5.3 Neuronal Communication Flashcards
What are sensory receptors?
detect stimuli and are energy transducers, converting one form of energy to electrical energy as a nerve impulse
What receptors respond to changes in pressure? How? What are some features?
Pacinian Corpuscles
- when pressure on skin changes, it deforms the ring. Sodium channels open, allowing them to diffuse into the cell and produce a generator potential.
- the movement across the membrane creates a change in potential difference.
- if a large enough stimulus is detected and enough gates are opened, the potential difference changes significantly = depolarisation and an action potential.
- rings of connective tissue around end of nerve cell
- fibroblasts producing connective tissue
What is the function of the motor neurone?
carry action potentials from CNS to an effector
What is the structure of a motor neurone?
- cell body with dendrites
- axon hillock
- very long axon with myelin sheath
- synaptic endings/ axon terminals
What is the function of the relay neurone?
connect sensory and motor neurones
What is the structure of a relay neurone?
- cell body with dendrites
- VERY short axon, with synaptic endings
What is the structure and function of sensory neurones?
- carry APS from receptors to CNS
- may have dendrites to sensory receptor
- dendron with myelin sheath
- cell body
- axon with myelin sheath
- synaptic endings
What are similarities in the structures of all the neurones?
- plasma membrane has many gated ion channels and Na+/K+ pumps
- cell body with nucleus, mitochondria and ribosomes
- ## dendrites carry impulses towards body, and axons away
What are the differences between the neurones?
- motor: cell body in CNS and has LONG axon to effector, sensory has LONG dendron to cell body which is outside of CNS. Short axon into CNS
- relay: short dendrities + short axon. CNS
- third of peripheral nervous system neurones MYELINATED. Rest in PNS, and the CNS, ARE NOT.
- sensory and motor contain nodes of ranvier, relay dont
What does myelination look like on a neurone? What is the purpose?
- Schwann cells that make up fatty sheath. Cytoplasm wrapped around axon.
- Gaps are nodes of Ranvier. The movement of ions can only occur here
- Jumping of impulse = rapid = increased speed of transmission over long distances
What occurs in non myelinated neurones? What is the purpose?
- AP moves along in a wave
- Schwann cells loosely wrapped around multiple axons instead
- Shorter distances, coordinating functions where speed isn’t so important
What is happening when neurones are at rest?
- Gated sodium ion channels are kept closed
- ATP is used to actively pump 3 Na+ ions out for every 2K+ ions inside
- Some potassium channels are open meaning the membrane is more permeable to them, and they tend to diffuse out.
- The cell cytoplasm also contains many large anions
- Membrane is POLARISED - more negative compared to outside = resting potential at about -60mV
What are the stages of an action potential?
- Resting potential
- A few sodium ion channels open (due to a stimulus in receptor cells, or diffusion in neurones) and some sodium ions diffuse into the cell
- Membrane reaches about -50mV - the THRESHOLD
- Positive feedback causes nearby voltage gated sodium ion channels to open and loads of ions flood in = DEPOLARISATION
- Potential difference reaches +40mV, where the sodium ion channels now close and potassium channels open.
- Potassium ions diffuse OUT of the cell bringing the potential difference back to negative = REPOLARISATION
- The potential difference overshoots slightly = HYPERPOLARISATION
- K+ ion channels close. Original resting potential is restored via the pump.
What is the refractory period?
after each AP it is impossible to reach another for some time -> allows the cell to recover and ensures APs are transmitted in one direction.
How are local currents formed?
- Sodium ion channel opens during depolarisation allowing Na+ to diffuse into the neurone
- Localised increase in conc of Na+ inside the neurone -> AP
- Sodium ions diffuse sideways along the axon/dendron away to a region of lower concentration.
- Causes slight depolarisation of the membrane further down and causes opening of Na+ channels further down the neurone at a node of Ranvier.
- The open channels allow rapid influx of sodium ions in, causing a full depolarisation along the neurone = another AP.
What is saltatory conduction? What are the advantages?
- the jumping of action potentials from one node to the next
- the local currents are elongated, and then the ionic movements occur at the Nodes
- speeds up transmission
How is the intensity of a stimulus transmitted?
