nervous coordination

• Main forms of coordination in animals

  • Nervous system
  • Hormonal system
• Nervous system

  • Use nerve cells to pass electrical impulses along their length
  • Stimulate target cells by secreting chemicals- neurotransmitters, directly on them
  • Rapid communication on specific parts of the body, response short-lived and restricted onto localised region
• Nervous coordination

  • Use nerve cells to pass electrical impulses along length
  • Such as reflex action (such as quick withdrawal from hot substances)
• Hormonal system

  • Produce chemicals, transported in blood plasma to target cells
  • Target cells have specific receptors on cell surface membranes and change in of conc of hormones stimulate them
  • Results in a slower, less specific form of communication between parts an organism
  • Responses are long-lasting and has a widespread effect
• Hormonal coordination


  • Is by chemicals transported in blood plasma
  • an example of hormonal coordination is control of blood glucose
• Neurones


  Nerve cells that are specialised to rapidly carry electrochemical changes called nerve impulses from one part of the body to another
• Mammalian motor neurone

  • Cell body containing all usual cell organelles, including the nucleus and large amounts of rough endoplasmic reticulum. Associated with the production of proteins and neurotransmitters
  • Dendrons - extensions of a cell body which subdivide into smaller branched fibres, called dendrites, carry nerve impulse towards cell body
  • Axon - single long fibre that carries nerve impulses away from the cell body
  • Schwann cells - surround the axon, protect and provide electrical insulation. Also, carry out phagocytosis (removal of cell debris) and play a part in nerve regeneration. They wrap themselves around the axon many times, so layers of membranes build in around it
  • Myelin sheath - forms a covering to axons and is made up of membranes of Schwann cells. Membranes are rich in lipid known as myelin. Neurones with myelin sheath are myelinated neurones
  • Nodes of Ranvier - constriction between adjacent Schwann cells where there is no myelin sheath
• • Neurones are classified according to their function:
  • Sensory neurones: transmit nerve impulses from a receptor to an intermediate or motor neurone. Have one dendron often very long. Carries impulse towards cell body and one axon that carries it away from the cell body
  • Motor neurones: transmit nerve from an intermediate or relay neurone to an effector (gland or muscle). Motor neurones have long axon and many short dendrites
  • Intermediate or relay neurones: transmit impulses between neurones, e.g. from sensory to motor neurones. Have many short processes
• Nerve impulse

  Self-propagating wave of electrical activity that travels along axon membrane. A temporary reversal of electrical potential difference across the axon membrane. Reversal between 2 states called: resting potential and action potential.
• Resting potential

  Movement of ions, such as sodium ions and potassium ions, across axon membrane is controlled in a number of ways:

  • phospholipid bilayer
  • channel proteins
  • some carrier proteins
• Phospholipid bilayer


  • Of axon plasma membrane prevent sodium and potassium ions diffusing across it
• Channel proteins


  • Span phospholipid bilayer. Protein have channels called ion channels and have gates which can be opened or closed so sodium or potassium ions can move through by facilitated diffusion
• Carrier proteins

  • Actively transport potassium ions into axon and sodium ions out of axon. Mechanism called: sodium-potassium pump
• As result of these controls: inside of an axon is negatively charged relative to the outside. Known as resting potential.

  Axon is polarised
• Establishment of potential difference (difference in charge between inside and outside of axon)

  1. Sodium ions actively transported out of axon by sodium-potassium pumps

  2. Potassium ions actively transported into the axon by sodium-potassium pumps

  3. Active transport of sodium ions is greater than that of potassium ions ( 3 Na ions move out for every 2 K ions move in)
• Na and K ions both positive; outward movement of Na ions greater than inward movement of K ions. Meaning, more sodium ions in tissue fluid surrounding axon than in cytoplasm, and more potassium ions in the cytoplasm than in tissue fluid. Creating an electrochemical gradient
• Na ions begin to diffuse back naturally into axon and K ions begin to diffuse out of axon
• Most of gates in channels that allow K ions to move through are open, whilst most of gates in channels that allow Na ions to move through are closed
• • Stimulus of sufficient  size detected by receptor in nervous system, energy causes temp reversal of charges either side of this part of axon membrane
  • If stimulus is great enough: negative charge inside membrane become positive charge- action potential, and part of axon membrane becomes depolarised
  • Depolarisation occurs since channels in axon membrane change shape and hence open or close, depending on voltage across membrane
  • These are voltage-gated channels
• label the different types of neurone

