Chapter 13

• The need for communication systems in multicellular organisms

  • as species have evolved, cells have become specialised to perform specific functions - as result organisms need to coordinate the function of diff cells & systems to operate effectively
  • when changes occur in an organism's internal or external environment the organism must respond to these changes in order to survive
• Coordination in animals vs plants

  • animals react through electrical responses (via neurones) & chemical responses (via hormones)
  • plants react through number of chemical communication systems including plant hormones
• What does coordination rely on?

  communication at a cellular level through cell signalling
• Cell signalling

  • one cell releases a chemical which has effect on another cell, known as target cell
  • transfer signals locally: between neurones at synapse using neurotransmitters
  • transfer signals across large distances: using hormones e.g. cells of pituitary gland secrete ADH which acts on cells in kidneys
• Coordination in plants

  • do not have nervous system like animals
  • to survive must still respond to internal & external changes in their environment
  • e.g. plant stems grow towards a light source to maximise rate of photosynthesis - achieved through plant hormones
• Homeostasis

  the different functions of organs must be coordinated to maintain a relatively constant internal environment
• Stimulus

  changes in the internal & external environment
• Neurones

  specialised nerve cells that transmit electrical impulses around the body to enable communication between cells in diff parts of an organism
• Nerves

  consist of many neurones bundled together
• Pathway of most nervous responses

  1. sensory receptor
  2. sensory neurone
  3. relay neurone
  4. motor neurone
  5. effector
• Sensory neurone

  • transmit impulses from sensory receptor cell to a relay neurone, motor neurone or the brain
  • one dendron which carries impulse to cell body
  • one axon which carries impulse away from cell body
• Label neurone

  sensory neurone

  A) dendron

  B) nucleus

  C) cell body

  D) myelin sheath

  E) node of ranvier

  F) axon

  
• Relay neurone

  • transmit impulses between neurones
  • clusters of dendrites each leading to a dendron
  • each dendron passes to central cell body
  • short axon carries impulses from cell body to synaptic endings
• Label neurone

  relay neurone

  A) cell body

  B) nucleus

  C) axon

  D) dendron

  E) dendrites

  
• Motor neurone

  • transmit impulses from relay neurone or sensory neurone to an effector
  • dendrites leading to cell body
  • one long axon
• Label neurone

  motor neurone

  A) nucleus

  B) cell body

  C) dendrites

  D) myelin sheath

  E) node of Ranvier

  F) axon

  
• How are myelin sheaths produced?

  • Schwann cells produce layers of membrane by growing around the axon many times
  • many layers of plasma membrane make up a myelin sheath
• Function of a myelin sheath

  • act as an insulating layer & allow these myelinated neurones to conduct the electrical impulse at a much faster speed
  • electrical impulse jumps from one node of Ranvier (gaps in the myelin sheath) to the next as it travels along the neurone allowing the impulse to be transmitted faster
• Non-myelinated neurones

  impulse is transmitted continuously along the nerve fibre - much slower
• Outline how body reacts to change in environment

  • body detects changes in its environment using sensory receptor
  • sensory receptors convert stimulus they detect into a nerve impulse
  • information then passed through nervous system & on to CNS - normally to brain
  • brain coordinates required response & sends impulse to an effector to result in desired response
• Features of sensory receptors

  • they are specific to a single type of stimulus
  • they act as a transducer - convert a stimulus into a nerve impulse
• How do receptors respond to pressure & movement as a stimulus?

  • mechanoreceptor detects stimulus
  • e.g. Pacinian corpuscle
  • sense organ - e.g. skin
• How do receptors respond to chemicals as a stimulus?

  • chemoreceptors detect stimulus
  • e.g. olfactory receptor (detects smell)
  • sense organ - e.g. nose
• How do receptors respond to heat as a stimulus?

  • thermoreceptors detect stimulus
  • e.g. end-bulbs of Krause
  • sense organ - e.g. tongue
• How do receptors respond to light as a stimulus?

  • photoreceptors detects stimulus
  • e.g. cone cell (detects diff light wavelengths)
  • sense organ - e.g. eye
• Sensory receptors role as a transducer

  convert stimulus into a nervous impulse called a generator potential
• How does the Pacinian corpuscle convert mechanical pressure into a nervous impulse?

