5 - MZH - Homeostasis 7 - Neuronal Communication Flashcards
What does the nervous system do?
- What type of response does it provide?
- Give 4 examples
Provides rapid responses to changes in both the internal and external environment
What does a general nervous pathway look like?
What does each stage do?
Define Stimulus, Receptor and Effector
Stimulus = Detectable change in the external or internal environment of an organism
Receptor = A cell, or protein on a CSM that detects specific stimuli
Effector = A muscle or gland which carries a body’s response to a stimulus
What are sensory receptors?
What are transducers?
Sensory receptor = Specialised cells that respond to stimuli. Most sensory receptors are known as transducers - they convert one form of energy into another.
In this case the transducers are converting energy to electrical impulses/ action potentials.
Give 4 examples of:
Stimulus - Sensory receptor - Energy change involved (what brings abou this every transfer change?)
Transducers bring about this transfer of energy
What are Pacinian Corpuscles?
Structure + Function?
Pacinian corpuscles = (type of mechanoreceptor) which detects changes in pressure on the skin and joints.
NOTE: NOT constant pressure.
Structure:
- Each pacinian corpuscle consists of a single nerve fibre of a dendrite of a sensory neurone wrapped around by layers of a membrane called lamellae.
- Layers of connective tissue are seperated by a jelly/ viscous gel like material.
Function:
- Pressure applied to skin deforms the lamaellae which pushes up against the dendrite triggering a generator potential.
- Stretch in membrane opens voltage gated Na+ ion channels. Inflow of Na+ ions raises membrane potential - depolarises it.
- The more intense the stimuli the larger the generator potential.
4.
Resting potential = ?
Resting potential = -70mV
Structure of a neurone:
Feature: Large cell body
Decription?
Contains the nucleus and many of the cell organelles
Structure of a neurone:
Feature: Dendrons
Decription?
Short extensions of the cell body which transmit impulses towards the cell body.
Structure of a neurone:
Feature: Dendrites
Decription?
- Each dendron has smaller extensions called dendrites
- Dendrites are stimulated by other neurones
Structure of a neurone:
Feature: Axon
Decription?
- It’s a single entension of the cell body
- Can be up to 1m long
- Always transmits nerve impulses away from cell body
- Ends in series of synaptic knobs
Structure of a neurone:
Feature: Myelin
Decription?
- In mammals many axons are surrounded by a sheath of fatty material = Myelin sheath
- Myelin enables the neurones to conduct action potentials rapidly
Structure of a neurone:
Feature: Schwann cell
Decription?
Specialised cells which produce myelin
Structure of a neurone:
Feature: Node of Ranvier
Decription?
- Small gaps between the myelin sheath where bare membrane is exposed.
- Nerve impulses travel along in a series of jumps from one node to the next - Advantage of myelination.
- This jumping is called Salutatory conduction
Name the different types of neurones (3)
What do they do?
Sensory neurone = Carries action potential from a sensory receptor → CNS
Motor neurones = Carries action potentials from the CNS → effector e.g. muscle or gland
Relay neurones = Connects the sensory and motor neurones together
Compare the structure of Sensory and motor neurones:
- Cell body
- Dendrites
- Axon
- Dendron
- Myelinated
Define resting potential and give it’s value
Resting potential = Charge difference across a neurone membrane that must be set up before action potentials can be generated
-70mV
How is the resting potential generated and maintained?
Resting potential = -70mV
There is an imbalance of Na+ and K+ across the membrane. More Na+ on the outside and more K+ inside neurone.
- Imbalance of ions is maintaned by a Na+/K+ pump.
- Na+/K+ pump uses ATP for active transport to remove Na+ and take up K+.
- Ratio of 3Na+ out : 2K+ in
- Both ions can diffuse back down their electrochemical gradient through open channel proteins. NOTE: There’s many more K+ channel protein than Na+ channel proteins.
- ∴ Many K+ that has been pumped into neurone will leave through their open channel proteins but very few Na+ can return back to neurone once pumped out. This further reinforces the imbalance of ions first established by Na+/K+ pump
At what potential difference is the membrane considered to be polarised?
