Neuronal Communication Flashcards
Pathway of an impulse
Sensory receptor - Sensory neurone - Relay neurone - Motor neurone - Effector
What is action potential
How the impulse is transmitted along neurones
- Carried as a rapid depolarisation of the membrane caused by the influx of sodium ions
Motor neurone
Carry the action potential from the CNS to an effector
- has dendrites; axon; cell body; myelin sheaf around axon
- have their cell body IN the CNS and have a long axon that carries the AP out to the effector
- Wrapped around each axon is a mylein sheaf and in-between each sheath is a node of Ranvier
Sensory neurone
Carry AP from Sensory receptor to CNS
- has dendrites; long dendron; short axon ; myelin sheaf; cell body
- Cell body outside CNS, which dendron transmits AP too
- Axon transmits AP to CNS
- Wrapped around each dendron/ axon is a mylein sheaf and in-between each sheath is a node of Ranvier
Relay neurone
Connects sensory to motor
- has short dendrites and can have several divisions of the axon + cell body
- Conducts impulses in coordinated pathways
Cell body
With a nucleus
dendrites / dendron / axon
D - to cell body
A- from cell body
What is a Schwann cell
Plasma membrane containing mylein
- wrap themselves around the axon (dendron) to create a mylein sheath
Why are neurones so long
So they can transmit an AP over a large distance
Structure/ function of myelinated neurones
- Many Schwann cells that make up fatty sheath called the myelin sheath
- wrapped tightly so several layers of membrane and thin cytoplasm of SC
- Intervals of 1-3mm gaps (nodes of Ranvier 2-3 um)
- Tightly wrapped prevents movement of ions across Neurone membrane and can only occur at N of R
- Impulse / AP jumps from one node to the next = rapid conduction
Non - myelinated neurones
- several neurones enshrouded with one loosely wrapped SC
- AP moves along rather than jumping
Advantages of Myelinated neurones
- transmits AP more quickly enabling more rapid response to stimulus which is ideal where there is a long distance that needs to be covered
- NM - short distance and used to coordinate bodily functions i.e. breathing so increased speed of transmission is not important
Resting potential
The potential difference across the membrane whilst the neurone is at rest
- Whilst at rest it is actively pumping ions across the plasma membrane of the neurone
What happens to a neurone at rest
1) Using the Sodium Potassium Pump (ATP) 3 NA+ out of plasma membrane and 2K+ in
2) Gated sodium channels are closed, however some potassium channels open
3) Some Potassium ions diffuse out of channels by facilitated diffusion - membrane more permeable to K+
4) Membrane also contains organic anions (negative ions)
5) Interior of the membrane becomes more negative than exterior = Polarized
6) Potential difference = -60 mV (not a set value can vary) which is the resting potential
In myelinated neurones where do ion exchanges occur
Only at the nodes of Ranvier
Action potential definition
A brief reversal of the potential across the membrane of a neurone causing a peak at +40mV compared to the resting potential at 60 mV
Positive Feedback
a mechanism that increases a change taking the system further way from the optimum
How is an ap created
1) The neurone is at rest
2) The sodium ion channels in the plasma membrane open. Sodium ions diffuse out into neurone (depolarisation)
3) It reaches the threshold value of -50mV
4) Positive feedback then causes the sodium ions voltage channels to open and more sodium ions to diffuse in (depolarisation)
5) A value of +40mV is achieved. This is an action potential. Sodium ion voltage gated channels close. Potassium ion voltage gated channels open
Stages of Action potential after achieved
1) A value of +40mV is achieved. This is an action potential. Sodium ion voltage gated channels close. Potassium ion voltage gated channels open
2) Potassium ions diffuse out of the neurone (repolarisation)
3) Hyperpolarisation occurs - this is where the potential difference over shoots slightly
4) The potassium ion voltage gated close
5) The sodium potassium ion pump restores to resting potential
What is important about the channels
Only become voltage gated after there has been a change in potential difference across the neurone membrane
Transducer
A cell that converts one form of energy to another
Sensory receptor
Cells/sensory nerve endings that respond to a stimulus in the external/ internal environment of an organism and create an action potential
Sensory receptor of the change in light intensity and the electrical change
Light sensitive cells (Rods/cones) in the retina
- light to electrical
Sensory receptor of the change in temperature and the electrical change
Temperature receptors in the skin and hypothalamus
-heat to electrical
Sensory receptor of the change in pressure on the skin and the electrical change
Pacinian corpuscle in the skin
-mechanical to electrical
Sensory receptor of the chemicals in the air and the electrical change
Olfactory cells in the epithelium lining the nose
- chemical to electrical
Sensory receptor of the chemicals in the food and the electrical change
Chemical receptors in the taste buds on the tongue
- Chemical to electrical
Structure of the Pacinian Corpuscle
Rings of connective tissue / fibroblast that produces the connective tissue / sensory nerve fibre
How does the sensory neurone create a generator potential
1)When pressure in the skin changes this deforms the rings of connective tissue, which push against the nerve ending
