5.3- Neuronal communication Flashcards
(43 cards)
1
Q
Neurones- outline the reflex arc
A
Sensory receptor - sensory neurone - relay neurone (CNS- brain/spinal cord) - motor neurone - effector (muscle or gland)
Impulse is transmitted as along neurones as an action potential
2
Q
Name and describe the functions of 3 different types of neurones
A
- motor neurones- carry an action potential from the CNS to an effector
- sensory neurones - carry the action potential from a sensory receptor to the CNS
- relay neurones- connect sesnory and motor neurones
3
Q
Describe the general structure of neurones
A
- many are very long so that they can transmit the action potential over a long distance
- a cell body contains the nucleus, many mitochondria, and ribosomes
- an axon carries impulses away from the cell body
- numerous dentrites connect to other neurones- carry impulses towards the cell body
4
Q
Sensory neurones- diagram, structure
A
- have a long dendron carrying the action potential from a sensory receptor to the cell body
- cell body is positioned just outside the CNS
- have a short axon carrying the action potential into the CNS
5
Q
Relay neurones- diagram, structure
A
- have many short dendrites and a short axon
- the number of dendrites and the number of divisions of the axon is variable
- relay neurones are an essential part of the nervous system- conduct impulses in coordinated pathways
6
Q
Motor neurones- diagram, structure
A
- have cell body in the CNS
- have a long axon that carries the action potential out to the effector
7
Q
Name 2 categories of neurones
A
-Myelinated and non-myelinated
8
Q
Describe myelinated neurones
A
- around 1/3 of the peripheral neurones in vertebrates are myelinated- insulated by an individual myelin sheath
- most sensory and motor neurones are associated with many Schwann cells- called the myelin sheath
- these Schwann cells are wrapped tightly around the neurone so the sheath consists of several layers of plasma membrane and thin cytoplasm from the Schwann cell
- at intervals of 1-3mm along the neurone there are gaps in the myelin sheath- the nodes of Ranvier- each node is very short (2-3 micrometres long)
9
Q
Describe non-myelinated neurones
A
- also associated with Schwann cells, but several may be enshrouded in one loosely wrapped Schwann cell
10
Q
Describe advantages of myelination
A
- myelinated neurones can transmit an action potential much more quickly than non-myelinated neurones can: 100-120 ms -1 vs 2-20 ms-1
- myelinated neurones carry action potentials from sensory receptors to the CNS and from the CNS to effectors - carry action potentials over long distances (longest neurone in humans can be around 1m)
- increased speed of transmission mean the action potential reaches the end of the neurone much more quickly - enables a more rapid response to a stimulus
- non-myelinated neurones tend to be shorter and carry action potentials over a short distance- often used in coordinating body functions such as breathing, and the action of the digestive system- increased transmission of speed not so important
11
Q
Describe resting potential in neurones
A
- Sodium/potassium ion pump uses ATP to actively transport 3 sodium ions are pumped out for every 2 potassium ions that are pumped in
- Gated sodium ion channels are kept closed
- Some of the potassium ion channels are open
- Means the plasma membrane is more permeable to potassium ions than to sodium ions- potassium ions tend to diffuse (by facilitated diffusion) out of the cell
- The cell cytoplasm contains large organic anions (negatively charged ions)
- Means the interior of the cell is maintained at a negative potential compared with the outside
- The cell membrane is said to be polarised
- The potential difference across the cell membrane is about -60mV
- This is called the resting potential
- In myelinated neurones, the ion exchanges described only occur at the nodes of Ranvier
12
Q
Describe the stages of an action potential
A
1:
- The membrane starts in resting state
- Polarised- inside of cell is -60mv compared to outside
- There is a higher concentration of sodium ions outside than inside
- There is a higher concentration of potassium ions inside than outside
2:
- Sodium ion channels open and some sodium ions diffuse into the cell
3:
- Depolarisation:
- becomes less negative with respect to the outside
- reaches the threshold value of -50mv
4:
- Positive feedback causes nearby voltage-gated sodium ion channels to open
- many sodium ions flood in
- as more sodium ions enter, the cell becomes positively charged compared to the outside
5:
- The potential difference across the plasma membrane reaches +40 mV
- the inside of the cell is positively charged compared to the outside
6:
- The sodium ion voltage-gated channels close
- potassium ion voltage-gated channels open
7:
- Repolarisation:
- Potassium ions diffuse out of the cell bringing the potential difference back to negative compared with the outside
8:
- The potential difference overshoots slightly, making the cell hyperpolarised
- The potassium ion voltage gated channels close
9:
- The original potential difference is restored by sodium potassium ion pumps so that the cell returns to its resting state
13
Q
Describe sensory receptors
A
- specialised cells that can detect changes in our surroundings
- most are energy transducers that convert one form of energy to another
14
Q
Describe transducers (sensory receptors)
A
- most are adapted to detect changes in a particular form of energy
- each change is called a stimulus- sensory receptors respond by creating a signal in the form of electrical energy- nerve impulse
15
Q
Name examples of sensory receptors, their stimulus, and energy changes involved
A
- Light sensitive cells (rods and cones) in the retina- detect change in light intensity- convert light to electrical
