C2.2 Neural Signalling

A neuron consists of…

The cell body (soma), one or more dendrites, and a single axon

The cell body (soma) has…

A nucleus and cytoplasm. It lacks centrioles because neurons do not multiply

Dendrites

Short and branched nerve fibres that project from the cell surface. They receive signals from other neurons

Axon hillock

A conical projection that connects the cell body with the axon. It is the site where an action potential is generated

Axon

A long single fibre moving away from the cell body

Schwann cell

A type of glial cell in the PNS that wraps around nerve fibers, producing the myelin sheath

What is myelin made up of?

Lipids (75%) and proteins (25%)

Myelin sheath

An insulating layer that wraps around parts of the axon

In the CNS, what is responsible for secreting myelin?

Oligodendrocytes

In the PNS, what is responsible for secreting myelin?

Schwann cells

Nodes of Ranvier

The unmyelinated gaps between schwann cells

Axon terminal (synaptic knob)

The end of an axon which transmits messages to other cells via the use of neurotransmitters

Resting membrane potential

The potential difference across a nerve cell membrane at rest. Approximately -70mV

What are the concentrations of potassium and sodium ions in an unstimulated neuron?

Potassium ions are maintained at high concentrations within the cell while sodium ions are maintained at high concentrations outside of the cell

Action potential

An all or nothing sequence of changes in membrane potential, has 2 main phases: depolarisation and repolarisation

Membrane potential

A measure of the difference in charge between the inside and outside of a neuron, created due to the unequal distribution of ions on both sides of the cell membrane

How does the presence of the myelin sheath speed up the transmission of signals?

It causes the nerve impulse to jump from one node of Ranvier to another

What does the sodium-potassium pump do?

It helps to transport sodium ions out of the cell and potassium ions into the cell via active transport

Active transport

A type of cell transport that requires the input of energy in the form of ATP

What happens in the process of active transport?

The ATPase enzyme breaks down ATP into ADP

Each time this happens, 3 sodium ions are pumped out in exchange for 2 potassium ions that enter into the cell against the concentration gradient

Due to the unequal movement of ions, the extracellular fluid has a higher concentration of positive charges than the intracellular fluid

A negative charge is developed inside the neuron and a positive charge is developed outside

Propagation

The passage of a nerve impulse through an axon

Nerve conduction velocity

The speed at which a nerve impulse travels down a neuron

What factors affect the speed of signal transmission?

Amount of myelination

Diameter of an axon

Temperature

Myelination

Insulation by the wrapping of Schwann cells around an axon

How does myelination affect the speed of impulse transmission?

When the neuron is myelinated, the action potential has to jump from node to node, covering a longer distance in a short time (saltatory conduction). When the neuron is unmyelinated, the action potential has to travel the entire length of the axon

How does the diameter of an axon affect the speed of impulse transmission?

In axons with a wider diameter, there is less leakage of ions, resulting in faster generation of action potential. In axons with a smaller diameter, the ions face a lot of resistance from other molecules get delayed in transmitting the impulse

Saltatory conduction

The method by which nerve impulses move down a myelinated axon with excitation occurring only at nodes of Ranvier

How does temperature affect the speed of impulse transmission?

The hotter the temperature, the faster the transmission of nerve impulses, due to an increase in kinetic energy

The synapse consists of…

The presynaptic neuron, synaptic cleft (20nm), and the postsynaptic neuron

Why is resting membrane potential negative?

Because there is an unequal distribution of ions across the membrane, causing the inside of the neuron to be negatively charged

Neuromuscular junction

A synapse between a motor neuron and a muscle fibre

Neuroglandular junction

A synapse between a neuron and glandular cells

Neuronal synapses

A synapse between two neurons, eg. synapses in the brain, spinal cord and peripheral nerves

Depolarisation

The cell membrane's charge becomes positive. This is caused by positive sodium ions going into the cell

Repolarisation

The cell membrane's charge returns to negative. This is caused by positive potassium ions moving out of the cell

The process of synaptic transmission

A nerve impulse (action potential) reaches the terminal end of the presynaptic neuron

Depolarisation causes voltage gated calcium channels to open. Calcium ions rush into the neuron

The calcium ions cause synaptic vesicles containing a neurotransmitter to move to the membrane and fuse

The neurotransmitter rapidly diffuses across the synaptic cleft

The neurotransmitter binds with a neuroreceptor (protein channels) in the postsynaptic membrane

Upon binding, the protein channels of the receptors open, allowing the influx of sodium ions

An excitatory action potential is initiated, and the nerve impulse is propagated along the postsynaptic neuron

Threshold potential

The level to which a membrane potential must be depolarised to initiate an action potential

Refractory period

A brief period where a neuron is unresponsive to another stimulus, it ensures that the action potential moves in a unidirectional manner

Oscilloscope

A graph-displaying device that draws a graph of an electrical signal. It is used to measure the membrane potential across an axon membrane

Exogenous compounds

Substances which enter the body from an outside source through ingestion, inhalation or absorption through the skin

Neonicotinoids

A class of chemicals similar to nicotine, they completely block synaptic transmissions by binding to acetylcholine receptors

Effect of cocaine on humans

Blocks the transfer of dopamine by binding to dopamine reuptake transporters. Dopamine builds up in the synaptic cleft causing the postsynaptic neuron to receive amplified signals

Hyperpolerisation

A change in the cell’s membrane potential that makes it more negative than its resting potential

What is hyperpolerisation caused by?

An efflux of potassium ions or an influx of chloride ions. This results in an increased polarisation of the cell membrane, making it more difficult to depolarise

What are examples of excitatory neurotransmitters?

Acetylcholine, glutamate, dopamine

What are examples of inhibitory neurotransmitters?

GABA, dopamine

Summation

The combined effect of the excitatory and inhibitory stimuli that are received from presynaptic neurons and transmitted to the postsynaptic neuron

Transmission of pain

Neurons with free nerve endings in the skin have specific ion channels that respond to stimuli. These channels can be triggered by factors like high temperature, acid or certain chemicals like capsaicin

When these channels open, positively charged ions flow into the neuron

When the threshold potential is reached, the impulse propagates along nerve fibres to the brain

Pain is perceived and processed

Consciousness

An example of an emergent property

Which area of the brain is responsible for consciousness?

Cerebral cortex

How do excitatory neurotransmitters affect action potential?

They increase the likelihood that the neuron will fire an action potential

How do inhibitory neurotransmitters affect action potential?

They decrease the likelihood that the neuron will fire an action potential