Understanding Neural Cells - AP Biology

Neuron function is highly dependent on ion concentrations. Sodium is required for depolarization, potassium for hyperpolarization and repolarization, and calcium is responsible for initiating neurotransmitter release. Vesicles of neurotransmitter are stored in the terminal end of the axon, opposite from the end with the axon hillock and cell body. When voltage-gated calcium channels open, the binding of calcium to these vesicles causes them to exocytose the neurotransmitter.

Myelin is the fatty substance that wraps around axons to create nodes of Ranvier. These nodes allow the depolarization signal to jump along the axon, rather than traveling fluidly. This method, known as saltatory conduction, allows the action potential to travel faster. Degeneration of myelin is associated with numerous neurological disorders.

It is true that the axon of one neuron transmits information to the dendrite(s) of another, but this contact is not direct. Neurons are separated by synapses, which are small gaps that neurotransmitters must cross in order to move from their original location to a new dendrite. The two neurons do not physically touch.

Depolarization results from the opening of voltage-gated sodium channels during the initiation of an action potential. Sodium has an electrochemical gradient that causes it to enter the cell when the channels open, resulting in a net flow of positive ions into the cell that increases the membrane potential. This increase is known as depolarization.

Potassium follows a gradient opposite to sodium. When voltage-gated potassium channels open, ions flow out of the cell and cause hyperpolarization.

Rods and cones are contained in the human retina, but they differ in shape and function.

The functional difference between the two is that rods are more sensitive to light, but do not distinguish colors. Comparatively, cones are able to sense color and are less sensitive to distinctions fo light and dark. There are three different types of cone photoreceptors that have different sensitivity across the visible spectrum, with optimal responses to red, blue and green light, respectively.

Both types of receptors use rhodopsin as the visual pigment that is activated when exposed to light. This initiates a signaling cascade that causes cellular depolarization through sodium ion channels and the release of the neurotransmitter glutamate by bipolar neuron cells to further transmit the visual signal.

The signaling cascade in response to light in the human eye is caused by two types of photoreceptors present in the retina—rods and cones. Rods provide dark and light vision (black and white) and cones are capable of discerning color along the visible spectrum.

Rhodopsin is the visual pigment in photoreceptors and is made up of a light-absorbing vitamin-like molecule (retinal) that is bound to a cell membrane protein called opsin. The absorbtion of light by rhodopsin results in chemical bond shifting and a change in the molecule's shape. This causes rhodopsin to activate. The active rhodopsin initiates a G-protein cascade that causes sodium channels on the receptor's cell membrane to close. This prevents sodium ions from entering the cell, resulting in a build-up of ions in the extracellular space in comparison to the cell interior. The membrane potential is directly related to this difference in concentration; a higher concentration of positive ions outside the cell will result in hyperpolarization.

In response to being in a hyperpolarized state, the photoreceptor cell will stop its release of the neurotransmitter glutamate across the cell synapse with retinal neurons. Glutamate is an inhibitory neurotransmitter; halting the release of glutamate will leave the neurons capable of stimulation by visual signals.

Neurons can be unipolar, bipolar, or multipolar, depending on how many projections are coming off of the cell body. Bipolar neurons are found in the retina and inner ear, and have a single dendritic extension as well as a single axonal extension. Most neurons are multipolar; they have numerous dendrites and a single axon.

During the initial phase of an action potential, voltage-gated sodium channels open and allow sodium ions to enter the cell. This causes the membrane potential to rise to a positive value, resulting in depolarization.

Next, voltage-gated potassium channels open and potassium ions rush out of the cell. This reduces the membrane potential, resulting in repolarization as the potential becomes negative.

As more and more potassium exits the cell, the membrane potential declines below the resting potential, resulting in the hyperpolarized state. The sodium-potassium pump then functions to import potassium ions and export sodium ions to reestablish the resting membrane potential.

The relative refractory period is a time frame near the end of an action potential where another action potential can be generated only if a larger than normal stimulus is encountered by the neural cell. The relative refractory period takes place during the hyperpolarization of the cell. Since the membrane potential during hyperpolarization is more negative than the resting potential, it requires a much larger stimulus in order to reach threshold.

The PNS is the part of the nervous system that is comprised of all the nerves located outside of the central nervous system. The PNS nerves are not protected by bone; therefore, they are susceptible to toxins and injury. The PNS can be further divided into the somatic and autonomic nervous systems.

Glial cells are important in maintaining nervous system homeostasis. The major functions of glial cells include insulating neurons, holding them in place, and supplying them with nutrients and oxygen. Glial cells also degrade pathogens and dead neurons.

Neuron cells are functional units of nervous tissue that transmit electrical signals. Neurons typically are composed of a soma, dendrites, and an axon. The soma is the body of the cell, the dendrites are branched projections that receive signals, and the axon conducts signals away from the cell body.

Axons are the parts of neurons that transmit electrical signals away from the cell body and to other neurons. The axon branches off of the soma and is long with a constant radius. Parts of the axon are myelinated, meaning that it is insulated to allow for salutatory conduction. Gaps in insulation are referred nodes of Ranvier.

In neurons and many other cells, electrical signal transmission requires action potential. Action potential can be defined as the rapid rise and fall of membrane potential that allows for signal propagation. In the case of neurons, reaching action potential allows the electrical signal to travel down to axon, which permits signal transduction.

Ganglia are described as clusters of nerve cells that are found in the autonomic nervous system.

Dendrites are branched projections of the neuron that receive electrical signals from other cells. The dendrites are connected to the soma—cell body.

The soma is the cell body of a neuron. The nucleus and many organelles are located within the soma. The soma has a specialized region called the axon hillock, which is where the soma transitions into the axon.

Neurotransmitters are chemicals that transmit signals from one neuron to another across the synapse. This transport is conducted through exocytosis and endocytosis of neurotransmitters in vesicles. Common neurotransmitters are norepinephrine, dopamine, epinephrine, and histamine.

The synapse is the junction between two nerve cells through which neurotransmitters conduct an electrical signal.

Schwann cells are a type of glial cell that wrap around the axon to form the myelin sheath. This insulation allows for saltatory conduction, which increases the speed of signal transmission through the neuron. Gaps between Schwann cell myelination on the axon are called nodes of Ranvier.

Grey matter is a part of the central nervous system that contains most of the cell bodies in the system and few myelinated axons. Grey matter is located in both the brain and spinal cord. Among other things, it is involved in muscle control, sensory perception, emotions, and self-control.