Neuronal communication

• cell body of a neuron

  contain all organelles of a normal cell

  nucleus is located here

  proteins and neurotransmitter chemicals are made here
• dendrons of a neuron

  carry action potentials to surrounding cells
• axon

  conductive long fiber that carriers the nervous impulse along the motor neuron
• myelinated neuron

  schwann cells which wrap around the axon to form a myelin sheath

  it is lipid based and so is impermeable to small ions

  gaps in the myelin sheath are call the nodes of ranvier

  action potentials therefore jump between the nodes by saltatory conduction
• why are myelinated neurons faster

  action potential is only generated at the nodes of ranvier instead of the entire length of the axon
• sensory neuron

  carry impulses from the sensory receptor to the relay neuron

  long dendron which carries the impulse from the receptor cell to the cell body

  axon carries the impulse from the cell body to the next neuron
• relay neuron

  multiple short axon and dendrons that look like tentacles

  carry impulses from a sensory to a motor neuron
• motor neuron

  carry the impulse from the sensory neuron to the effector

  have one long myelinated axon and multiple short dendrons
• sensory receptors

  they are transducers -> convert different stimuli into electrical nervous impulses
• pacinian corpuscle

  located deep in the skin

  they contain special channel proteins known as stretch medium sodium channels

  these open and allow Na+ to enter the sensory neuron where they are stretched and deformed

  when pressure is applied it deforms the neuron plasma membrane and widening the protein channels which allows a generator potential to be established
• resting potential

  at rest there is a difference in charge between the inside and outside of a neuron

  there are more positive ions such as Na+ and K+ outside compared to inside therefore the inside is said to be more negatively charged

  it is resting at -70 mV
• establishing a resting potential

  maintained by a sodium-potassium pump

  it pumps 2 K+ into the neuron and 3 Na+ out of the neuron

  this creates electrochemical gradients causing K+ to diffuse out and Na+ to diffuse in

  the membrane is more permeable to K+ so more positive charge is moved out of the membrane than is diffusing in
• action potential

  when the neurons voltage increases beyond a set point from the resting potential generating a nervous impulse

  an increase in voltage known as depolarisation is due to the membrane becoming more permeable to Na+ ions
• threshold voltage

  -55 mV
• action potential voltage

  +40 mV
• refractory period voltage

  less than -70 mV
• stimulus

  provides energy which causes the voltage-gated sodium ion channels to open

  Na+ diffuses into the neuron which causes more voltage-gated channels to open

  causing the inside of the neuron to depolarize as the charge is overall more positive
• action potential is reached

  sodium ion voltage-gated channels close

  but the K+ stay permanently open which causes repolarization as the overall charge inside the neuron becomes more negative

  more K+ channels open causing more K+ to diffuse out which can lead to hyperpolarization where the overall charge is more negative than the resting potential
• at each node of ranvier, once an action potential has been generated it can open the sodium ion voltage-gated channels at the next node
• all or nothing principle

  if the depolarisation does not exceed -55 mV an action potential is not produced

  however if it does exceed it, it will always peak at the same maximum voltage of +40 mV
• bigger stimuli increase the frequency of action potentials
• purpose of the all or nothing principle

  any stimulus that exceeds -55 mV will always peak at the maximum voltage of +40 mV

  bigger stimuli don't give a higher voltage

  instead it is the frequency of stimuli that would increase

  this is important as it makes sure animals only respond to large enough stimuli rather than every slight change in the environment
• refractory period

  where the membrane cannot stimulate another action potential because the sodium ion voltage-gated channels are recovering and cannot be opened
• importance of the refractory period

  1. discrete impulses are produced -> makes sure that each action potential is seperate as they can't be generated quickly after each other
  2. action potentials travel in one direction -> ensure a response
  3. limits number of transmissions -> prevents over reaction of a stimulus
• synapses

  gaps between the end of an axon of one neuron and the dendrite of another neuron
• the gap between a synapse

  synaptic cleft
• function of a synapse - presynaptic

  an action potential arrives at the synaptic knob

  depolarisation causes the opening of Ca2+ channels where the calcium ions can diffuse into the synaptic knob

  vesicles containing acetylcholine move towards and fuse with the presynaptic membrane releasing acetylcholine into the synaptic cleft
• function of a synapse - postsynaptic

  neurotransmitters diffuse down a concentration gradient across the synaptic cleft to the postsynaptic membrane

  they bind to complementary receptors there which causes Na+ ion channels to open and allowing sodium ions to diffuse into the postsynaptic membrane

  if this reaches above the -55 mV threshold then depolarization will occur and an action potential will be generated

  the neurotransmitters broken down by acetylcholinesterase and the Na+ channels close
• unidirectional synapse

  vesicles containing neurotransmitters only on the presynaptic neuron

  receptors for neurotransmitters only on the post synaptic neuron

  acetylcholinesterase present in the synaptic cleft will break down the acetylcholine to prevent another action potential from being generated
• summation

  rapid build up of neurotransmitters in a synapse to help generate an action potential
• spatial summation

  many different presynaptic neurons all converging at one postsynaptic neuron

  combine the neurotransmitters they release to exceed the threshold value
• temporal summation

  one neuron releases a neurotransmitter repeatedly over a short period of time to exceed the threshold potential value
• inhibitory synapse

  causes chloride ions to move into the postsynaptic neuron and potassium ions to move out

  this causes hyperpolarisation making an action potential really unlikely