- Temperature receptors in skin and hypothalamus- detect change in temperature- heat to electrical
- Pacinian corpuscles in the skin- detect change in pressure on skin- mechanical to electrical
- Vibration receptors in cochlea of ear- detect change in sound- sound to electrical
- Olfactory cells in epithelium lining in the nose, chemical receptors in taste buds on tongue- detect chemical changes in the air and food- chemical to electrical
16
Q
Describe Pacinian corpuscle’s
A
- pressure sensor that detects changes in pressure on skin
- oval-shapes structure that consists of a series of concentric rings of connective tissue wrapped around the end of a nerve cell
- Pressure on skin causes the rings of connective tissue to deform
- Causes sodium ion channels to open- sodium ions enter the sensory neurone- generator potential
- Reaches threshold potential- sodium ion gated channels open- starts action potential
- only sensitive to changes that deform the rings of connective tissue- if pressure is constant they stop responding
17
Q
What causes an action potential to move along a neurone
A
Local currents in the cytoplasm of the neurone
18
Q
Describe the movement of local currents
A
- Sodium ion channels open at that point in the neurone- sodium ions diffuse in- less negative then eventually positive inside cell- depolarisation at that point
- Sodium ions diffuse sideways along the neurone away from the area of increased concentration- local current
- Local current cases a slight depolarisation further along the neurone- becomes slightly less negative- changes membrane potential
- causes voltage gated sodium ion channels to open – causes action potential (full depolarisation) further along the neurone- moved across
- Will continue to move in same direction across neurone as the concentration of sodium ions behind the action potential is still high
19
Q
Describe what happens at the previous area of the neurone once the action potential has passed
A
After the local current has passed, the potassium ion voltage gated channels open and the sodium ion voltage gated channels close- causes repolarisation, hyperpolarisation , and then the resting potential is restored
20
Q
What allows the neurone to recover from an action potential, what does this also ensure
A
- refractory period
- action potentials are only transmitted in one direction
21
Q
Describe the refractory period
A
- the concentration of sodium/potassium ions must be restored by the sodium potassium ion pumps
- means it is impossible to stimulate an action potential in the same region of the neurone directly after firing
22
Q
How does action potential transmission occur in myelinated neurones
A
Saltatory conduction
23
Q
Describe saltatory conduction
A
- The myelin sheath is impermeable to sodium and potassium ions as it insulates the neurone
- Means ionic movement only occurs at the nodes of Ranvier
- Local currents are therefore elongated and sodium ions diffuse along the neurone from one node of Ranvier to the next- action potential appears to jump from one node to the next- saltatory conduction
- Means speed of action potential transmission increases- myelinated neurones can transmit action potentials at a speed of up to 120ms-1
24
Q
What is the rule about action potentials
A
- “all or nothing rule”- all action potentials are the same size/magnitude and each one produces a depolarisation of +40 mV
- can still detect stimuli of different intensities due to different frequencies of transmission
25
Describe frequency of transmission
- We can detect stimuli of different intensities from the frequency of action potentials arriving in the sensory region of the brain- higher frequency means more intense stimulus
- When the stimulus is at a higher intensity, more sodium channels are opened in the sensory receptor- produces more generator potentials – more frequent action potentials in the sensory neurone- more frequent action potentials entering the CNS
- Maximum frequency is limited because of the refractory period- sodium potassium ion pumps need to restore resting potential after hyperpolarisation
26
Briefly describe synapses
- Synapse is a junction between 2 or more neurones where one neurone can communicate with, or signal to, another
- Space between them is the synaptic cleft (approx. 20 nm wide)
- Neurotransmitter is released as action potential can't bridge gap
- Synapses that use acetylcholine as the neurotransmitter are called cholinergic synapses
- presynaptic neurone ends in swelling called presynaptic bulb
27
Describe the presynaptic bulb
- Many Mitochondria- ATP for exocytosis
- Lots of SER- packages neurotransmitter into vesicles
- Many vesicles containing neurotransmitter (e.g. acetylcholine) which will diffuse across synaptic cleft
- Number of voltage gated calcium ion channels on cell surface membrane
28
Describe the post-synaptic membrane
- Contains specialist sodium ion channels that can respond to the neurotransmitter
- These channels consist of 5 polypeptide molecules- 2 have receptor site that is specific to acetylcholine
- Receptor sites have complimentary shape to shape of acetylcholine molecule
- When acetylcholine is present in the synaptic cleft it binds to the 2 receptor sites and causes the sodium ion channel to open
29
Describe transmission across the synapse
1) An action potential arrives at the presynaptic bulb.
2) Action potential changes potential difference in cell- causes voltage-gated calcium ion channels to open and
3) calcium ions diffuse into the synaptic bulb
4) The calcium ions cause the synaptic vesicles to move to, and fuse with the presynaptic membrane
5) The acetylcholine is released by exocytosis
6) The acetylcholine diffuses across the synaptic cleft.
