5.3 Neuronal communication Flashcards
(25 cards)
What features are common to all sensory receptors?
Act as energy transducers which establish a generator potential
-Respond to specific stimuli
Describe the basic structure of a Pacinian corpuscle
Single nerve fibre surrounded by layers of connective tissue which are separated by viscous gel and contained by a capsule
-Stretch mediated Na+ channels on plasma membrane
-Capillary runs along base layer of tissue
What stimulus does a Pacinian corpuscle respond to? How?
1) Pressure deforms membrane, causing stretch-mediated Na+ ion channels to open
2) If influx of Na+ raises membrane to threshold potential, a generator potential is produced
3). Action potential moves along sensory neurone
Describe the features of all neurones
Cell body: contains organelles and high proportion of RER
Dendrons: Branch into dendrites which carry impulses towards the cell body
Axon: Long, unbranched fibre carries nerve impulses away from cell body
Describe the structure and function of a sensory neurone
They have a long axon, with dendrites at one end. The cell body is in the middle of the axon. At the other end there are axon terminals
-A sensory neurone is usually unipolar,
-They transmit impulses from receptors to CNS
Describe the structure and function of a relay neuron.
-They have highly branched dendrites. The cell body is closer to the dendrites along the axon
At the other end, there are highly branched axon terminals
-They are usually bipolar, they transport impulses between neurones
Describe the structure and function of a motor neurone
They have dendrites at one end. The cell body is at the end of the axon. At the other side is an axon terminal
-They are usually multipolar, they transmit impulses from relay neurones in the CNS to effectors
Describe the additional features of a myelinated neurone
-Schwann cells: wrap around axon many times
-Myelin sheath: Made from myelin rich membranes of Schwann cells
-Nodes of Ranvier: very short gaps between neighbouring Schwann cells where there is no myelin sheath
Name three processes Schwann cells are involved in
-electrical insulation
-Phagocytosis
-Nerve regeneration
Explain why myelinated axons conduct impulses faster than un-myelinated axons
Due to Saltatory conduction.
This is where the impulse jumps from one node of ranvier to another. Depolarisation cannot occur where myelin sheath acts as electrical insulator
-This is so impulse does not travel along the whole axon length
Where are myelinated and non-myelinated neurones found in the body
Myelinated: Most neurones are in central and peripheral nervous systems e.g spinal reflexes
Non-myelinated: These are group c nerve fibres involved in transmitting secondary pain
What is resting potential?
The potential difference (Voltage) across neurone membrane when not stimulated (-50 to -90mV, usually -70mV in humans)
How is resting potential established
1) Membrane is more permeable to K+ than Na+
2) Sodium-potassium pump actively transports 3Na+ out of cell and 2K+ into cell
This establishes an electrochemical gradient: the cell contents are more negative than the extracellular environment
Name the stages in generating an action potential
1) De-polarisation
2) Re-polarisation
3) Hyper-polarisation
4) Return to resting potential
What happens during depolarisation?
1) The stimulus created facilitated diffusion of Na+ into cell down an electrochemical gradient
2) Potential difference across membrane becomes more positive.
3) If membrane reaches threshold potential (-50mv), voltage-gated Na+. channels open. this is due to the positive feedback mechanism
4) The significant influx of Na+ ions reverses potential difference to +40mV
What happens during re-polarisation?
1) Voltage-gated Na+ channels close and voltage-gated K+ channels open
2) Facilitated diffusion of K+ ions out of cell down their electrochemical gradient
3) The potential difference across membrane becomes more negative
What happens during hyperpolarisation?
1) There is an ‘Overshoot’ when K+ ions diffuse out, causing the potential difference to become more negative than resting potential
2) There is then a refractory period, where no stimulus is large enough to raise membrane potential to threshold
3) Voltage-gated K+ channels close and sodium-potassium pump re-establishes resting potential
Explain the importance of the refractory period
-No action potential can be generated in hyper-polarised sections of membrane
This ensures unidirectional and discrete impulses. It also limits the frequency of impulse transmission
Why is the frequency of impulse transmission significant?
Enables organism to distinguish size of stimulus, although. all action potentials have same magnitude
-Larger stimuli result in higher frequency of transmission since they overcome hyper-polarisation more quickly
What is the function of synapses?
-Electrical impulse cannot cross junction
-Neurotransmitters send impulses between neurones/from neurones to effectors for excitatory or inhibitory response
-Summation of sub-threshold impulses
-New impulses can be initiated in several different neurones for multiple simultaneous responses
Describe the structure of a synapse
-Pre-synaptic neurone ends in synaptic knob. This contains lots of mitochondria, endoplasmic reticulum and vesicles of neurotransmitter
-Synaptic cleft: 20-30nm gap between neurones
-Postsynaptic neurone: has complementary receptors to neurotransmitter
What happens in the presynaptic neurone when an action potential is transmitted between neurones?
1) Wave of depolarisation travels down presynaptic neurone, causing voltage-gated Ca2+ channels to open
2) Vesicles more towards and fuse with presynaptic membrane
3) Exocytosis of neurotransmitter into synaptic cleft
How do neurotransmitters cross the synaptic cleft?
Simple diffusion
What happens in the postsynaptic neurone when an action potential is transmitted between neurones
1) Neurotransmitter binds to specific receptor on postsynaptic membrane
2) Ligand-gated Na+ channels open
3) If influx of Na+ ions raises membrane to threshold potential, action potential is generated.
