Neuron Physiology

Stimulating! How We Feel the Burn

The nervous system's main responsibility is to maintain homeostasis, that is to keep controlled conditions within limits that maintain health, by employing nerve impulses or action potentials that quickly respond to stimuli and permit the brain to adjust body processes accordingly. The three main functions are (1) sensory, (2) integrative, and (3) motor. The sensory function refers to sensing changes in both the internal environment (ie: bladder expansion) and external environment (ie: heat from capsaicin). The integrative function is responsible for not only analyzing sensory information, but also for storing certain aspects, and then executing decisions concerning appropriate behavior. Motor function responds to stimuli by initiating muscular contractions or glandular secretions.

To understand the pathway of an action potential sent to the brain, one must understand basic anatomy of nervous tissue. Nerve cells or neurons function in thinking, memory, muscular activity and glandular regulation. However they cannot undergo mitosis and therefore have no regenerative ability. Every single neuron in our body has a soma, or "cell body" that contains a nucleus surrounded by cytoplasm that includes lysosomes, mitochondria and a Golgi complex. Dendrites and axons are often collectively called nerve fibers because they both participate in the conduction of an action potential to the brain. Dendrites are the receiving portion of the neuron, characteristically short, tapered and highly branched. The axon, a long cylindrical projection that joins soma and axons, propagates nerve impulses toward another neuron, muscle fiber or gland cell.

Nerve cells have electrical properties due to the presence of a membrane potential and various ion channels. A cell's membrane potential is defined as the resting electrical difference across the cell's membrane. It exists because there is a difference in the electrical charge measured inside the cell as compared to the outside of the cell, and is called a potential because with proper stimulation it can suddenly produce an action potential. The neuron action potential exists because neurons are excitable cells that can generate signals by a change in membrane potential. A local change in the electrical potential difference occurs across the plasma membrane triggered by a chemical or mechanical gated channel stimulated by another neuron's synaptic knob. This is called a "graded potential". If the membrane is depolarized to a critical level, called the threshold potential, voltage gated sodium channels in that region will also open. This increases sodium permeability in that region of the membrane, and sodium will rush into the cell. As a result, the cell becomes electrically positive within. As fast as they open, sodium channels close and potassium starts exiting the membrane through leak channels and voltage gated channels. The membrane potential quickly returns to normal as potassium starts to leak out of the cell and return the cell to an inside negative condition. The response in its entirety is "all or none"-if the cell reaches threshold, it depolarizes. In summary, the potential difference is negative at rest, dominated by potassium permeability.

The impulse's final destination is the brain, where it can be processed into a specific response to the stimuli. But how does it get there? The answer is through synaptic transmission. The action potential reaches the synaptic knob of the cell and depolarizes the membrane. This causes the release of a neurotransmitter into the synaptic cleft of the neuron. Chemically gated channels open on the postsynaptic cell and its potential changes due to the entry of sodium and calcium. This change in the cell membrane can be either depolarizing or hyperpolarizing. This will increase or decrease the cell's chance of reaching the threshold potential. If the effect is depolarizing the response will be potentially excitatory, bringing the cell membrane closer to its threshold. If the effect is hyperpolarizing, the response will be potentially inhibitory, taking the cell farther from threshold. If the net effect is depolarization beyond threshold, an impulse will be generated. If, however, the net effect is hyperpolarization below the specific threshold potential, an impulse will be inhibited.