Nerves
- Created by: imanilara
- Created on: 10-06-16 19:04
Sensory Receptors+ Motor Neurones
Sensory receptors= ENERGY TRANSDUCERS
Light sensitive cells in eye-- light intensity + wavelengths of light
Olfactory cells in nose- volatile chemicals
Tastebuds- soluble chemicals
Pressure receptors on skin- pressure on skin
Sensory+Motor:-
- both long to transmit AP over distance
- CSM has gated ion channels for Na, K or Ca
- Na/K pumps- use ATP to actively transport Na out
- Myelinated sheath- made of schwann cells- nodes of ranvier
- both have cell body
Sensory+Motor contrast
- Sensory - receptor-CNS, motor CNS-effector
- Sensory- cell body outside CNS, motor, cell body inside CNS
- Sensory - short axon, motor- long axon
Resting potentials+action potentials
Resting potential= -60mV- 3 Na pumped out, 2 K diffuse in - polarised- cytoplasm also contains anions (neg charged ions)
Action potential= at rest, Na+ kept closed- if some open, Na will diffuse quickly down concentration gradient- opened by energy changes in environment - gates further along opened by the change in p.d. - voltage gated channels.
ALL OR NOTHING:
- once generator potentials are large enough to reach threshold potential (-40mV) - causes large influx of sodium ions as the gates open -depolarisation to +40mV= action potential
Ionic movements
- resting potential- -60mV - 3 Na+ out, 2 K in
- some sodium channels open due to change in environement- sodium diffuses in
- causes a depolarisation, and if large enough, reaches threshold pot. to cause depolarisation (-40mV)
- causes voltage gated channels to open- detect change in p.d.-large influx of sodium diffuses in = +40mv= ACTION POTENTIAL
- K+ channels open and Na+ channels close - hyperpolarisation where it overshoots too negatively
- original p.d. restored
REFRACTORY PERIOD- imposs to restimulate the membrane - allows cell to recover and for signal to be transmitted in ONE DIRECTION
Key words: resting potential, polarised, sodium ion channels, generator potential, threshold potential, voltage gated ion channels, all or nothing, depolarisaiton, hyperpolarisation,
Transmission of action potentials
LOCAL CURRENTS:
- AP causes sodium channels to open
- sodium diffuses in down conc gradient
- conc rises at the point where the channels opened
- causes sodium to diffuse sideways in the neurone - LOCAL CURRENT
Voltage gated sodium ion channels:
- decrease in p.d causes the voltage gates to open - allowing AP to move along
Myelin sheath:
- fatty sheath- non conducting of Na+ and K+- keeps AP in neurone
- nodes of ranvier, gaps between the schwannn cells- allow saltatory conduction - ionic exchanges happen here - local currents elongated + sodium ions diffuse in at nodes of ranvier
- Myelinated much faster than non-myelinated- 120 m/s
Local currents- sideways- p.d. decrease- v. gated channels- saltatory conduction
Nerve junctions
CHOLINERGIC SYNAPSES- use acetylcholine as NT
presynaptic knob:
- mitochondria
- vesicles containing acetylcholine
- voltage gated calcium ion channels in membrane
postsynaptic knob:
receptors complementary to acetylcholine - once binded causes sodium channels to open
Transmission across synapse:
- AP arrives at synpatic knob - calcium voltage gated channels open- calcium diffuses in
- causes vesicles to fuse to membrane- exocytosis- acetycholine diffuses across synaptic cleft
- bind w receptors on post synaptic membrane- cause sodium channels to open
- generator potential created- threshold- voltage gated channels open etc... AP created
Role of acetylcholinesterase
In synaptic cleft - hydrolyses acetylcholine into choline and ethanoic acid-stops transmission +synpase doesnt produce more action potentials . Products diffuse in and packaged to form acetylcholine again - uses ATP- recycled.
Signals and messages
The roles of synapses:
- several presynpatic converge into one postsynaptic- different parts create the same response - useful in danger
- one pre synpatic diverge into many post synpatic- allows to be transmitted to different parts of the NS- eg reflex arc - informs the brain + elicits response
- Allow transmission in one direction
- filter out low level signals
- or low level signals amplified by summation -when low level persistent generates successive AP's in presynaptic - release of many vesicles=generator potential in postsynaptic
- acclimatisation- run out of vesicles- means u get used to bad smells
Frequency of transmission -
generator potentials = more action potentials = more vesicles released= higher frequency of AP's in postsynaptic = more intense stimulus
Myelinated vs non myelinated
Advantage:
- Quicker- 120m/s compared to 20m/s
- more rapid response to stimuli
- longer than non-myelinated- but they dont need to be long - in digestive system + breathing
Comments
No comments have yet been made