NA. 5.3 Neuronal Communication Flashcards
What features are common to all sensory receptors?
- Act as energy transducers which establish a generator potential.
- Respond to specific stimuli.
What stimulus does a Pacinian corpuscle respond to? How?
- Pressure deforms membrane, causing
stretch-mediated Na+ ion channels to open. - If influx of Na+ raises membrane to threshold potential, a generator potential is produced.
- Action potential moves along sensory neuron
Describe the basic structure of a Pacinian corpuscle
Single nerve fibre surrounded by layers of connective tissue in an oval shape separated by a gel
Describe the features of all neurons.
Cell body: contains organelles & 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 function of a sensory neurone
Transmits impulses from sensory receptors to CNS.
Describe the function of a motor neurone
Transmits impulses from relay neurons in the CNS to effectors.
Describe the structure and function of a relay neuron.
Transmits impulses between neurons
List some structural adaptations of neurons
- many are long so can transmit over distance
- maintain the potential difference accros their plasma membrane
- myelin sheath
Describe the additional features of a Myelinated neuron.
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 3 processes Schwann cells are involved in.
- electrical insulation
- phagocytosis
- nerve regeneration
What are the advantages of Myelination?
quicker transmission of an electrical impulse(5-50 times faster)
Differences between Myelinated and Non-myelinated neurons?
- myelinated: tends to carry impulses over longer distances, faster transmission speed used for receptor to CNS and CNS to effector
- non-myelinated : tends to be over short distances, used in coordinating body function
Explain why Myelinated axons conduct impulses faster than unmyelinated axons.
Saltatory conduction: Impulse ‘jumps’ from one node of Ranvier to another. Depolarisation cannot occur where myelin sheath acts as electrical insulator. So impulse does not travel along whole axon length.
What is resting potential?
Potential difference (voltage) across neuron membrane when not stimulated (-50 to -90 mV, usually about -70 mV in humans).
What is an action potential?
a brief reversal of the potential across the membrane of a neurone causing a positive peak (as compared to the negative resting potential)
How is resting potential established?
1.Membrane is more permeable to K+ than Na+.
2.Sodium-potassium pump actively transports 3Na+ out of cell & 2K+ into cell.
establishes electrochemical gradient: cell contents more negative than extracellular environment.
Name the stages in generating an action potential
- Depolarisation
- Repolarisation
- Hyperpolarisation
- Return to resting potential
What happens during Depolarisation?
- Stimulus → facilitated diffusion of Na+ into cell down electrochemical gradient.
- p.d. across membrane becomes more positive.
- If membrane reaches threshold potential (-50mV), voltage-gated Na+ channels open. (positive feedback mechanism).
- Significant influx of Na+ ions reverses p.d. to +40mV.
What happens during Repolarisation?
- Voltage-gated Na+ channels close and voltage-gated K+ channels open.
- Facilitated diffusion of K+ ions out of cell
down their electrochemical gradient. - p.d. across membrane becomes more negative.
What happens during Hyperpolarisation?
- ‘Overshoot’ when K+ ions diffuse out = p.d. becomes more negative than resting potential.
- Refractory period: no stimulus is large enough to
raise membrane potential to threshold. - Voltage-gated K+ channels close &
sodium-potassium pump re-establishes resting potential.
Explain the importance of the Refractory period.
No action potential can be generated in hyperpolarised sections of membrane.
•allows cell to recover after the action potential
• Ensures unidirectional impulse.
• Limits frequency of impulse transmission; larger stimuli have higher frequency.
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 hyperpolarisation more quickly.
What is a synapses?
neurons generate potentials these Electrical impulse cannot cross junction between neurones so Neurotransmitters send transmit impulses between neurons/ from neurons to effectors for excitatory or inhibitory response
What do synapses enable?
- Summation of sub-threshold impulses.
* New impulses can be initiated in several different neurons for multiple simultaneous responses.
Describe the structure of a synapse
- Presynaptic neuron ends in synaptic knob: contains lots of mitochondria, endoplasmic reticulum & vesicles of neurotransmitter.
- Synaptic cleft: 20-30 nm gap between neurons.
- Postsynaptic neuron: has complementary receptors to neurotransmitter (ligand-gated Na+ channels).
What happens in the presynaptic neuron when an action potential is transmitted between neurons?
- Wave of depolarisation travels down presynaptic neuron, causing voltage-gated Ca2+ channels to open.
- Vesicles move towards & fuse with presynaptic membrane.
- Exocytosis of neurotransmitter into synaptic cleft
How do neurotransmitters cross the synaptic cleft?
Simple diffusion
What happens in the postsynaptic neuron when an action potential is transmitted between neurons?
- Neurotransmitter binds to specific receptor on postsynaptic membrane.
- Ligand-gated Na+ channels open.
- If influx of Na+ ions raises membrane to threshold potential, action potential is generated.
What happens in an inhibitory synapse?
- Neurotransmitter binds to and opens Cl- channels on postsynaptic membrane & triggers K+ channels to open.
- Cl- moves in & K+ moves out via facilitated diffusion.
- p.d. becomes more negative: hyperpolarisation so no action potential is generated
What is a Cholinergic synapse?
A synapse that uses Acetylcholine as its neurotransmitter. can be excitatory or inhibitory
What is the role of Acetylcholinesterase?
An enzyme found in the synaptic cleft that hydrolyses acetylcholine into acetic acid(ethanoic acid) and choline, thus preventing it from generating continually producing action potentials
What may be found in the pre-synaptic bulb?
- Many mitochondria-produce ATP as this a active process
- lots of vesicles
- Large amount of smooth endoplasmic reticulum to package hella vesicles
- many voltage gated calcium ion channels
Define summation
Neurotransmitter from several sub-threshold impulses accumulates to generate action potential
What are the 2 types of summation?
- temporal summation(many to 1)
* spatial summation (1 to many)
What is the difference between Temporal and Spatial summation?
- Temporal: one presynaptic neuron releases neurotransmitter several times in quick succession.
- Spatial: multiple presynaptic neurons release neurotransmitter.
What happens after the breakdown of Acetylcholine by Acetylcholinesterase
they diffuse back into the presynaptic membrane and ATP is used to reform acetylcholine, ready for storage/use
what does it mean to get ‘habituated’ to something
the nervous system no longer react to a stimulus due to prolonged exposure which causes the synapse to run out of vesicles containing the neurotransmitter
