Biological Psychology/Nerve Cells and Nerve Impulses
The cells of the nervous system are generally of two types, neurons or glial cells, with each contributing different functions to the nervous system. Neurons act to receive, sustain, and transmit electrochemical signals throughout the nervous system, that is from one neuron to another. Glial cells, which outnumber neurons on a 10 to 1 bases, increase speed and efficiency of signals being sent along a neuron as well as acting as a passage from the blood to the nervous system and responding to injury and disease. These two types of nerve cells have many different variations which serve different functions in the nervous system.
Classes of Neurons
There is no single appearance for neurons, with varying numbers of axons and dendrites extending from the cell body. These two types of extensions are called processes and through looking at the number processes a neuron has helps to form a way of categorising neurons. When there are more than two process extending directly from the cell body a neuron is said to be a mulitpolar neuron. These neurons may have an axon and many branches of dendrites and these make up a majority of neurons. A similar class of neuron is the multipolar interneuron which is essentially the same as a multipolar neuron except these neurons do not have axons, only many dendrites and act to concerntrate neural activity within a structure of the nervous system rather than send signals to other regions of the nervous system. Two other types of neurons include unipolar neurons which have only one process extending from its cell body of which may branch to contain both dendrite and axons, and the bipolar neuron which has two processes extending from its cell body.
Classes of Glial Cells
The list of functions glial cells perform in the nervous system are differentiated between different classes of neuroglia. The first two classes of neuroglia are oligoendrocytes and schwann cells. Oligodendrocytes are the glial cells which help speed up and improve the effectiveness of axonal conduction in the central nervous system. This is done through the cell wrapping extensions of a fatty insulating substance known as myelin around the axons of some neurons. A similar function is performed for the peripheral nervous system by another type of neuroglia known as schwann cells. However these differ from oligodendrocytes in that each schwann cell is a single myelin segment along a neuron whereas oligodendrocytes can create multiple myelin along the same or many different axons of neurons; and in that only schwann cells provide axonal regeneration after damage.
A third class of neuroglia are astrocytes which are star shaped cells which attach to both blood vessels and central nervous system neuron cell bodies offering a variety of support functions such as assistance in maintaining structure and the traversing of nutrients and chemicals from blood to the nerve cell. Microglia, a fourth class of glial cell, aiding the nervous system by responding to injured or diseased neurons through consuming cellular debris and prompting an inflammatory response. Finally, during early fetal development another class of glial cell called radial glial cells form a temporary network which aids neural migration in the developing neural tube.
There is a difference in electrical charge between the inside and the outside of the nerve cell which varies due to positively and negatively charged ions, which add up to result in the voltage of the neuron and its membrane potential. In a neuron's resting state, known as resting potential, the cells electrical charge is typically between -60 and -80 millivolts and in this state it is not transmitting signals, it is said to be polarized. Several factors can influence the neuron's electrical charge resulting in a further decrease electrical charge within the cell, or an increase of electrical charge within the cell which when large enough causes an action potential. The state of the neuron is a result of the number of positive or negatively charged ions, and the stimuli that open or close the cells ion channels.
Ions, Ion Pumps and Ion Channels
Due to the nature of ions they tend to be distributed in a consistent manner within the neural tissue as they move to areas of less ionic concentration than high concentration. Electrostatic pressure also causes to spread out the positive or negative charged ions relatively evenly throughout neural tissue as the ions repel like charges in the same area and are attracted to opposing charges in different areas. The distribution of ions on the inner and outer wall of the cell membrane is not equal and potassium, sodium, and chloride ions can pass through the cell membrane of the resting neuron easily due to electrostatic pressure differences driving them either in or out. Potassium ions are continuously being driven out of the nerve cell by 20 mV of pressure and sodium ions are pushed inside the cell membrane despite resistance due to 120mV of pressure. As shifts in the levels of ions in the neuron lead it to action potential there are active processes in place within the neuron to keep levels of potassium and sodium at a balance, known as sodium potassium pumps, which pump three sodium ions inside the neuron for two potassium ions outside the neuron. Chloride ions on the other hand naturally pass inside and out of the neuron depending on the electrostatic pressure until an equilibrium is reached. Another manner in which ions pass in and out of the neuron is a type of through gated ion channels which open and close in response to a stimuli associated with that gate type. Stretch-gated ion channels are found in stretch sensing cells, ligang-gated ion channels are opened or closed in response to a certain chemical, often a neurotransmitter, and are located at the cells synapse. A third type is Voltage-gated ion channels which are usually found in axons but sometimes dendrites and open or close when the membrane potential changes. Gated ion channels are the means by which nerve cells create action potentials and therefore create communication through the nervous system.
Axonal conduction as Signals for Action Potential
Stimuli that affect the gated ion channels of neurons influence the membrane potential of the neuron creating one of three potentials: hyper polarization, depolarization, and action potential. In hyperpolarization the cells membrane potential is decreased in charge (from -70 to -72) and in depolarization the charge is increased (from -70 to -68), larger or more frequent stimuli produce larger polarization in the appropriate direction and it is possible for both hyperpolarized and depolarized stimuli to affect the neuron which can cancel each other out to the level of their charge, likewise multiple depolarized (or hyperpolarized) stimuli can affect a neuron, a process called summation. Finally, polarization that is caused from the firing of another neuron is called postsynaptic polarization giving depolarization the name excitatory postsynaptic potential (EPSP) and hyperpolarization the name inhibitory postsynaptic potenial (IPSP) because they each increase and decrease the likelihood of causing an action potential, respectively.
Depolarization and hyperpolarization can increase or decrease the membrane potential of the neuron taking it close or further to what is called the threshold of excitation which is often -65 mV. When the membrane potential reached the particular threshold of excitation for that cell an action potential is generated resulting in a momentary shift in energy lasting only 1 millisecond, where the membrane potential is then inverted from -70 mV to around +50 mV. Action potentials are often referred to as all-or-none responses because when they occur they occur fully or not at all.