C2.2 Neural signalling

What is an ion?

A molecule with a positive or negative charge

What is diffusion?

The net movement of molecules from a region of high concentration to a region of low concentration

What is ATP?

A molecule produced by respiration which is a very good energy source

What is a concentration gradient?

The difference between concentrations

What is polar molecule?

A molecule which has a positive charge in one region and a negative charge in the other

What are protein channels?

Allow molecules to move through the cell membrane passively

What are protein pumps?

Move molecules through the cell membrane by using ATP (energy)

What is the resting potential of a neurone?

The electrical potential difference across the plasma membrane of a neurone that isn’t conducting an impulse is approximately -70mV

What is membrane potential?

Difference in a charge across a plasma membrane

Charge outside the neurone?

Positively charged

Charge inside the neurone

More negatively charged relative to outside

Central nervous system?

Consists of brain and spinal cord

Two parts of nervous system?

CNS and Peripheral nervous system

What is peripheral nervous system?

The nerves outside of the spinal cord

Structure of nerves?

Nerves are composed of bundles of nerve fibres which in turn are made up of specialised cells called neurones

What causes the negative resting potential in neurones?

Negatively charged molecules in the cytoplasm (proteins, ATP, pyruvates). Sodium potassium pump. Permeability of membrane.

Sodium potassium pump.

More positive charges are being pumped out of the cytoplasm than are being pumped in. 3 Sodium ions out for every 2 potassium ions in.

Permeability of membrane (sodium)?

Sodium ions that have been pumped out into the tissue fluid can’t diffuse back into the cytoplasm as the membrane is impermeable to sodium ions (voltage gated sodium channels are closed)

Permeability of membrane (potassium)?

The membrane is slightly permeable to potassium ions so some potassium ions which have been pumped into the cytoplasm can diffuse out into the tissue fluid resulting in a relatively negative charge in the cytoplasm.

Three stages of Action Potential?

Depolarisation, Repolarisation, Refractory period

Depolarisation?

In response to a signal initiated at a dendrite, sodium channels open. Sodium ions are more concentrated outside the neurone which causes a passive influx of sodium ions. This makes the inside of the axon membrane more positive

Repolarisation?

After influx of sodium ions, potassium channels open and sodium channels shut. Due to higher concentration of potassium ions inside the neurone, there is a passive eflux of potassium ions. Inside of the membrane returns to negative potential.

What is hyperpolarisation?

Happens after Repolarisation. A delay in closing of the potassium ion channels where the axon becomes more negative than the resting potential.

What is the reason of hyperpolarization?

To ensure everything is propagating in the same direction

What is the potential at hyperpolarization?

-80mV

Refractory period

Resting potential must be restored via the action of the sodium-potassium pumps before the neurone can fire again

Charge across plasma membrane after depolarisation?

+40mV

What is a nerve impulse?

Action potentials that move along the length of an axon as a wave of depolarisation

What causes propagation of action potentials?

Occur because depolarisation of one part of the neurone will trigger depolarisation in the neighbouring part

Two factors that cause propagation of action potentials?

Sodium channels, Local currents

How do sodium channels affect propagation of action potentials?

Sodium channels are voltage-gated. Depolarisation in one part of the axon will cause a sodium channel to open in the next segment of the axon

Local currents impact on propagation of action potentials (1)?

The depolarised part of the axon has a different sodium ion concentrations to its neighbouring part that has not yet depolarised. There is a higher concentration of sodium ions inside the depolarised axon, therefore sodium will diffuse along the axon to that part that has not yet depolarised.

Local currents impact on propagation of action potentials (2)?

Lower concentration of sodium ions outside the depolarised axon so sodium ions from the non-depolarised part will move to the depolarised part. This reduces the sodium ions in the non-depolarised part and means that the charge outside the axon is less positive than before. Local currents reduce the concentration gradient in the part of the neurone that has not yet depolarised.

When is the potential across the membrane -50mV?

The part to be depolarised next. Much closer to the threshold potential

What is the threshold potential

An action potential occurs when the membrane depolarises to a certain threshold. If this threshold is not reached the action potential will not be triggered. -55mV

What are local currents?

The diffusion of sodium ions along the inside and outside of the membrane in particular directions

Main result of local currents?

The difference in concentrations of sodium inside and outside the cell decreases which causes the membrane potential to increase to -50mV and triggering the sodium channels in the next part of the axon to open for depolarisation

Correlation between axon diameter and conduction speed of an impulse?

Positive correlation because larger diameters result in less resistance

What is a myelin sheath?

Composed of Schwann cells which wrap around the axon of a neurone and produce a lipid-rich substance called myelin.

What does myelin do?

Acts as an electrical insulator

what are nodes of Ranvier?

