What is the difference between membrane irritability and membrane conductivity

They are masses of rough endoplasmic reticulum involved in the synthesis of neuronal proteins. They occur in clumps that are bounded from the rest of the cytoplasm by neurofibrils - portions of the cytoplasmic cytoskeleton. Although mature neurons don't divide they are extremely metabolically active, as evidenced by the amount of Nissl substance they contain.

It is easy to understand this in that the protein synthesis occurring in the cyton must supply the cyton itself with needed materials and also the long neuronal fibers that extend outward from the cyton. Dendrites - extensions of the cyton that are generally short and numerous. They possess many synapses on their surface and conduct signals, emanating from other neurons, into the cyton.

Dendrites, like the cyton, have no myelin sheath. Axon - a single extension of the cyton arising from a slightly swollen area on the cyton called the axon hillock. The axon may vary in length from a few millimeters to a meter and carries nerve impulses away from the cyton toward other neurons, muscles or glands.

Its membrane is called the axolemmaand its contents are referred to as axoplasm. If an axon is myelinated, the Schwann cell wraps itself around the axon, as much as times, with each Schwann cell sequentially occupying an adjacent portion of the length of the axon. The tight Schwann cell wrappings, encircling the axon, are called the myelin sheath. The myelin sheath speeds up nerve impulse conduction along the axon. Areas covered by myelin are called internodes.

Areas between Schwann cell- wrapped regions possess no myelin and are called Nodes of Ranvier. Unmyelinated axons in the PNS are still covered by Schwann cells, just without the multiple coils of membranous wrapping. The oligodendrocyte forms myelin sheaths in the CNS. It is believed that oligodendrocytes do not support regeneration of neurons, whereas Schwann cells do: this may be the reason that neuronal regeneration after damage is possible in the PNS peripheral nerves , but is not commonly observed in the brain or spinal cord.

This overall process of movement of materials within the axon is called axon transport or axon flow. The power for this movement along the neurotubules is based on the activity of two motor proteins: kinesin powers anterograde movements and dynein powers retrograde movements.

Sodium-Potassium Pump Action: a pump called the sodium-potassium pump in the cell membrane of a neuron uses the energy of ATP to pump potassium ions into the neuron and pump sodium ions out of the neuron. Chloride ions follow sodium and phosphates and negatively charged intracellular proteins tend to pair up with potassium. So, neurons are bathed in a sodium chloride solution on the outside and a potassium phosphate solution on the inside. Other ions are present and are important for certain activities, but these ions are most important for the membrane potentials.

For each ATP molecule utilized, the sodium-potassium pump pulls 2 potassium ions into the cell and throws 3 sodium ions out of the cell.

Permeability of the cell membrane: the cell membrane permits the diffusion of relatively large amounts of potassium ions out of the cell and only a small amount of sodium ions into the cell. These diffusive movements are simply due to these ions moving down their concentration gradients after their active transport by the sodium-potassium pump..

Development of the Resting Potential: The net charge on either side of the neuronal cell membrane is called the resting potential and is experimentally measured as about millivolts as measured from the inside where it is negative.

This resting membrane potential RMP is due mainly to the the outward diffusion of potassium ions that result in a net positive charge developing on the outside of the cell membrane potassium carries a single positive electrical charge and a net negative charge on the inside of the cell membrane due to the negative phosphate ions and proteins left inside when potassium diffused out of the cell. The permeability properties of the membrane do not allow phosphates or proteins to follow potassium when it diffuses out of the cell.

If the outward diffusion of potassium ions were the only influence on the RMP, it would be about mv. But there are 2 other influences: 1 The unequal pumping action of the sodium-potassium pump: the pump brings only 2 positively charged potassium ions into the cell for each 3 positively charged sodium ions it ejects.

The result is that the pump tends to make the RMP about -3mv lower than would be if the pump were not working. So if the outward diffusion of potassium and the sodium-potassium pump were the only factors at work in the establishment of the RMP, then the RMP value would be about mv.

This effect of sodium is not as strong as might be expected because some negatively charged chloride ions accompany sodium when it diffuses into the neuron. So the net result of potassium diffusing out of the neuron, sodium and chloride diffusing into the neuron, and the action of the sodium-potassium pump is mv. Local potentials, action potentials, and the generation of a nerve impulse 1 Local potentials: The dendrites and the cyton of a single neuron in the central nervous system may have as many as 10, synapses on its combined surface area.

