C2.2 - Neural Signalling

what are neurons?

specialized cells that carry electrical impulses

what are sensory neurons?

neurons that transmit nerve impulses from sense receptors to the central nervous system

what are interneurons? and where are they located?

located within the central nervous system. Interneurons transmit nerve impulses between neurons

what are motor neurons?

neurons that transmit nerve impulses from the central nervous system to an effector

what are the three components of all neurons?

dendrites, cell body (soma) and axon

what are dendrites of neurons?

short nerve fibres which receive nerve impulses from receptors, or other neurons

what are cell body (soma) of neurons?

- contains the cell's nucleus and most of its cytoplasm

- cell body determines if a signal coming from dendrites is to be passed onto the axon

what are the axon of neurons?

Nerve signals in the form of an action potential travel down the axon to axon terminals

what is the resting potential of a neuron (explain)?

Resting potential is the electrical potential difference (voltage) of the cytoplasm of a neuron, relative to its surroundings, when not stimulated or involved in passage of a nerve impulse

what is the resting potential of a neuron (value)?

approximately -70mV

why is the resting potential of a neuron negative?

due to the distribution of ions inside and outside the plasma membrane of a neuron

There is a high concentration of __________ within the cytoplasm of neurons

organic negative ions

what does the sodium potassium pump do and what does it use? (broad explanation)

The sodium-potassium pump uses ATP energy to maintain the resting potential of a neuron

the sodium potassium pump maintains the resting potential by pumping more ______ out than pumping _______ in

more Na+ out than K+ in

how is the potential difference across the membrane of the axon of the neuron maintained in terms of ions?

by sodium ions (Na+) being outside the axon of a neuron, and potassium ions (K+) and chlorine ions (Cl-) being inside the axon

what does the sodium potassium pump pump in and out of the neuron?

transports Na+ out of the axon, and transports K+ into the axon

The sodium potassium pump transports Na+ and K+ against

their concentration gradients (active transport)

explain the steps of the action of the sodium potassium pump:

1. Three Na+ attach to the sodium ion binding sites on the sodium potassium pump protein

2. ATP attaches to the sodium potassium pump

3. ATP is hydrolyzed, with a phosphate remaining attached to the protein pump. ADP is released

4. The phosphate causes the pump to change shape, moving the sodium across the axon membrane, releasing Na+ outside the axon

5. Two K+ attach to the potassium ion binding sites on the sodium potassium pump protein

6. The phosphate is released from the pump.

7. The pump returns to its original shape moving the K+ into the axon.

The process can be repeated.

what is the sodium potassium pump an example of?

example of an exchange transporter, as the sodium ions and potassium ions are transported in opposite directions

what are nerve impulses?

action potentials that are propagated along nerve fibres

what does the action potential of nerve fibers involve?

- the flow of sodium (Na+) and potassium (K+) ions.

is the action potential of a nerve fiber electrical?

The action potential is electrical as it involves the flow of ions along nerve fibres such as axon

what is the threshold potential in a neuron?

the minimum potential difference required across a membrane to initiate an action potential

at what threshold potential do voltage-gated sodium channels open and what does this allow?

open at the threshold potential of -55mV, allowing sodium ions (Na+) to enter a nerve fibre

nerve fibers can be ____ or ______

dendrites, axons

what is depolarisation?

an increase to potential difference across the membrane of a cell

what happens when a neuron, at resting potential (-70mv), receives a stimulus and reaches a threshold potential?

voltage-gated sodium channels open causing sodium ions to diffuse into the nerve fibre, increasing the potential difference (voltage) to +30mv

what is repolarization?

the restoration of potential difference to resting potential across the membrane of a cell

what happens when the potential difference reaches +30mv?

voltage-gated sodium channels close, and voltage gated potassium channels open

what is hyperpolarization?

when potassium ions (K+) move into the cell, reducing the potential difference below resting potential (-70mV)

how does the sodium-potassium pump restore resting potential?

by actively pumping sodium out of the cell and potassium into the cell

what happens in terms of ions during depolarization and where?