Spaces between the Schwann cells which allow electrical impulses to travel along the axon differently to an unmyelinated neurone

What is saltatory conduction?

Type of conduction occurring in a myelinated neurone

Possible adaptions found in large animal neurones?

The axons in large animals likely to have a wide diameter which allows the impulse to travel faster. Likely to be myelinated allowing for saltatory conduction. Results in a quicker reaction time.

What is a neurotransmitter?

A chemical which diffuses across a synapse to trigger an action potential in the next neurone

Post-synaptic?

Located after the synapse

Synapse (or synaptic cleft)

A junction between two neurones or between a neurone and a receptor cell/effector cell

Pre-synaptic

Located before the synapse

How long does Acetylcholine bind to receptors?

Only binds to receptors for a short period of time due presence of acetylcholinesterase in the synaptic cleft.

What does acetylcholinesterase do?

breaks down the acetylcholine and the products of this reaction are reabsorbed by the pre-synaptic membrane so they can be repackaged into vesicles and used again

How is acetylcholine released into the synaptic cleft?

Via exocytosis

what does acetylcholine do once released into synaptic cleft?

Diffuse to the post-synaptic membrane and binds to receptors on the membrane

What does binding of acetylcholine to receptors on membrane cause?

Adjacent sodium channels open in the post-synaptic membrane. Sodium ions move into the post-synaptic neurone allowing the post-synaptic membrane to reach the threshold potential required for an action potential to be triggered

Where does the action potential arrive at?

The pre-synaptic membrane

What does depolarisation trigger in presynaptic membrane?

Calcium channels to open. Calcium ions diffuse into the pre-synaptic neurone and stimulate vesicles containing acetylcholine to move towards the synaptic cleft

What is summation

Summation refers to the process by which multiple signals (neurotransmitters) from presynaptic neurons are combined at the postsynaptic neuron to determine if it will fire an action potential.

What are the two types of summation?

Temporal and spatial

Temporal summation

Occurs when multiple nerve impulses from a single presynaptic neuron arrive at the same synaptic knob within a short period, and their effects add up over time to reach the threshold.

Spatial summation

Occurs when multiple presynaptic neurons release neurotransmitters at the same time, and their effects combine to reach the threshold on the postsynaptic neuron, triggering an action potential.

How do inhibitory neurotransmitters prevent an action potential/

Inhibitory neurotransmitters bind to the receptors on post-synaptic membrane. This causes hyper polarisation because either Chlorine ions enter the post synaptic neurone or potassium ions diffuse out. Membrane potential becomes more negative meaning it is less likely that threshold potential is reached.

What are exogenous chemicals?

Chemicals originating outside an organism

Examples of exogenous chemicals that affect synaptic transmission in insects?

Neonicotinoid pesticides

Examples of exogenous chemicals that affect synaptic transmission in humans?

Cocaine

Structure of neonicotinoid pesticides?

Similar in structure to acetylcholine

How do neonicotinoid pesticides result in paralysis and death of insects?

Bind to the acetylcholine receptor in cholinergic synapses in CNS of insects. Acetylcholinesterase doesn’t break it down so binding is irreversible. Acetylcholine can’t bind as receptor is blocked and so synaptic transmission is prevented. Leads to paralysis and death of insects

Why do neonicotinoid pesticides not affect humans and mammals?

Insects have a higher proportion of cholinergic synapses in their CNS. Neonicotinoid bind more strongly to receptors in insects than in mammals

What does Cocaine stimulate?

Transmission at brain synapses that use dopamine

Reward pathway due to cocaine?

Dopamine builds up in synapse as cocaine blocks its return to pre-synaptic neurone. Causes continuous stimulation of the ‘reward pathway’ neurones giving feelings of pleasure.

Symptoms of high doses of cocaine?

Can raise the body’s temperature, cause convulsions a heart attack and heart failure.

Snorting cocaine?

Seriously damages the cartilage of the nose

What is the membrane protein that cocaine binds to?

Dopamine reuptake transporters embedded in membrane of receiving neurone

What is summation?

The process that determines whether or not an action potential will be generated by the combined effects of excitatory and inhibitory signals, both from multiple simultaneous inputs (spatial summation) and from repeated inputs (temporal summation).

What is consciousness?

The qualitative feeling that is associated with perceptions such as colour, temperature or sound, together with the deeper processes of reflection, communication and thought.

Role of the left cerebral hemisphere?

Receives sensory inputs from sensory receptors in the right side of the body and the right side of the visual field of both eyes. Controls muscle contraction in the right side of the body.

Role of the right cerebral hemisphere?

Receives sensory inputs from sensory receptors in the left side of the body and the left side of the visual field of both eyes. Controls muscle contraction in the left side of the body.