Earth Science: Biology – Humans – Nervous System
The human nervous system does not only control the vital processes of the body, it is also a mediator to the world around us, as it can pick up, evaluate, and store stimuli.
It can be subdivided into the central nervous system (CNS), including spinal cord and brain, and the peripheral nervous system (PNS) with all nerve fibers of the body.
Earth Science: Biology – Humans – Nerve Cells and Signal Transfer
Stimuli picked up from the environment are transferred to the brain through nerve cells. This information is then processed by the brain and, if required, a reaction (for example, a muscle contraction) is triggered via nerves.
The functional and structural units of the nervous system are the nerve cells (neurons). The human body contains about 100 billion of these. They consist of a cell body (soma), where several short, usually finely branched processes, called dendrites, emerge. There is also a long, thin projection, referred to as axon or neuraxon.
The latter functions in impulse transmission to other cells, and it can be surrounded by a myelin sheath that provides insulation and nutrition. The neurons are in contact with other cells via synapses. However, there is no direct contact, because the cells are kept apart by a two-nanometer-wide gap (synaptic gap).
This prevents the action potential from a stimulated cell from crossing over directly to a cell that is at rest. Chemical carrier substances (transmitters) are used for the transfer, meaning that a conversion from electrical to chemical signals occurs.
Resting and action potential
The pickup and transfer of stimuli is brought about by changes in the membrane potential, for example, the unequal distribution of electrical charges within and outside of the nerve cell.
Prerequisites for this are the selective permeability of the membrane and the activity of membrane-localized ion pumps that assure an unequal distribution of potassium (K*), sodium (Na), chloride ions (CI), and large protein anions (A ). This way the concentration of K” and A ions within the neuron is high, while there are more Na~ and CI -ions present on the outside.
Although there is a concentration gradient, only potassium ions diffuse through the membrane to the outside in unexcited cells, that is, until the increasingly positive charge on the outside and negative charge on the inside no longer permit this. With a reverse potential of -70 to -90 mV (inside) equilibrium is reached. This state is referred to as a resting potential.
When a nerve cell is stimulated, certain pores within the membrane change their conformation so that, for two milliseconds, these become permeable for Na’ ions and the cell interior becomes temporarily positive relative to the exterior medium (depolarization). K” ions will then flow out again and so compensate for the Na diffusion potential.
During the Na inflow and the K” outflow the potential reaches a peak value of +30 mV and a nerve impulse occurs. Brief changes of membrane potential is called action potential.
BASICS
NERVE CELL length varies from a micron to more than 3 ft (1 m) A neuron can have up to 10,000 synapses.
Transfer speed of nerve impulses can be up to 393 ft/s.