Receptors & Heart Rate Control

• Pacinian corpuscle structure:

  • Capsule
  • Lamellae
  • Axon with myelin sheath
• Pacinian corpuscles are specific to pressure only
• Pacinian corpuscle and pressure:

  • Pressure on the skin deforms stretch-mediated Na+ channels
  • Deformation of the membrane causes stretch-mediated Na+ channels to open
  • More positive ions so depolarisation
  • This depolarisation is called a generator potential
• A generator potential does not equal an action potential
• Generator potential and action potentials:

  • Greater pressure
  • More stretch-mediated Na+ channels OPEN
  • Greater generator potential
  • If generator potential reaches THRESHOLD then an action potential is generated (nerve impulse propagated along sensory neurone)
  • Frequency of action potentials related to the intensity of the stimulus
  • Maximum frequency of the action potentials is limited by the refractory period
• The two types of receptor cell in the eye are rods and cones
• Features of rods:

  • RHODOPSIN is the light sensitive pigment
  • Evenly distributed throughout the macula
  • Sensitive to ALL wavelengths of light
  • High visual sensitivity to low levels of light intensity
  • Low visual acuity (poor resolution)
  • Retinal convergence due to several rods sharing a single bipolar neurone
• Features of cones:

  • Densely packed in the fovea
  • Three types of cone (for humans) detects specific wave lengths of light (red, green and blue)
  • Three types of IODOPSIN
  • Iodopsin is less sensitive than rhodopsin so requires higher light intensity
  • High visual acuity
  • Each cone is connected to a single bipolar nuerone
• The pigment in cones is called iodopsin
• The pigment in rods is called rhodopsin
• Photons causes the pigment to break down, altering the chemical structure and leads to a generator potential
• Any light breaks down rhodopsin
• Specific wavelengths of light break down iodopsin
• Visual acuity in cones:

  • Temporal summation
  • Requires more light intensity to reach threshold to trigger cone
  • High visual acuity (high resolution)
• Visual acuity in rods:

  • Spatial summation (retinal convergence)
  • Low light intensity is absorbed by multiple rods connected to one bipolar neurone
  • Lower visual acuity (low resolution)
• Every cone synapses with one bipolar neurone
• Multiple rods synapse with one bipolar neurone
• The heart muscle is myogenic, meaning it can initiate its own contraction
• Control of heart rate process:

  • SAN sends wave of electrical activity (depolarisation) across both atria
  • Both atria contract
  • Layer of non-conductive tissue prevents wave reaching ventricles
  • Wave of electrical activity reaches the AVN
  • 0.1 second delay allowing atria to empty all blood
  • Wave of electrical activity sent from AVN
  • Down the bundle of His to the base of the ventricle
  • Up the purkinje fibres
  • Causing the ventricle to contract from the apex of the heart upwards
• The heart controls the contraction of atria and ventricles:

  • SAN -> AVN -> bundle of His or Purkinjie fibres
  • Electrical activity over atria
  • Atria contract
  • Non-conducting tissue between atria and ventricle so ventricle does not contract with atria
  • Delay at AVN to ensure atria are empty
  • Ventricles contract from the apex upwards
• Heart rate is under the control of the autonomic nervous system
• Sympathetic system:

  • Stimulates effectors
  • Speeds up heart rate
  • Fight or flight
  • Neurotransmitter is noradrenaline
• Parasympathetic system:

  • Inhibits effectors
  • Controls activity at rest
  • Neurotransmitter is acetylcholine
• The sympathetic and parasympathetic branches are antagonistic
• Heart rate is controlled by the medulla oblongata in the brain
• Receptors in the control of heart rate:

  • Baro-receptors
  • Chemo-receptors
• Baro-receptors and chemo-receptors are found in the walls of the aorta and carotid arteries
• Blood pressure increases above normal:

  • Detected by baroreceptors
  • More frequent impulses sent to the medulla oblongata
  • More frequent impulses from the inhibitory centre in the medulla to SAN via the parasympathetic nerve
  • Lower frequency of impulses from SAN across atria
  • Heart rate decreases
• Blood pressure decreases below normal:

  • Detected by baroreceptors
  • More frequent impulses sent to the medulla oblongata
  • More frequent impulses from the acceleratory centre in medulla to SAN via sympathetic nerve
  • More frequent impulses from SAN across atria
  • Heart rate increases
• Blood pH decreases below normal:

  • Detected by chemoreceptors
  • More frequent impulses sent to medulla oblongata
  • More frequent impulses from acceleratory center in the medulla to SAN
  • More frequent impulses from SAN across atria
  • Heart rate increases
• Describe how a Pacinian corpuscle produces a generator potential:

  • More pressure deforms stretch-mediated Na+ channels
  • Sodium channels open and more Na+ ions flow in axon
  • Depolarisation leads to a generator potential
• The three different cone types allow vision using the fovea to be in colour
• Suggest how muscles could cause a pupil to constrict:

  • Circular muscle contracts
  • Radial muscle relaxes
• Explain how more cones in the fovea enable an organism to see its prey in detail:

  • High visual acuity
  • Each cone is connected to a single neurone
  • Cones send separate set of impulses to the brain
• Explain how more rods enable an organism to hunt in the dark:

  • High visual sensitivity
  • Several rods connected to a single bipolar neurone
  • Spatial summation to reach threshold
• Why having only rods and no cones causes difficulty seeing in detail:

  • No cones so only rods
  • Several rods connected to a single bipolar neurone
  • Rods send single sets of impulses to the brain
• Why red-green colour blindness makes it hard to distinguish between red, green and other colours:

  • Green sensitive cones are non-functional
  • Three different types of cones
  • Different colours seen due to stimulation of more than one cone
• How both ventricles contract after initiation of the SAN:

  • Electrical activity only through AVN
  • Wave of electrical activity passes over more ventricles at the same time
• How fall in heart rate and metabolic rate are linked:

  • Less CO2 in blood due to lower metabolism
  • Detected by chemoreceptors
  • More impulses sent down parasympathetic nerves
  • From medulla to SAN