Resting potential at -70mV
When is a action potential triggered?
- Action potentials begin when a particular stimulus causes the membrane in one part of the neurone to suddenly change its permeability.
- The depolarisation of the membrane triggers a generated potential and if this generated potential is equal to or exceeds the threshold value then a action potential is triggered.
State the 5 steps in the events of an action potential
- Resting potential
- Depolarisation
- Repolarisation
- Hyperpolarisation
- Resting potential
Do it all over again!
Describe the sequence of events that occur when an action potential is triggered (+ use figures)
- Resting potential - Initial membrane potential = -70mV
- Depolarisation - A stimulus occus and the voltage gated Na+ channels open. Na+ diffuse into neurone down their electrochemical gradient. Depolarisation causes more channels to open and ∴ results in further depolarisation.
- So many Na+ diffuse into of neurone that the inside of neurone becomes positive (+40 mV).
- Repolarisation - Na+ channels close and voltage gated K+ channels open. K+ diffuses down it’s electrochemical gradient out of the neurone making the inside of the membrane become more negative.
- Hyperpolarisation - So many K+ ions rush out of membrane that the membrane potential falls below the resting potential (undershoot).
- Resting potential - Na+/K+ pump restores resting potential by pumping 3Na+ out for every 2K+ in. From hyperpolarisation back to resting potential is called the refactory period.
What is the purpose of the refactory period?
Period between hyperpolarisation and restoration of resting potential.
Refactory period is split into 2 parts:
- Absolute refactory period = Second action potential can’t occur.
- Relative refactory period = Second action potetnial can occur with a large enough stimulation.
- After an action potential, the neurone cell membrane can’t be excited again straight away
- This is because the ion channels are recovering and they can’t be made open.
- It also acts as a time delay to ensure that action potentials don’t overlap and are unidirectional i.e. only travel in one direction.
What are local currents and their purpose?
Local currents allow action potentials to be transmitted as a nerve impulse
Describe how the transmission of action potentials occur along a neurone?
- Action potential at one Node of Ranvier and a resting potetnial at the next node causes a localised circuit of electricity.
- This electrical energy opens the voltage-gated Na+ channels at the next node ∴ Na+ rush into axon caused membrane to be depolarised. K+ rush out and a new action potential is generated.
- Previous node returns to its resting potential due to Na+/K+ pump.
- Action potential continues to travel down the neurone in the same fashion.
Give 4 distinct characteristics of action potentials
- They’re only generated if the stimulus triggers a generated potential greater than a certain threshold value
- Action potentials have a all or nothing law where all action potentials are the same size
- Strong stimuli cause many action potentials to be generated i.e. increased frequency
- They always travel in one direction
Comparison betwwen myelinated and non-myelinated neurones:
- What neurones are included in each type
- Do they have a myelin sheath?
- Length of axons and dendrons
- Whether neurones are insulated
- Transmission speed
- Type of transmission involved
Advantages of myelination (2)
Why is speed not as big of a concern in non-myelinated neurones?
- Much faster transmission of action potentials from 2-20ms-1 in non-myelinated neurones to 100-200ms-1 in myelinated neurones
- Greater speed of transmission ∴ result in a more rapid response as in sensory & motor neurones action potentials may have to travel relatively long distances
Non myelinated nerves tend to be shorter ∴ an increased speed in transmission is not that important
State and explain 2 other factors that may affect the speed of conduction of action potentials aside from myelination
Temperature - Higher temperatures will increase the rate of diffusion of ions and thus increase the rate of conduction
Diameter - An axon with a larger diameter will transmit action potentials faster but this doesn’t necessarily apply to all animals
What is a synapse?
It’s purpose and what type of synapses are involved with action potentials?
Why is it called that?
Synapse = The junction between 2 neurones or between a neurone and effector
- The gap created by the synapse is too wide for electrical impulses to cross ∴ a chemical neurotransmitter is needed.
