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Chapter 3

PSYB65 Chapter 3.doc

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Zachariah Campbell

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PSYB65 – Chapter 3 Neuroanatomy: Finding Your Way around the Brain: Describing Locations in the Brain: - The body is symmetrical o Ipsilateral – structures that lie on the same side o Contralateral – structures that lie on opposite sides o Bilateral – one of them lies in each hemisphere o Proximal – structures that are close to one another o Distal – structures that are far from one another - Movement away and toward the brain: o Afferent (sensory pathways) – movement toward a brain structure o Efferent (motor pathways) – movement away from a brain structure A Wonderland of Nomenclature: - Precentral gyrus – a part of the brain responsible for motor ability o Also called:  Gyrus percentralis – the motor strip  Jackson’s strip – after Hughlings-Jackson suggesting that the representation of the body in the brain also is orderly  Primary motor cortex or M1 – referred by electrophysiologists to distinguish it from other motor regions of the cortex  Somatomotor strip  The motor homunculus  Area pyramidalis – Gall found that the pyramidal tract that extends from the cortex into the spinal cord comes mainly from this cortical region - Longest name for a brain structure is nucleus reticularis tegmenti pontis Bechterewi o Known as NRTP An Overview of Nervous System Structure and Function: - CNS – consists of the brain and spinal cord - PNS – encompasses everything else o SNS – consists of all the spinal and cranial nerves to and from the sensory organs, muscles, joints and skin  Produces movement and transmits incoming sensory information to the CNS o ANS – balances the body’s internal organs to rest and digest through the parasympathetic (calming) nerves  Or to fight and flee through the sympathetic (arousing) nerves Support and Protection: - The brain and spinal cord are supported and protected from injury and infection in 4 ways: o Brain is enclosed in the skull and spinal cord is encased in a bony vertebrae  PNS lacks bony protection but can renew itself after injury by growing new axons and dendrites o Meninges – triple layered set of membranes  Dura mater – outer layer, tough double layer of tissue  Arachnoid membrane – middle, very thin sheet of delicate tissue  Pia mater – inner, moderately tough tissue that clings to the surface of the brain o Brain and spinal cord are cushioned from shock and sudden changes of pressure by the cerebrospinal fluid  Circulates in the 4 ventricles inside the brain, spinal column and within the subarachnoid space  If outflow is blocked, hydrocephalus (water brain), severe mental retardation or death can result o Blood-brain barrier – protects the brain and spinal cord from many chemical substances circulating the rest of the body  Cells of the capillaries forms this barrier, which are very small blood vessel that form tight junctions with one another Blood Supply: - Brain receives its blood supply from 2 internal carotid arteries and 2 vertebral arteries that course up each side of the neck o The cerebral arteries branch off into several smaller arteries that irrigate the brainstem and cerebellum and give rise to 3 arteries that irrigate the forebrain.  Anterior cerebral artery (ACA) – irrigates the medial and dorsal part of the cortex  Middle cerebral artery (MCA) – irrigates the lateral surface of the cortex  Posterior cerebral artery (PCA) – irrigates the ventral and posterior surfaces of the cortex - The veins of the brain are classified as external and internal cerebral and cerebellar veins Neurons and Glia: - Neural stem cell – a single undifferentiated cell o Produces the various specialized cells that make up the adult brain - A stem cell has extensive capacity for self-renewal o Forms a brain by dividing and producing 2 stem cells, both of which can divide again o In the adult, one stem cell dies after each division, so the mature brain contains a constant number of dividing stem cells  Adult stem cells serve as a source of new neurons for certain parts of the adult brain; plays a role in brain repair after injuries such as stroke - In the developing embryo, stem cells give rise to progenitor cells that migrate and act as precursor cells o Gives rise to non-dividing, primitive types of nervous system cells called blasts  Blasts differentiate into: • Neurons • Glia - Neurons: o Sensory neuron – a bipolar neuron, consists of a cell body with a dendrite on one side and an axon on the other o Somatosensory neurons – project from the body’s sensory receptors into the spinal cord  Modified so that the dendrite and axon are connected, which speeds information conduction o Interneurons – link up sensory and motor neuron activity in the CNS o Motor neurons – located in the brainstem project to facial muscles  Motor neurons together are called the final common path since all behaviour produced by the brain is produced through them - Glia cells: (table 3.1) o Ependymal cell, Astrocyte, Microglial cell, Oligodendroglial cell, Schwann cell Gray, White and Reticular Matter: - Gray matter – acquires its characteristic gray-brown color from the capillary blood vessels and neuronal cell bodies that predominate there - White matter – consists largely of axons that extend from these cell bodies to form connections with neurons in other brain areas o Axons are covered with an insulating layer of glial cells that are composed of lipids (that make milk look white) - Reticular matter – contains a mixture of cell bodies and axons from which it acquires its mottled gray and white or netlike appearance Layers, Nuclei, Nerves and Tracts: - Tract or fiber pathway – a large collection of axons projecting to or away from a nucleus or layer in the CNS o They carry information from one place to