There is an influx of sodium ions (Na+) into a nerve fibre during depolarisation

what happens when sodium ions enter a nerve channel by diffusion?

they create a local channel

where are local channels?

within the axon

what does the movement of sodium ions in local channels within the axon cause?

decreases the potential difference to threshold potential, allowing the action potential to be propagated along the nerve fibre

what is an oscilloscope?

a device used by scientists to measure the changes in potential difference of axons during an action potential

what are the four stages of an oscilloscope tracing of an action potential (describe voltage and appearance of line)?

1. the neuron is at resting potential, line is flat (-70mv)

2. the nerve fibre undergoes depolarisation, line is increasing (plateau at +40mv)

3. nerve fibre undergoes repolarisation, line decreases

4. nerve fiber undergoes hyperpolarisation, line dips below -70mv and then increases back to -70mv

explain what happens when the neuron is at resting potential:

The sodium-potassium pump is actively transporting sodium ions out of the cell, and potassium ions into the cell

explain what happens when the neuron is depolarizing:

The threshold potential has been reached, and the voltage gated sodium channels open.

Sodium ions (Na+) diffuse into the nerve fibre, increasing potential difference across the membrane

explain what happens when the neuron is repolarizing:

The voltage-gated sodium channels close, and the voltage gated potassium channels open.

Potassium ions (K+) diffuse out of the nerve fibre, reducing the potential difference across the membrane

explain what happens when the neuron is in hyperpolarization:

The potential difference goes below resting potential before the voltage gated potassium channels close.

The sodium-potassium pump restores the resting potential of the nerve fibre

what are myelinated neurons?

neurons with a layer of myelin wrapped around the axon

what are the spaces between myelin sheaths called?

nodes of Ranvier

what are schwann cells?

A type of cell that forms insulating myelin sheaths around the axons of neurons in the peripheral nervous system

what are myelinated neurons capable of?

saltatory conduction

what is saltatory conduction?

Saltatory conduction describes the way an action potential jumps between nodes of Ranvier as it moves down an axon

what happens in myelinated vs non myelinated neurons?

saltatory conduction occurs in myelinated neurons and is faster than non-saltatory conduction which occurs in non-myelinated neurons

where are sodium-potassium pumps and voltage-gated channels clustered?

at the nodes of Ranvier

why can action potentials only occur at nodes of Ranvier?

because myelin sheaths insulate the rest of the neuron

what do action potentials do when traveling down an axon?

they jump from node to node

why does saltatory conduction require less energy than non-saltatory conduction?

because the sodium-potassium pumps are only active at the nodes of Ranvier

Saltatory conduction is much ______than non-saltatory conduction

faster

Generally as animal size increases

the speed of nerve impulses decreases

what is animal size and nerve impulse speed an example of?

negative correlation

large animals have ______ to increase speed of nerve impulses

adaptations

what are adaptations of large animals to increase the speed of nerve impulses?

- wider diameter of axons

- myelination of axons

why did scientists use giant axons of squid?

to develop an understanding of the transmission of nerve impulses

what happens as the diameter of an axon increases?

the speed of the nerve impulse increases

what is myelin?

a multilayer of phospholipids and proteins, which surrounds axons

what does myelin act as and what does that do?

acts as an insulating layer, and increases the speed of nerve impulses

what is the speed of nerve impulses in myelinated axons?

as high as 150 ms-1 compared to unmyelinated axons of 0.5 to 10 ms-1

what are synapses?

junctions between two neurons or a neuron and an effector, such as muscles or glands

what is the presynaptic membrane?

the end of the axon terminal where neurotransmitters are released and taken back up

what is the postsynaptic membrane?

the membrane of the next neuron or the muscle fiber

Nerve impulses can only pass from the _______ to the ________

presynaptic membrane, postsynaptic membrane

how are nerve impulses transmitted across synapses?

using chemical signalling chemicals known as neurotransmitters

explain the steps that the arrival of an action potential at an axon terminal initiates:

- Voltage-gated calcium channels open: Calcium ions (Ca2+) diffuse into the axon terminal.

- Movement of vesicles: The entry of calcium ions into the axon terminal triggers the movement of vesicles containing neurotransmitters to the presynaptic membrane.

