IB Bio Topic 6.5 - Neurons and Synapses

Neuron

another term for a nerve cell that transmits signals as electrical impulses

Myelin Sheath

An insulating layer surrounding the axons of some neurons. They speed up the transmission of a nerve impulse

Schwann Cells

Cells of mostly lipids (with some proteins). They are responsible for making the myelinated sheath.

Axon

The part of a neuron along which the nerve signal travels (toward the terminal)

Node of Ranvier

The gap between adjacent Schwann cells

Saltatory Conduction

Refers to myelination speeding up the rate at which signals travel across a neuron by forcing them to jump between nodes of Ranvier.

Continuous Conduction

Refers to the form of conduction for nerve signals on neurons that are not myelinated. It is slower than saltatory conduction.

Cell Body

where the nucleus of the neuron is located

Resting Potential

the resting electrical imbalance between the inside and outside of a neuron resulting from the gradient of sodium and potassium ions across its membrane. There are more sodium ions outside the neuron and more potassium ions inside the neuron

Membrane Potential

the broad term for the electrical difference across the membrane of a cell. In resting nerve cells, this value is -70 mV because the membrane is more permeable to potassium ions (K) and also pumps out three Na⁺ ions per two K⁺ ions for each turn of the sodium potassium pump.

Depolarization

refers to stimulated dendrites causing voltage-gated Na+ (sodium ion) channels to open in a neuron, resulting in the rapid diffusion of sodium ions down a concentration gradient into the neuron

Repolarization

refers to the opening of voltage-gated K+ (potassium ion) channels opening when the membrane potential reaches +30 mV

Hyperpolarization

refers to a brief period after repolarization where the membrane potential is at a lower value than the resting potential

Refractory Period

refers to a period following hyperpolarization where a neuron’s Na+ channels are deactivated, preventing the cell from being depolarized

Action Potential

the membrane potential of a neuron during depolarization and repolarization. It notably peaks at +30 mV

Oscilloscope

a device that registers electrical voltages and displays them on a screen as time passes

Threshold Potential

the minimum level to which a membrane potential must be depolarized to trigger an action potential. It varies between -55 mV and -40 mV

All or Nothing Principle

The principle that aneuron MUST be depolarized to reach above around -50 mV for action potential to be initiated. Otherwise, nothing happens.

Synapse

a structure connecting a neuron to a gland, muscle, sensory cell (receptor), or another neuron

Presynaptic Neuron

Refers to the neuron before the synapse

Postsynaptic Neuron

Refers to the neuron after the synapse

Neurotransmitters

chemicals that diffuse from the presynaptic neuron to the postsynaptic neuron. They may cause action potentials in postsynaptic neurons, allowing nerve signals to travel between multiple neurons

Postsynaptic Potentials

the possible changes in the membrane potential of a postsynaptic neuron due to neurotransmitters

Excited

Refers to when neurotransmitters cause a cell’s membrane potential to increase (depolarized)

Inhibited

Refers to when neurotransmitters cause a cell’s membrane potential to decrease (hyperpolarized)

Acetylcholine

a neurotransmitter often found in synapses connecting nerves and muscle fibers. It is notable for its secretion ensuring action potential is propagated to the postsynaptic neuron.

Choline Acetyltransferase (Choline Acetyl Transferase)

An enzyme that combines an acetyl group with choline to form acetylcholine, which is stored in vesicles in presynaptic neurons

Acetylcholinesterase

An enzyme that breaks down acetylcholine into acetyl and choline after it is secreted from the presynaptic neuron, so they can be reabsorbed and reused.

Modulation

refers to synapses’ function of controlling the intensity and duration of the nerve signals they send

Neonicotinoids

a group of compounds that are widely used as insecticides due to their ability to prevent nerve signals of insects from being propagated to their postsynaptic nerves

Cholinergic Synapses

any synapse that uses acetylcholine as its neurotransmitter