C2.2 Neural Signalling Flashcards
state the structure + function of the dendrites
structure:
- short and highly branched nerve fibres projecting from the cell surface
function:
- to receive information
- to help in making connections with other neurons
- their branches increase the surface area for receiving signals
number of dendrites may differ in neurons
state the structure + function of the axon
structure:
- long single fibre moving away from the cell body
- (in myelinated neurons) it is covered by Schwann cells
function:
- to conduct nerve impulses to the terminal knobs
state the structure + function of the cell body
structure:
- has a nucleus and cytoplasm with the typical cytoplasmic organelles
- lacks centrioles because the neurons do not multiply
function:
- to contain the nucleus (genetic information) and the cell organelles
cell body is AKA “soma”
Identify the cell body, axon and dendrites in this diagram of a typical sensory neuron
Identify the cell body, axon and dendrites in this diagram of a typical motor neuron
What are the two main parts of the nervous system?
- The central nervous system (CNS)
- The peripheral nervous system (PNS)
What organs make up the central nervous system (CNS)?
What protects these organs?
The brain and spinal cord
(protected by bone)
What is the function of the CNS?
It receives and processes bodily information and activity
What type of cell is the nervous system made of?
Neurons
What are neurons?
What is their main function?
- Nuerons = the functional and structural cells of nerve tissue that transmits information throughout the body
- Function = to carry rapid electrical impulses
What is the peripheral nervous system (PNS)? What does it do?
All nerves outside the CNS (these nerves run to and from the CNS)
It connects the CNS to organs, muscles, and sensory receptors.
How do neurons communicate?
by transmitting a nerve impulse (an electrical signal)
There are three types of neurons in the nervous system:
______ neurons
______ neurons
______neurons
- sensory neurons
- relay neurons (AKA interneurons / connector neurons)
- motor neurons
What is the function of sensory neurons?
What is a key structural feature of sensory neurons?
- function = to transmit nerve impulses from sensory receptors to the CNS
- key feature = long axons
What is the function of motor neurons?
What is a key structural feature of motor neurons?
- function = to transmit nerve impulses from the CNS to effectors
- key feature = long axons
What is the function of relay neurons?
What is a key feature of relay neurons?
- function = to help transmit signals between the sensory and motor neurons
- key feature = are much smaller cells than sensory and motor neurons + have many interconnections with other neurons
What is the function of the nervous system?
- Regulates our actions and allows us to interact with our environment
- Along with the endocrine system, it coordinates all bodily functions
What is the axon hillock?
- the conical projection that connects the cell body with the axon
- the site where an action potential is generated
What is the myelin sheath?
- an insulating cover formed by Schwann cells in the PNS and by oligodendrocytes in the CNS
- made up of lipids and proteins
what are the nodes of ranvier?
- unmyelinated gaps between the myelinated sections of axons
What are axon terminals or synaptic knobs?
- the terminal endings of the axon
- they are called axon terminals or synaptic knobs because of their knob-like appearance
- here is where nerve impulses are transmitted to the next neuron/effector
label the different parts of a neuron
Define membrane potential.
What causes it?
- the difference in electrical charge between the inside and outside of a neuron’s membrane (measured in volts)
(membrane = polarized because the outside is + and inside is -)
- caused by uneven distribution of ions across the membrane
(Na⁺ is more concentrated outside, K⁺ is more concentrated inside)
at rest, it’s called “resting membrane potential”
Define resting potential.
What is the voltage of the resting potential?
- the electrical potential across the plasma membrane, when the neuron is not stimulated (not transmitting a signal)
- about -70 millivolts
Why is the inside of the neuron negative even though K⁺ (a positive ion) is concentrated inside?
- Some K⁺ leaks out, taking positive charge with it
- Big negative molecules stay trapped inside
- The sodium-potassium pump pushes more Na⁺ out than K⁺ in (creates net loss of positive charge)
- This makes the inside more negative than the outside
Outline three mechanisms that create the resting potential in a neuron
Sodium-Potassium Pump
- Actively transports 3 Na⁺ ions out and 2 K⁺ ions in using ATP
- Maintains the concentration gradients of Na⁺ and K⁺
- Creates a negative charge inside the neuron
Diffusion of Ions
- K⁺ ions diffuse out of the neuron through potassium leak channels
- As positive charge leaves, it adds to the negativity inside the neuron
Selective Permeability
- The neuron’s membrane is more permeable to K⁺ than Na⁺, allowing more K⁺ to leave
- unequal permeability helps maintain a negative internal environment
Outline the six steps of sodium-potassium pump action
1. Binding of Sodium Ions (Na⁺)
- 3 Na⁺ ions from inside the neuron bind to the pump protein
2. ATP Hydrolysis:
- The pump uses ATP to hydrolyze a molecule of ATP, releasing energy
3. Conformational Change:
- The energy from ATP causes the pump protein to change shape, which moves the Na⁺ ions to the outside of the cell
4. Binding of Potassium Ions (K⁺):
- After Na⁺ ions are transported out, 2 K⁺ ions from the outside of the neuron bind to the pump
5. Pump Returns to Original Shape:
- The binding of K⁺ causes the pump to return to its original shape, which releases the 2 K⁺ ions into the inside of the neuron
6. Ready to Start Over:
- The pump is now ready to bind 3 Na⁺ ions from the inside of the neuron and begin the process again
- It will do this to maintain ion gradients (Na⁺ high outside, K⁺ high inside)
What is the role of Na⁺ ions in the resting potential and action potential?
