Topic 6

• Stimulus:

  • Detectable change in the environment
  • detected by cells called receptors
• Nervous system structure:

  • Central nervous system = brain and spinal cord
  • peripheral nervous system = receptors, sensory and motor neurones
• Importance of simple reflexes:

  • Rapid - short pathway only three neurones & few synapses
  • autonomic - conscious thought not involved - spinal cord coordination
  • protect from harmful stimuli e.g., burning
• Specific tropisms:

  • Response to light: phototropism
  • response to gravity: gravitropism
  • response to water: hydrotropism
• Indoleacetic acid:

  • Type of auxin (plant hormone)
  • controls cell elongation in shoots
  • inhibits growth of cells in roots
  • made in tips of roots / shoots
  • can diffuse to other cells

  
• Phototropism in shoots:

  • Shoot tip produces IAA
  • diffuses to other cells
  • IAA accumulates on shaded side of shoot
  • IAA stimulates cell elongation so plant bends towards light
  • positive phototropism

  
• Phototropism in roots:

  • Root tip produces IAA
  • IAA concentration increases on lower (darker) side
  • IAA inhibits cell elongation
  • root cells grow on lighter side
  • root bends away from light
  • negative phototropism
• Gravitropism in shoots:

  • Shoot tip produces IAA
  • IAA diffuses from upper side to lower side of shoot in response to gravity
  • IIAA stimulates cell elongation so plant grows upwards
  • negative gravitropism
• Gravitropism in roots:

  • Root tip produces IAA
  • IAA accumulates on lower side of root in response to gravity
  • IAA inhibits cell elongation root bends down towards gravity and anchors plant
  • positive gravitropism
• Taxis:

  • Directional response by simple mobile organisms
  • move towards favourable stimuli (positive taxis) or away from unfavourable stimuli (negative taxis)
• Kinesis:

  • When an organism changes its speed of movement and rate of change of direction in response to a stimulus
  • if an organism moves to a region of unfavourable stimuli it will increase rate of turning to return to origin
  • if surrounded by negative stimuli, rate of turning decreases - move in straight line
• Receptors: Responds to specific stimuli
• Pacinian corpuscle:

  • Receptor responds to pressure changes
  • occur deep in skin mainly in fingers and feet
  • sensory neurone wrapped with layers of tissue
• Pacinian corpuscle structure

  A) outer capsule

  B) Lamellae

  C) Sensory neurone

  D) Schwann Cell

  
• How pacinian corpuscle detects pressure:

  1. When pressure is applied, stretch-mediated sodium ion channels are deformed
  2. sodium ions diffuse into sensory neurone
  3. influx increases membrane potential - establishment of generator potential
• Rod cells:

  • Concentrated at periphery of retina
  • contains rhodopsin pigment
  • connected in groups to one bipolar cell (retinal convergence)
  • do not detect colour
• Cone cells:

  • Concentrated on the fovea
  • fewer at periphery of retina
  • 3 types of cones containing different iodopsin pigments
  • one cone connects to one neurone
  • detect coloured ligh
• Rods are more sensitive to light
• Cones give higher visual acuity
• Rods have a lower visual acuity
• cones allow colour vision
• Why rods have high sensitivity to light:

  • Rods are connected in groups to one bipolar cell
  • retinal convergence
  • spatial summation
  • stimulation of each individual cell alone is sub-threshold but because rods are connected in groups more likely threshold potential is reached
• Why cones have low sensitivity to light:

  • One cone joins to one neurone
  • no retinal convergence / spatial summation
  • higher light intensity required to reach threshold potential
• Why rods have low visual acuity:

  • Rods connected in groups to one bipolar cell
  • retinal convergence
  • spatial summation
  • many neurones only generate 1 impulse / action potential -> cannot distinguish between separate sources of light
• Why cones have high visual acuity:

  • One cone joins to one neurone
  • 2 adjacent cones are stimulated, brain receives 2 impulses
  • can distinguish between separate sources of light
• Why rods have monochromatic vision:

  • One type of rod cell
  • one pigment (rhodopsin)
• Why cones give colour vision:

  • 3 types of cone cells with different optical pigments which absorb different wavelengths of light
  • red-sensitive, green-sensitive and blue-sensitive cones
  • stimulation of different proportions of cones gives greater range of colour perception
• Myogenic: When a muscle (cardiac muscle) can contract and relax without receiving signals from nerves
• Sinoatrial node: Located in right atrium and is known as the pacemaker
• Sinoatrial node: releases wave of depolarisation across the atria, causing muscles to contract
• Atrioventricular node: Located near the border of the right / left ventricle within atria
• Atrioventricular node: releases another wave of depolarisation after a short delay when it detects the first wave from the SAN
• Bundle of His: Runs through septum
• Purkyne fibres: In walls of ventricles
• Purkyne fibres:

  • spread wave of depolarisation from AVN across bottom of the heart
  • the muscular walls of ventricles contract from the bottom up
• Role of nonconductive tissue:

  • Located between atria and ventricles
  • prevents wave of depolarisation travelling down to ventricles
  • causes slight delay in ventricles contracting so that ventricles fill before contraction
• Importance of short delay between SAN and AVN waves of depolarisation:

  • Ensures enough time for atria to pump all blood into ventricles
  • ventricle becomes full
• Role of the medulla oblongata:

  • Controls heart rate via the autonomic nervous system
  • uses sympathetic and parasympathetic nervous system to control SAN rhythm
• Chemoreceptors:

  • Located in carotid artery and aorta
  • responds to pH / CO2 conc. changes
• Baroreceptors:

  • Located in carotid artery and aorta
  • responds to pressure changes