VERTEBRATE NERVOUS SYSTEMS
Vertebrate nervous systems include a pronounced trend toward cephalization and a dorsal, hollow nerve cord.
The vertebrate brain follows a three part plan of forebrain, midbrain and hindbrain. The cerebrum and surrounding cortex of the forebrain reach its greatest development in mammals.
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The Human Central Nervous System
The human nervous system is divided into the central nervous system (CNS) - the brain and the spinal cord - and the peripheral nervous system (PNS) - the somatic and autonomic systems.
The somatic system is largely composed of sensory receptors and sensory neurons and the voluntary motor nerves that move skeletal muscle, while the autonomic system is motor and involuntary, dedicated mainly to homeostasis. The central nervous system arises in the embryo from the neural tube.
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The Spinal Cord
The spinal cord includes pathways between the brain and much of the peripheral nervous system. The white regions are myelinated neurons, while the gray are nerve cell bodies. Many reflexive acts occur at the cord level.
The cord emerges from the brain through the foramen magnum and lies within the vertebral canal. It is the bathed in the cerebrospinal fluid and surrounded by the meninges, three layers of supporting connective tissue.
Motor neurons arise from cell bodies in the cord to pass out through the ventral motor root of the spinal nerves. Sensory neurons enter the the cord via the dorsal sensory root from rows of ganglia outside, where their cell bodies cluster.
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The Human Brain
The human brain weighs about 1.4 kg, has a volume of 1300 to 1500 cc contains some 100 billion neurons and 10 times that number of glial cells. Cell bodies form its gray outer regions, while myelinated fibers make up its white inner mass. Cerebrospinal fluid and the meninges cushion and protect the brain, respectively.
The hindbrain consist of
the medulla oblongata (or medulla), which controls breathing and heart rate and contains many pathways, including some form cranial nerves;
the cerebellum, concerned with balance, equilibrium and voluntary muscle coordination;
the pons, which contains tracts traveling between forebrain and cord and to and from the cerebellum.
The midbrain forms connections between the hindbrain and forebrain and receives sensory input from auditory and visual receptors.
The forebrain is involved in conscious thought, sensory reception, voluntary movement, and other voluntary acts. It consists of the following:
The thalamus, which is a relay structure, connects the various parts of the brain and includes the reticular system, which taps incoming and outgoing communications. It also acts as an alarm system and suppresses irrelevant stimuli, thus permitting sleep.
The hypothalamus, which monitors many functions, acts as a homeostatic regulator (heart rate, blood pressure, body temperature, thirst, hunger, sex drive). It also stimulates hormonal activity in the pituitary and is subject to negative feedback.
The limbic system, containing a number of specific nuclei, includes the hypothalamus, thalamus and some cortical pathways. It links the fore brain and midbrain, and is involved in emotion (for example, when areas of the amygdala are stimulated, we may experience rage).
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The cerebrum, the largest region of the forebrain, is divided into left and right cerebral hemispheres, which are covered by the wrinkled cerebral cortex. The cortex contains some 15 billion cell bodies and dendrites. The white matter below includes the myelinated fibers.
The importance of the cerebrum to common voluntary acts is minimal in frogs but increasingly important in rats, cats, dogs, monkeys, and humans.
The cerebrum is made up of a number of lobes. The occipital lobe, receives and analyzes visual information, while the temporal lobe (bounded by the fissure of Sylvius, or lateral sulcus) processes auditory input and some visual information. The parietal lobe processes sensory information, including body position. It is separated from the frontal lobe by the fissure of Rolando or central sulcus.
The processing of sensory input and the instigation of voluntary muscle action occurs in specific regions of the cortex.
The cerebral hemispheres are functionally distinct and many learned patterns take place in just one hemisphere. The left hemisphere predominates in the right handed persons and in the more common type of left handedness, but in the rare form of left handedness, dominance is in the right. Speech is primarily a left hemisphere function, as is analytical thought. Damage to this hemisphere can provide a loss of language abilities.
Connections between the hemispheres occur through the corpus callosum