- all action potentials are the same intensity
- FREQUENCY of APs arriving at the sensory region of the brain.
- higher frequency = more intense stimulus
What are cholinergic synapses?
- junction between two neurones -> synaptic cleft
- a synapse that uses acetylcholine as the neurotransmitter released from the pre-synaptic neurone which diffuses across the cleft to the post-synaptic neurone
What specialised features does the pre-synaptic bulb contain?
- many mitochondria for ATP
- SER to package neurotransmitter into vesicles
- large numbers of vesicles with acetylcholine
- number of voltage gated calcium ion channels
How is the post-synaptic membrane specialised?
- many sodium ion channels that respond to neurotransmitter, with 5 polypeptide molecules
- two of these have a special receptor site specific to acetylcholine, complementary
- binding causes Na+ ion channels to open
Describe the transmission across a synapse process
- AP arrives at synaptic bulb
- Voltage gated Ca2+ ion channels open
- Ca2+ ions diffuse into the synaptic bulb
- They cause the vesicles to move to and fuse with the pre-synaptic membrane
- Acetylcholine is released by exocytosis and diffuses across cleft
- It binds to receptor sites on sodium ion channels on post-synaptic membrane and causes them to open
- Sodium ions diffuse into the post-synaptic neurone
- EPSP is created (excitatory post synaptic potential)
- If threshold then an AP is generated, which passes down.
What is the role of acetylcholinesterase?
- Found in synaptic cleft
- Hydrolyses the acetylcholine to ethanoic acid and choline, which are recycled.
- they reenter the bulb by diffusion and endocytosis and are recombined to acetylcholine using ATP
- Stops over-transmission of signals
What is an excitatory post-synaptic potential vs an inhibitory post-synaptic potential?
- EPSP: the small depolarisation across the post-synaptic membrane, when a relatively small number of acetylcholine molecules diffuse and bind
- IPSP: reduce the effect of summation and prevent an AP in the post-synaptic neurone
What is summation? Describe the two types
when the effects of several impulses are added together
several APs in same pre-synaptic neurone = TEMPORAL
AP’s arriving from several different pre-synaptic neurones = SPATIAL
How is communication controlled at synapses?
- several pre-synaptic neurones might converge to one post-synaptic neurone -> could be useful when diff stimuli are warning us of danger
- one pre-synaptic neurone might diverge to several post-synaptic neurones -> useful in reflex arc
- synapses ensure APs go one way
- synapses can filter out low level, unwanted signals, or summation can amplify them
- after repeated stimulation a synapse might run out of neurotransmitter = habituation.
The sensory neurone was stimulated electrically and the FURA-2 fluorescence in the
synaptic bulb was measured. At the same time, an electrode recorded the membrane
potential in a postsynaptic neurone.
The results of this study are shown i
Strength of electrical
stimulation
low
medium
high
FURA-2 fluorescence in
synaptic bulb
low
medium
high
Highest membrane
potential in postsynaptic
neurone (mV)
–60
+40
+40
The intensity of FURA-2 fluorescence is proportional to the concentration of Ca2+ ions.
The scientists concluded that changes in the concentration of Ca2+ ions in the presynaptic
neurone caused an action potential in the postsynaptic neurone.
Evaluate and explain the scientists’ conclusion.
.
conclusion is valid because:
1 concentration of Ca2+ is proportional to strength
of stimulus
2 Ca2+ change from low to , medium / high , causes increase in (membrane) potential
3 action potential in , presynaptic neurone / synaptic
bulb leads to , opening of Ca2+ channels / entry of Ca2+
4 Ca2+ , causes / AW , release of neurotransmitter
5 neurotransmitter causes , Na+ / sodium ion channels to open in (post-synaptic) neurone
6 if threshold is exceeded this causes action potential in (postsynaptic) neurone / depolarises (postsynaptic) membrane
conclusion may not be valid because:
7 changes in Ca2+ concentration may not be thecause of (postsynaptic) action potential
8 Ca2+ change from medium to high but no change in (membrane) potential
