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Chapter 3

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University of Toronto Scarborough
Zachariah Campbell

Chapter 3 organization of the nervous system Portrait - Stroke: an interruption of blood to the brain that kills brain cells and causes sudden appearance of neurological symptoms. - Ischemia a deficiency in blood flow to the brain due to functional constriction or the actual obstruction of a blood vessel such as by a clot. - Tissue okasnubigen activator: breaks up clot Intro - Roughly 1000 neurons and up to 15000 connections - Nuclei: group of cells. Can be seen with special stains to see functional groups. Some are folded, or have distinctive shapes or colours. Neuroanatomical: Finding your way around the brain  Describing Locations in the Brain Brain structures in relation to their location to body parts. (E.g.rostum=beak). - Rostrum=forward, - Dorsal (superior)=up, - Caudal= back, - Ventral down (inferior). - Anterior or frontal =front, - Posterior=back, - Lateral=side, - Medial= center or between. Cut section: - Coronal: vertical cut (crown down), - Horizontal (cut along the horizon) - Sagittal= cut lengthwise (in between eyes). - (Figure 3.1 A, B & C) Symmetrical side names: - Ipsilateral: if structures lie on the same side - Contralateral: lie on the opposite side - Bilateral: if each structure lies in a different hem. - Proximal: close together - Distal: far apart - Afferent: movement towards the brain structure (sensory pathways) - Efferent: away from the brain structure (motor pathways)  A wonderland of Nomenclature (many terms for the same thing) - precenteal gyrus: responsible for diminished motor ability (has many other names) An overview of Nervous System Structure and Function (figure 3.2) - Central Nervous System (CNS): brain and spinal cord - Somatic Nervous System (SNS): spinal cord, cranial nerves, to and from the sensory organs and the muscles, joints and skin. It produces movement and transmits incoming sensory info to the CNS (vision, hearing, pain, temp, touch and position of body movements. - The autonomic nervous system (ANS): balances internal organs to “rest and digest” through the parasympathetic (calming) nerves or to “flight and flee” through the sympathetic nerves (arousing).  Support and Protection - The brain and spinal cord are protected from injury and infection in four ways: 1) The skull encloses the brain and the interlocking vertebrae enclose the spine. The PNS lies outside the skull and vertebrae so it is more vulnerable to injury however can grow new axons and dendrites to renew itself. Self repair (as in PNS) is limited in the CNS. 2) Within bony case enclosing the CNS there is a triple layer membrane the Meninges (figure 3.3) the outer Dura membrane: tough double layer of tissue enclosing the brain in a loose sac. The middle arachnoid membrane: very thin sheet of delicate tissue that follows the contours of the brain resembling a spider web). And the inner Pia mater: moderately tough tissue that clings to the surface of the brain. 3) Cerebrospinal fluid: (CFS) cushions brain and spinal cord from shock and sudden changes of pressure. It circulates in four ventricles inside the brain, spinal column and within the subarachnoid space. It is continuously made and drained off into the circulatory system. - Hydrocephalus: the outflow for the CFS is blocked can cause mental retardation or death 4) The brain and spinal cord are protected from many chemicals that are circulating the rest of the body by the Blood-brain barrier. Capillaries: very small blood vessels. To form the blood brain barrier capillaries form tight junctions with one another.  Blood Supply - The brain receives its blood supply from 2 internal carotid arteries and two vertebral arteries that go up each side of the neck. The four arteries connect at the base of the brain where they enter the skull. From there the cerebral arties branch off into several smaller arteries that supply the brainstem and the cerebellum and give rise to 3 arteries that supply the forebrain. - (Figure 3.4) - Anterior cerebral artery (ACA): supplies the medial and dorsal part of the cortex. - Middle cerebral artery (MCA): supplies the lateral surface of the cortex. - Posterior cerebral artery (PCA): supplies the ventral and posterior surfaces of the cortex. - External or internal cerebral and cerebellar veins: blood spent returns to the heart, it doesn’t NEC. Follow major arteries.  