Question

choose any topic related to biology and write a review article (4 pages max, double spaced, Times new roman, font size 12)

Answer 1

Neural conduction and firing of action potential

Neuronal pathways play major role in sensing the environment and how the brain reacts to it. Lets us take pain perception as an example to understand sensory neurons communicate to brain and how brain processes it.

The pain perception is also called as nociception involves relaying a painful stimulus to the central nervous system. It involves several processes:

· The stimuli can be mechanical such as (pressure, punctures and cuts) or chemical (burns). By contacting the stimuli, the free nerve ending sense the stimuli.

· Specialized nerve system sends that stimuli as a signal to CNS which involves several neurons within CNS.

· Brain is the pain centre which recievesthe information for further processing and action.

Pain receptors are called as nociceptors which differs from normal sensory receptors and they always fire into action first. Noniceptors are classified as

· A δ mechanosensitive receptors - slightly myelinated, conduct faster, respond to mechanical stimuli

· A δ mechanothermal receptors - slightly myelinated, cnduct faster, respond to mechanical and thermal stimuli

· Polymodal nociceptors (C fibers) -- unmyelinated, conduct slower, respond to a variety of stimuli.

These receptors helps to sense the pain from the environment and the nociceptor neurons travel in peripheral sensory nerves. As an example, when there is a cut, the signal for the intense pain is conducted rapidly by the A δ-type nociceptors, followed by a slower, prolonged, dull ache, which is conducted by the slower C-fibers.

The soma of nociceptor neurons lie in the dorsal root ganglia of peripheral nerves just inside the spine where they make synapses on neurons within the dorsal horn (the top half of the butterfly-shaped gray matter). The signal is transmitted upward via the secondary neurons through an area of the spinal cord's white matter called the spinothalamic tract which then travel up the spinal cord through the medulla (brain stem) and synapse on neurons in the thalamus, the brain's relay center.

Once it is processed in the brain, the response signal hits the motor cortex which then travels down the spinal cord and eventually to motor neurons. The impulse of motor neuron would trigger the muscle contraction to move out of whatever is causing the pain.

Now look at the events of action potential, phenomenon by which the nerve impulse are conducted.

The graph depicts the events of action potential with respect to voltage change. Resting membrane potential is a steady state dynamic process that is balanced by ion leakage and ion pumping. To start with the action potential, the membrane potential has to change. When a stimulus is applied to the neural cells, the ‘attaining threshold’ occurs. Depolarization is an important event to progress the action potential. Membrane threshold is the minimum change in membrane potential for a stimuli to cause, in order to generate an action potential for that particular stimuli. In other words, the stimuli that has to depolarizes the membrane to -60mV or threshold to generate membrane potential by opening large number of channels on the membrane. The membrane threshold is attained with the help of voltage-gated Na+ channel, which depolarizes membrane from -70mV to -60mV. Since the NA+ is 10 fold higher outside the cell than inside at rest, opening of these channels cause change in potential to greater extent, making the cell less negative known as ‘depolarization’. The phase continues with the concentration gradient for Na+ till the membrane attains +30mV when the repolarization begins. The period when the membrane is inactive for stimulus is called ‘refractory period’. There are two phases of refractory period includes: absolute and relative refractory period. This is due to the inactivation gate of voltage gated Na+ channel. Another AP can be triggered only after the membrane attains resting state. The phase of repolarization and hyperpolarization are attained with the help of channels specific for the potassium ion creating the concentration gradient acts on K+ when the membrane voltage moves back toward the -70 mV. But it actually overshoots causing hyperpolarization because of K+ channels are slightly delayed in closing, accounting for this short overshoot. The action potentials are always “all or none.” Either the membrane reaches threshold and everything happens as depicted above or nothing else happens when the membrane does not reach threshold.