B15 nervous coordination + muscles

• neurones are specialised cells adapted to rapidly carrying nerve impulses from one part of the body to another
• a motor neurone has the following components:

  cell body, dendrons, axon, schwann cells, myelin sheath, nodes of ranvier
• the cell body of a motor neurone contains all the usual organelles and is associated with production of proteins and neurotransmitters
• the dendrons of a motor neurone are extensions of the cell body which carry nerve impulses towards the cell body, they divide into smaller branches called dendrites
• the axon of a motor neurone is a single long fibre which carries nerve impulses away from the cell body
• the schwann cells of a motor neurone surround the axon in layers, protect axon and provide insulation, carry out phagocytosis, and are involved in nerve regeneration
• the myelin sheath of a motor neurone covers the axon and is made up of schwann cell membranes rich in myelin
• the nodes of ranvier of a motor neurone are constrictions between adjacent schwann cells where there is no myelin sheath
• the three types of neurone are:

  motor, sensory, intermediate/relay
• sensory neurones transmit nerve impulses from a receptor to an intermediate or motor neurone

  motor neurones transmit nerve impulses from an intermediate or relay neurone to an effector

  intermediate/relay neurones transmit nerve impulses between neurones
• sensory neurones have one long dendron that carries impulse to cell body and one axon that carries impulse away from cell body

  motor neurones have a long axon and many short dendrites

  intermediate/relay neurones have a very short axon and many dendrites
• a nerve impulse is a wave of electrical activity that travels along the axon membrane, caused by a temporary reversal of the electrical potential difference across the axon membrane
• the phospholipid bilayer of the axon plasma membrane prevents Na+ and K+ ions from freely diffusing across it, so they have to cross it by facilitated diffusion using gated channel proteins which can be opened or closed to control the movement of ions, or by active transport using a sodium-potassium pump carrier protein
• the free diffusion of Na+ and K+ ions leads to the inside of an axon being relatively negative compared to the outside, making it polarised, this is the resting potential of an axon
• resting potential is established by the following process:

  • 3 Na+ ions are actively transported out of the axon by the sodium-potassium pump
  • 2 K+ ions are actively transported into the axon by the sodium-potassium pump
  • both ions are positive and more Na+ ions move out than K+ ions move in so an electrochemical gradient is created
  • voltage-gated sodium channels are closed so few Na+ ions diffuse back into the axon
  • voltage-gated potassium channels are open so K+ ions diffuse out of the axon
• action potential process:

  • at resting potential some K vgc are open but all Na vgc are closed
  • some Na vgc open so Na+ ions diffuse into the axon along their electrochemical gradient
  • Na+ ions are positive, so potential difference across the membrane is reversed
  • as Na+ ions diffuse in, more Na vgc open, until a threshold is reached, then Na vgc close and K vgc open
  • as K+ ions diffuse out, more K vgc open, this starts repolarisation of the axon
  • resting potential is restored
  • K vgc close slowly, causing hyperpolarisation
  • sodium-potassium pump reverses hyperpolarisation
  • resting potential is restored
• once an action potential is created, it moves rapidly down an axon, but the size of it remains the same from one end to the other, and technically nothing actually moves down the axon
• as one region of the axon produces an action potential then becomes depolarised, it acts as a stimulus for the depolarisation of the next region, causing a travelling wave of depolarisation
• in an unmyelinated axon, the wave of depolarisation moves to the adjacent resting region where sodium ions trigger a change in potential difference, stimulating the next action potential
• in a myelinated axon, no action potential can be generated in a section which has myelin as it is an electrical insulator, so the action potential jumps between gaps in the myelin which are called nodes of ranvier, through a process called saltatory conduction
• a nerve impulse is the transmission of an action potential along the axon of a neurone without it changing in size
• factors which affect speed of action potential travel:

  • presence of myelin sheath
  • axon diameter
  • temperature
• presence of a myelin sheath affects speed of an action potential because if the axon is myelinated, saltatory conduction occurs which is much faster than generating an action potential at every point along the axon
• axon diameter affects speed of an action potential because the greater the axon diameter, the faster the conduction, as there is less leakage of ions
• temperature affects speed of an action potential because if temperature increases, the ions will diffuse more rapidly, and rate of respiration increases so the amount of ATP produced increases, so the sodium-potassium pump can work faster
• the period after an action potential is the refractory period, in which a new action potential cannot be generated because the sodium channels enter a recovery stage and are closed
• the refractory period prevents the movement of the wave of depolarisation backwards down the chain, because an action potential cannot be generated in a neurone in the refractory period, it also means discrete impulses are produced as a new impulse cannot be formed directly behind the first one, so they stay separate, and it limits the number of action potentials produced and therefore the strength of the stimulus that can be detected
• the all-or-nothing principle means that either an action potential is produced, or it is not, and a threshold value must be reached for an action potential to be created, and all action potentials will be of the same strength
• a synapse is a point where one neurone communicates with another neurone, or with an effector, by transmission of chemicals called neurotransmitters
• neurones are separated by a synaptic cleft
• only the presynaptic neurone releases neurotransmitters
• the axon of the presynaptic neurone ends in a swollen portion called the synaptic knob, which has many mitochondria and lots of endoplasmic reticulum, required for production of neurotransmitters in the axon
• the neurotransmitters are stored in synaptic vesicles which fuse with the membrane of the neurone and release them into the synaptic cleft, where they diffuse across to the postsynaptic neurone which has specific receptor proteins on its membrane for them
• synapses can only pass information in one direction, from the presynaptic neurone to the postsynaptic neurone, this prevents action potentials from travelling backwards
• if action potentials are low-frequency, they can lead to insufficient concentrations of neurotransmitter to trigger a new action potential in the postsynaptic neurone, this can be solved in two ways, spatial summation or temporal summation
• spatial summation is when many different presynaptic neurones together release enough neurotransmitter to reach the threshold value of the postsynaptic neurone
• temporal summation is when one presynaptic neurone releases neurotransmitter many times over a short time period to reach the threshold value of the postsynaptic neurone
• a cholinergic synapse is a synapse which uses the neurotransmitter acetylcholine
• acetylcholine is made up of ethanoic acid and choline
• cholinergic synapse transmission:

  • ap arrives at pre-n., causes Ca vgc to open, Ca2+ ions enter sk by facilitated diffusion
  • synaptic vesicles fuse with the pres-m, releasing acetylcholine into the sc
  • acetylcholines diffuse across the sc, bind to receptors on Na vgc in the post-m
  • this causes Na vgc to open, Na+ ions diffuse into the post-n
  • this generates a new ap in the post-n
  • acetylcholinesterase hydrolyses acetylcholine into ch and ea, which diffuse back across the sc into the pre-n
  • ea and ch are combined into acetylcholine using ATP
  • Na vgc close due to no acetylcholine in their receptors