2) Sodium channels are sensitive so when deformed the sodium channels open
3) Sodium ions diffuse in creating a generator potential
- As this continues, threshold potential is reached, then normal process of generating an AP
What are the ‘failed initations’
Stimulus is to weak so not enough sodium ion channels open, so threshold potential is not reached
Polarised
When the cell in inactive
- Negatively charged inside compared to outside
Depolarisation
The outside becomes more negative than the inside of the cell
Why after a period of time, will a constant sound become unnoticed
- Sensory receptor responds to a change in the environment
- Constant sound no longer a change
- Sensory neurone will become habituated to the sound and the impulse will no longer reach the brain
How is an action potential transmitted along a non-myelinated neurone
1) When AP occurs sodium ion channels open at any point in the neurone
2) Sodium ions diffuse in down the concentration gradient
3) Sodium ions diffuse sideways along the neurone down the concentration gradient - local current
4) Local current causes slight depolarisation, and changes potential difference across membrane, causes sodium ion volted gated channels to open
5) This enables more Na+ to diffuse in full depolarisation generating AP
AP has moved across neurone
Local Current
Movement of ions along the membrane
Why do the sodium ion volted gated channels to open when the action potential is moving across the membrane
Due to the change in potential difference, as the sodium ions make the membrane less negative
Can the AP be reversed in the opp. direction across the neurone
No - diffuse down conc gradient and sodium ion conc sill high behind the AP
What happens behind the action potential once it has moved across the membrane
- Sodium ion channels close, potassium ion volted gated channels open
- K+ ions diffuse out of the cell bringing pd back to negative (repolarisation)
- pd across membrane overshoots slightly (hyperpolarisation)
- Sodium potassium ion pump works to restore neurone back to resting potential
Why following an ap is it impossible to have another ap immediately after in the same region of the neurone
Sodium potassium ion pump needs to restore to ap and needs a period of time for ions to be redistributed - refractory period
Myelinated sheath
Insulating layer of fatty material composed of Schwann cells tightly wrapped around the neurone - sodium/potassium ions cannot diffuse through
How is an action potential transmitted along a myelinated neurone
Saltatory conduction
In-between Schwann cells are nodes of ranvier - ionic movements can occur here, and channels can open
- Local currents are elongated and sodium ions diffuse from one node of ranvier to the next
-Ap appears to jump from one node of ranvier to another
All or nothing rule
the idea that a neurone can only have a full response (fire an action potential) or no response to a stimulus due to its threshold.
- all ap are the same size/magnitude
Frequency of transmission
- Can detect stimulus at different frequencies (loud/quiet)
- Brains determine intensity of stimulus from frequency of ap along the sensory neurone
- Higher frequency of ap - more intense stimulus - how much more ap occurs across neurone
What does a higher intensity stimulus do
More sodium channels open in the sensory receptor - produces more generator potentials - more frequent ap in the sensory neurone - more frequent ap entering CNS
Why is the maximum frequency of ap limited
Short refractory period so sodium potassium ion pump can restore the neurone back to resting potential
Synapse
The junction between two or more neurones, where one neurone can communicate with, or signal too, another neurone
Synaptic cleft
Gap between the two neurones
- approximately 20nm
What is the presynaptic bulb and what does it consist of:
Pre-synaptic neurone ends in a swelling called the pre-synaptic bulb contains:
-many mitochondria - for active process that require ATP
-many SEM - packages neurotransmitter into vesicles
-many vesicles containing acetylcholine - transmitter will diffuse across the synaptic cleft
- Number of calcium ion volted gated channels open
Post synaptic membrane
Contains specialised sodium channels that respond to the neurotransmitter
- channels consist of five polypeptide molecules
- Two of the molecules have a specific and complimentary receptor site to acetylcholine
- When acetylcholine is present in the synaptic cleft it binds to the two receptor sites and causes the sodium ion channels to open
Cholinergic synapse
Synapse that uses acetylcholine as its neurotransmitter
Neurotransmitter
Chemical used as a signalling molecule between two neurones in a synapse
Transmission of a neurone across a synapse until release of acetylcholine
- An AP arrives at synaptic bulb
- Calcium ion volted gated channels open
- Calcium ions diffuse into the synaptic bulb
- Calcium ions cause the synaptic vesicles to move and fuse with the pre-synaptic membrane
- Acetylcholine is released by exocytosis
Transmission of a neurone across a synapse across synaptic cleft
- Acetylcholine molecules diffuse across the cleft
- Acetylcholine binds to receptor sites of the sodium channels in the post-synaptic membrane
- Sodium ion channels open
Transmission of a neurone across a synapse how the ap is made
- Sodium ions diffuse across the post-synaptic membrane into the post-synaptic neurone
- Generator potential/ excitatory post-synaptic potential (EPSP) is created
