brasheranatomyCNS,PNS

THE CENTRAL NERVOUS SYSTEM

The brain begins to form about 4 weeks after fertilization of the egg occurs.

The brain has 4 major regions: 1) Cerebral Hemispheres 2) Diencephalon 3) Brain Stem 4) Cerebellum

1) Cerebral Hemispheres A) Anatomy of the cerebrum a) gyri--raised ridges on the surface of the cerebrum b) sulci--grooves on the surface of the cerebrum c) fissures--deep depressions on the surface of the cerebrum d) gray matter--darker tissue located mainly on the outside of the cerebrum which consists of cell bodies of neurons e) white matter--lighter tissue located on the inside of the cerebrum, consisting of fiber tracts f) corpus callosum--a fiber tract of white matter that connects the cerebral hemispheres
 * This is the largest part of the brain, which encircles most of the rest of the brain.
 * The left side of the cerebrum regulates the right side of the body.
 * Longitudinal fissure--divides the cerebrum into left and right halves
 * Central fissure--divides the cerebrum into anterior and posterior sections
 * Lateral sulcus--divides the cerebrum along the lateral sides into superior and inferior portions

B) Physiological Areas of the Cerebrum a) Broca's Area--Area located at the base of the precentral gyrus. Vocalization of words is regulated here. This area is typically present only in the left hemisphere b) Intellectual reasoning--Regulated by areas in the anterior part of the frontal lobe c) Complex memories--Regulated by the temporal and frontal lobes d) Speech-- Regulated at the junction of the temporal, parietal, and occipital lobes e) Language comprehension--Regulated by the frontal lobe f) Basal Nuclei--These are areas of gray matter located deep within the cerebrum, which help regulate voluntary motor activities. (They modify instructions sent to the skeletal muscles.)


 * Huntington's Chorea is a genetic disease affecting the basal nuclei. It causes jerky movement. Doctors use dopamine (neurotransmitter) blockers to help the symptoms.**
 * Parkinson's Disease is caused by a lack of dopamine. Patients have tremors and have trouble initiating movements.

2) Diencephalon A) Thalamus--This area is a relay station for sensory impulses passing upward into the sensory areas of the cerebrum. As impulses enter the thalamus, we can detect if the sensation is pleasant or unpleasant. B) Hypothalamus--This area helps regulate body temperature, water balance, and metabolism. The hypothalamus is a part of the //limbic system,// or emotional-visceral brain, which helps regulate thirst, appetite, sex drive, pain, and pleasure. C) Pituitary Gland--An endocrine gland which hangs from the floor of the hypothalamus by a stalk. D) Mammillary Bodies--Reflex centers involved in the sense of smell. E) Epithalamus--The roof of the third ventricle contains the pineal body, which is an endocrine gland, and the choroid plexus of the third ventricle, which is a mass of capillaries that forms CSF.
 * The structures of the diencephalon surround a cavity called the third ventricle.

3) Brain Stem A) Midbrain a) Cerebral Aqueduct--A canal connecting the 3rd ventricle with the 4th ventricle. b) Cerebral Peduncle--A pair of fiber tracts which convey ascending and descending impulses. c) Corpora Quadrigemina--This area consists of 4 rounded bodies that are reflex centers involved with vision and hearing. B) Pons--This area consists mostly of fiber tracts, some of which help control breathing. C) Medulla Oblongata--This area contains centers that control heart rate, blood pressure, breathing, swallowing, and vomiting.
 * Contains many small areas of gray matter which control breathing and blood pressure.
 * The Reticular Formation is gray matter involved in motor control of the visceral organs.
 * The Reticual Activating System regulates consciousness and sleep/wake cycles. Damage can cause permanent coma.

4) Cerebellum
 * This portion of the brain has two hemispheres.
 * This area coordinates skeletal muscle activity, controls balance, and controls equilibrium.
 * Damage to the cerebellum causes ataxia, where individuals exhibit clumsy and disorganized movements.

Protection of the brain and spinal cord is provided by the meninges (membranes) that surround them. They cushion the brain and also contain the cerebrospinal fluid (CSF). The outmost membrane is called the dura mater and is very protective. The middle layer, called the arachnoid mater, has a web-like appearance. The pia mater is a delicate layer that lies next to the brain and the spinal cord. Between the arachoid mater and the pia mater is a space called the subarachnoid space. CSF is found in this space and circulates around the brain and spinal cord. CSF is produced by tiny capillaries within the ventricles (spaces) of the brain and later returns to the bloodstream.

The spinal cord is an extension of the brain. It lies within the vertebral column and forms a solid cord until it reaches the level of the 1st lumbar vertebra. At the point, the cord separates to become a bundle of nerves called the cauda equina (horse's tail), which fills the remainder of the vertebral column. In the spinal cord, white matter is located on the outside of the cord, with gray matter found on the inside.

While the brain and spinal cord form the central nervous system (CNS), all nerves that extend from the brain and spinal cord form the peripheral nervous system (PNS). Extending from the brain are 12 pairs of nerves called cranial nerves. The first letter of each word in the following sentence helps students to remember the order of the nerves: On (olfactory) Old (optic) Olympus (Oculomotor) Towering (Trochlear) Tops (Trigeminal), A (Abducens) Finn (Facial) And (Auditory) German (Glossopharyngeal) Viewed (Vagus) Some (Spinal Accessory) Hopps (Hypoglossal). These nerves mainly innervate areas of the face and neck. The Vagus nerves innervate the internal organs.

Extending from the spinal cord are 31 pairs of spinal nerves. They exit the spinal cord through the intervertebral foramina of the vertebral column. There are 8 pairs of cervial nerves, 12 pairs of thoracic nerves, 5 pairs of lumbar nerves, 5 pairs of sacral nerves and one pair of coccygeal nerves. All of these nerve fibers join together after they leave the spinal cord to form bundles that are called plexi (plexus is singular). The cervical plexus, brachial plexus, lumbar plexus, and sacral plexus innervate the arms and legs. The thoracic nerves do not form a plexus.