A signal arriving at any one of those synapses may be stimulatory or inhibitory. Stimulatory means that the signal opens a ligand-gated channel the neurotransmitter is the ligand that opens the gate allowing sodium ions to enter the neuron. Inhibitory means that the signal opens a ligand-gated channel allowing potassium ions to leave the neuron. So, in every millisecond of existence in the nervous system, tens of thousands of signals influence the electrical charge on the dendrites and cyton of every neuron.

These are referred to as local potentials. The net result of all the stimulatory and inhibitory signals local potentials , each based on either stimulatory or inhibitory neurotransmitters attaching to the neuronal cell membrane, reaches the area of the axon hillock - the area of the cyton where the axon begins. This region is the first part of the neuron to possess voltage-gated ion channels not the ligand-gated channels of the dendrites and cyton.

If the signal is beyond the threshold value, meaning that the RMP has been changed from to about mv, then an action potential occurs. We say that the membrane has been depolarized at that spot. Then the sodium gate closes and the usual outward diffusion of potassium occurs and the membrane potential returns to the resting potential of and may go lower to as there is often a temporary overshoot in outward diffusion of potassium.

This return to the RMP is called repolarization. So an action potential is a depolarization followed by a repolarization and takes a total of about 1 millisecond. And then the area next to that one has an action potential and so on down the axon all the way to the axon terminals with no reduction in the intensity of the action potentials. The reason for this propagation of the action potential is that as sodium ions diffuse into the axon during the depolarization portion of the first action potential near the axon hillock, some of these sodium ions diffuse over to the next adjacent part of the axon and open the voltage-gated ion channels that are found there and all along the axon.

The stimulation of the voltage-gated ion channel then lets sodium ions enter, followed by potassium ions exiting and we have just generated a new action potential - sodium diffusing in followed by potassium diffusing out.

And the process continues all the way down the length of the axon. So we see that what we call a nerve impulse, or bioelectrical signal, is a series of action potentials occurring sequentially down the length of the axon. Nerve impulse propagation down unmyelinated and myelinated fibers 1 The nerve impulse moves down unmyelinated nerve fibers slowly with each region of the axon going through depolarization and repolarization cycles utilizing voltage-gated sodium channels.

In this process, regions only fractions of a millimeter apart must each depolarize and repolarize as the signal is propagated. The reason is what is called saltatory conduction: the points of depolarization and repolarization are not every adjacent spot on the axon, but only at nodes of Ranvier which are about 1 mm apart along the length of the axon. So the impulse jumps along the axon from one node of Ranvier to the next like a smooth, flat stone skipping across the water of a smooth lake; saltation means jumping along.

Synapses 1. In general, synapses are points of communication between neurons. The space separating one neuron from the next one, the synaptic cleft, is only about millimicrons in width. The majority of synapses in the nervous system are chemical synapses that operate through the release of neurotransmitters from the presynaptic neuron that stimulate or inhibit the postsynaptic neuron.

These electrical synapses allow more rapid communication between cells. Overall Structure - The presynaptic neuron of chemical synapses possesses a synaptic knobfilled with synaptic vesicles attached to the cytoskeleton that are moved into and out of position to discharge their neurotransmitters through the presynaptic membrane into the synaptic cleft. The postsynaptic neuron possesses proteins on its postsynaptic membrane which serve as receptors for ligand-gated channels to permit movements of ions into and out of the postsynaptic membrane.

Calcium's Role: An electrical signal traveling down a nerve fiber reaches the axon terminal and causes the opening of voltage-gated calcium channels permitting diffusion of calcium ions from the surrounding fluid to enter the synaptic knob.

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What is the defference between membrane irritability and membrane conductivity? Unanswered Questions. Compare and contrast between Wordsworth P. Which of the following is released from the host cell in response to the presence of lipid A? The depolarization results in one of two types of reversal in polarity, or potentials:. Graded potential A graded potential simply represents a change in polarity that will vary in degree depending upon the strength of the stimulus.

Graded potentials occur on the dendrites and cell bodies of neurons. Action potential An action potential occurs when the change in polarity reaches a certain level called the threshold. When the threshold is reached, the depolarization becomes self-propagating and a nerve impulse travels along the axon of the neuron.

Propagation of the Action Potential. This is an example of what is called the all-or-none response. Repolarization The depolarization, whether a graded potential or an action potential, ends when the plasma membrane returns to its original resting potential with the inside of the cell being more negative than the outside again.

This is called repolarization. Until repolarization occurs the axon cannot conduct another nerve impulse.