Neuronal communication

• we need to respond to a stimuli in order to ensure that enzyme activity is maintained, cellular function is maintained and to ensure an organisms survival
• homeostasis is maintaining a constant internal environment. despite changes both internally and externally
• negative feedback is when the response is to reverse the effect of the stimulus
• positive feedback is when the response is to increase the effect of the stimulus
• neuronal: communicates via electrical impulses and synapses, conscious and unconscious, very fast response, short term effect, one direction along a neurone

  hormonal: communicates via chemicals in bloodstream, always unconscious, relatively slow response, longer term effect, many directions
• sensory neurone carries impulses from receptor to the CNS, has long dendrites
• relay neurone allows sensory and motor neurones to communicate, found in the CNS, no axon
• motor neurone carry impulses from CNS to effector, long axon, nucleus in middle
• schwann cell maintains peripheral nervous system, it wraps around the inner axon laying down a myelin sheath around the neurone
• node of ranvier a gap in the myelin sheath, 0.7-1.4 um long, they speed up action potentials
• symptoms of MS (multiple sclerosis) include fatigue, difficulty walking, vision problems
• MS is an autoimmune disease - the body's immune system attacks its own tissues - breaks down myelin sheath
• a stimulus is a change in the environment
• chemoreceptors detect chemical changes
• photoreceptors detect changes in light
• sensory receptors act as transducers as they change the stimulus to an electrical signal
• synapses are found between axons and dendrites
• the pre-synaptic knob contain numerous mitochondria and ER as they are required to manufacture neurotransmitters
• an action potential causes the neurotransmitter to be released into the synaptic cleft (gap)
• neurotransmitters diffuse across the cleft and bind to receptor proteins on the post synaptic neurone
• the most common neurotransmitter found in the nervous system is Acetylcholine - role in brain functions
• the receptor for Acetylcholine is a G-protein
• it is important for an enzyme to break down neurotransmitters found in the cleft to prevent constant stimulation of the post-synaptic cell and excessive firing of action potentials
• synapses ensure that impulses are only transmitted in one direction as nerve cells only have one transmission site
• ions get in and out of cells through ion channels
• 4 types of ion channels: ligand-gated, mechanically-gated, voltage-gated, two way pump
• ions move in voltage-gated channels due to an potential difference
• ions move through a ligand-gated channel as a molecule to bind/interact with a receptor site to open the channel
• ions move through a mechanically gated channel as a pressure/stretch on the membrane to physically open it
• the resting membrane potential is -70mv
• the threshold membrane potential is -55mv
• an action potential propagated along a neurone by:

  • neurone begins at resting membrane potential (-ve inside, +ve outside)
  • action potential (AP) occurs = for a short period of time is -ve outside and +ve inside
  • Na+ ions flow along the cytoplasm down an electrochemical gradient
  • depolarises the adjacent cell membranes = more Na+ voltage-gated channels open = another AP
• to increase the speed of an action potential:

  • increased temperature (= increased KE)
  • decreased axon length (= steeper electrochemical gradient) increased axon width (=more space to pass organelles)
  • myelinated (nodes of Ranvier present = only place where AP can occur - jumps along neurone - saltatory conduction)
• AP can vary in speed and frequency but not in size (always 100mv)
• Steps of synaptic transmission:

  • Action potential (AP) reaches the presynaptic membrane, activating Ca+ voltage-gated channels to open
  • Ca+ ions move through channels down their electrochemical (EC) gradient into the axon terminal in the presynaptic neuron
  • Ca+ triggers the movement of synaptic vesicles
  • Synaptic vesicles fuse with the presynaptic axon terminal membrane, releasing neurotransmitters by exocytosis into the synaptic cleft
  • Neurotransmitters diffuse and bind onto specific receptors on ligand-gated Na+ channels, opening them
• • Na+ channels open, allowing Na+ to diffuse into the postsynaptic cell, depolarizing it to -55mV
• summation is adding up of ion channels
• two types of summation: temporal summation and spatial summation
• EPSP: Excitatory post synaptic potential
• IPSP: Inhibitory post synaptic potential