  • when pressure is applied the corpuscle changes shape & stretch-mediated sodium ion channels widen
  • sodium ions can now diffuse into the neurone
  • influx of positive sodium ion channels changes the potential of the membrane - becomes depolarised - results in a generator potential
  • generator potential creates an action potential that passes along the sensory neurone
  • action potential will then be transmitted along neurones to the CNS
• What is resting potential?

  • when a neurone is not transmitting an impulse
  • outside of membrane is positively charged & inside of axon is negatively charged - membrane is polarised
  • potential difference is normally about -70mV
• How is a resting potential created?

  • 3 sodium ions are actively transported out of the axon for every 2 potassium ions actively transported in
  • there are more Na+ ions outside the axon than inside whereas there are more K+ ions inside the cytoplasm than outside
  • most of the gated sodium ion channels are closed, preventing the movement of sodium ions back into the axon, whereas many potassium ion channels are open allowing potassium ions to diffuse out of the axon
  • even more positive charge builds up outside the axon than inside the cell creating a resting potential of -70mV
• Stages of changes in potential difference during an action potential

  A) polarised (resting potential)

  B) depolarised (action potential)

  C) depolarisation

  D) hyper-polarisation

  E) repolarisation

  F) repolarised (resting potential)

  
• Polarised

  • neurone has a resting potential - is not transmitting an impulse
  • some potassium ion channels (mostly not voltage-gated) are open but sodium voltage-gated ion channels are closed
• Depolarisation

  • energy of the stimulus triggers some voltage-gated ion channels to open, making membrane more permeable to sodium ions - sodium ions diffuse into axon down electrochemical gradient - inside of neurone less negative
  • change in charge causes more sodium ion channels to open allowing more sodium ions to diffuse into axon (positive feedback)
  • when potential difference reaches +40mV voltage-gated sodium ion channels close & voltage-gated potassium ion channels open - sodium ions can no longer enter axon but membrane is now more permeable to potassium ions
• Repolarisation

  • potassium ions diffuse out of axon down electrochemical gradient - this reduces charge, resulting in inside of axon becoming more negative than outside
  • initially lots of potassium ions diffuse out of axon = inside of axon more negative than resting state - hyperpolarization
  • voltage-gated potassium channels now close
  • sodium-potassium pump causes sodium ions to move out of cell & potassium ions to move in - axon returns to resting potential - repolarised
• Propagation of action potentials

  1. at resting potential - more positive outside axon - polarised
  2. influx of sodium ions depolarises the axon membrane
  3. inc +ve charge opens sodium voltage-gated channels further along the axon - influx of Na+ ions in this region causes depolarisation - behind new region of depolarisation sodium voltage-gated channels close & potassium ones open - K+ ions diffuse out of axon
  4. action potential is propagated along axon - diffusion of K+ out continues until axon membrane behind action potential has been repolarised
  5. membrane returns to resting potential
• Refractory period 


  • period after an action potential when the axon cannot be excited again
  • voltage-gated sodium ion channels remain closed, preventing movement of sodium ions into the axon
• Importance of refractory period


  • prevents propagation of an action potential backwards along the axon as well as forwards
  • makes sure action potentials are unidirectional
  • ensures action potentials do not overlap & occur as discrete impulses
• Saltatory conduction 


  • myelinated axons transmit impulses at a faster rate
  • because depolarisation of the axon membrane can only occur at nodes of Ranvier where myelin not present
  • action potential 'jumps' from one node to another in processes known as saltatory conduction
  • this is faster than a wave of depolarisation along the whole length of the axon membrane
  • also more energy efficient - repolarisation uses ATP in the sodium pump so by reducing amount of repolarisation needed saltatory conduction makes transmission more efficient
• 3 factors affecting speed of nerve impulses

  • axon diameter
  • temperature
  • presence of myelin
• How does axon diameter affect speed of impulses

  • bigger the axon diameter, faster the impulse is transmitted
  • because there's less resistance to the flow of ions in the cytoplasm
• How does temperature affect the speed of nerve impulses?

  • higher temp, faster nerve impulse
  • ions diffuse faster at higher temps
  • however, only occurs up to 40°C as higher temps cause proteins (such as sodium-potassium pump) to denature