Resting Potential: Na⁺ is concentrated outside the membrane
Action Potential: Na⁺ rushes inside the membrane when the neuron is stimulated, causing depolarization
What is the role of K⁺ ions in the resting potential and action potential?
Resting Potential: K⁺ is concentrated inside the membrane
Action Potential: K⁺ rushes outside the membrane after depolarization, causing repolarization
What is the role of ion channels in the resting potential and action potential?
Resting Potential: Na⁺ and K⁺ ion channels are closed (at rest)
Action Potential:
- Na⁺ channels open first, allowing Na⁺ to rush in (depolarization)
- Na⁺ channels close just as K⁺ channels open, allowing K⁺ to rush out (repolarization)
What is the role of the sodium potassium pump in the resting potential and action potential?
- The pump moves Na⁺ ions outside the neuron and K⁺ ions inside the neuron, maintaining the ion gradients
- This process requires ATP and helps to restore the resting potential after an action potential
(works continuously in both potentials)
define nerve impulse
a temporary reversal in the electrical potential across the membrane of a neuron
define action potential
- a rapid change in the membrane potential of a neuron
- occurs when the neuron is stimulated, causing depolarization followed by repolarization
What does the “all or nothing” principle of action potentials mean?
means that action potentials are either fully triggered or not triggered at all
What is the magnitude of an action potential?
All action potentials are the same magnitude, which is +40mV
(doesn’t matter how strong the stimulus is, it is always the same)
Once an action potential is created at one part of the neuron, it _________
Once an action potential is created at one part of the neuron, it continues to propagate to the end of the neuron, traveling along the axon
In which direction can action potentials travel?
They can only travel in one direction, from the dendrites to the axon terminals
When does a generator potential occur?
when a few sodium channels open, causing a small depolarization
(By itself, it’s not enough to generate an action potential, but many generator potentials can combine to trigger an action potential)
How does the axon diameter affect conduction speed?
Larger axons conduct impulses faster because they have less resistance to ion flow
(smaller axons have more resistance and slower conduction)
What is the difference in nerve impulse speed between myelinated and unmyelinated fibres?
- Myelinated fibres conduct nerve impulses faster due to saltatory conduction (where the impulse jumps from node to node)
- Unmyelinated fibres transmit impulses slower, as the impulse moves continuously along the axon (instead of jumping)
How does the conduction speed of nerve impulses correlate with animal size?
- Larger animals generally have larger axon diameters, which results in faster conduction speeds
- Faster conduction = needed to cover longer distances between body parts
- Larger animals usually have adaptations for faster nerve impulse transmission
Which ion plays a key role in generating an action potential?
sodium (Na+)
because it causes the opening of Na+ channels which inflow and cause an imbalance of charges
Sequence of Events of an Action Potential along a Neuron? (12 steps)
describe what happens before it reaches the axon terminal
1. Stimulus triggers sodium channel opening – A stimulus causes some voltage-gated sodium (Na+) channels to open, allowing sodium ions to flow into the neuron.
2. Membrane potential becomes less negative – The influx of Na+ ions causes the membrane potential to rise, making the inside of the neuron less negative.
3. Threshold is reached and action potential is triggered – If the membrane potential reaches the threshold (-50mV), an action potential is triggered, initiating a rapid electrical signal.
4. Na+ voltage-gated channels open – The action potential causes Na+ voltage-gated channels to fully open, allowing a large influx of sodium ions into the neuron.
5. Neuron becomes depolarized – The influx of Na+ ions causes the neuron’s membrane potential to become positive, reversing the polarity (depolarization).
6. Impulse travels down the axon – Depolarization triggers the opening of Na+ channels further down the axon, propagating the action potential toward the axon terminals.
7. Potassium channels open – At the peak of depolarization, voltage-gated potassium (K+) channels open, allowing K+ to flow out of the neuron.
8. Repolarization occurs – The efflux of K+ ions restores the membrane potential, making the inside of the neuron more negative again (repolarization).
9. Sodium-potassium pump restores resting potential – The sodium-potassium pump actively transports Na+ out of the cell and K+ back in, restoring the neuron’s resting potential.
10. Hyperpolarization occurs – The efflux of K+ causes the membrane potential to briefly become more negative than the resting potential (hyperpolarization).
