?The nervous system consists of the brain, spinal cord, and a complex network of neurons. This system is responsible for sending, receiving, and interpreting information from all parts of the body. The nervous system monitors and coordinates internal organ function and responds to changes in the external environment. This system can be divided into two parts: the central nervous system and the peripheral nervous system. Let’s take a look at the central nervous system. Central Nervous System
The central nervous system (CNS) is the processing center for the nervous system. It receives information from and sends information to the peripheral nervous system. The two main organs of the CNS are the brain and spinal cord. The brain processes and interprets sensory information sent from the spinal cord. Both the brain and spinal cord are protected by three layers of connective tissue called the meninges. Within the central nervous system is a system of hollow cavities called ventricles.
The network of linked cavities in the brain (cerebral ventricles) is continuous with the central canal of the spinal cord. The ventricles are filled with cerebrospinal fluid which is produced by specializedepithelium located within the ventricles called the choroid plexus. Cerebrospinal fluid surrounds, cushions, and protects the brain and spinal cord from trauma. It also assists in the circulation of nutrients to the brain. Central Nervous System: Brain The brain is the control center of the body. It consists of three main components: the forebrain, the brainstem, and the hindbrain.
The forebrain is responsible for a variety of functions including receiving and processing sensory information, thinking, perceiving, producing and understanding language, and controlling motor function. The forebrain contains structures such as the thalamus and hypothalamus which are responsible for such functions as motor control, relaying sensory information, and controlling autonomic functions. It also contains the largest part of the brain, the cerebrum. Most of the actual information processing in the brain takes place in the cerebral cortex. The midbrain and the hindbrain together make up the brainstem.
The midbrain is the portion of the brainstem that connects the hindbrain and the forebrain. This region of the brain is involved in auditory and visual responses as well as motor function. The hindbrain extends from the spinal cord and contains structures such as the pons andcerebellum. These regions assist in maintaining balance and equilibrium, movement coordination, and the conduction of sensory information. The hindbrain also contains themedulla oblongata which is responsible for controlling such autonomic functions as breathing, heart rate, and digestion.
The brain plays a central role in the control of most bodily functions, including awareness, movements, sensations, thoughts, speech, and memory. Somereflex movements can occur via spinal cord pathways without the participation of brain structures. The cerebrum is the largest part of the brain and controls voluntary actions, speech, senses, thought, and memory. The surface of the cerebral cortex has grooves or infoldings (called sulci), the largest of which are termed fissures. Some fissures separate lobes.
The convolutions of the cortex give it a wormy appearance. Each convolution is delimited by two sulci and is also called a gyrus (gyri in plural). The cerebrum is divided into two halves, known as the right and left hemispheres. A mass of fibers called the corpus callosum links the hemispheres. The right hemisphere controls voluntary limb movements on the left side of the body, and the left hemisphere controls voluntary limb movements on the right side of the body. Almost every person has one dominant hemisphere.
Each hemisphere is divided into four lobes, or areas, which are interconnected. The frontal lobes are located in the front of the brain and are responsible for voluntary movement and, via their connections with other lobes, participate in the execution of sequential tasks; speech output; organizational skills; and certain aspects of behavior, mood, and memory. The parietal lobes are located behind the frontal lobes and in front of theoccipital lobes. They process sensory information such as temperature, pain,taste, and touch.
In addition, the processing includes information about numbers, attentiveness to the position of one’s body parts, the space around one’s body, and one’s relationship to this space. The temporal lobes are located on each side of the brain. They process memory and auditory (hearing) information and speech and language functions. The occipital lobes are located at the back of the brain. They receive and process visual information. The cortex, also called gray matter, is the most external layer of the brain and predominantly contains neuronal bodies (the part of the neurons where the DNA-containing cell nucleus is located).
The gray matter participates actively in the storage and processing of information. An isolated clump of nerve cell bodies in the gray matter is termed a nucleus (to be differentiated from a cell nucleus). The cells in the gray matter extend their projections, called axons, to other areas of the brain. Fibers that leave the cortex to conduct impulses toward other areas are termedefferent fibers, and fibers that approach the cortex from other areas of the nervous system are termed afferent (nerves or pathways).
Fibers that go from the motor cortex to the brainstem (for example, the pons) or the spinal cord receive a name that generally reflects the connections (that is, corticopontine tract for the former and corticospinal tract for the latter). Axons are surrounded in their course outside the gray matter by myelin, which has a glistening whitish appearance and thus gives rise to the term white matter. Cortical areas receive their names according to their general function or lobe name. If in charge of motor function, the area is called the motor cortex.
If in charge of sensory function, the area is called a sensory or somesthetic cortex. The calcarine or visual cortex is located in the occipital lobe (also termed occipital cortex) and receives visual input. The auditory cortex, localized in the temporal lobe, processes sounds or verbal input. Knowledge of the anatomical projection of fibers of the different tracts and the relative representation of body regions in the cortex often enables doctors to correctly locate an injury and its relative size, sometimes with great precision. Central Nervous System: Spinal Cord
The spinal cord is a cylindrical shaped bundle of nerve fibers that is connected to the brain. The spinal cord runs down the center of the protective spinal column extending from the neck to the lower back. Spinal cord nerves transmit information from body organs and external stimuli to the brain and send information from the brain to other areas of the body. The nerves of the spinal cord are grouped into bundles of nerve fibers that travel in two pathways. Ascending nerve tracts carry sensory information from the body to the brain.
Descending nerve tracts send information pertaining to motor function from the brain to the rest of the body. Central Nervous System: Neurons Neurons are the basic unit of the nervous system. All cells of the nervous system are comprised of neurons. Neurons contain nerve processes which are “finger-like” projections that extend from the nerve cell body. The nerve processes consist of axons and dendrites which are able to conduct and transmit signals. Axons typically carry signals away from the cell body. They are long nerve processes that may branch out to convey signals to various areas.
Dendrites typically carry signals toward the cell body. They are usually more numerous, shorter and more branched than axons. Axons and dendrites are bundled together into what are called nerves. These nerves send signals between the brain, spinal cord, and other body organs via nerve impulses. Neurons are classified as either motor, sensory, or interneurons. Motor neurons carry information from the central nervous system to organs, glands, and muscles. Sensory neurons send information to the central nervous system from internal organs or from external stimuli. Interneurons relay signals between motor and sensory neurons.
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