Nervous co-ordination

• Proteins and polypeptides - the attachment of hormones to a receptor triggers the production of a second chemical inside the cell that completes the function.
• Steroids like oestrogen bond with the receptor forming a new compound that has the ability to pass through the cell membrane.
• The central nervous system is composed of specialised cells called neurons and nerve cells.
• Nerve cells and neurons are bound together by connective tissue to form nerve fibres which electrical impulses travel down.
• Hormonal system:

  • Communication is by chemicals called hormones
  • Transmission by blood system
  • Hormones travel to all parts of the body
  • Transmission is slow
  • Response is long lasting
  • Effect may be permanent and irreversible
  • Response is widespread
• Nervous system:

  • Communication by nerve impulses
  • Transmission by neurons
  • Transmission and response is very rapid
  • Response is short lived
  • Nerve impulses travel to specific parts of the body
  • Response is localised
  • Effect is temporary and reversible
• 1. Sensory neurone - carry impulses from receptors to the CNS (brain or spinal cord)
  2. Relay (intermediate) neurone - found entirely within the CNS and connect sensory and motor neurons.
  3. Motor neurone - carry impulses from the CNS to effectors (muscles or glands)
• Motor Neurone:

  
• Dendrites - Carry impulses from other nerve cells to the cell body.
• Axon - long membrane covered cytoplasmic extensions that transmits impulses away from cell body, covered in myelin sheath.
• Myelin Sheath - made of phospholipid membrane of schwann cells which covers axon, speeds up impulses.
• Nodes of Ranvier - constrictions between adjacent Schwann cells.
• Axon terminal - where axon divides and synapses appear.
• Resting potential - the potential difference between the inside and outside of a membrane when a nerve impulse is not being conducted.
• Resting potential - in a resting axon (one that is not transmitting impulses), the inside of the axon always has a negative electrical potential compared to outside the axon.
• This potential difference when there are no impulses is usually about -70mV (ie. the inside of the axon has an electrical potential about 70mV lower than the outside)
• The active transport of sodium ions and potassium ions:

  • Carrier proteins called sodium-potassium pumps are present in the membranes of neurones
  • These pumps use ATP to actively transport 3 sodium ions out of the axon for every 2 potassium ions that they actively transport in
  • This means that there is a larger concentration of positive ions outside the axon than there are inside the axon
  • The movement of ions via the sodium-potassium pumps establishes an electrochemical gradient
• A differential membrane permeability:

  • The cell-surface membrane of neurones has selective protein channels that allow sodium and potassium ions to move across the membrane by facilitated diffusion
  • The protein channels are less permeable to sodium ions than potassium ions
  • This means that potassium ions can diffuse back down their concentration gradient, out of the axon, at a faster rate than sodium ions
• Resting potential of axon and how it is maintained

  
• Sodium-Potassium pump and how it maintains a resting potential.

  
• The Axon being polarised is the same as resting potential.
• Action potentials are caused by the rapid movement of sodium ions and potassium ions across the membrane of the axon.
• An action potential occurs when the membrane becomes depolarised due to stimulation from another nerve or muscle fibre.
• Action Potential:

  1. Stimulus
  2. Depolarisation
  3. Repolarisation
  4. Hyperpolarisation
  5. Resting state

  
• Resting potential

  Sodium voltage gates are closed
• Action potential

  1. Energy of the stimulus changes the permeability of the axon membrane

  2. Some sodium voltage gated channels open

  3. Sodium ions diffuse into the membrane

  4. Charge is raised

  5. More sodium gated channels open

  6. Sodium ions flood the axon via diffusion
• Depolarisation


  Inside of the axon becomes less negative
• Action potential peak

  1. Sodium voltage gated channels close

  2. Voltage gated potassium channels open

  3. Potassium ions diffuse out of axon

  4. Membrane becomes more positive

  5. Inside becomes more negative
• Hyperpolarization

  Axon becomes hyperpolarized
• Restoring resting potential

  1. Potassium gates close

  2. Membrane restores axon to resting potential by the sodium potassium pump
• How an impulse is transmitted in one direction along the axon of a neuron and why a nervous impulse does not move back along the axon.

  
• When receptors (such as chemoreceptors) are stimulated


  They are depolarised
• If the stimulus is very weak or below a certain threshold

  The receptor cells won't be sufficiently depolarised and the sensory neuron will not be activated to send impulses
• If the stimulus is strong enough to increase the receptor potential above the threshold potential

  The receptor will stimulate the sensory neuron to send impulses
• All-or-nothing principle

  An impulse is only transmitted if the initial stimulus is sufficient to increase the membrane potential above a threshold potential
• Rather than staying constant, threshold levels in receptors often increase with continued stimulation, so that a greater stimulus is required before impulses are sent along sensory neurons
• Refractory period

  • Ensures action potentials are discrete events, stopping them from merging into one another
  • Ensures 'new' action potentials are generated ahead (further along the axon), rather than behind the original action potential, as the region behind is 'recovering' from the action potential that has just occurred
• Refractory period

  Means the impulse can only travel in one direction, which is essential for the successful and efficient transmission of nerve impulses along neurones
• Refractory period

  Means there is a minimum time between action potentials occurring at any one place along a neurone
• Length of the refractory period

  Key in determining the maximum frequency at which impulses can be transmitted along neurones (between 500 and 1000 per second)