6.5 - Neurones and Synapses

electricity vs nerve impulse

An impulse is always carried from the dendrite end of a neurone along the membrane of the cell body down the axon, and results in a release of a neurotransmitter. The impulse does not travel in the opposite direction because neurotransmitter molecules cannot be released from the dendrite end of neurones, and the 'message' would simply stop at that point.

Nerve impulse; resting potential

when it is not sending an impulse. https://i.imgur.com/1H8QH7L.png

motor response

If a response is needed, some portion of the brain or spinal cord initiates a response that is called a motor response.

Action potential reaches terminal buttons

1. Action potential results in calcium ions (Ca2+) diffusing into the terminal buttons 2. Vesicles containing the neurotransmitter fuse with the plasma membrane and release the neurotransmitter. 3 The neurotransmitter diffuses across the synaptic gap (or cleft) from the presynaptic neurone to the postsynaptic neurone. 4 The neurotransmitter binds with a receptor protein on the postsynaptic neurone membrane. 5 This binding results in an ion channel opening and sodium ions diffusing in through this channel. 6 This initiates the action potential to begin moving down the postsynaptic neurone because it is now depolarized (the action potential is now self-propagating). 7 The neurotransmitter is degraded (broken into two or more fragments) by a speci c enzyme(s) and neurotransmitter is released from the receptor protein. 8 The ion channel closes to sodium ions . 9 Neurotransmitter fragments diffuse back across the synaptic gap to be reassembled in the terminal buttons of the presynaptic neurone (often called reuptake) v

Threshold potential

A nerve impulse is only initiated if the threshold potential is reached.

action potential

An action potential consists of depolarization and repolarization of the neurone

Depolarisation ; sending impulse

An action potential is often described as a self-propagating wave of ion movements in and out of the neurone membrane. The movement of the ions is not along the length of the axon, but instead consists of ions diffusing from outside the axon to the inside, and from inside the axon to the outside. The resting potential requires active transport (the Na/K pump) to set up a concentration gradient of both sodium and potassium ions. As sodium ions are actively transported to the outside of the membrane, they diffuse in when a channel opens. This diffusion of sodium ions is the 'impulse' or action potential, and results in the inside of the axon becoming temporarily positive in relation to the outside. It is a nearly instantaneous event that occurs in one area of an axon, and is also called a depolarization. This depolarized area of the axon then initiates the next area of the axon to open up the channels for sodium, and thus the action potential continues down the axon. This is the self-propagating part of an action potential; once you start an impulse at the dendrite end of a neurone, that action potential will self-propagate to the axon end of the cell, where the synaptic terminals are located. https://i.imgur.com/2ie5b7J.png

Cranial nerves

Cranial nerves emerge from an area of the brain known as the brainstem. One well known example is the optic nerve pair, which carries visual information from the retina of the eyes to the brain. There are 12 pairs of cranial nerves.

Spinal nerves

Spinal nerves emerge directly from the spinal cord. They are mixed nerves, as some of the neurones within them are sensory and some are motor. There are 31 pairs of spinal nerves.

explain achieving minimum threshold in depolarisation

Each action potential must reach a minimum threshold in order to be self-propagated. This begins at the first receptor neurone that began the chain of events. A receptor neurone is a neurone that is modified to begin the sequence of events by transducing (converting) a physical stimulus of some kind into the first action potential. For example, some of the cells that make up the retina of your eyes are receptor cells. Each type of retinal cell has a minimum physical stimulus magnitude that is required in order to begin the impulse. For some retinal cells this is a minimum intensity of light. If that minimum intensity is not reached, no action potential begins. If the minimum is reached, an action potential is initiated and begins to self-propagate. There is no such thing as a strong impulse or a weak impulse: if the minimum threshold for that type of receptor is reached, an impulse begins. When a nerve impulse is being self-propagated along a neurone, that is happening because each successive area of the neurone membrane has reached its threshold and is causing the next area of the membrane to also reach its threshold. https://i.imgur.com/2ie5b7J.png

Saltatory conduction by neurones that have a myelin sheath

Many neurones of an organism with an advanced nervous system have axons with a myelin sheath; they are said to be myelinated. Saltatory conduction is the term used to describe the phenomenon whereby an action potential of myelinated axons skips from one node of Ranvier to the next as the impulse progresses along the axon towards the synaptic terminals. - The impulse travels much faster compared with an impulse in non-myelinated bres, because the in/out ion movements characteristic of an impulse take time, and saltatory conduction allows areas of the membrane to be skipped. This is very important for the ef cient neural processing characteristic of organisms with a high functioning nervous system. - Less energy in the form of ATP is expended for the transmission of impulses, as the only locations where the Na/K pump needs to re-establish resting potentials is at the nodes of Ranvier.

A new class of insecticides based on blocking synaptic transmission

Neonicotinoid insecticides are a relatively new class of insecticide that are chemically similar to nicotine. This type of insecticide works by binding to postsynaptic receptors that normally accept the neurotransmitter acetylcholine. When acetylcholine binds to the receptor protein, the result is the normal continuation of the action potential along the postsynaptic neurone. When neonicotinoid molecules bind to the same receptor proteins, the action potential is not propagated. In addition, the neonicotinoid molecules are not broken down by the enzyme acteylcholinesterase and thus the receptor becomes permanently blocked. This leads to a paralysis of the affected insect, and eventually death.