- Cholinergic synapses are used for the transmission of electrical impulses. It’s called so as it uses the neurotransmitter Acetylcholine (ACh)
Describe the sequence of events that occur during synaptic transmission (8)
- Action potential arrives at the pre-synaptic bulb
- Voltage gated Ca2+ gates in the CSM of the pre-synaptic bulb to open. Ca2+ enter the pre-synaptic bulb.
- Vesicles containing the neurotransmitter ACh in the pre-synaptic bulb move to the pre-synaptic membrane and fuse with it releasing ACh into the synaptic cleft by exocytosis.
- ACh diffuses across synaptic cleft and attach to ACh receptor molecule present on post-synaptic membrane.
- Neurotransmitter/receptor complex triggers the opening of Na+ gated channels on the CSM of the post-synaptic bulb.
- If sufficient ions enter the post-synaptic bulb an action potential is generated in the post-synaptic neurone.
- ACh bound to receptor proteins on the post-synaptic bulb are hydrolysed. ACh → Choline + Ethanoic acid - enzyme = acetylcholine esterase. Choline and ethanoic acid detach from receptor sites and diffused back to pre-synaptic membrane.
- Choline and ethanoic acid molecules enter pre-synaptic bulb and are reformed into ACh (uses ATP) which is stored into vesicles for future use.
Explain the all or nothing law with action potentials
- Once the threshold is reached, an action potential will always fire with the same chance in voltage i.e. amplitude doesn’t change.
- The stronger the stimuli the greater the number of action action potentials fired.
Stronger stimuli = Increased frequency of action potentials (like binary)
Synaptic transmission:
Unidirectional (3)
- Transmission at a synapse can only flow in one direction
- Neurotransmitters are only released from the pre-synaptic membranes
- Neurotransmitter receptors are only present on the post-synaptic membranes
What is the difference between synaptic convergence and synaptic divergence?
Synaptic convergence = Many pre-synaptic neurones release neurotransmitters to one post-synaptic neurone.
MANY TO ONE
NOTE: Synaptic convergence allows spatial and temportal summation as well as integration)
Synaptic divergence = One pre-synaptic neurone release neurotransmitters to many post-synaptic neurones.
ONE TO MANY
Synaptic transmission:
Summation
Weak stimuli leads to not enough neurotransmitter released to reach threshold level ∴ depolarisation of post-synaptic membrane doesn’t occur.
2 Different types of summation:
-
Spatial summation (down to convergence) - ≥2 presynaptic neurones converge and release their neurotransmitters at the same time onto the same post synaptic neurone.
- Small amounts of neurotransmitter is released from each presynaptic neurone. They add up to reach the threshold level.
-
Temporal summation (down to divergence) - ≥2 nerve impulses arrive in qiuck succession from the same presynaptic neurone.
- Makes action potential more liekly as more neurotransmitter is released into the synaptic cleft
Synaptic transmission:
Inhibition
MIMIC, BLOCK or INHIBIT (in 2 ways)
- MINIC - Chemicals with the same shape as the neurotransmitters. These drugs are called “agonists”
- e.g. Nicotine mimics ACh and binds to certain types of cholinergic receptors in brain.
- BLOCK - Chemicals that block receptors so they can’t be activated by neurotransmitters. Fewer receptors (if any) can be activated.
- e.g. Curare blocks ACh by blocking certain cholinergic receptors at certain neuromuscular junctions, so muscle cells can’t be stimulated ∴ muscle becomes paralysed.
- INHIBIT BY PREVENTING HYDROLYSIS THE NEUROTRANSMITTER - Results in more neurotransmitters in the synaptic cleft to bind to receptors. They remain there for longer.
- e.g. Nerve gases stop ACh from being hydrolysed in the synaptic cleft. This leads to loss of muscle control.
- INHIBIT BY RELEASING LESS NEUROTRANSMITTER FROM PRESYNAPTIC NEURONE - So fewer receptors are activated.
- e.g. Opioids block Ca2+ channels in the presunaptic neurone. Means that fewer vesicles fuse with the presynaptic membrane ∴ less neurotransmitter is released.