7) Acetylcholine binds to receptor sites on the sodium ion channels on the postsynaptic neurone
8) This makes the sodium ion channel open
9) sodium ions diffuse across the post-synaptic membrane into the post-synaptic neurone
10) this initiates a generator potential (Excitatory post-synaptic potential- EPSP)
11) If sufficient GPs combine then the potential difference across the postsynaptic membrane reaches threshold potential and new action potential is created in the post synaptic neurone
- once an action potential is achieved it will pass down the post-synaptic neurone
30
What will happen is acetylcholine is left in the synaptic cleft, describe what occurs to prevent this
- If acetylcholine is left in the synaptic cleft it will continue to open the sodium ion channels in the post synaptic membrane- will continue to cause action potentials
- Acetylcholinesterase- enzyme found in synaptic cleft- hydrolyses the acetylcholine to ethanoic acid (acetic acid) and choline
- This stops the transmission of signals, so that the synapse doesn’t continue to produce action potentials in the post synaptic neurone
- The ethanoic acid and choline are recycled- re-enter the synaptic bulb by diffusion and are recombined to acetylcholine using ATP (from respiration in mitochondria)- stored in synaptic vesicles for future use
31
Name 2 types of neurone junctions
- divergent
- convergent
32
Describe divergent neurone junctions
- One pre-synaptic neurone may diverge into several post synaptic neurones
- Can allow one action potential to be transmitted to several parts of the nervous system
- This can be useful in a reflex arc- one post-synaptic neurone elicits the response, while another informs the brain
33
Describe convergent neurone junctions
- Several pre-synaptic neurones may converge on one post-synaptic neurone
- Can allow action potentials from different parts of the nervous system to contribute to generating an action potential in one post-synaptic neurones- creating a particular response
- could be useful when several different stimuli are warning us of danger
- AKA spatial summation
34
Describe summation, name 2 types
- when one axon potential passes down the axon to the synapse it will cause a few vesicles to move to, and fuse with, the presynaptic membrane
- the relatively small number of acetylcholine molecules diffusing across the cleft produces a small depolarisation- excitatory post-synaptic potential (EPSP)
- It may take several generator potentials to reach the threshold and cause an action potential
- The effects of several GPs combine together to increase the membrane depolarisation until it reaches the threshold- summation
- 2 types- temporal and spatial
35
Describe temporal summation
- Several Action potential in the same pre-synaptic neurone
36
Describe spatial summation
- Action potentials arriving from different pre-synaptic neurones
- Convergent neurones
37
Name two types of post-synaptic potentials
- excitatory post synaptic potentials (EPSP)
- inhibitory post synaptic potential (IPSP)
38
Describe EPSPs
- When one action potential passes down an axon o the synapse it will cause a few vesicles to move to, and fuse with, the presynaptic membrane
- The relatively small number of acetylcholine molecules diffusion across the cleft produces a small depolarisation
- This is an excitatory post-synaptic potential (EPSP)
39
Describe IPSPs, give examples
- Some pre-synaptic neurones can produce inhibitory post-synaptic potentials (IPSPs)
- These can reduce the effect of summation and prevent an action potential in the postsynaptic neurone
- In many synapses, EPSPs and IPSPs compete with each other and determine whether or not the post-synaptic membrane will generate an action potential
- GABA and Glycine are examples of neurotransmitters involved in IPSPs
- An IPSP can be achieved by opening chloride ion channels that allow chloride ions into the post-synaptic neurone (GABA), or by opening potassium ion channels that allow potassium ions out of the cell
- Both cause a temporary hyperpolarisation to be produced
40
How many directions can synaptic transmission occur in, why
- only 1
- Only the pre-synaptic bulb contains vesicles containing the neurotransmitter (acetylcholine)
- Only the post-synaptic membrane has the receptors that are complementary to these vesicles
41
Describe low-level signals at synapses
- Synapses can filter these out
- If a low-level stimulus creates an action potential in the presynaptic neurone it is unlikely to pass across a synapse to the next neurone, because several vesicles of acetylcholine must be released to create and action potential in the post-synaptic neurone
- Low level action potentials can be amplified by summation
42
Describe neurone habituation/fatigue
- After repeated stimulation a synapse may run out of vesicles containing the neurotransmitter
- The synapse is said to be fatigued
- Means the nervous system no longer responds to the stimulus = we have become habituated to it
- Explains why we get used to a smell or background noise
- May also help to avoid overstimulation to an effector, which could cause damage
43
Describe the creation and strengthening of specific neuronal pathways
- This occurring in the nervous system is thought to be the basis of conscious thought and memory
- Synaptic membranes are adaptable
- In particular, the post-synaptic membrane can be made more sensitive to acetylcholine by the addition of more receptors
- Means that a particular post-synaptic neurone is more likely to fire an action potential, creating a specific pathway in response to a stimulus