- Exocytosis: The vesicles fuse with the presynaptic membrane releasing the neurotransmitters into the synaptic cleft

what are neuromuscular junctions?

synapses between axon terminals of motor neurons and muscle fibres

what is acetylcholine?

an example of a neurotransmitter which exists in many types of synapses, including neuromuscular junctions

what does the arrival of an action potential at an axon terminal result in?

the release of acetylcholine into the synaptic cleft

explain the generation of postsynaptic action potentials using acetylcholine as an example:

- Acetylcholine diffuses across the synaptic cleft

- Acetylcholine binds to transmembrane acetylcholine receptors on the postsynaptic membrane, which opens voltage gated sodium channels

- Sodium ions (Na+) flow into the neuron, generating an excitatory postsynaptic action potential

what are exogenous chemicals?

chemicals that come from sources outside of living things

what can exogenous chemicals interfere with?

synaptic transmission

Two examples of chemicals that interfere with synaptic transmission are:

- Neonicotinoids, which bind to cholinergic protein receptors and prevent the binding of acetylcholine

- Cocaine, which prevents the reuptake of dopamine

what are neonicotinoids?

a group of chemicals that are used in pesticides around the world

there is evidence that supports that neocotinoids are killing ________ such as _______

non target insects, bees

where does the neurotransmitter acetylcholine bind?

to nicotinic cholinergic receptors in the postsynaptic membrane

where do neocotinoids bind?

to nicotinic cholinergic receptors in the postsynaptic membrane

what do neonicotinoids have a similar structure to?

nicotine and acetylcholine

what do neonicotinoids do and what does this cause? (in insects)

- The neonicotinoids bind irreversibly to the acetylcholine receptors of insects

- This blocks synaptic transmission, resulting in paralysis and death of the insect

compare shape of acetylcholine receptors in humans and insects and what this causes?

- the shape of the acetylcholine receptors in humans is different than insects

- therefore neonicotinoids do not bind to human receptors as strongly as they do to insect receptors

- neonicotinoids are less toxic to humans.

what is dopamine?

a neurotransmitter which is responsible for feelings of pleasure and motivation

what do dopamine transporters do?

remove dopamine from synaptic clefts between neurons in the brain

what does cocaine do (simple)?

causes a feeling of euphoria/high that makes the person hyper and fidgety

explain the action of cocaine:

- Cocaine binds to and blocks the dopamine transporters, preventing the reuptake of dopamine

- High concentrations of dopamine remain in the synaptic cleft, and continue binding to the dopamine receptors in the postsynaptic membrane

- The excess dopamine creates a euphoric feeling, due to the continued excitation of the reward pathway

what do excitatory neurotransmitters do?

open sodium channels allowing sodium ions to enter the neuron, increasing the likelihood of an action potential being generated at the postsynaptic neuron

what do inhibitory neurotransmitters do?

- bind to protein receptors that allow negative ions to enter the neuron, resulting in the neuron being hyperpolarized

- the potential difference is lower than the resting potential of the neuron

- this decreases the likelihood of an action potential being generated at the postsynaptic neuron

Neuron have many synapses with

other neurons

where may inhibitory and excitatory neurotransmitters be released?

at different synapses, sending inhibitory and excitatory signals to the cell body

what does the summation of the effects of excitatory and inhibitory neurotransmitters in a postsynaptic neuron refer to?

the process by which the combined influence of these neurotransmitters determines whether the neuron will generate an action potential or not

there is an _____________ consequence of the summation of neurotransmitters, which means that

all or nothing

either the threshold potential is reached and an action potential propagated, or not

what are nociceptors and what are the responsible for?

a group of pain receptors found in the skin, which respond to a range of stimuli, including high temperature, acid, or certain chemicals such as capsaicin in chilli peppers

when do sodium channels on a nociceptor open? what does this cause?

when the nociceptor is stimulated

- Sodium ions flow into the nociceptor, causing the threshold potential to be reached

- An action potential is generated, which travels to the brain, where pain is perceived

what are emergent properties?

arise from the interaction of component parts

The whole is greater than the sum of its parts

what is consciousness an example of?

an emergent property of neurons in the brain

what is consciousness?

the state of being aware of self and surroundings

What is the function of ion channels in the propagation of nerve impulses?

To transmit the electrical signal along the nerve fibre

What is the main difference between resting potentials and action potentials based on their oscilloscope traces?

Action potentials have a rapid depolarisation phase