Anatomy of the Neuron
The human body can be described on many levels. Macroscopic descriptions would focus on how our organ systems, such as our skin, heart, intestines, lungs, and others, work together to accomplish a specific task. As we descend to the microscopic level, we see how different tissues make up individual organs, and how those tissues themselves are made up of dense assemblages of cells. In the body, the cell is the central unit for biological study. Schwann first proposed the cell theory in 1839, which, stated that, "all body organs and tissues are composed of individual cells" (Shepard, 2001). Psychology owes a huge debt of gratitude to neurology and developmental biology for the amount of information that has been gained concerning the exact processes and structures of the cell.
The neuron is the cell of the nervous system and brain. Neurons are different then the other cells of the body in numerous ways, but it is their information-processing and transmitting ability (Carlson, 2004) that enables consciousness, communication, and indeed life to progress at all. Like other specialized cells of the body, neurons perform numerous tasks. Therefore they come in variegated shapes and sizes depending upon their function (Kalat, 2001). Neurons share the same basic structure; this includes the (1) cell body, otherwise known as the soma; (2) dendrites, (3) axons, and (4) terminal buttons (Carlson, 2004). Neurons might be described to look like tiny spindly sea stars, arms dangling delicately out in any direction that a connection with another neuron might be found.
The soma is the command center of the cell; it contains the nucleus, which houses the life operations of the cell including the cytoplasm, the mitochondria, the nucleus, the endoplasmic reticulum among other structures. The nucleus directs cell functioning via chemical messengers which pass through the nuclear membrane and deliver different chemical triggers to other parts of the cell (Mycek, Harvey, & Champe, 2000).
The word dendron comes from the Greek word for tree, and the dendrites of a neuron very much resemble branch like forms (Carlson, 2004). Dendrites are long, thin spider web-like structures that branch off from one end of the soma. They are the receivers of messages being sent from other neurons, through the synapse, the small gap between neurons. Axons, look like long slender tubes, that more often than not, are covered by a protective coating called myelin (Koch, 2004). Myelin, which is roughly 80 percent lipid and 20 percent protein, is made by oligodendrocytes (Rosenzweig, Leiman, & Breedlove, 1999), specialized cells that provide support to the axon. The myelin sheath surrounding axons serves to speed up transmission down the axon. The sheath of myelin is not continuous but rather segmented into small sections approximately 1-2 micrometers long with a gap in between the next segment of myelin. This gap is known as the node of Ranvier (Kalat, 2001). The axon is responsible for carrying information from the soma to the terminal buttons.
At the end of the axon are a collection of small branches which end in button-like structures called terminal buttons (Sternberg, 2003). Terminal buttons receive the information from the soma via the axon in the form of an action potential. This causes an electro-chemical change in the terminal button, which then releases certain chemicals into the synaptic gap, signaling other nearby dendrites of the message.
Neurons are typically classified according to the way in which the axons and dendrites are branched out from the soma. Multipolar neurons have numerous dendritic trees yet only one axon; bipolar neurons have only one dendritic tree and axon, while unipolar neurons consist of one axon that splits in two directions, both receiving and sending information to the central nervous system (CNS) (Carlson, 2004).
The Action Potential
On the most basic level in neuroscience is the acknowledgement that the action potential is "the primary means of conveying information rapidly form one neuron to the next" (Koch, 2004). Diffusion is the mechanism by which different molecules of a substance tend to spread out evenly, bouncing off of each other, until they are evenly spaced within a mixture (Carlson, 2004). When electrolytes are dissolved in water they break into component parts, separating into ions. For instance, NaCl (sodium chloride) will break into Na+ and Cl-, where sodium is now a positively charged ion, chlorine a negative charged ion. Because like charges repel each other, sodium ions push away from other sodium ions and chlorine ions push away from other chlorine ions. The net effect of diffusion and ionization of electrolytes in a cell is to create an even distribution of charges. Because the intracellular and extracellular fluid contain different ions, this contributes to the membrane potential, or the difference in electrical charge in and out of a cell (Carlson, 2004; Pinker, 1997). The difference in charge (about 70 millivolts) is caused by a higher concentration of Potassium ions (K+) outside the cell and a higher concentration of both Sodium (Na+) and Chlorine (Cl-) inside the neuron. Because like charged ions have a natural inclination to push away from each other, and diffusion serves to push ions to areas of higher concentration to areas of lower concentration, electrostatic pressure builds up on the membrane of the neuron.
The neural membrane is made up of linked molecules of lipids. This membrane has openings which are controlled by "gates" that allow the transport of different ions into and out of the neuron. Ion channels are controlled by a lock and key mechanism, that is, only certain chemical shapes can fit onto the outer structure of the membrane and thus activate the channel to open. When ion channels are opened, the electrostatic pressure forces ions through the channel causing a depolarization in the neuron. This depolarization of the membrane potential triggers an electric pulse down the neuron, this is known as an action potential (Kalat, 2001). Specific gates in the membrane actively pump out Sodium ions, which results in a low concentration of intracellular sodium. Because the membrane is not permeable to sodium ions (unless the gates are open) there exists a much higher level of Na+ outside the cell than inside. When the neuron is stimulated by an outside event, such as by pain receptors, the gates are opened up and the rush of sodium ions into the cell changes the polarization and thus begins the action potential.
Action potentials generally start in the dendrites spines, although they can begin in the axon itself, and travel through the soma, down the length of the axon. Passage of an action potential through a myelinated axon is achieved by a process called salutatory conduction (Kalat, 2001). Myelinated axons have two distinct advantages over non-myelinated axons. First, the myelin decreases the ability of Na ions to enter the cell since they may only enter a myelinated axon at the node of Ranvier. This means that the cell spends less energy pumping ions into and out of the axon (Carlson, 2004). Secondly, myelin speeds up the rate of transmission, reaching speeds of up to 100 meters per second (Sternberg, 2003).
When the signal finally reaches the terminal button, synaptic vesicles bind themselves to calcium channels on the synaptic membrane (Rosenzweig, Leiman, & Breedlove, 1999) causing their contents, neurotransmitters, to be released through the membrane into the synaptic cleft. These neurotransmitters travel small distances to the postsynaptic neuron where they dock with specific receptor sites and then trigger other ion channels to open, leading to yet another action potential. After some molecules of the neurotransmitters have docked to the postsynaptic neuron, a process of reuptake pulls back leftover chemicals into the cytoplasm of the terminal button for reuse (Carlson, 2004).
Released neurotransmitters can produce either excitory or inhibitory responses which lead to depolarizations (EPSPs) or hyperpolarizations (IPSPs) (Carlson, 2004). The specific combinations of EPSPs and IPSPs that occur thus determine the firing rate of neurons. Neurons communicate through a process of chemical and electrical changes. When the action potential (electrical) reaches the terminal button it activates channels that are voltage dependent. These channels open to release Calcium ions, which are able to bind to the synaptic vesicles and thus allow them to break open, spilling their contents into the synaptic cleft. Neurotransmitters then bind to sites on the postsynaptic membrane causing the opening of ion channels, which then cause another depolarization or hyperpolarization depending upon which ion channels are opened. The presynaptic neuron then releases molecules that retrieve left over neurotransmitters and return them to the cytoplasm for recycling (Gandhi & Stevens, 2003), this process normally prevents the re-stimulation of an action potential when the postsynaptic neuron resets. Other chemicals such as peptides, neuromodulators, and hormones can also trigger action potentials by proxy of second messengers (Mycek, Harvey, & Champe, 2000).
By j.w.gibson copyright 2006
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