Nervous System: Neuron Structure and Impulse Transmission

Ans - 2

Schwann cells are the primary glial cells responsible for myelinating peripheral nerve fibers in the peripheral nervous system (PNS).

Neurons are indeed surrounded and supported by glial cells (glia). Glia provide structural support, nutrition, and insulation for neurons in the nervous system. This is a fundamental fact of neurobiology.

Negatively charged proteins inside the neuron are large and trapped intracellularly, meaning they cannot easily cross the plasma membrane.

In a resting neuron, the plasma membrane is significantly more permeable to potassium ions (K+) than to sodium ions (Na+). This is due to many more leak channels being open for K+ at rest, which is fundamental to establishing the resting membrane potential.

To measure transmembrane potential (voltage difference across a membrane), you need two electrodes: one inside the cell and one outside in the extracellular fluid. A single electrode cannot measure a voltage difference - it requires two reference points.

The resting membrane potential in neurons typically falls between -40 and -90 mV, with -70 mV being the most commonly cited value. This negative charge exists because the inside of the cell is more negatively charged than the outside due to ion concentration gradients.

The sodium potassium pump is an electrogenic active transport mechanism that expels 3 sodium ions out of the cell for every 2 potassium ions it brings in.

Nerve impulse conduction refers to the propagation of an action potential along a nerve fiber (axon). This is the process by which the electrical signal travels from the point of initiation down the length of the neuron, not the initiation itself or its transfer to target tissues.

The typical resting membrane potential for most neurons is approximately -70 mV, though this can vary between -60 and -80 mV depending on the cell type. This value represents the electrical potential difference across the cell membrane when the cell is not actively transmitting signals.

Nerve signal transmission occurs in four distinct phases: resting (polarized), depolarization, repolarization, and hyperpolarization (refractory period). These phases represent the sequential changes in membrane potential during an action potential.

Depolarization and repolarization are extremely rapid processes during an action potential, typically occurring within milliseconds. Depolarization involves the rapid influx of sodium ions, while repolarization involves the rapid efflux of potassium ions to restore the resting potential.

Depolarization is caused by a rapid increase in membrane permeability to sodium ions. Voltage-gated Na+ channels open suddenly in response to threshold stimulus, allowing Na+ to rush into the cell and rapidly change the membrane potential from negative to positive.

Depolarization and repolarization phases of an action potential occur very rapidly, typically taking only 1-5 milliseconds total. This speed allows neurons to transmit signals quickly over long distances, enabling rapid communication within the nervous system.

The minimum stimulus required to open enough voltage-gated Na+ channels to initiate depolarization is called the threshold stimulus (or threshold potential). This critical value must be reached for the all-or-nothing action potential to be triggered.

The threshold stimulus must be more positive (less negative) than the resting membrane potential. Typically, the resting potential is around -70 mV, and threshold is around -55 mV, meaning the membrane must depolarize by about 15 mV to reach threshold and trigger an action potential.

For typical excitable cells like neurons, the threshold potential required to trigger an action potential is approximately -55 to -60 mV.

Multipolar neurons have one axon and two or more dendrites (i.e. more than two processes arise from cell body, hence multipolar). Cells of retina have bipolar neurons with one axon and one dendtrite (i.e. 2 processes in total, hence bipolar). Unipolar neurons have only an axon attached to the cell body and are usually found in the embryonic stage.

Ans - 3

A spike refers to the nerve impulse or action potential itself - the rapid, transient change in membrane potential that propagates along a neuron. While nerves, axons, and dendrites are anatomical structures, the spike is the electrical signal that travels through them.

Ans - 4

A neuron at resting membrane potential has a negatively charged interior relative to the exterior. This resting potential is typically around -70mV and is maintained by ion concentration gradients and selective membrane permeability. The neuron is not actively firing (at work) during this state.

Transmembrane potential is an electrical measurement that requires an instrument capable of detecting voltage changes. A cathode ray oscilloscope (CRO) displays voltage signals over time and can be used with microelectrodes to measure membrane potentials. Microscopes visualize structure, electroscopes detect static charge, and neither measures dynamic bioelectric potentials.

1. Glia

4 major functions of Glial cells are: 

 1. keep neurons in place (physical support). 

 2. provide nutrients and oxygen. 

 3. insulation, and, 

 4. Phagocytosis of dead neurons and destry pathogens