Question

describe the locations and functions of neuroglia

classify neurons on the basis of their structure and function.

describe a resting potential and an action potential. explain the sequence of events following threshold.

Answer 1

First question:

Location and functions are neuroglia:

• Glial cells in the Central nervous system:

Astrocytes: It maintains the blood-brain barrier, regulate ion, nutrient, and dissolved gas concentrations, Form scar tissue after injury.

Oligodendrocytes: It forms myelin around CNS axons.

Microglia: It removes cellular debris, and pathogens in CNS by phagocytosis.

Ependymal cells: It lines the ventricles of the brain and central canal of the spinal cord, assists in production, circulation, and monitoring of cerebrospinal fluid.

• Glial cells in the Peripheral nervous system:

Satellite cells: It surrounds the nerve cell bodies in peripheral ganglia.

Schwann cells: It surrounds all axons in PNS, it is responsible for myelination of axons in PNS, it participates in the repair process after injury.

Second question:

A neuron has a cell body, dendrites, and axon. In the CNS, the myelin for the axon is produced by oligodendrocyte and in the PNS by Schwann cell.

Type of nerve cells:

Structural classification:

• Typical unipolar brush cells are neurons specific to the cerebellum and the granule region of the dorsal cochlear nucleus.
• Pseudounipolarneurons have dendrite and axon emerging from the same process. It is found in the spinal ganglia and sensory ganglia of cranial nerves V, VII, IX, and X, all mixed-cranial nerves.
• Bipolar neurons have axon and single dendrite on opposite ends of the soma. Olfactory neurons are bipolar and unmyelinated, Bipolar cell of the retina Vestibulocochlear nerve has the bipolar neuron type in both the spiral (cochlear) ganglion as well as Scarpa’s (vestibular) ganglion.
• Multipolar neurons have two or more dendrites, separate from the axon, and are the largest population of nerve cells in the nervous system. It includes motor neurons, neurons of the autonomic nervous system, interneurons, and Purkinje cells of the cerebellar cortex.

Functional classification:

• Sensory or afferent neuron: These neurons carry action potential towards the CNS.
• Motor or efferent neuron: These neurons carry action potential away from the CNS.
• Interneuron: They connect the motor and sensory neurons and transmit the impulse between them.

Third question:

Action potential is defined as the transient change in membrane potential of about 100 millivolts, which is conducted along the axon in an all-or-none fashion. It is also known as impulse or spike potential.

Resting potential is the difference in membrane potential of the neuronal membrane at the resting state.

Phases of action potential:

Phase of depolarization: it is recorded as a sharp upward wave during which the membrane potential approaches zero and then attains a positive value. It consists of slow depolarization to the threshold, rapid rising phase, overshoot, and peak. During overshoot, membrane potential crosses the zero or isopotential level, and then at peak, it reaches a maximum potential of +35 mV. During depolarization, a massive influx of sodium ions occurs.

The phase of repolarization is recorded as a downward stroke during which the membrane potential returns to the resting level. It includes a rapid falling phase and the slow terminal part called the after depolarization. During this phase, the sodium channels close and potassium channels open and there is the efflux of potassium ions into the cell.

The phase of repolarization is followed by an after-hyperpolarization phase during which the membrane potential overshoots (becomes more negative) and then returns back to the resting level. The membrane potential becomes more negative than the resting membrane potential giving rise to prolong the and slow undershoot which is called a phase of after-hyperpolarization. Finally, the potassium channels completely close, restoring the membrane potential back to the resting level.