Central Nervous System Help (page 2)
Central Nervous System
The central nervous system (CNS) consists of the brain and spinal cord. The functions of the CNS include body orientation and coordination, assimilation of experiences (learning), and programming of instinctual behavior. The CNS contains gray and white matter. The gray matter consists of either nerve cell bodies and dendrites, or of unmyelinated axons and neuroglia. It forms the outer convoluted cerebral cortex and cerebellar cortex in the brain and forms the inner portion of the spinal cord. The white matter consists of aggregations of myelinated axons and forms nerve tracts, within the CNS.
There are five regions of the brain, some with multiple structures:
Brain region Structures Telencephalon Cerebrum Diencephalon Thalamus, Hypothalamus, and Pituitary gland Mesencephalon Superior colliculus, Inferior colliculus, and Cerebral peduncles Metencepahlon Cerebellum and Pons Myelencephalon Medulla oblongata
The cerebrum consists of five paired lobes within two convoluted cerebral hemispheres. The hemispheres are connected by the corpus callosum. The cerebrum is responsible for higher functions, including perception of sensory impulses, instigation of voluntary movement, memory, thought, and reasoning. The outer convoluted surface, the cerebral cortex, is composed of gray matter. Elevated folds of the convolutions are the gyri (gyrus, singular) and the depressed grooves are the sulci (sulcus, singular). The convolutions greatly increase the surface area of the gray matter. Beneath the cerebral cortex is the thick white matter, the cerebral medulla.
The diencephalon, a major autonomic region of the forebrain, is almost completely surrounded by the cerebral hemispheres. It contains the:
- Thalamus. The thalamus is a paired organ immediately below the lateral ventricle. It is a relay center for all sensory impulses, except smell, to the cerebral cortex.
- Hypothalamus. The hypothalamus consists of several nuclei interconnected to other parts of the brain. Most of its functions relate to regulation of visceral activities including: cardiovascular regulation, body-temperature regulation, water and electrolyte balance, gastrointestinal activity and hunger, sleeping and wakefulness, sexual response, emotions, and control of endocrine functions through stimulation of the anterior pituitary.
- Epithalamus. The pineal gland extends from the epithalamus. It secretes the hormone melatonin, which may play a role in the onset of puberty.
- Pituitary Gland. The pituitary is divided into the anterior pituitary, the adenohypophysis, and the posterior pituitary, the neurohypophysis. This gland has endocrine functions.
The mesencephalon, or midbrain, is a short section of the brain stem between the diencephalon and the pons. It contains the superior colliculi, concerned with visual reflexes, the inferior collicui, responsible for auditory reflexes, and the cerebral peduncles, which contain sensory and motor fibers and are involved with coordinating reflexes.
The metencephalon contains the:
- Pons. The pons consists of fiber tracts that relay impulses from one region of the brain to another. Many cranial nerves originate here. Also the apneusitic and pneumotaxic centers involved with regulating respiratory rate are located in the pons.
- Cerebellum. The cerebellum consists of two hemispheres and is responsible for involuntary coordination of skeletal-muscle contractions in response proprioreceptors within muscles, tendons, joints, and sensory organs.
The medulla oblongata connects to the spinal cord and makes up much of the brain stem. It is composed primarily of white matter tracts that communicate between the spinal cord and the brain. Three areas that control autonomic functions are: the cardiac center, sending inhibitory and accelerator fibers to the heart; the vasomotor center, which causes the smooth muscle of arterioles to contract; and the respiratory center, which controls the rate and depth of breathing.
Ventricles of the brain
The ventricles of the brain consist of a series of cavities that are connected to one another and to the central canal of the spinal cord.
Three connective tissue membranes cover the entire CNS.
The three meninges are (from outermost to innermost): Dura mater, Arachoid, Pia mater.
The dura mater forms a tough tubular sheath around the spinal cord. The epidural space is a vascular area between the sheath and the vertebral canal. It is the location where epidural anesthesia is injected. The subarachnoid space is located between the arachnoid and the pia mater. It contains cerebrospinal fluid. Cerebrospinal fluid (CSF) is a clear, lymph-like fluid produced continually by active transport of substances from blood plasma by specialized capillaries in the roof of the ventricles, the choroid plexuses. CSF forms a protective cushion around and within the CNS; it also buoys the brain. CSF circulates around the CNS through the ventricles of the brain, the central canal of the spinal cord, and the subarachnoid space.
The blood-brain barrier (BBB) is a structural arrangement of the capillaries that surround connective tissue and the vascular processes of astrocytes (a type of neuroglia in the CNS) that cling to the capillaries. The BBB selectively determines which substances can move from the blood plasma to the extracellular fluid of the brain. Fat-soluble compounds readily pass through the BBB, as do H2O, O2, CO2, and glucose. Certain chemicals such as alcohol, nicotine, and anesthetics also readily pass through. Inorganic ions pass more slowly and other substances, such as macroproteins, lipids, certain toxins, and most antibiotics are restricted.
There are over 200 neurotransmitters synthesized and secreted by neurons within the brain. The most important are listed below.
The spinal cord extends through the vertebral canal of the vertebral column to the level of the first lumbar vertebra (L1). It is continuous with the brain through the foramen magnum of the skull. The spinal cord consists of centrally located gray matter involved in reflexes, and peripheral ascending and descending tracts of white matter that conduct nerve impulses to and from the brain. Thirty-one pairs of spinal nerves arise from the spinal cord. The gray matter in cross section has a four-horned appearance (Figure 10-1). The posterior (dorsal) horns receive the axons of sensory fibers that enter the spinal cord; the anterior (ventral) horns contain the dendrites and cell bodies of motor neurons that leave the spinal cord. At the thoracic and lumbar level there are lateral horns that contain the preganglionic sympathetic neurons that leave via the anterior root. The white matter is composed primarily of myelinated fibers that form spinal tracts. The tracts are separated by the horns of gray matter into three regions: posterior, lateral, and anterior funiculi.
Practice problems for these concepts can be found at: Central Nervous System Practice Problems
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