- If sufficient generator potentials combine then the potential across the post-synaptic membrane reaches the threshold potential
-A new AP is created in the post-synaptic neurone - once ap achieved it will move down post synaptic neurone
What happens if acetylcholine is left in the synaptic cleft
Will continue to open Na+ channels in post-synaptic membrane and will continue to cause AP
Acetylcholinesterase
Enzyme found in the synaptic cleft
- hydrolyses the acetylcholine to ethanoic acid ( acetic acid) and choline
- Stops the transmission of signals, so synapse does not continue to produce ap in post synaptic neurone
What happens after acetylcholine is hydrolysed
Ethanoic acid and choline recycled
- re-enter the synaptic bulb by diffusion and recombine to acetylcholine using ATP
- recycled acetylcholine is stored in the synaptic vesicle for later use
Summation
Occurs when the effects of several excitatory post-synaptic potentials (EPSP’s) are added together
Convergence / spatial summation
Several pre-synaptic neurones may converge on the post-synaptic neurone
- this allows ap from different parts of the nervous system to contribute to generating ap in one post synaptic neurone - particular response
- useful when different stimuli are warning of danger
Temporal summation
Several post synaptic neurones may converge on one pre synaptic neurone
- one ap in pre- synaptic neurone does not produce an ap in post-synaptic neurone - requires series of ap
IPSP
Combination of several EPSPs could be prevented from an ap from one IPSP (Inhibited post synaptic potentials)
- reduces the effect of summation and prevents an ap in the post synaptic neurone
IPSP eg.
GABA and glycine
How does an IPSP work
Achieved by opening chloride ion channels that allow chloride ions into the post-synaptic neurone or by opening potassium ion channels that allow potassium ions out of the cell
- temporary hyper- polarisation produced
Divergence
One pre-synaptic may diverge to several post -synaptic neurones
- allows one ap to be transmitted to several parts of the nervous system
-useful in a reflex arc
- one post synaptic neurone elicits a response whilst another informs the brain
In which direction does an ap occur
Synapses ensure the ap can only occur in one direction, only the pre-synaptic has the vesicles containing neurotransmitter whilst the post synaptic has complimentary receptors
How do synapses filter out low level, unwanted signals
If a low level stimulus creates an ap in the pre-synaptic neurone, unlikely to pass across the synapse to the next neurone
AP frequency too low - not enough acetylcholine released
How are low level ap amplified by summation
If low-level persistent it will generate several successive ap in the pre-synaptic neurone
- Release of many vesicles of acetylcholine over a short period will enable the post-synaptic neurone EPSP’s to combine to produce an ap
What can happen after repeated summation
The neurone may run out of the neurotransmitter
-synapse = fatigued
- Nervous system no longer responds to the stimulus, we become habituated too it
- explains why we get used to certain smell/ noise after a period of time
- May also help to avoid overstimulation of an effector, which could cause damage
Creation and strengthening of specific pathways within ns is thought o be the basis of conscious thought and memory
Synaptic membrane - adaptable
- post synaptic membrane - more sensitive to acetylcholine by the addition of more receptors
- particular post -synaptic neurone is more likely to fire an ap if more receptors creating an ap in a specific pathway in the response to a stimulus
What does the synaptic cleft do
Ap arrives in one direction
Convergence
Divergence
Filter out low level noises
One neurone can transmit impulses to many neurones
Differences between the structure of the sensory neuron
and the motor neuron
Motor Neuron:
- cell body in CNS
- Dendrites connect directly to the cell body
-Longer axon
- Ends at motor plate
-Does not have dendron
- Cell body located at the end of the neuorone
Sensory neurone:
- Cell body not in the CNS
- Shorter axon
- Connects to sensory receptor
- has a dendron
- cell body in middle of neurone
What do the schwann cells produce
mylein
What does the mylein sheath prevent
Prevents movement of ions out of the neuron and prevents depolarisation
Why can a person not feel pain or move when they have an autoimmune disease
Damage to myelin shaeth
removes insulation
slows down conduction as prevents saltatory conduction in the sensory neurone
What happens when temp is to high to a neurone
proteins that make up the ion channels denature and the fluidity of the phospholipid bilayer is disrupted
Why after a long period of time would you not be able to feel your coat
No Longer a change in the environment so sodium channels open and not in the correct place for sodium ion movement / not enough sodium ions enter to reach the threshold potential
Charge of an inactive neurone
Polarised
Excitatory
If threshold potential is reached due to enough EPSPs combining then the it will cause a full depolarisation of the neurone
Inhibitory
Action potential prevented caused by hyperpolarisation
Spatial/Temporal
Spatial - many pre synaptic neurones converging onto one post-synaptic neurone
Temporal - Action potential triggered many time over a short period on a single pre synaptic neurone
What affects the speed of a neurone
Axon diameter and temperature
How does the NS increase heart rate
Impulses along nerve endings, which release acetylcholine
