5.1.3 - Neuronal Communication Flashcards
What do sensory receptors do?
Detect a stimulus/changes in the internal and external environments of an organism
How do receptors act as a transducer?
They convert different types of stimuli into electrical nerve impulses
Which receptors detect light?
Photoreceptors (rods and cones)
Which receptors detect heat?
Thermoreceptors (skin)
Which receptors detect pressure?
Mechanoreceptor (Pacinian corpuscle in skin)
Where are the Pacinian corpuscles found?
Deep in the skin mainly in fingers and feet or in joints
What is the structure of a Pacinian corpuscle?
It has a single neurone, the ending of which is surrounded by layers of tissue separated by a gel
How does the Pacinian corpuscle respond to the stimulus of change in pressure?
- The membranes of the pacinian corpuscle have stretch mediated sodium channels
- When pressure is applied it deforms the neurone plasma membrane and stretches and widens the sodium channels so sodium ions diffuse in which leads to the establishment of a generator potential
What are the 3 types of neurones?
- Sensory
- Relay
- Motor
What are the common features of all 3 neurones?
- Cell body: contains the organelles found in a typical animal cell including the nucleus and proteins and neurotransmitter chemicals are made here
- Dendrons: carry the action potentials to surrounding cells
- Axon: conductive, long fibre that carries the nervous impulse along the neurone
What is the function of a sensory neurone?
Transmits electrical impulses from sensory receptors to other neurones (relay, motor or brain)
What is the structure of a sensory neurone?
They have a long dendron which carries the impulse from the sensory receptor cell to the cell body of the neurone and then an axon to carry the impulse from the cell body to the next neurone
What is the function of a relay neurone?
Transmits electrical impulses from sensory neurones to motor neurones
What is the structure of a relay neurone?
They have multiple short axons and dendrons
What is the function of a motor neurone?
Transmits electrical impulses from the CNS to the effector (muscles or glands)
What is the structure of a motor neurone?
They have one long axon and multiple short dendrons
What are myelinated neurones?
They have Schwann cells which wrap around the axon to form the myelin sheath which is a lipid and so does not allow charged ions to pass through it
What is the function of a myelin sheath?
It acts as an electrical insulator speeding up the conduction of electrical impulses
What are the gaps in the myelin sheath called?
nodes of Ranvier
Why is the transmission of electrical impulses faster in myelinated neurones than in non-myelinated neurones?
The action potential jumps from node to node (saltatory conduction) which means the action potential travels along the axon faster as it doesn’t have to generate an action potential along the entire length, just at the nodes of Ranvier
What is the resting potential?
- When a neurone is not conducting an impulse there is a difference between the electrical charge inside and outside of the neurone
- There are more positive ions, Na+ and K+ outside compared to inside so the inside of the neurone is comparatively more negative
What is the potential difference when the neurone is in resting state?
-70mV
How is the resting potential established and maintained?
- The resting potential is maintained by a sodium potassium pump involving active transport and ATP
- The pump moves 2K+ ions in and 3Na+ ions out
- This creates an electrochemical gradient causing K+ to diffuse out and Na+ to diffuse in
- The membrane is more permeable to K+ so more are moved out resulting in the -70mV
What is needed for an action potential to generate?
Stimuli need to reach a threshold potential
What is the threshold potential?
-50mV
What is the all or nothing principle?
- If the depolarisation does not exceed -50mV an action potential and impulse are not produced (nothing)
- Any stimulus that does trigger depolarisation to -50mV will always peak at the same maximum voltage (all)
- Bigger stimuli just increase the frequency of action potentials
Why is the all or nothing principle important?
It ensures that animals only respond to large enough stimuli rather than responding to every slight change in the environment
What is depolarisation?
- When the distribution of the charge reverses as more sodium ions flow into the axon due to the membrane becoming more permeable to Na+ ions
- The action potential is reached as more sodium ions flow across the membrane which continues until all voltage gated sodium ion channels are open which is an example of positive feedback
What is repolarisation?
- When sodium ions stop flowing into the axon because membrane permeability for Na+ ions decreases
- The membrane permeability for K+ ions increases so they flow out of the axon returning the potential difference to -70mV
What is hyperpolarisation?
When the cell’s membrane potential becomes more negative than the resting potential
What is the refractory period?
Where there is no membrane permeability to sodium and potassium ions and the neurone is unresponsive
Why is the refractory period important?
- It ensures that discrete impulses are produced. An action potential cannot be generated immediately after another which makes sure that each is separate
- It ensures that action potentials travel in one direction. This stops the action potential from spreading out in 2 directions which would prevent a response
- It limits the number of impulse transmission which is important to prevent over reaction to a stimulus
What happens after the refractory period?
Neurone returns to its resting potential
What is the gap between 2 neurones called?
Synapse/synaptic cleft
What transmits an action potential across a synapse?
Neurotransmitter
What are cholinergic synapses?
Synapses which use acetylcholine as the neurotransmitter
How does an action potential travel across a synapse?
- An action potential arrives at the end of the presynaptic axon (synaptic knob). This causes an influx of calcium ions which cause vesicles containing ACh to fuse with the membrane and release ACh into the cleft by exocytosis
- ACh diffuses across the cleft and binds to receptors on the postsynaptic membrane delaying the impulse by about 0.5ms
- An influx of sodium ions occurs as ion channels open, depolarising the cell. If the threshold potential is reached an action potential will be generated
- An enzyme (acetylcholinesterase) on the postsynaptic membrane hydrolyses ACh. The breakdown product choline is reabsorbed into the synaptic knob and recycled into ACh
What is summation?
The rapid build-up of neurotransmitters in the synapse to help generate an action potential
What is spatial summation?
When impulses from different presynaptic neurones collectively reach the threshold to cause an action potential
What is temporal summation?
When several impulses arrive in quick succession from a single presynaptic neurone causing an action potential
What do excitatory inputs do?
Cause depolarisation and action potentials
What do inhibitory inputs do?
Cause the neurotransmitters to bind with the postsynaptic neurone allowing negative chloride ions to enter it which hyperpolarises the postsynaptic neurone making depolarisation less likely
