General Neuron Physiology - Anatomy

An action potential across a cell membrane has five phases:

1. The resting membrane potential is a negative membrane potential established by the sodium-potassium pump and maintained by potassium leak channels.

2. Depolarization involves opening of voltage-gated sodium channels and results in a rapid influx of positively-charged sodium ions into the cell, creating a positive membrane potential.

3. Overshoot occurs during the maximal value (peak) of the action potential.

4. Hyperpolarization occurs when sodium channels close and potassium channels open, allowing potassium to leak out the cell, and establishing a negative membrane potential below the resting potential.

5. Repolarization occurs when voltage-gated potassium channels eventually close and the membrane potential returns to the resting value via action of the sodium-potassium pump.

Potentiation refers to the phenomenon when nerves become more effective at transmitting signals due to extensive use of the same pathway.

When at rest, the neuron initially has a negative membrane potential. At the beginning of an action potential, voltage-gated sodium channels open, allowing sodium ions to enter the cell. This causes the cell to become positively charged compared to the outside of the cell. This process is called depolarization.

After depolarization occurs, the sodium channels close, initiating the absolute refractory period. Voltage-gated potassium channels then open and potassium ions exit the cell. This results in hyperpolarization and the relative refractory period. The potassium channels then close and the sodium-potassium pump returns the cell to its resting potential by removing sodium and collecting potassium.

Once an action potential has been created, the membrane has a period of time during which it cannot be stimulated to create another action potential. The absolute refractory period occurs when the voltage-gated sodium channels initially close. The first gating mechanism of these channels cannot be overcome by an electrical stimulus, and the sodium channels will remain closed even if a large electrical stimulus is present. During this period, even a very large stimulus cannot result in neural depolarization.

Following this, the secondary gating mechanism for the channel becomes active. This mechanism is sensitive to electrical stimuli, but keeps the channels closed when the neuron is at rest. The relative refractory period results when sodium channels are capable of opening, but the cell is hyperpolarized, making it very difficult to initiate a stimulus that reaches the action potential threshold.

The neuron has a resting potential. In its resting state, the neuron has a resting potential with a slightly negative interior compared to the exterior. Sodium ions enter the cell and alter the membrane potential. Through voltage-gated channels, enters and makes the interior less negative therefore decreasing the membrane potential difference, which is known as depolarization. The membrane potential depolarizes all the way up to the threshold level. After enough enters, the threshold membrane potential is reached. This opens more channels. An action potential is fired, which means that the depolarization spreads down the neuron's axon. This travels down the entire axon, eventually reaching the dendrite and signaling to other neurons.

The voltage-gated channels along the axonal plasma membrane open and close in response to changes in voltage, and may exist in three distinct states: deactivated, activated, and inactivated. While the axon is at rest, these channels are said to be deactivated; they are impermeable to sodium ions since their activation gates are closed. Once the neuron gets depolarized to the threshold of the voltage-gated sodium channels, the activation gates open, allowing the influx of sodium down its concentration gradient into the cell. During this time the channels are in their activated state. At the peak of the action potential the activation gates are still open, but the inactivation gates close, stopping the flow of sodium through the channels. The channels are in the inactivated state due to the cell becoming depolarized. Once the membrane potential drops back down towards resting, the inactivation gates open, and the activation gates close, thereby deactivating the channels again, until another action potential depolarizes the membrane.

To support the general function of the nervous system, neurons must communicate within the cell (intracellular signaling) and between other cells (intercellular signaling). In order to achieve long distance and rapid communication, neurons have special abilities for sending electrical signals (action potentials) along axons. This mechanism is called conduction, and it is how the neuron's cell body communicates with its own terminals via the axon. Communication between neurons is achieved at synapses by the process of neurotransmission.

At resting potential, the cell membrane is about 25 times more permeable to potassium ions than it is to sodium ions. During an action potential, the membrane becomes much more permeable to sodium ions than potassium ions, causing the membrane potential to become more positive, as sodium flows down its concentration gradient into the cell. Note that this concentration gradient is largely set up by the action of the sodium-potassium ATPase, which pumps three sodium ions out of the cell in exchange for two potassium ions into the cell.

A synapse is a specialized junction between cells. It is involved in the integration and converging of signals between neurons. At a synaptic junction, the membranes of the pre- and post- synaptic neurons are separated by a gap called a synaptic cleft, which is the site of neurotransmitter release.

Glutamate opens sodium channels in the plasma membrane of the receiving neuron, moving the action potential towards (depolarize) the sodium Nernst potential (81mV). GABA is an inhibitory neurotransmitter which opens chloride channels in the plasma membrane of the receiving neuron, making the neuron more difficult to excite (hyperpolarized).

The action potential of a neuron is generate at the axon hillock and is propagated down the axon and to the terminal branches where it will synapse with the dendrites of the next neuron.

Depolarization is when the neuron becomes more positive by gaining positively charged ions, specifically sodium ions. During depolarization the sodium ion channels open and sodium ions enter the neuron, reducing the membrane potential to roughly +35 mV.

In order to have an action potential, you must have a depolarization. Na channels must close before K channels open

Myelin helps to increase resistance along the axon, which helps to propagate the action potential along the axon.

The influx of positive sodium ions into the neuron is known as depolarization. This is the loss of negative charge that occurs when positive sodium passes through the neural membrane and enters the neuron.

The resting membrane potential is based on the difference in electrical charges of the ions that flow through the membrane. The membrane potential has a greater permeability to potassium when at rest which causes a shift in its potential. Thus, potassium has the strongest affect on the RMP and causes it to be closer to potassium's reversal potential. Side note: This potential is strongly held by the sodium potassium pump.

If an already positive membrane potential becomes more positive, it is becoming hyperpolarized because the electrical difference between the inside and outside of the cell is getting larger. On the other hand, a stimulus that moves the potential difference closer to 0 is depolarizing because it is decreasing the difference in electrical potential between the inside and outside of the cell.

The pineal gland secretes melatonin, it is also thought to have some role in regulating the pituitary gland. The thalamus is the center of pain perception and does not secrete hormones. The pituitary gland, also known as the "master gland" secretes many hormones except for melatonin, some example secretions are thyroid stimulating hormone, follicle-stimulating hormone, and growth hormone. Lastly, the hypothalamus secretes tropic hormones that regulate the activity of other glands.

The limbic system includes the amygdala, hippocampus, hypothalamus, and thalamus. Each of these structures secretes hormones that assist in processing emotions such as fear, and anger. The brainstem is responsible for basic life support. The cerebrum is the area dedicated to higher thinking. Lastly, the parietal lobes are where auditory signals are processed and they contain the primary somatosensory cortex.

Melatonin is the hormone secreted by the pineal gland, it controls the circardian rhythm and is responsible for feeling sleepy. Corticotropin, released by the adrenal cortex is associated with cortisol release. Insulin is the most notable hormone associated with glucose metabolism and is released by the pancreas. Gastrin, cholecystokinin, and secretin assist digestion.

Oligodendrocytes myelinate central nervous system (CNS) axons. Each oligodendrocyte can myelinate up to thirty axons, stretching between neurons. Oligodendroctyes are the predominant type of glial cell in white matter, with the myelin giving them a white appearance. Schwann cells myelinate axons in the peripheral nervous system (PNS), and can only be associated with one neuron per cell.