C2.2 neural signalling

neuron

1. cell body - cytoplasm&nucleus , elongated nerve fibre- conduct electrical impluse

2. dendrities - shorter fibres, projecting from cell body

3. nucleus

4. schwann cell

5. node of ranvier

6. axon terminal

Different Types of Neuon

Motor Neuron

• Short Dendrones

• Long Axons

Sensory Neuron

• Long Axon

• Short Dendrones

• Middle Cell Body

Relay Neuron

• Short Dendrones

• Short Axon

sodium potassium pump

1. Na+ binds to the sodium potassium pump → stimulates phosphorylation by ATP → Causes protein to change its conformation → expels Na+

2. Extracellular K+ binds to protein → triggers release of phosphate group → restores protein original conformation → K+ released, Na+ sites is receptive again → cycle repeats

Nerve impulses as action potentials that are propagated along nerve fibres

Membrane potential is the voltage inside the neuron relative to the outside

When the membrane potential is negative, we describe the membrane as polarized

• Depolarization: the membrane potential becomes positive 

• Repolarization: the restoration of a negative membrane potential

depolarization & repolarization

During depolarization Na⁺ diffuses into the cell

• making the membrane potential positive

During repolarization K⁺ diffuses out of the cell

• restoring a negative membrane potential

Resting Potential ( Also known as: Polarization)

• This is when the neuron is not stimulated. The potential difference is -70mV

• Sodium-potassium pump actively pumps sodium ions out of the axon, whilst potassium ions can diffuse back in

Action potential (Also known as: Depolarization ) → Na IN

• Stimulus had reached to threshold, this activate the neuron.

• Sodium- potassium pump is turned off

• Sodium ion channels open, this cause an influx of sodium ions back to the axon

• This reverse the potential difference to +40mV

Repolarization → K OUT

• Potassium channels open.

• Sodium channels close, sodium-potassium pumps closed

• Potassium ions diffuse out of the neuron down the concentration gradient

• Axon is negatively charged again

Hyperpolarization

• The potential difference is now more negative than -70mV

• No nerve impulses can be fired before the neuron returns to resting potential.

• Ensuring electrical impulse will only pass in one direction

→ The sodium-potassium pump return the cell to its resting potential, prepared for a new action potential.

saltatory conduction → Increase nerve impluse

• The Schwann cell produced myelinated sheath and contains fatty substances → wrap axon with it

• Insulate electrical impulses

• As only the node of Ranvier is the site of depolarization and the electrical impulse will jump from node to node.

• By doing so, this speeds up the electrical conduction along a neuron

• This electrical conductivity is called saltatory propagation

factors affecting speed of impulse

1) Temperature

• The higher the temperature, the faster the speed

• Warm-blooded animals have faster responses than cold-blooded ones

2) Axon diameter

• The larger the diameter, the faster the speed ( less resistance )

• Marine invertebrates, which live at temperatures close to 0°C, have developed thick axons to speed up their responses.

3) Schwann cells

• The Schwann cell is made of a fatty substance called the myelinated sheath.

• It is used to insulate electrical impulses

• By doing so, this speeds up the electrical conduction along a neuron

Action Potential 

1.  Signal from pre-synaptic neuron causes some Na⁺ to enter the post-synaptic neuron

2. If threshold potential is reached, voltage-gated sodium channels open, causing an influx of sodium ions

3. Membrane potential becomes positive (depolarization)

4. Ion movements from the depolarised section of the nerve fibre depolarize the next part of the fibre

5. At the peak of the action potential, voltage-gated sodium channels close and voltage-gated potassium channels open, causing an efflux (moving out) of potassium ions

6. A negative membrane potential is restored (repolarization)

7. The sodium-potassium pump restores membrane potential by re-establishing the concentration gradients of potassium and sodium ions

8. There is a refractory period after depolarization, where the membrane cannot immediately depolarize again. This ensures that action potentials do not propagate backwards along the neuron.

Propagation of nerve impulses as a result of local current

• An action potential in one part of the axon triggers an action potential in the next part, called propagation of the nerve impulse.

• Na⁺ ions diffuse from a region with an action potential to the next region that is still at the resting potential.

• Diffusion of Na⁺ ions inside the membrane causes local currents.

Local Current:

• voltage gated sodium channel opens

• Diffusion of Na⁺ ions

• It changes the voltage across the membrane from the resting potential (-70mV) to the threshold potential (-50mV), causing an action potential.

synapse

A signal can only pass across a synapse in one direction - from the presynaptic neuron to the postsynaptic neuron

• This is the gap between two neurons.