  A) cell body

  B) dendrites

  C) axon

  D) nucleus

  E) cell body

  F) myelin sheath

  G) node of ranvier

  H) dendron

  I) axon

  J) nerve endings

  K) dendrites

  L) dendron

  M) axon

  
• Action potential: movement of Na ions inwards due to diffusion, is passive process
• label the nerve cell

  A) axon terminal

  B) axon

  C) nucleus

  D) nucleolus

  E) dendrites

  F) cell body

  G) myelin sheath

  H) node of ranvier

  I) schwann cells

  J) axon

  K) myelin sheath

  
• Resting potential: movement of Na ions inwards maintained by active transport, is active process
• • Action potential means axon membrane is transmitting a nerve impulse
  • Resting potential means axon membrane is not transmitting a nerve impulse
• Passage of an action potential

  • Action potential: moves rapidly along an axon
  • Size of an action potential remains the same from one end of the axon to another
  • As a region of axon produce an action potential and becomes depolarised, it acts as a stimulus for depolarisation of next region of axon
  • Action potentials generated along each small region of axon membrane
  • Action potential: a travelling wave of depolarisation
  • Whilst, previous region of membrane returns to resting potential, it undergoes repolarisation
• Myelinated axon

  Fatty sheath of myelin around axon acts as electrical insulator, prevents action potentials forming
• Myelinated axon


  • Breaks in myelinated sheath called Nodes of Ranvier
  • Action potentials can occur on these points
• Passage of an action potential along myelinated axon

  1. Localised circuits arise between adjacent Nodes of Ranvier

  2. Action potential jumps from node to node in the process- called saltatory conduction
• Action potential pass along myelinated neurone


  Faster than along unmyelinated sheath of same diameter
• In an unmyelinated sheath, events of depolarisation have to take place all the way along the axon taking up more time
• Myelin sheath

  • Electrical insulator
  • Prevents action potential forming in part of axon covered
  • Action potential jumps from one node of Ranvier to another (saltatory conduction)
  • Speeds up conduction more than an unmyelinated one
• Diameter of axon

  • Increasing diameter= faster speed of conductance
  • Less leakage of ions from large axon
  • Membrane potentials harder to maintain
• Temperature


  • Affects rate of diffusion of ion
  • Increasing temp= faster nerve impulse
  • Affects energy of active transport from respiration (sodium-potassium pump controlled by enzymes)
  • Enzymes function increase at increasing temps
  • Above certain temps enzyme and plasma membrane protein denature and impulse fail to conduct
  • Affects speed and strength of muscle contraction
• All or nothing principle


  • At certain levels of stimulus (threshold value), which trigger action potential
  • Below threshold value=no action potential= no impulse generated
  • Stimulus exceeding threshold value will generate action potential so nerve impulse is generated
  • Action potentials around same every time generated, so size and strength not detected by size of action potentials
• all or nothing principle

  Describes a nerve impulses,

  at certain threshold values can trigger an action potential

  • below threshold= no action potential
• Size of stimulus detected

  • Number of impulses in given time: larger stimulus= greater impulse generated in a given time
  • Having different neurones with different threshold values: brain interpret number and type of neurones that pass impulses as result of given stimulus and so determines size
• Once action potential generated in region of axon, period afterwards when inward movement of Na ion prevented since Na voltage-gated channels are closed. So no further action potential are generated called refractory period
• Purposes of the refractory period


  1. Ensures action potentials are propagated in one direction only

  2. Produces discrete impulses

  3. Limits the number of action potentials
• Ensures action potentials are propagated in one direction only


  • Action potentials only pass from one active region to a resting region
  • Action potentials cannot be propagated in a region that is refractory- only move in forward direction
  • Prevents action potentials spreading out in both directions
• Produces discrete impulses

  • Due to refractory period, new action potentials cannot be formed immediately behind first one
  • Ensures action potentials are separated from one another