another within the CNS  Ex, optic tract carries information from the retina of the eye to other visual centers in the brain - Nerves – fibers and fiber pathways that enter and leave the CNS The Origin and Development of the Central Nervous System: - Levels of function - three part scheme splits into forebrain, brainstem and spinal cord: (table 3.8) - Brain begins as a tube and after folding and maturing, its interior remains hollow o Ventricles – 4 prominent pockets created by the folding of this hollow interior in the brain  Numbered 1-4 • Lateral ventricles – first and second, form C-shaped lakes underlying the cerebral cortex • 3 and 4 ventricles – extend into the brainstem and spinal cord  All are filled with cerebrospinal fluid, produced by ependymal glial cells located adjacent to the ventricles th • Flows from the lateral ventricles out through the 4 ventricle and eventually drains into the circulatory system The Spinal Cord: Spinal-Cord Structure and the Spinal Nerves: - Within the body is a tube of nerve cells that divides into segments that receives nerve fibers from afferent sensory receptors in the part of the body adjacent to it o It sends efferent fibers to control the muscles of that part of the body - The spinal cord lies inside the bony spinal-column vertebrae, which are categorized into 5 regions from top to tail o 30 spinal cord segments:  8 cervical (CS)  12 thoracic (T)  5 lumbar (L)  5 sacral (S) - These segments are called dermatomes, that encircle the spinal column as a stack of rings - Each spinal segment is connected by SNS spinal nerve fibers to the body dermatome of the same number o Cervical segments – control forelimbs o Thoracic segments – control the trunk o Lumbar segments – control the hind limbs - Dorsal root – a strand of fibers that are formed from spinal nerve fibers converging as they enter the spinal cord o Afferent fibers enter the dorsal part of the spinal cord, brings information from the sensory receptors of the body - Ventral root – a similar strand of spinal nerves that are formed from efferent fibers o Efferent fibers leave the ventral parts of the spinal cord, carrying information from the spinal cord to the muscles Spinal-Cord Function and the Spinal Nerves: - Francois Magendie – Found that cutting the dorsal roots caused loss of sensation and cutting the ventral roots caused loss of movement - Charles Bell – eleven years earlier, suggested the opposite functions for each of the roots, basing his conclusions on anatomical information and the results from somewhat inconclusive experiments on rabbits o Today the principle that the dorsal part of the spinal cord is sensory and the ventral part is motor is called the Bell-Magendie law - Charles Sherrington – showed that the spinal cord retains many functions even after it has been separated from the brain o Paraplegic – persons whose spinal cords are cut so that they no longer have control over their legs o Quadriplegic – If the cut is higher on the cord, making them unable to use their arms either - Fibers in the spinal tracts do not re-grow in adult mammals o Idea that new growth is prevented by the presence of certain inhibitory molecules on the tracts of the cord below the cut - Pain and tactile receptors made in the SNS communicate with fibers in many other segments of the spinal cord and can produce appropriate adjustments in many body parts o Ex. one leg is withdrawn in response to a painful stimulus, the other leg must simultaneously extend to support the body’s weight - The spinal cord contains all the SNS connections required for allowing an animal to walk o SNS consists of all the spinal and cranial nerves that produce movement and transmit incoming sensory information to the CNS o Sensory information plays a central role in eliciting different kinds of movements organized by the spinal cord  Reflexes – movements dependent only on spinal-cord function, specific movements elicited by specific forms of sensory stimulation  Size of the spinal nerve fiber coming from each kind of receptor is distinctive • Pain and temperature fibers are smaller o Usually produces flexion movements that bring the limb inward, toward the body and away from injury • Touch and muscle sense are larger o Usually produces extension movements, which extend the limb outward, away from the body Connections between Central and Somatic Nervous Systems: - The somatic nervous system is monitored and controlled by the CNS o The spinal cord oversees the spinal nerves o The brain oversees the 12 pairs of cranial nerves  Cranial nerves can have: • Afferent functions, such as for sensory inputs to the brain • Efferent functions, such as for motor control Autonomic Nervous System Connections: - ANS regulates the internal organs and glands by connections through the SNS to the CNS o Sympathetic and Parasympathetic work in opposition  Sympathetic system (fight or flight) – arouses the body for action • Ex. stimulating the heart to beat fast  Parasympathetic system (rest and digest) – calms the body down • Ex. slowing the heartbeat - ANS interacts with the rest of the nervous system o Activation of the sympathetic system starts in the thoracic and lumbar spinal-cord regions  The spinal cord is connected to a chain of autonomic control centers, collections of neural cells called sympathetic ganglia that function like a primitive brain, controlling the internal organs o A part of the parasympathetic system also is connected to the spinal cord, specifically to the sacral region  The greater part of the parasympathetic system derives from 3 cranial nerves: • Vagus nerve – calms most of the internal organs • Facial nerve – controls salivation • Oculomotor nerve – controls pupil dilation and eye movements  The PNS connects with parasympathetic ganglia near the target organs  Pain in these organs is perceived
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