C2.2 Neural Signaling Flashcards
Define:
Nervous System
Body system used for internal communication. It consists of nerve cells.
Define:
Nerve Cells
Neurons. They help with internal communication by transmitting nerve impulses.
About 8.5 billion neurons in the human nervous system.
Define:
Nerve Impulses
Electrical Signal; An action potential that starts at one end of a neuron and is propagated along the axon to the other end of the neuron.
Describe:
Neuron Structure
Neurons have:
* A cell body with cytoplasm and a nucleus.
* Nerve fibres, which nerve impulses travel along.
* Dendrities
* Axons
Define:
Nerve Fibres
Narrow Outgrowths
Describe:
Dendrites
Short Branched Nerve Fibres
Describe:
Axons
Elongated Nerve Fibres
Define:
Membrane Potential
Voltage exists across the membrane, due to an imbalance between the net charge (negative or positive) of cytoplasm and the fluid outside.
When neurons transmit an impulse, its membrane potential changes suddenly.
Why is the Membrane Potential Negative?
Cytoplasm is usually electrically negative compared to outside fluid. Therefore, membrane potential is expressed as a negative value (e.g. -40 mV for liver cells).
Define:
Resting Potential
The membrane potential when a neuron is not transmitting an impulse; -70mV
What factors contribute to the resting potential?
- Sodium-Potassium Pumps In Membrane: For every 3 Na+ pumped out, only 2 K+ pumped in.
- Pumped Ions Leak Back Across Membrane By Diffusion: Membrane 50x more permeable to K+ than Na+, so leakage of K+ ions is faster.
- Negatively Charged Proteins In Nerve Fibres (Organic Anions)
- Creates a charge imbalance and concentration gradients between both ions.
- Increases the difference between the Na+ and K+ concentration gradients, increasing the overall charge imbalance across the membrane.
- Contributes to the charge imbalance.
Define:
Electronode
Can be used to monitor the membrane potential at one position along a nerve fibre.
Define:
Action Potential
An all-or-nothing sequence of changes in membrane potential with two main phases—depolarization and repolarization.
Define:
Depolarization
Change in membrane potential from negative to positive.
Both due to movement of positively charged ions (Na+ and K+) across the membrane—NOT to movement of electrons.
Define:
Repolarization
Change in membrane potential back from positive to negative.
Both due to movement of positively charged ions (Na+ and K+) across the membrane—NOT to movement of electrons.
How does depolarization occur?
Depolarization is due to opening of sodium channels in the membrane, allowing Na+ ions to diffuse into the neuron down the concentration gradient.
* Concentration of sodium ions outside about 10x higher than inside.
* Entry of Na+ ions reverses the charge imbalance across the membrane, resulting in the inside to be positive relative to outside.
* Raises membrane potential from -70mV to +30mV.
How does repolarization occur?
Repolarization is due to closing of sodium channels and opening of potassium channels in the membrane, K+ ions diffuse out the neuron down the concentration gradient and no more sodium ions diffuse in.
* Occurs rapidly after depolarization.
* Sodium channels close (no more Na+ ions diffuse into neuron).
* Inside of neuron becomes negative again relative to outside.
* Potassium channels remain open until the membrane potential has fallen to close to -70mV.
What happens after repolarization?
Diffusion of potassium repolarizes the neuron, but does not fully restore the resting potential as the concentration gradients of sodium and potassium ions have not yet been re-established.
* To restore resting potential, Na+ is actively pumped out and K+ in.
* Takes a few milliseconds.
* Must occur before another action potential.
How is the action potential propogated?
Action potentials are propagated along nerve fibres, as the ion movements that depolarize one part of the fibre trigger depolarization in the neighboring part of the fibre.
This is how neural signals pass along nerve fibres.
Why do nerve impulses always move in one direction?
Nerve impulses always move in one direction, as it is only initiated at one terminal of a neuron and can only be passed on to other neurons or different cell types at the other terminal.
There is a refractory period after depolarization that prevents propagation of an action potential backwards along an axon.
Refractory Period
A state of recovery that occurs after a neuron has fired an action potential.
During this period, another action potential cannot be easily produced. This encourages unidirectional flow of action potentials because they cannot travel backwards to inactive neurons.
How does diameter affect the speed of nerve impulses?
Larger diameters of nerve fibres reduce resistance, so impulses are transmitted quicker (e.g. squids). However, not all animals have this space available.
Define:
Myelination
Coating of nerve fibres consisting of a series of Schwann cells, with gaps between them called nodes of Ranvier.
How does myelination affect the speed of nerve impulses?
Modification of nerve fibres that increase the speed of nerve impulses. In myelinated nerve fibres, the impulse can jump from one node of Ranvier to the next, speeding up the transmission to as much as 100 metres per second.
Define:
Synapse
Junction between two cells in the nervous system. Signals only pass in one direction across a synapse.
List:
Three Main Types of Junctions
- Synapses between sensory receptor cells and neurons, in sense organs.
- Synapses between neurons, in both the brain and spinal cord.
- Synapses between neurons and muscle fibres or gland muscles.
Muscles and glands are called effectors, because they effect (carry out) response to a stimulus.
Define:
Presynaptic Neuron
Brings the signal to the synapse in the form of a nerve impulse or action potential.
Define:
Postsynaptic Neuron
Carries the signal away from the synapse, again in the form of a nerve impulse.
Define:
Neurotransmitters
Chemicals that carry signals across a narrow fluid-filled gap (synaptic cleft) between the presynaptic and postsynaptic neurons.
Describe:
Release of Neurotransmitter From a Presynaptic Membrane
Synapse Transmission: Occurs rapidly, in this sequence:
* Nerve impulse is propagated along the presynaptic neuron until it reaches the end of the neuron and the presynaptic membrane.
* Depolarization of the presynaptic neuron causes calcium ions (Ca2+) to diffuse through the channels in the membrane into the neuron.
* Influx of Ca2+ causes vesicles containing neurotransmitters to move to the presynaptic membrane and fuse with it.
* Neurotransmitter is released into the synaptic gap by exocytosis.
Describe:
Triggering of an Action Potential in the Postsynaptic Neuron
Release of neurotransmitter from a presynaptic neuron leads to a series of events that trigger an action potential in the postsynaptic neuron:
* Neurotransmitter molecules diffuse across the synaptic gap.
* Neurotransmitter binds to receptors in the postsynaptic membrane, causing ion channels to open.
* Ions diffuse down their concentration gradient into the postsynaptic neuron, causing the membrane potential to change.
* Mostly, potential rises (becomes less negative). This is called an excitatory postsynaptic potential.
* If excitatory postsynaptic potential is strong enough, it triggers an action potential which propagates away from the synapse.
* Neurotransmitter rapidly broken down and removed from the synaptic gap.
Describe:
What is the role of acetylcholine as a neurotransmitter?
- When it binds to receptors in the postsynaptic membrane, a channel opens in the receptor. Sodium ions diffuse through this channel and into the postsynaptic membrane, causing an excitatory postsynaptic potential.
- Acetylcholine only remains bound to the receptor for a short time and only one action potential is initiated in the postsynaptic neuron.
- Because enzyme (acetylcholinesterase) is present in the synaptic gap and rapidly breaks acetylcholine down into choline and acetate.
- The choline is reabsorbed into the presynaptic neuron, where it is converted back into acetylcholine by recombining with an acetyl group.
In the presynaptic neuron, choline (absorbed from the diet) is combined with an acetyl group produced by aerobic respiration. This produces acetylcholine, which is loaded into vesicles and then released into the synaptic gap during synaptic transmission.
