6.5 IB Biology- Neurons and Synapses

outline the mechanism of neuron depolarisation

In response to signals initiated by dendrites, sodium channels open within the membrane of the axon. As Na+ ions are more concentrated outside of the membrane, the opening of the sodium channels creates a passive influx of sodium ions. the influx of sodium ions causes the membrane potential to become more positive.

describe how nerve impulses are propagated through along the neuron axon

Depolarisation occurs when ion channels open up and cause a change in membrane potential. the ion channels that occupy the length of the axon are voltage gated (open as response to changes to membrane potentials) Hence depolarisation at one point of the axon triggers the opening of ion channels in the next segment of the axon. This causes depolarisation to spread along the length of an axon as an 'undirectional wave."

Compare the speed of nerve impulse conduction myelinated and non-myelinated neurons.​

In myelinated neurons, the action potential propagate sequentially along the nodes of Ranvier. in unmyleinated sheaths, action potentials propagate sequentially along the axon in a continuous wave of depolarisation.

State the role of neurotransmitters.​

Neurotransmitters are chemical messengers released from neurons and function to transmit signals across synaptic cleft.

Outline the mechanism and benefit of saltatory conduction.

Saltatory condition increases the speed of electrical transmission by upto 100 fold.

Define synapse, synaptic cleft and effector.

Synapse: physical gaps that separate neurons from other cells. Synaptic cleft: neurotransmitters are released and try to cross the synaptic cleft. Effector: other cells, not neurons.

outline causes and consequences of the refractory period after depolarisation

The causes of refractory period, refers to the time following a nerve impulse before the neuron is able to fire again. In a normal resting state, sodium channels are predominantly outside the neuron and potassium ions mainly inside. Following depolarisation (sodium influx) and repolarisation ( potassium efflux), this ionic distribution is largely reversed. Before a neuron can fire again, the resting potential must be restored via the anti port action of the sodium potassium pump.

Explain movement of sodium ions in a local current.

The flow of Na+ ions from one region of the axon to the neighbouring region which then increases the charge of the neighbouring region so that its threshold is reached and its action potential occurs.

explain three mechanisms that together create resting potential in a neuron

The maintenance of a resting potential is an active process that is controlled by sodium potassium pumps. the sodium potassium pump is an active pump that actively exchanges sodium and potassium ions. it expels 3 Na+ ions for every 2 K+ ions admitted. This creates an electrochemical gradient whereby the cell interior is relatively negative compared to the extracellular environment. The exchange of sodium and potassium ions requires the hydrolysis of ATP.

Explain how the movement of sodium ions disseminates an action potential along an axon.

action potentials are generated within the axon.

define nerve impulse

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

state the voltage of the resting potential

approx= -70 m/v

define action potential

are rapid changes in charge across the membrane when a neuron is firing. They are when membrane potential of specific axon rapidly change.

State the function of the nervous system.

converts sensory information into electrical impulses within the nervous system.

Annotate a neuron drawing with the name and function of the following cell parts: dendrites, axon and cell body

dendrite: are short branched fibres that convert chemical information from other neurons or receptor cells into electrical signals. Axon: an elongated fibre that transmits electrical signals to terminal regions for communication with other neurons and effectors. Soma: A cell body containing the nucleus and organelles, where essential metabolic processes occur to maintain cell survival.

What are the stages of action potentials

depolarisation, re-polarisation and refractory period

Define membrane potential

difference in electric potential between the interior and the exterior of a biological cell.

outline the mechanism of neuron repolarisation

following the influx of sodium, potassium channels open within the membrane of the axon. As K+ ions are more concentrated within the membrane, opening potassium channels cause a passive influx of potassium ions. The efflux of potassium causes the membrane potential to return to a more negative internal differential.

define resting potential

is the difference in charge across the membrane when a neuron is not firing

define depolarisation

it is a rapid change in membrane potential usually from a relatively negative to positive internal charge.

State the role of Schwann cells in formation of myelin.

mylein is a mixture of proteins and phospholipids produced by schwann and glial cells.

define neuron repolarisation

refers to the restoration of a membrane potential following depolarisation.

Outline the mechanism of synaptic transmission, including the role of depolarization, calcium ions, diffusion, exocytosis, neurotransmitters, receptors, sodium ions, sodium channels, threshold potential and action potential.

synaptic transmission: depol: when action potential reaches the axon terminal, it triggers the opening of voltage gated calcium channels. Calcium 2+ ions diffuse into the cell and promote the fusion of vesicles containing neurotransmitters with the cell membrane. neurotransmitters are released from the axon terminal by exocytosis and cross the synaptic cleft. neurotransmitters bind to specific receptors on the post-synaptic membrane and open ligand-gated ion channels the opening of ion channels generates an electrical impulse in the post synaptic, propagating the pre-synaptic signal The neurotransmitters released into the synapse are either recycled (by reuptake pumps) or degraded (by enzymatic activity).

Describe that cause of and effect of membrane potential reaching the threshold potential.

the cause of threshold potential which is the minimal stimulus is the level required to open voltage gated ion channels. Threshold potentials are triggered when the combined stimulation from the dendrites exceeds a minimal level of depolarisation. the effect is that if the overall depolarisation from the dendrites is sufficient to activate voltage-gated ion channels in one section of the axon, the resulting displacement of ions should be enough to trigger activation of voltage gated ion channels in the next section of axon.

Outline the structure and function of myelin.

the mylein sheath surrounds the axon as an insulating layer. It improves the conduction of nerve impulses.

What is the dissemination of nerve impulses

the result of local currents that cause each successive part of the axon to reach the threshold potential.