Question

A neuron has three main parts: dendrites, cell body, and axon. A neuron may send signals to other neuron or other tissues (effectors). Describe the physiology of how the signal moves from one point to the other and the other organ or tissue responds.

Answer 1

The process of sending these signals takes place in two steps: along the cell (action potential) and between cells (neurotransmitters).

• Information is sent as messages called action potentials

• Action potentials travel down a single neuron cell as an electrochemical cascade, allowing a net inward flow of positively charged ions into the axon ie sodium ions .

• Within a cell, action potentials are triggered at the cell body, travel down the axon, and end at the axon terminal

• The axon terminal has vesicles filled with neurotransmitters ready to be released

• The space between the axon terminal of one cell and the dendrites of the next is called the synapse.... It acts as a coonection between two neurons or one neuron and effector organ....

• Chemicals known as neurotransmitters are stored in membrane-bound vesicles at the axon terminal of neurons

• Membrane proteins on the vesicle bind membrane proteins at the axon terminal to tether the vesicles in place

• A protein known as complexin acts like a brake, and stop the vesicles from fusing into the membrane and releasing their contents [remember: complexin complicates the process of vesicle fusion]

• The vesicle protein synaptotagmin can bind and release complexin in the presence of calcium

As the action potential travels down the axon, positive ions continue to flood the cell. Eventually, this influx reaches the very end of the neuron – the axon terminal. When this happens, the positive ions trigger voltage-gated calcium channels to open and let calcium ions into the cell. The calcium ions can then activate synaptotagmin to release the brake, and the vesicles fuse with the cell membrane, and the vesicle contents are released into the synaptic cleft...

Excitatory neurotransmitters cause the signal to propagate - more action potentials are triggered. Inhibitory signals work to cancel the signal....

• The major excitatory neurotransmitter is glutamate

• The major inhibitory neurotransmitter is GABA (gamma-aminobutyric acid)

The post-synaptic terminal are the dendrites and cell body of a cell. Dendrites are projections specially designed to create more surface area and receive more information.

• The post-synaptic terminal is full of neurotransmitter receptors

• The neurotransmitter receptors are, for the most part, ligand-gated ion channels, that open in response to being bound by the neurotransmitter they are specific for

• Whether a neurotransmitter is excitatory or inhibitory depends on the type of channel they open – excitatory neurotransmitters bind channels that let in positive ions like Na^++start superscript, plus, end superscript, inhibitory neurotransmitters open up Cl^-−start superscript, minus, end superscript channels

• Some receptors set off a signal cascade when activated, and lead to the transcription of new proteins or the insertion of more receptors into the cell membrane

Each vesicle packet released from the pre-synaptic neuron will contain a set amount of neurotransmitters, which will then bind to some of the receptors on the post-synaptic cell. If the neurotransmitter is excitatory, the influx of positive ions will depolarize (bring closer to zero) the cell body. If the neurotransmitter is inhibitory, it will hyperpolarize the cell body. However, a single vesicle of neurotransmitter isn’t enough to depolarize the cell body. Most of the time, all the vesicles released from an action potential aren’t enough to trigger an action potential in the following cell. This is why the brain uses neuron networks to send many signals to a single cell, or why a neuron may have to fire a couple times before it can pass the message along. There might even be competition among the neurons, with a single post-synaptic neuron receiving glutamate from one pre-synaptic neuron and GABA from its neighbor. The post-synaptic cell will only send the message along if it gets enough excitatory input to depolarize across the threshold, and open the voltage-gated ion channels in its axon....

For example- skeletal muscular contraction-

when u initiate an action

action potential is generated this will travel down the nerve supplying the muscle to be contracted

this action potential on reaching the motor end plate release acety choline neurotransmitter fron presynaptic neuron...

acetyl choline is released in synaptic cleft and bind to post synaptic acety choline receptor present on muscle membrane..

this binding opens the sodium gated channel and influx of sodium leads to release of calcium from sarcoplasmic reticulum...

this calcium binds to actin and myosin leading to muscular contraction..