IB Biology - 6.5 (Neurons and Synapses)

Axon

Carries nerve impulses away from the cell body.

Motor End Plate

Releases the neurotransmitter substances.

Neonicotinoids

- Neonicotinoid pesticides have the ability to irreversebly bind to nicotinic acetylcholine receptors and trigger a sustained response - Neonicotinoids cannot be broken down by acetylcholinesterase, leading to permanent overstimulation - Insects have a different composition of acetylcholine receptors which bind to neonicotinoids more strongly, meaning it is more toxic to themand thus they make a highly effective pesticide Evaluation: - Neonicotinoids have successfully protected crops from pest species - They have also been linked to a reduction in bird and honey bee populations - Consequently, there is a resticrted use of neonicotinoids in certain countries (including the EU)

Acetylcholine

- Used by the CNS and PNS - Released at neuromuscular junctions and binds to receptors on muscle fibers to trigger muscle contraction - Produced in the pre-synaptic neuron by combining choline with an acetyle group - After it is released it activated the post-synaptic cell by binding to a class of a specific receptor (nicotinic or muscarinic) - An overstimlation can lead to fatal convulsions and paralysis, so it continually removed from the synapse - The anzyme acetylcholinesterase breaks down acetylcholine to its two parts - The choline returns to the pre-synaptic neurons so acetylcholine can be reformed.

Synaptic Transmission

- Voltage-gated calcium ion (Ca²⁺) channels open when an action potential reaches the synapse, causing a calcium ion influx - The calcium ions cause the synaptic vesicles to fuse with the cell, releasing the neurotransmitter by exocytosis - The neurotransmitter diffuses across the synaptic cleft - The neurotransmitter binds to the neuroreceptors in the post-synaprtic membrane, which may initiate an action potential - The neurotransmitter is broken down by a specific enzyme in the synaptic cleft - The breakdown products of the neurotransmitter are re-absorbed by the pre-synaptic neuron through endocytosis to re-synthesize the neurotransmitter. Neurotransmitters either trigger (excitatory) or prevent (inhibitory) a response.

Neurotransmitter

A chemical substance produced and released by a neuron that passes across the synapse to carry a nerve impulse.

Myelin Sheath

A fatty white layer of tissue, made up of protein and phospholipids, which functions as an insulating layer and speeds up the transmission of nerve impulses along the axon

Synapse

A junction that physically separated a neuron from another cell (other neurons, receptor cells, or effector cells). Neurons transmit information across synapses by converting the electrical signal to a chemical signal.

Synaptic Cleft

A narrow gap (20 nm) between two neurons across which action potentials are transmitted.

Action Potential

A rapid change in membrane potential consisting of two phases, depolarization (a change from negative to positive) and repolarization (a change from positive to negative). Depolarization: - Voltage-gated Na⁺ ions open upon stimulation, allowing Na⁺ ions to diffuse into the neuron down hte concentration gradient. - This causes the charge imbalance to be reeversed, meaning the inside of the membrane is briefly positively charged, at about +30 mV Repolarization: - Occurs rapidly after depolarization due to the closing of Na⁺ ions and the opening of voltage-gated K⁺ channels - This causes K⁺ to diffuse out of the neuron, down the concentration gradient - This restores the negative charge of the membrane, lower than resting potential (-80 mV) Hyperpolarization: - Short refractory perios that occurs after repolarization, when an action potential cannot be generated - The Na⁺/K⁺ pump re-establishes the concentration gradient, thus re-establishing resting potential

Oscilloscope

A scientific instrument that is used to measure the membrane potential across a neuron membrane. Data is displayed in a graph with time (ms) on the x-axis and membrane potential on the y-axis. - At resting potential the neuron is at a state of rest - At depolarization, there is a rising spike in membrane potential - At repolarization, there is a falling spike in membrane potential - At the refractory period, membrane potential returns to a state of rest

All-or-none Principle

An action potential of the same magnitude will always occurs provided a minimum electrical charge.

Nerve Impulse

An electrical signal or impulse that is generated and conducted by a neuron through positively charged ions being pumped across the axon.

Cell Body

Contains the nucleus and most of the cytoplasm; there are extensions leading off of it.

Nodes of Ranvier

Junctions along the axon where two Schwann cells meet; assist in the propagation of nerve impulses

Schwann Cells

Line the axon and form the myelin sheath

Neuron

Nerve cell, the building block of the nervous system.

Sensory Neurons

Neurons with long axons that transmit nerve impulses from sensory receptors all over the body to the CNS.

Motor Neurons

Neurons with long axons that transmit nerve impulses from the CNS to effectors (muscles and glands) all over the body.

Dendrons and Dendrites

Provide a large surface area for neuron connections and carry nerve impulses towards the cell body.

Nervous Systen

Responsible for internal communication. Consists of nerve cells called neurons (about 85 million).

Interneurons

Smaller neurons with many interconnections that connect neurons with other neurons. Also known as connector neurons or relay neurons.

Membrane Potential

The difference in electrical charge/voltage between the interior and exterior of a neuron. Also known as membrane voltage.

Resting Potential

The electrical potential across the membrane of a neuron that is not conducting an impulse. Usually, the inside of the membrane is -70 mV. - Nerve cell membranes have several Na⁺/K⁺ pumps that help establish resting potential. - Na⁺ ions are pumped out while K⁺ ions are pumped in. - For every three Na⁺ ions that are pumped out, two K⁺ ions are pumped in, creating a concentration gradient for both ions - The membrane is more permeable to K⁺ than Na⁺, so K⁺ ions leak back faster than Na⁺ ions - The concentration gradient of Na⁺ is steeper than the concentration gradient of K⁺, which creates a charge imbalance - The presence of organic negatively charged ions increases the change imbalance

Threshold Potential

The minimum membrane potential required to open voltage-gated ion channels that initiate an action potential. Usually at -55 mV. - Threshold potentials are triggered when the combined stimulation from dendrites exceeds a minimum level of depolarization - If the overall depolarization from the dendrites is sufficient to activate voltage-gates ions channels in one section of the axon, the resulting displacement of ions will tirgger the activation of the channels in the next axon section.

Myelanation

The ymelin sheath shows up as white matter while neruronal cell bodies, dendrites, synapses, and support cells show up as grey matter. - Myelanation improves speed of propagation - Myelanation takes up significant spave

Propagation of Nerve Impulses

Unmyelinated Neurons: The action potential is propagated sequeantially along the axon in a continuous wave of depolarization Myelinated Neurons: The action potential is propagated via saltatory conduction, meaning the action potential "hops" from one Node of Ranvier to the next.