Neurons and Glia - Neural stem cell (germinal cell): they produce carious specialized cells that make up the adult brain, and produce additional stem cells that persist into adulthood. It has extensive capacity for self-renewal. (Figure 3.5). Initially to form a brain it keeps dividing. In adults when one divides, one dies so there is a constant. - Stem cells give rise to progenitor cells: migrate and act as precursor cells. Which also creates a primitive nervous system cell called blasts: some turn into neurons and other glia. - New neurons are produced after birth and even continue to produce throughout adulthood in some cases. - Different types of neurons: 1) Sensory neuron and bipolar neuron (retina) consist of a cell body, with dendrite on one side and an axon on the other 2) Somatosensory neurons: project from the body’s sensory receptors to the spinal cord. The dendrite and axon are connected which speeds info transfer because it doesn’t have to go through the cell body. 3) Interneurons: within brain and spinal cord. They link sensory and motor activity in CNS. There are many different kinds. Many kinds have many dendrites and branches. A brain or spinal cord interneuron only has one axon although it can branch as well. 4) Motor neurons: located in the brain stem projects to facial muscles. And other muscles through the spinal cord. - (Figure 3.6) and (table 3.1).  Gray, White and Reticular matter - Gray matter: gray-brown colour from the capillary blood vessels and neuronal cells that predominate there. - White matter: consists mostly of axons that extend from these cell bodies to form connections in other areas. The axons are covered in an insulating layer of glial cells composed of fatty substances that give it a white colour. - Reticular mater: mixture of cell bodies and axons, which is why it appears gray and white or net like. - (Figure 3.7)  Layers, nuclei, nerves and tracts - A well defined group of cell bodies can form a layer or a nuclei - Tract (fiber pathway another name): a large collection of axons projecting to or away from the nucleus or layer in the CNS. They carry info from one place to another in the CNS (e.g. optic tract carries info from the retina (area in brain) to other parts of the brain). - Nerves: fiber pathways that enter and leave the CNS. (E.g. auditory nerve or vagus nerve, after they leave the central nervous system they to are called tracts. The origin and Development of the Central Nervous System - As the embryo grows the front and back will expand greatly and subdivide further, making 5 regions in all. Three regions of a primitive creature: - Prosencephalon: (front brain): responsible for olfaction. - Mesencephalon: (middle brain): vision and hearing - Rhombencephalon (hindbrain): controls movement and balance. (At this part of development the spinal cord is considered part of the hindbrain. Mammalian Embryo - The prosencephalon divides into a) Telencephalon: (end brain): the prosencephalon develops further to form the cerebral hemisphere (the cortex and related structures) b) Diencephalon: (between brain): the remaining part of the prosencephalon and includes the hypothalamus. - The mesencephalon remains. - The back brain also divides into: a) Metencephalon: (across brain): includes large cerebellum b) Myelencephalon: (spinal brain): lower region of the brain stem. - Levels of function: newer levels partly replicating the work of older ones. Each region adds a different dimension to behaviour. - The brain starts as a tub and begins to fold and mature. The interior remains “hollow” - Ventricles: four pockets created by folding of the hollow interior.  Lateral ventricles: first and second, 3 and fourth extend into brain stem.  All are filled thth cerebrospinal fluid, which drains through the lateral out through the 4 and eventually drains into circulatory system.  (Figure 3.9) The Spinal Cord  Spinal cord structure and the spinal nerves - Nerve cells are divided into segments. Each segment receives nerve fibers from afferent sensory receptors. And sends efferent fibers to control muscle of that part of the body. - 30 spinal cord segments; 8 cervical (C), 12 thoracic (T), 5 Lumbar (L) and 5 sacral (S) - (Figure 3.10) - Dermatomes: segments that encircle the spinal column as a stack of rings. - Dorsal Root: afferent fibers entering the dorsal part of the spinal cord, bring info to the sensory receptors of the body. The spinal cord nerve fibers converge forming a strand of fibers called the dorsal root. - Ventral Root: afferent fibers leaving the ventral (anterior) part of the spinal cord carrying info from spinal cord to the muscles, which forms a sim strand of spinal nerves. - (3.11)  Spinal cord function and the Spinal Nerves - Bell-Magendie Law: principle that states that the dorsal part of the spinal cord is sensory and the central part is motor. - Spinal cord injury  Paraplegic: spinal cord is cut so they no longer have control of their legs.  Quadriplegic: if the cut is high enough and they cannot use their arms down. - Although in some species the fibres of the spinal cord can regrow in adult mammals they cannot. - (Figure 3.2) - Reflexes: movements dependent only on spinal cord functions - Sensory nerves: there are nerves for pain, temperature, muscle and joint movement. The size of the nerves varies. Pain and temp are normally smaller. Touch and muscle sense are larger. - Flexion movements: movements that bring the limb inwards; towards body and away from injury (temp and pain). - Extension movements: bring the limb outwards away from the body to maintain contact with stimulus.  Connection between central and Somatic Nervous Systems - Somatic nervous system is monitored and controlled by the CNS. - Spinal cord oversees the spinal nerves and brain oversees 12 pairs of cranial nerve
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