• The messenger is in chemical form called neurotransmitter.

Synapse - neurotransmitter

1. An electrical impulse arrived at the end of the presynaptic neuron. → Depolarisation of presynaptic terminal→ opens calcium ion channel → influx of calcium

2. Calcium trigger release of neurotransmitters from vesicles

3. A neurotransmitter is released to the synapse by exocytosis

4. Neurotransmitters diffuse across the synaptic

5. Neurotransmitter reached and binds to the postsynaptic receptor

6. This will cause the sodium ion channel to open, which depolarizes the postsynaptic neuron and can generate a new electrical impulse

excitatory and inhibitory channels

excitatory:

• neuroreceptors that are sodium (Na⁺) channels

• channels open, positive ions diffuse in, causing a depolarization

• excitatory postsynaptic potential (EPSP) -stimulate an action potential

acetylcholine, glutamate.

inhibitory 

• neurotransmitter that are chloride(Cl-) chanels

• channel open - negative ion diffuse in - hyperpolarization 

• inhibitory postsynaptic potential - supress action potential 

• impluse in one neuron inhibit the impluse in the next

GABA

acetylcholine - neurotransmitter 

exist in many synapse - inculding neuromusular junctions 

• break down ACh in synaptic cleft

• Ch ( choline) - absorb in presynaptic neuron - regenerate ACh

Effects of exogenous chemicals on synaptic transmission

• Exogenous chemicals: Any chemical substance that alters the physiological state of a living organism.

• There are 2 types of psychoactive drugs:

  • Excitatory: Cocaine

  • Inhibitory: Such as alcohol, THC

Drugs can affect the synapses in the following ways:

1. Mimic the neurotransmitter and act as a competitive inhibitor → neonicotinoids & acetylcholine

2. Prevent the breakdown or re-uptake of neurotransmitter. Therefore, constantly activate the receptor → cocaine

3. Prevent the release of neurotransmitter → GABA

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block synaptic transmission: neonicotinoids → blocks acetylcholine

promote synaptic transmission: cocaine → blocks dopamine reuptake

→ Both are excitatory

Neonicotinoids → Effects of exogenous chemicals on synaptic transmission

Normal Neurotransmitter:

• acetylcholinsterase break down acetylcholine

• Prevent overstimulation & blockage of acetylcholine receptor 

Neonicotinoids Toxicity:

• Neonicotinoid pesticides bind to acetylcholine receptors in the post-synaptic membranes (of cholinergic synapses in insects)

• Acetylcholinesterase cannot break down neonicotinoids → blockage of acetylcholine receptor 

• Therefore blocking the synaptic transmission, this ultimately kills the insect.

• Honeybees are killed along with insect pest that are the intended target of neonicotinoids

how cocaine works 

• bind to dopamine reuptake receptor → inhibit reuptake of dopamine 

• dopamine accumulation in synaptic cleft 

• Increase the likelihood to Parkinson disease because the dopamine receptors loses the sensitivity to dopamine

• This also explains why cocaine users will need to increase dosage

inhibitory neurotransmitter - GABA 

1. The binding of GABA opens ligand-gated Cl^- channels

2. This makes the postsynaptic membrane potential more negative (hyperpolarized)

3. This hyperpolarization makes it harder for the postsynaptic neuron to reach threshold potential, therefore inhibiting nerve impulses

Summation of the effects of excitatory and inhibitory neurotransmitters in a postsynaptic neuron

• The sum of the excitatory and inhibitory potentials

• Summation combines the effects of excitatory and inhibitory potentials to determine whether the postsynaptic neuron will fire

• This is a result of the levels of inhibitory and excitatory neurotransmitters released into the synaptic cleft influencing the postsynaptic membrane potential

Perception of pain by neurons with free nerve endings in the skin

• The impulse from the sensory neuron travels to the CNS (central nervous system)

1. Sensory regions of the cerebral cortex (brain) enable us to ‘feel’ the sensation of pain 

2. The prefrontal cortex allows us to evaluate the situation

3. Protective reflex reactions, like retracting our hand from a hot surface, do not involve the brain [spinal reflex arc]

Consciousness as a property that emerges from the interaction of individual neurons in the brain

Consciousness is broadly defined as awareness.

• It is an example of an emergent property, as a single neuron does not have awareness, consciousness arises from the interaction of many individual neurons*.