Nervous Communication

• Sensory receptor:

  • located in sense organs 
  • convert stimulus into nerve impulse 
  • information passed to CNS 
  • specific to a single type of stimulus 
  • act as a transducer 
• Transducer:

  • detects a stimulus 
  • converts stimulus into nervous impulse called a generator potential 
• Pacinian corpuscle:

  • detect mechanical pressure 
  • located deep in skin 
  • located in fingers, soles of feet and joint 
• structure of pacinian corpuscle: 

  • end of sensory neurone found in centre of corpuscle 
  • surrounded by layers of connective tissue with each layer of tissue separated by a layer of gel 
  • within the neurone membrane there are stretch-mediated sodium ion channels which change shape when pressure is applied 
  • when pressure is applied, the permeability changes 
• Pacinian corpuscle as a transducer: 

  • stretch-mediated sodium ion channels are too narrow to allow Na+ ions in (resting potential)
  • pressure is applied to the corpuscle, corpuscle changes shape, membrane surrounding neurone stretches, sodium channels widen and sodium ions diffuse into neurone 
  • the membrane becomes depolarised which results in a generator potential 
  • the generator potential creates an action potential which travels along the sensory neurone to the CNS 
• stimulus - changes in the internal and external environment 
• response - information is processed and an appropriate response is triggered 
• neurones - specialised nerve cell, transmits electrical impulses around the body
• Cell body: 

  • nucleus surrounded by cytoplasm
  • cytoplasm contains large amounts of endoplasmic reticulum and mitochondria as they are involved in the production of neurotransmitters 
• dendrons:

  • short extensions which come from the cell body 
  • divide into dendrites 
  • responsible for transmitting impulses towards the cell body 
• axon: 

  • singular, elongated nerve fibres that transmit impulses away from cell body 
  • can be very long 
  • fibre is cyclindrical in shape with a narrow region of cytoplasm surrounded by a membrane 
• Sensory neurone: 

  • carry action potential away from sensory receptor to CNS 
• relay neurone/intermediate neurone: 

  • found in CNS, adapted to carry impulses to and from numerous other neurones 
• motor neurone: 

  • carry action potential from CNS to an effector 
• Myelin sheath: 

  • insulating layer made of fatty material 
  • produced by Schwann cells 
  • Schwann cell produces myelin sheath by growing around axon in a spiral, enclosing it in many layers of plasma membrane 
  • mostly made of lipids and proteins 
  • there are gaps between the schwann cells called the nodes of Ranvier, every 1-3mm and 2-3micrometers in size 
• Myelin sheath role in transmission:

  • sodium and potassium ions cannot diffuse through fatty layer 
  • the ionic movements that create an action potential cannot occur over the length of the neurone 
  • ionic charges that cause an action potential only occur at nodes of ranvier 
  • action potential then jumps from one node to the next (saltatory conduction)
• Resting potential: 

  • when a neurone is not transmitting an impulse 
  • area outside of membrane is more positively charged than inside axon 
  • cell surface membrane is polarised 
  • potential difference across the membrane is -70mV 
  • sodium ions are actively transported out of axon 
  • potassium ions are actively transported into axon 
  • movement of ions occurs via sodium-potassium pumps 
  • for every 3 Na+ pumped out, 2 K+ go in 
  • gated sodium channels are closed, potassium channels are open 
  • potassium ions diffuse out of cell, sodium ions remain outside 
  • resting potential is created. 
• Action potential:

  • axon is depolarised
  • energy of stimulus triggers sodium channels to open
  • sodium ions diffuse into axon down gradient
  • the neurone is less negative
  • the change in charge causes more sodium ion channels to open
  • when pd reaches +40mV, sodium channels close and potassium channels open
  • potassium ions diffuse out of the cell causes the charge to become more negative (hyperpolarisation)
  • hyperpolarisation prevents another action potential from being generated
  • potassium channels close, Na+ is pumped out of cell, K+ is pumped into cell
  • axon is repolarised
• Propagation of action potential:

  • action potential is triggered in sensory receptor
  • first region of neurone is depolarised
  • sodium ions are attracted to the negative charge and concentration gradient ahead in the axon
  • this causes them to move down the axon
  • next region is stimulated to depolarise
• refractory period:

  • voltage-gated sodium channels closed
  • prevents propagation of action potential backwards
  • prevents action potentials overlapping
• synapse - junction between two neurones
• neurotransmitter - chemicals that transmit impulses across a synapse
• synaptic cleft - the gap which separates the axon of one neurone from the dendrite of the next
• presynaptic neurone - the neurone along which the impulse has arrived
• postsynaptic neurone - neurone that receives neurotransmitters
• synaptic knob - swollen end of presynaptic neurone, contains many mitochondria and large amounts of ER to manufacture neurotransmitters
• synaptic vesicles - vesicles containing neurotransmitters; fuse with presynaptic membrane and release contents into synaptic cleft
• neurotransmitter receptors - receptor molecules which the neurotransmitter binds to the postsynaptic membrane
• Excitatory neurotransmitters:

  • result in the depolarisation of the postsynaptic neurone
  • if the threshold is reached in the postsynaptic membrane, an action potential is triggered
• Inhibitory neurotransmitters:

  • these cause hyperpolarisation of the postsynaptic membrane
  • this prevents an action potential from being triggered
  • it binds to specific transmembrane receptors in the plasma membrane of both pre- and postsynaptic neurones
  • this causes the opening of ion channels to allow the flow of chloride ions into the cell or potassium ions out of the cell, causing hyperpolarisation
  • this reduces excitability through the nervous system and is used to maintain muscle tone
• transmission of impulses across a synapse:

  • action potential reaches end of presynaptic neurone
  • depolarisation causes calcium ion channels to open
  • calcium ions diffuse into synaptic knob
  • synaptic vesicles fuse with the synaptic membrane
  • neurotransmitters are released and diffuse to receptors on the postsynaptic membrane
  • this causes sodium ion channels to open and sodium ions enter the postsynaptic neurone
  • an action potential is triggered
  • enzymes that breakdown the neurotransmitter are present in the synaptic cleft to prevent another impulse being sent
• Acetylcholine - acetic acid and coline
• acetylcholinesterase - breaks down acetyl choline into acetic acid and choline, which diffuses back across the synaptic cleft to the presynaptic neurone
• Spatial summation:

  • a number of presynaptic neurones connect to one postsynaptic neurone
  • each presynaptic neurone releases neurotransmitters which build up to trigger an action potential in the postsynaptic neurone
• temporal summation:

  • a single presynaptic neurone releases neurotransmitters several times over a short period
  • this builds up in the synapse until the quantity is sufficient enough to trigger an action potential