

  - corpuscle temporarily springs into elongated shape , deforming membrane again + setting off another burst of impulses
• What is the function of the gel-filled lamellae

  - filter stimuli

  
  

  -if pressure applied slowly gel flows away from stimulus and membrane not deformed
• What is the eye an example of ?

  sense organ
• Where are rods and cones located?

  - Cones mainly located in fovea (where light is focused)

  
  

  
  

  - Rods don't appear at fovea but area all around periphery of retina
• Optic nerve?


  -All sensory neurones bundled together + form optic nerve

  
  

  
  

  - which transmits impulses from the retina to the brain
• Rod cells


  - contain pigment rhodopsin - very sensitive to light

  
  

  - breaks down + absorbs light which triggers events that lead to action potentials in connection bipolar neurones

  
  

  - and then in sensory neurone of optic nerve

  
  

  -then impulses transmitted to brain

  
  

  - absence of further stimulation rhodopsin reforms -requiring ATP

  
  

  
  

  · In bright light breakdown happens faster than the rhodopsin can reform so rod cells do not function and are said to be bleached

  
  

  
  

  -In this state your eyes are said to be light-adapted.
• Why does it take a while before you can see reasonably well after entering a darkened room?

  It takes time for the rhodopsin to reform.
• Cone cells


  - sensitive to HIGH light intensities + absorb light of dif wavelengths

  
  

  - 445 nm BLUE

  -535nm GREEN

  -575nm RED

  
  

  - pigment - iodposin which doesn't break down in dim light

  
  

  >idea of colour vision relying on 3 types of cone, with each type of cone containing a different form of iodopsin which absorbs different wavelengths of light, is known as the trichromatic theory

  
  

  
  

  > brain receives impulses from these cone cells in the same way as from rod cells

  
  

  
  

  > brain interprets colour according to the proportion of each type of cone stimulated
• Red-Green colour blindness


  - overlap in range of wavelengths of light absorbed especially by red sensitive and green sensitive cones

  
  

  
  

  · In absence of red sensitive cones, red light will still be detected by the green sensitive cones

  
  

  
  

  >brain not able to distinguish between red and green however, as it has no input from red sensitive cones

  
  

  
  

  
  

  >Inability to produce either red or green sensitive cones causes red-green colour blindness, resulting from the inheritance of a RECESSIVE allele carried on the X chromosome

  
  

  
  

  
  

  - sex linked disease.
• Rod cells show retinal convergence. Explain what this means:

  
  

  How is this different for cone cells?

  Many rod cells connect to the same bipolar neurone

  
  

  
  

  each cone cell connects to an individual bipolar neurone (1:1 ratio)
• if several rods are stimulated at the same time, summation occurs

  
  

  Q. What type of summation is this and explain what happens

  >spatial summation and as more depolarisation of the postsynaptic cell gradually occurs, the threshold will be reached and a generator potential will result
• Cone cells are only sensitive to high light intensity and they have high visual acuity,

  
  

  Q. What is meant by high visual acuity and how is this brought about?


  >When an image has high resolution and fine detail can be seen

  
  

  
  

  Each cone synapses with a separate bipolar neurone, which in turn then generates impulses in a separate sensory neurone in the optic nerve
• Why do cones give high acuity

  >fovea contains very high density of cone cells, with most of the cones packed into this tiny area, and no rods
• Why do rods give poor acuity?


  > brain cannot distinguish which one of several rod cells has been activated
• What is the overall function of rod cells?


  - Vision in dim light
• What is the overall function of cone cells?

  - Vision in bright light

  
  

  - colour vision

  
  

  - Allow detail to be seen
• Give 2 reasons why rods provide greater sensitivity than cones


  · Rhodopsin is broken down at low light intensities

  
  

  
  

  · Rods show retinal convergence
• Give 2 reasons why cones provide greater acuity than rods

  · 1:1 ratio of cones: bipolar cells

  
  

  
  

  · High density at the fovea
• What is the function of the ANS?

  - regulates internal glands + muscles- normally beyond our conscious control

  
  

  
  

  - made up of two antagonistic systems, the sympathetic and parasympathetic divisions.
• What is the neurotransmitter for the sympathetic nerve, the general effect + when is it dominant during ?

  - noradrenaline

  
  

  - excitatory

  
  

  -stress, danger + excitement
• What is the neurotransmitter for the parasympathetic nerve, general effect + when's it dominant during?

  - acetylcholine

  
  

  - inhibitory

  
  

  -rest
• Explain why they are described as antagonistic:

  - Actions normally oppose each other

  
  

  -sympathetic prepares for activity

  
  

  -parasympathetic slows down + conserves energy
• How is heart rate controlled

  - Sinoatrial node SAN natural pacemaker of the heart
• what is the average heart rate of a healthy adult during rest?

  70 - 75 b.p.m
• What is the advantage to an athlete of an increase in heart rate during exercise?

  -Increased cardiac output

  
  

  -increased blood flow to muscles

  
  

  -more oxygen & glucose to muscle cells

  
  

  
  

  - increased respiration rate

  
  

  -more ATP for muscle contraction
• what are changes to the heart rate controlled by

  - cardiac centre in a region of brain called medulla oblongata
• where does the cardiac centre send impulses to ?

  SAN via two different nerves of the autonomic nervous system
• What is a sympathetic nerve responsible for ?

  - increasing heart rate
• What is a parasympathetic nerve responsible for

  - decreasing heart rate
• wha results in a faster heart rate?

  >More frequent impulses in sympathetic nerve and/or fewer impulses in the parasympathetic nerve will result in a faster heart rate,

  
  

  
  

  · Conversely, more frequent impulses in the parasympathetic nerve and/or fewer impulses in the sympathetic nerve will result in a slower heart rate,
• what two main sources does the cardiac centre receive information from?

  >Chemoreceptors in walls of the carotid arteries (that supply blood to the brain) + aorta

  
  

  > receptors clustered to form structures called the carotid and aortic bodies

  
  

  >These chemoreceptors sensitive to changes in blood pH resulting from changes in carbon dioxide concentration

  
  

  
  

  · Pressure receptors (baroreceptors), also found in the carotid and aortic bodies, which are sensitive to changes in blood pressure
• changes in pH by CO2

  - CO2 dissolves in water to form a weak acid which can dissociate to form hydrogen ions