11. Refractory period begins – During the refractory period, the neuron cannot fire another action potential until the resting potential is fully restored.
12. Neuron is ready for another action potential – Once the resting potential is fully re-established, the neuron is ready to transmit another action potential.
Annotate the diagram to explain the different stages of the action potential.
Define synapse
A junction between two nerve cells, consisting of a minute gap across which impulses pass by diffusion of a neurotransmitter
Define synaptic cleft
The space between the end of one neuron and the target cell
Define effector
any cell or organ that carries out a response to a stimulus
List examples of effector cells
- skeletal muscle fibres
- gland cells
a signal can only pass in ______ across a typical synapse.
a signal can only pass in one direction across a typical synapse
What is the role of neurotransmitters?
to transmit signals across a synapse from a presynaptic neuron to a postsynaptic neuron or an effector cell
(they enable communication in the nervous system to coordinate responses + regulate body functions)
neurotransmitters can either be ______ or ______
neurotransmitters can either be excitatory or inhibitory
what are the three rules of synaptic transmission?
- Messages pass CHEMICALLY between neurons, but ELECTRICALLY within each neuron
- Neurons cannot touch each other
- Each neuron has thousands of neurotransmitters waiting to cross the synapse
Outline the sequence of events that occur at a synapse during transmission of an action potential
describe what happens once the action potential reaches the axon terminal
- An action potential arrives at the axon terminal of the presynaptic neuron.
- This causes depolarization of the presynaptic membrane.
- Voltage-gated calcium channels open in response to depolarization.
- Calcium ions enter the presynaptic neuron from the extracellular space, down their concentration gradient.
- Calcium functions as a chemical signal, triggering exocytosis of neurotransmitter-containing vesicles.
- The synaptic vesicles fuse with the presynaptic membrane and release neurotransmitters (e.g., acetylcholine) into the synaptic cleft.
- Neurotransmitters diffuse across the synaptic cleft toward the postsynaptic membrane.
what is acetylcholine?
- one of the most common neurotransmitters in both invertebrates and vertebrates
- is used as the neurotransmitter in many synapses including between neurons and muscle fibres
how is acetylcholine made?
where is it stored?
how does it activate a post-synaptic cell?
- made in axon terminal by combining choline with an acetate group
- stored in vesicles within the axon terminal until release via exocytosis in response to a nerve impulse
- activates a post-synaptic cell by binding to a specific receptor
Outline the mechanism of synaptic transmission occurring at a post-synaptic cell
(include the role of neurotransmitters, diffusion, receptors, gated ion channels, threshold potential and action potential)
- Neurotransmitters are released into the synaptic cleft via exocytosis from the presynaptic neuron.
- They diffuse across the synaptic cleft toward the postsynaptic membrane.
- Neurotransmitters bind to specific receptors on the postsynaptic membrane.
- This causes ligand-gated ion channels (e.g., Na⁺ channels) to open.
- Sodium ions enter the cell, depolarizing the membrane.
- If depolarization reaches the threshold potential (~ -50 mV), an action potential is generated.
- The nerve impulse continues down the axon of the postsynaptic neuron.
in the photo, this neurotransmitter = acetylcholine
Outline the digestion of acetylcholine by acetylcholinesterase
- acetylcholine is released by the presynaptic motor neuron at the neuromuscular junction.
- To prevent overstimulation, acetylcholine must be quickly removed from the synapse.
- The enzyme acetylcholinesterase, found in the synaptic cleft or on the postsynaptic membrane, breaks down acetylcholine.
- acetylcholine is hydrolyzed into acetate and choline.
- Choline is reabsorbed by the presynaptic neuron via endocytosis.
- Using energy from mitochondria, acetylcholine is resynthesized from choline and Acetyl coenzyme A
What is the role of calcium ions in synaptic transmission?
- When an action potential reaches the presynaptic terminal, it causes voltage-gated calcium channels to open
- Next, calcium ions flow into the presynaptic neuron, which triggers exocytosis of synaptic vesicles
What is the difference between an excitatory postsynaptic potential and an inhibitory postsynaptic potential?
Excitatory Postsynaptic Potential:
- Neurotransmitter binding opens sodium channels + sodium ions enter the cell
- Results in depolarization
- Increases likelihood of an action potential
Inhibitory Postsynaptic Potential:
- Neurotransmitter binding opens chloride or potassium channels
- Chloride ions flow into OR potassium ions flow out of the postsynaptic neuron
- Causes hyperpolarization (more negative inside the cell)
- Decreases likelihood of firing an action potential
What is the function of the neuromuscular junction? What is it?
- It’s a specialized synapse between motor neuron and muscle fiber
- It transmits signals from the motor neuron to the muscle to trigger muscle contraction
HOW IT’S DONE:
- Acetylcholine is released from the presynaptic terminal + binds to receptors on the muscle cell membrane
- This causes depolarization of muscle cell + triggers the release of calcium ions causing muscle contraction