Nerve impulse

Nerve impulses are action potentials propagated along the axons of neurones. An impulse is always carried from the dendrite end of a neurone along the membrane of the cell body down the axon, and results in a release of a neurotransmitter. The impulse does not travel in the opposite direction because neurotransmitter molecules cannot be released from the dendrite end of neurones, and the 'message' would simply stop at that point.

Repolarisation : returning to resting potential explained

Neurones do not send just one action potential; one neurone may send dozens of action potentials in a very short period of time. When one area of an axon has opened a channel to allow sodium ions to diffuse in, that area cannot send another action potential until ions have been restored to the positions characteristic of the resting potential. Diffusion cannot do this, thus active transport is required to pump ions to their resting potential positions. immediately following an action potential (depolarization), membrane proteins open to potassium ions and allow them to diffuse out of the axon. This is the first step of repolarization because it separates many of the sodium and potassium ions on different sides of the membrane. The problem is that these two ions are on the opposite side of the membrane in relation to where they need to be for the resting potential. The good news is that they are now in a position that allows the Na/K pump to once again begin actively transporting them across the membrane at the ratio characteristic of this pump (three sodium ions pumped out for every two potassium ions pumped in). This entire series of events, beginning with potassium ions diffusing out of the localized area of the membrane, is called repolarization. All of this is necessary for that local area of the membrane to be ready to send another impulse. https://i.imgur.com/9DviLGT.png

sodium / potassium ions in neurones

Neurones pump sodium and potassium ions across their membranes to generate a resting potential.

Propagation of nerve impulses

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

Synapses

Synapses are junctions between neurones and between neurones and receptor or effector cells. When presynaptic neurones are depolarized they release a neurotransmitter into the synapse.

Synapse motor neurone

Synapses can also occur where a motor neurone adjoins muscle tissue. This type of synapse is called a motor end plate or neuromuscular junction. The mechanism for this type of synapse is almost the same as a neurone-neurone synapse, although the end result leads to the muscle undergoing a contraction. Another place for a synapse is between a receptor neurone (cell) of the nervous system and the first sensory neurone.

Na/K Pump

The Na/K pump works by transporting three sodium ions 'out' for every two potassium ions 'in'. In addition, there are negatively charged organic ions permanently located in the cytoplasm of the axon

CNS

The brain and spinal cord comprise the central nervous system (CNS)

myelination

The myelination of nerve fibres allows for saltatory conduction. The myelin sheath is actually a series of cells, called Schwann cells, that have each wrapped themselves around the axon multiple times, creating multiple layers of the same cell membrane. The Schwann cells are spaced evenly along any one axon, with small gaps between them; these gaps are called nodes of Ranvier.

Resting potential explained

The resting potential is created by the active transport of sodium ions (Na+) and potassium ions (K+) in two different directions. The vast majority of the sodium ions are actively transported out of the axon cell into the intercellular fluid, and the majority of the potassium ions are transported into the cytoplasm. This active transport of sodium and potassium in opposite directions is an active transport mechanism called the sodium-potassium (Na/K) pump. The net result of the position of the charged ions leads to a net positive charge outside the axon membrane (positive in relation to the inside) and a net negative charge inside the axon membrane

Neurones explained

The three main subparts of a single neurone are its dendrites, cell body and axon. At the end of the axon are synaptic terminal buttons, which release chemicals called neurotransmitters that continue the impulse chemically to the next neurone(s) or possibly a muscle

Resting potential

The time period during which an area of a neurone is ready to send an action potential, but is not actually sending one, is called the resting potential, and this area of the neurone is said to be polarised

Weak vs strong impulse

There is no such thing as a strong impulse or a weak impulse: if the minimum threshold for that type of receptor is reached, an impulse begins.

Nerve

When many individual neurones group together into a single structure, that structure is called a nerve

Synapses: chemical communication between neurones

When one neurone communicates with another, the communication is chemical and occurs where two (or more) neurones adjoin each other in an area called a synapse. The two neurones always align with each other so that the axon's synaptic terminals of one neurone adjoin the dendrites of another neurone The chemical, called a neurotransmitter, is always released from the synaptic terminal buttons of the first neurone, and typically results in a continuation of the impulse when the neurotransmitter is received by the dendrites of the second neurone. The neurone that releases the neurotransmitter is called the presynaptic neurone, and the receiving neurone is called the postsynaptic neurone. At the distal end of axons, as part of the synaptic terminals, are swollen membranous areas called terminal buttons. Within these terminal buttons are many small vesicles lled with the chemical neurotransmitter. There are many examples of neurotransmitters; a very common example in humans is acetylcholine.

Neurone voltages LEARN

https://i.imgur.com/L6jT2Vs.png

Draw neurones

https://i.imgur.com/aplKz4q.png

Nerve impulse carried along a neurone explaiend

the individual neurones within the nerve are each capable of carrying the impulse. The axons of neurones in some organisms (including humans) that have a very highly developed nervous system, have surrounding membranous structures collectively called the myelin sheath. The myelin sheath greatly increases the rate at which an action potential passes down an axon. In order to study the nature of an action potential, it is best to study an axon that does not have a myelin sheath, otherwise known as a non-myelinated neurone.

Neurones

transmit electrical impulses Sensory neurones bring information in to the CNS, and motor neurones carry response information to muscles.