TEORIA DE LA EXTRACCIÓN CON SOLVENTES .1 Principios de Solubilidad

The nervous system of vertebrates is functionally divided between the cen-tral nervous system, consisting of the brain and spinal cord, and the pe-ripheral nervous system, including all neurons that do not have their cell bodies within the brain or spinal cord. Primarily, the nervous system is com-posed of four cell types: neurons, Schwann cells, oligodendrocytes, and as-trocytes.

Neurons are the information transfer cells that perform the primary ac-tivity of the nervous system. Schwann cells, oligodendrocytes, and astrocytes are support cells for neurons. Schwann cells are located only in the periph-eral nervous system, but they have the same function as oligodendrocytes, which are located solely within the central nervous system. Both cell types wrap a fatty myelin sheath around the axon, the electrical signal, to insulate it and thereby increase the speed of conduction. This axonal covering is white, whereas the neuron is gray, so that nerves composed primarily of ax-ons look white because of the myelin, and regiax-ons formed mostly by cell bodies look gray.

Support cells can also absorb excess neurotransmitter and provide cer-tain precursor molecules that the neurons will use to construct essential pro-teins and metabolites. Astrocytes appear only in the central nervous system, and their function is to absorb nutrients from the bloodstream and conduct them to the neurons. Data suggests that support cells are also instrumental in directing immature neurons into their correct location during develop-ment, as well as ensuring the integrity of synapses and guiding regrowth of axons after injury.

The brain and nervous system are composed of grouped functional sys-tems. This means that neurons can be categorized based on what kind of information they convey. These like-neurons are organized into pathways of conduction punctuated by processing nodes. The conduction pathways

axons cytoplasmic extensions of a neuron that transmit impulses away from the cell body

synapses spaces the cells and tissues of an animal the base of the brain to the body

are formed of nerves or fibers containing primarily neuronal axons. The nodes, called ganglia or brain nuclei, are mostly composed of neuronal cell bodies and dendrites. Because information is transferred along pathways, and each node processes the information in a characteristic manner, the ner-vous system is referred to as a labeled-line system.

The nervous system is also called a parallel pathway system, because sensations such as sounds and visual inputs are transferred to the brain in an organized manner within separate nerves. For instance, sounds are di-vided up into their respective frequencies, and each frequency travels in its own fiber, parallel to the other frequency fibers grouped together within the auditory nervous system. The different sounds thus remain segregated until they are processed in the cortex. Finally, although distinct regions of the brain perform unique tasks, many overall concepts that are important psychologically to humans are not located in any one region of the brain.

Memory, emotion, intelligence, and personality are all examples of emer-gent properties, meaning that they result from the coordinated activities of many brain regions.

Neurons carry information in the form of an action potential, which is a rapid (several milliseconds long) change in the electrical conduction of the cell membrane. When a neuron produces an action potential, it is de-scribed as firing, and a single action potential is called a spike. Action po-tentials are the primary form of communication between neurons, and the entire nervous system is mediated by this signal.

One may then wonder how perception can be so complex. This is be-cause many factors contribute to the information encoded by the action po-tentials, including the frequency of action popo-tentials, the probability of an action potential in any particular cell, the morphology (shape) of the neu-ron, the number and location of neurons that contribute the information, Nervous System

A. Sensory homunculus B. Motor homunculus

Lateral Medial

Medial Lateral

g

B A

Homunculus. Redrawn from Kandel, et. al, 2000.

dendrites branched extensions of a nerve cell that transmits impulses to the cell body

action potential a rapid change in the electric charge of the cell mem-brane

the number and location of neurons that receive the information, the type of neurotransmitter it uses, and the contributions of support cells. Further-more, although each individual neuron can only produce an action poten-tial for communication, this signal can have a different shape and character for different neurons.

The opposite of an action potential is a hyperpolarizing potential. This is instigated by inhibitory neurons, which release a neurotransmitter that decreases the probability that the neuron will fire. There may be thousands of inputs to a single neuron, or just one, and the contributions of all the factors listed above allow the combinatory activity of all the neurons in the ordered nervous system to produce consciousness, cognition (knowing), be-havior, sensation, and homeostasis (maintenance of an organism’s general health) in animals.

Peripheral nervous system. In vertebrates the peripheral nervous system is composed of both motor neurons, which instigate muscle movement and activity, and sensory neurons, which convey information about the external and internal state of the organism. Furthermore, interneurons are impor-tant intermediates in both sensory and motor pathways, because they con-nect different circuits and can modify a signal as it follows a particular course.

All subdivisions of the peripheral nervous system are comprised of these three neuronal types. The peripheral nervous system can be further divided into the autonomic nervous system and the somatic nervous system. Be-cause it mediates the activity of heart muscle, smooth muscle, and exocrine glands, the autonomic nervous system is also referred to as the involuntary nervous system. The somatic nervous system is called voluntary because it controls the skeletal muscles.

Autonomic nervous system. The autonomic nervous system is made up of the sympathetic, parasympathetic, and enteric divisions. The enteric sys-tem is a subsection of the peripheral nervous syssys-tem located in the

gas-This micrograph depicts three human nerve cells of the cerebral cortex and their branching fibers.

hyperpolarizing poten-tial any change in membrane potential that makes the inside of the membrane more negatively charged

somatic nervous system a part of the nervous system that controls the voluntary movement of skeletal muscles

homeostasis a state of equilibrium in an ani-mals internal environ-ment that maintains optimum conditions for life

trointestinal tract of the gut and is responsible for mediating digestive re-flexes. The high number and dense compaction of neurons in this system, and its autonomy with respect to the brain, cause some scientists to qualify it as a primitive “second brain.”

The sympathetic and parasympathetic divisions of the peripheral ner-vous system are functional opposites. Whereas the parasympathetic division is responsible for homeostatic activities, such as maintaining a basal respi-ratory pattern, heartbeat, and normal metabolism, the sympathetic division governs the body’s reaction to extreme situations. It instigates emergency measures in response to stress from strong emotions, athletic exertions, bat-tle, severe temperature change, and blood loss. The sympathetic division thus increases activity in the heart and other organs, the sweat glands, the vascular system, and certain smooth muscle groups. Because the autonomic division controls day-to-day bodily functions, it has been characterized as controlling “rest and digest” activities, whereas the sympathetic division is responsible for “fight or flight” reactions.

Somatic nervous system. The somatic nervous system allows verte-brates to monitor and control skeletal muscle output and to consciously sense aspects of the environment. Sensations originating at the skin or muscles of the trunk and limbs of an animal are called somatosensory information. Neurons located in the skin, muscle, joints, and ligaments of the body are specialized for transmitting somatosensory information to the central nervous system. Conveying the position of the limbs, mus-cle exertion, joint stress, temperature, tickle, pain, and tactile informa-tion, these sensory neurons enter the spinal cord via the dorsal root ganglia. The term “dorsal” means “toward the back of the body,” whereas

“ventral” means “toward the front of the body.” Ganglia are congrega-tions of neuronal cell bodies located outside of the brain. All sensory in-formation enters the spinal cord from the dorsal side and then travels up to the brain.

Motor nerves controlling muscle movement descend from the brain and send axons out of the ventral side of the spinal cord. There are ventral roots that contain motor axons, but, unlike the somatosensory nerves, there are no motor ganglia. Motor neurons in the somatic nervous system innervate (connect with) skeletal muscles and can be controlled by a mix of voluntary and involuntary impulses.

Sensation and motor control of the face, head, and neck do not enter