Nervous System
The neuron (nerve cell) is the basic information-processing unit of the nervous system and structural and functional unit that specializes in transmitting electrical impulses. Several hundred billion of these cells integrate into a functional mosaic with billions of interconnections, which make the recognition and interpretation of a myriad of sensory stimuli (understanding), retention of experience (memory), and the elaboration of an enormous range of human movement (locomotion) and emotional responses (behavior) possible.
The physiologic construct of a neuron consists of a cell body, the central part of the neuron containing the nucleus and a number of processes or branches (natural outgrowth) with functional consequences. Neurons have four functional subdivisions; (1) an input portion that receives information from other neurons, tissues, or the environment, (2) a short-range transmission portion, that transmits from the input portion to another location less than a maximum distance of about 5mm, (3) a long-range transmission portion (the conductive portion) by which information can be propagated more than a meter from the site of impulse initiation, and (4) an output portion, where the cell transmits information to another neuron, to muscle cells, to secretory cells, etc. There are different cellular mechanisms for each separate function of these classifications.
Dendrites are the input portion for receiving information, a cell body where the action of the cell takes place to activate an action potential. Dendrite-neuroanatomic usage; any neuronal process that carries information toward the cell body. neurophysiological usage; a neuronal process involved in short range information transfer, often toward the cell body and/or information processing. The term is normally confined to those processes that appear to be relatively unspecialized extrusions of the cell body; frequently these contain Nissl substance.
The dendrites spread out from the cell body like the branches of a tree and greatly increase the surface area of the neuron, thereby making more space available to receive contacts from other neurons. Dendrites depend on the cell body for their survival, range from a few tens of micrometers to as much as several millimeters in length. Like antennas, dendrites collect and receive incoming (afferent) information from other neurons or sensory receptors. The dendrites receive these incoming impulses directly on their surface membrane or on tiny projections of membrane called dendritic spines which project outward for a few micrometers like forests of tiny lollipops. The afferent information reaches the neuronal cell body in the form of membrane electrical disturbances. Impulses are conducted away from the neuronal cell body by a single process called the axon, with the exception of a few neuron types that are known to function without axons.
The axon emerges from the cell body as a conical protrusion called the axon hillock. The course the axon then follows depends on the nature of the parent neuron and the target element to be reached.. Axons can be very short (100 micrometers or less), while others have very long courses (up to more than 1 meter). Axons are uncovered (bare) or covered by a insulating lipoprotein cellular sheathe called myelin (Schwann cells in the PNS, oligodendrocytes in the CNS). The long fibers or axons of neurons are also often grouped. In the CNS, bundles of axons carrying information or motor commands of one kind are called tracts. Tracts make up the white matter of the CNS. In the PNS discrete bundles of axons bringing information to the CNS from peripheral structures and conducting motor commands to muscles and glands are called nerves.
Most axons develop side branches along the route called an axon collateral which help to bring information to several parts of the nervous system simultaneously. When the target neural elements are reached, the axon (and collateral) end in a small number of terminal branches. An axon terminal or synaptic bouton is the physical end-point of axon transmission which develops the synapse, the region of functional contact between two nerve cells.
The physiologic construct of a neuron consists of a cell body, the central part of the neuron containing the nucleus and a number of processes or branches (natural outgrowth) with functional consequences. Neurons have four functional subdivisions; (1) an input portion that receives information from other neurons, tissues, or the environment, (2) a short-range transmission portion, that transmits from the input portion to another location less than a maximum distance of about 5mm, (3) a long-range transmission portion (the conductive portion) by which information can be propagated more than a meter from the site of impulse initiation, and (4) an output portion, where the cell transmits information to another neuron, to muscle cells, to secretory cells, etc. There are different cellular mechanisms for each separate function of these classifications.
Dendrites are the input portion for receiving information, a cell body where the action of the cell takes place to activate an action potential. Dendrite-neuroanatomic usage; any neuronal process that carries information toward the cell body. neurophysiological usage; a neuronal process involved in short range information transfer, often toward the cell body and/or information processing. The term is normally confined to those processes that appear to be relatively unspecialized extrusions of the cell body; frequently these contain Nissl substance.
The dendrites spread out from the cell body like the branches of a tree and greatly increase the surface area of the neuron, thereby making more space available to receive contacts from other neurons. Dendrites depend on the cell body for their survival, range from a few tens of micrometers to as much as several millimeters in length. Like antennas, dendrites collect and receive incoming (afferent) information from other neurons or sensory receptors. The dendrites receive these incoming impulses directly on their surface membrane or on tiny projections of membrane called dendritic spines which project outward for a few micrometers like forests of tiny lollipops. The afferent information reaches the neuronal cell body in the form of membrane electrical disturbances. Impulses are conducted away from the neuronal cell body by a single process called the axon, with the exception of a few neuron types that are known to function without axons.
The axon emerges from the cell body as a conical protrusion called the axon hillock. The course the axon then follows depends on the nature of the parent neuron and the target element to be reached.. Axons can be very short (100 micrometers or less), while others have very long courses (up to more than 1 meter). Axons are uncovered (bare) or covered by a insulating lipoprotein cellular sheathe called myelin (Schwann cells in the PNS, oligodendrocytes in the CNS). The long fibers or axons of neurons are also often grouped. In the CNS, bundles of axons carrying information or motor commands of one kind are called tracts. Tracts make up the white matter of the CNS. In the PNS discrete bundles of axons bringing information to the CNS from peripheral structures and conducting motor commands to muscles and glands are called nerves.
Most axons develop side branches along the route called an axon collateral which help to bring information to several parts of the nervous system simultaneously. When the target neural elements are reached, the axon (and collateral) end in a small number of terminal branches. An axon terminal or synaptic bouton is the physical end-point of axon transmission which develops the synapse, the region of functional contact between two nerve cells.