Stimuli

• Generator Potential: depolarisation of the membrane of a receptor cell as a result of a stimulus

  • If the GP reaches threshold an AP is initiated
• Sensory receptors function

  • Act as transducers - covert the energy they sense into a GP leading to passage of a nervous impulse
  • Detect changes in the surroundings
• Pacinian Corpuscle

  • Mechanoreceptors found deep in the skin and detect strong pressures
  • Transduce mechanical energy of stimulus into a GP
• Where is the pacinian corpuscle found ?

  Fingers, Soles of feet, external genitalia
• Process that occurs in Pacinian Corpuscle

  • Membrane of the sensory nerve ending contains stretch-mediated Na+ ion channel proteins

  1. When pressure is applied the membrane becomes deformed and stretched
  2. Sodium channels are open and Na+ ions diffuse into the neurone
  3. Na+ ions change the potential of the membrane at it becomes depolarised producing a GP
  4. The GP initiates an AP if threshold is reached
• Describe how a Pacinian Corpuscle produces a GP when stimulated (3)

  • Increased pressure deforms stretch-mediated Na+ ion channels
  • Na+ ion channels open and Na+ ions flow in
  • Depolarisation leading to GP
• Explain how applying pressure to the Pacinian Corpuscle produces changes in membrane potential (3)

  • Pressure causes membrane to become deformed
  • Na+ ion channels open and Na+ ions enter
  • Greater pressure more channels open
• The membrane potential at Q was same whether medium or heavy pressure was applied to the fingertip. why ? (2)

  • Threshold has been reached
  • All or nothing principle - once threshold is reached causes maximal response
• What two types of photoreceptors does the retina contain ?

  • Rods
  • Cones
• Where are there no photorecptors ?

  • Blind spot where optic nerve attached so no light can be detected
• Where is there a particularly high number of cone cells ?

  The Fovea
• Visual acuity: visual clarity, resolution of vision
• Sensitivity to light: quantity of light that can be detected

  • Can detect fewer light waves = high sensitivity
  • Can detect many light waves at once = low sensitivity
• • Rod and Cone cells transduce light energy into GP
  • If threshold is reached GP initiates AP
• Rod Cells Structure

  • Cannot distinguish between different wavelengths of light - black and white vision
  • Show retinal convergence - multiple rod cells connected in groups of up to 100 in a single bipolar neurone
  • Low visual acuity due to retinal convergence - separate sources of light perceived as a single image
  • High sensitivity due to spatial summation
• Rod Cell Mechanism of Action

  • To create GP pigment rhodopsin is broken down by light energy
  • There needs to be enough energy from the low light intensity to cause breakdown
  • Enough pigment has to be broken down for the threshold to be met in the bipolar cell
  • Threshold can be reached as multiple rod cells connect to a single neurone so spatial summation occurs
• Cone Cells

  • Red, Green and Blue cone cells which contain different types of iodopsin pigment
  • Absorb different wavelengths of light
  • Perceive coloured images which depends on the proportion of each cone cell that is stimulated
  • Iodopsin only broken down if there is a high light intensity
• Cone Cell Mechanism of Action

  1. Light is focused by lens on the fovea
  2. Each type of cone cell has a specific type of iodopsin
  3. High light intensity breaks down iodopsin
  4. If threshold is reached this stimulates an AP in the bipolar cell
• Cone cell Structure

  • One cell connects to one bipolar cell so no spatial summation occurs so can only respond to high intensity light
  • High visual acuity as each cell connected to one bipolar cell so brain can distinguish between separate sources of light
  • Low sensitivity as can detect many light waves at once
• Distribution

  • Most cone cells found near the fovea as respond to high light intensities
  • Rod Cells further ways as respond to lower light intensities
• Explain why object appear sharper and in colour during the day (3)

  • Cone cells stimulated by bright light during the day
  • Cone cells have high visual acuity
  • Cone cells detect colour (using iodopsin)
• Explain why objects appear brightly coloured at the centre of your vision but grey at the periphery (3)

  • Fovea at the back of eye
  • Only cone cells found in the fovea
  • Only cone cells detect colour
• Explain why brightly coloured objects often appear grey and speckled in dim light (4)

  • Rod cells have high sensitivity
  • Rod cells have low visual acuity
  • Only rod cells are stimulated
  • Only cone cells detect colour
• Sinoatrial node is located in the right atrium and is known as the pacemaker
• Atrioventricular node is located between the atria
• Bundle of his runs through the septum
• Purkinje fibres in the walls of the ventricles
• Process of rate of Contraction

  1. A wave of electrical activity spreads out from the SAN across both atria and atria contract
  2. A layer of non-conductive tissue prevents the wave crossing to the ventricles
  3. Wave passes through a second group of cells AVN
  4. After a short delay AVN conveys an electrical wave between ventricles
  5. Bundle of his conducts the wave through the septum to the base of the ventricles where it branches into purkinje fibres
  6. Wave causes ventricles to contract from apex upwards
• The wave of electrical activity which coordinates the heart beat is delayed slightly at AVN - why ?

  • Allows blood to pass into ventricles from atria
  • Before ventricles contract
• Why is it important the electrical activity being passed to the base of the ventricles ?

  • Ventricle contracts from base/upwards
  • Blood pushed through the arteries so that all blood ejected
• The heart controls and coordinates the regular contraction of the atria and ventricles. Describe how (6)

  • SAN sends impulses over atria
  • Atria contract
  • Non conducting tissue (between atria and ventricles)
  • Delay at AVN ensures ventricles fill before ventricles contract
  • Ventricles contract from apex upwards
  • SAN - AVN - bundle of His - Purkinje fibres
• The cardiac cycle is controlled by the SAN and AVN. Describe how

  1. SAN acts as a pacemaker
  2. SAN sends wave of electrical activity across atria causing atrial contraction
  3. AVN delays electrical activity
  4. AVN sends wave of electrical activity down Bundle of his
  5. Causing ventricles to contract from base up
• Medulla Oblongata: in the brain and controls the heart rate via the autonomic nervous system
• Baroreceptors: detect blood pressure in the carotid arteries and aorta
• Chemoreceptors: detect conc. of Co2 (PH) in blood flowing through carotid arteries and aorta
• Blood pressure higher than normal mechanism:

  1. Detected by baroreceptors
  2. More impulses pass along sensory neurones to medulla oblongata
  3. More impulses to SAN via parasympathetic NS motor neurones
  4. Rate of electrical wave production decreases at SAN
  5. Decreases HR
• Blood pressure lower than normal mechanism:

  1. Detected by baroreceptors
  2. More impulses pass along sensory neurones to medulla oblongata
  3. More impulses to SAN via sympathetic NS motor neurones
  4. Rate of electrical wave production increases at SAN
  5. Increases HR
• PH decrease mechanism

  1. Detected by chemoreceptors
  2. More impulses pass along sensory neurones to medulla oblongata
  3. More impulses to SAN via sympathetic NS motor neurones
  4. Rate of electrical wave production increases at SAN
  5. Increases HR