neurons and neural communication- lecture 3 Flashcards
what are neurons (nerve cells)
-specialised cells that recieve and transmit info throughout the central nervous system
- vary in size but all have the same basic structure
parts of a neuron
soma
dendrites
axon
presynaptic terminals
plasma membrane
soma
- contains cell nucleus (houses chromosomes and DNA)
bulk of the cell is made of the cytoplasm which houses variety of structures
dendrites
-all info recieved here
- recieved from other neurons across synapse (line the surface of the dendrites)
- outgrowths called dendritic spines increase SA available for synaptic communication
axon
- all info set along the axon
- info sent as electrical impuse called action potential
- covered with layer of insulating material called myelin sheath (vertebrates only)
- in vertebrates there are breaks in myelin sheath called nodes of ranvier
presynaptic terminals
- action potential passes from cell body along the axon to presynaptic terminals
- when action potential reaches terminal they secrete neurotransmitters across synapse to next neuron in next chain, this then either excites or inhibits the post synaptic receptors on dendrites o another neuron
efferent axons
send info out from nervous system e.g. motor neurons
afferent axons
send info in e.g. sensory neurons
plasma membrane
separates the inside of the cell
support cells in the nervous system
glia/glial cells provide support to development and activity of neurons
features of support cells
- astrocytes
- schwann cells
- oligodendrocytes
astrocytes
wrap around related neurons helping to synchronise activity, preventblood-borne toxins enterig neurons (e.g. blood-brain barrier)
schwann cells
build myelin sheaths in peripheral nervous system, help neuron regrowth and guide to appropriate targets
oligodendrocytes
build myelin sheaths in brain and spinal cord (CNS)
electrical activity in the neuron
the inside of an inactive axon is more negatively charged than outside
this is called resting potential (-70mV)
probing a squid
microelectrodes are placed inside and outside the axon
both are connected to a voltmeter and voltage inside and out recorded
resting potential
- caused by concentrations of ions inside and outside cell
- more sodium outside the cell
- causes a negative charge (-70mV) inside the cell
movements of ions in and out the cell
- axon membrane is selectively permeable to potassium and sodium
- at rest- potassium and sodium difuse slowly through ion channels in axon membrane
- but sodium tries to sneak into the cell to balane concentration and charge
- the balance is maintained by ion pumps- 3 sodium out 2 potassium in
maintaining resting potential
- neuron is polrised during resting potential (a difference in electrical charge between inside and outside)
- polarisation is essential (ensures neuron is ready to fire when it recieves impulse)
all or nothing principle
neurons have diff thresholds of excitation (the strength of the trigger required to respond)
but once threshold is reached an action potential is triggered
passing info- the action potential
- a positive charge applied to the inside of axon makes it more positive
- membrane potential rapidly reversed, inside becomes strongly positive- up to 40 mV
- membrane potential quickly returns to normal but 1st it briefly overshoots
- this all takes 2 milliseconds
0- resting potential (-70 mV)
more sodium outsie than in so inside is more negative, inside and outside is polarised which is maintained by ion pumps
1- depolarisation
when stimulated past threshold sodium channels open and sodium rushes into axon causing a region of positive charge
2- repolarisation
sodium channels close, potassium channels open and potassium exits axon
3- hyperpolarisation
potassium continues to exit after repolarisation causing brief undershoot in charge (too negative) so ion pumps restore resting balance (sodium out, potassium in)
after action potential
the axon cant cope with repeated excitation as the sodium-potassium pump cant keep up
absolute refractory period
after action potential sodium channels remain closed for about 1ms during this time no stimulus can excite the neuron
relative refractory period
sodium channels open ut potassium channels remain closed for 2-4ms only an extremely strong stimulus can excite the neuron
movement of action potential
- unmyelinated axons (intervertebrates and small neurons)
- each oint along the axon membrane generates axon potential
- the next area of membrane is depolarised, reaches its threshold and generates another action potential
- so the action potential passes down the axon like a wave
where are myelinated axons
only in vertebrates
advantages of myelinated axons
- energy saving- sodium potassium pumps are only required at specific points along the axon
- speed