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Chapter 13 - Central Nervous System (CNS)
Appl Human Anat W/Lab (APK 2100C)
University of Florida
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A. INTRODUCTION a. CNS has been likened to central switchboard of telephoen system that interconnects and routes dizzying number of incoming and outgoing calls i. Past b. Now it is a supercomputer c. As basis of each person’s unique behaviour, personality and itellect, the workings o the human brain are a marvel and understanding it's complexity, a challenge B. The Brain a. The function of the brain range from mundane, but essential, life-sustaining activities to the most complex neural functions i. Brain controls heart rate, respiratory rate and blood pressure 1. Maintains internal environment hrough control of autonomic nervous system and endocrine system ii. Through cranial nerves that attach to it, brain is involved in peripheral innervation
- Innervation of the head, neck and throacic and abdominal viscera iii. Brain performs high-level takss - those associated w/ intelligence, consciousness, memory, sensory-motor integration, emotion, behvaiour and socilisation b. Human brian is approx 2 large handfuls of pinkish gray tissue, consistency of cold outmeal c. Avergae adult human brain weighs about 1500g or 3 pounds d. Basic PArts and Organisation of the Brain i. Classify brain in 4 parts 1. Brain stem 2. Cerebellum 3. Diencephalon 4. Cereblum ii. Gray mayyer contains short, nonmyelinated neurons and neuron cell bodies iii. White matter is composed of myelinated and nonmyelinated axons iv. Brain has inner region of gray mayyer adjacent to hollow ventricles
- Surrounded by white matter v. Brian has additional regions of gray matter that are not present in spinal cord vi. Difference occurs because during brain development, certain groups of neurons migrate externally to form collections of gray matter in regions that otherwise consist of grey matter
- Occur in cerevellum and cerebrum, wehre sheets of gray matter lie at surface of brain
- Each external sheets of gray matter, cortex; sheet covering cerebellum is cerebellar cortex, and one covering cerbrum is cerebral cortex vii. All other gray matter in brain is in form of cluster of neuron cell bodies (brain nuclei)
viii. Functionally, large amount of gray matter in brain allows part of CNS to perform very neural functions 1. Because gray matter contains many small interneurons that process information e. Ventricles of the brain i. Paired lateral ventricles (first and seconf ventricleS) lie in cerebral hemispherese
- Horsehsow shape reflects bending of hemispheres during development
- Two lateral ventricles lie close togther, separated by only thin media membrane, spetum pellucidum ii. The third ventricle lies in the diencephalon. Anteriorly, it connects to each lateral ventricle through an interventricular foramen. iii. In the midbrain, the thin tubelike central cavity is called the cerebral aqueduct, which connects the third and fourth ventricles. iv. The fourth ventricle lies in the brain stem, dorsal to the pons and the superior half of the medulla oblongata. Three openings occur in the walls of the fourth ventricle: the paired lateral apertures and the median aperture in its roof. These holes connect the ventricles with the subarachnoid space, which surrounds the whole CNS
- Allows cerebrospinal fluid to fill both the ventricles and the subarachnoid space. v. Fourth ventricle connects caudally to the central canal of the inferior medulla and spinal cord. f. The Brain Stem i. Most caudal of four major parts of the brain is the brain stem ii. Caudal to rostral, the three regions of the brain stem are mdeulla oblongata, pons and midbrain iii. Each region is an inch long, and together they make up 2% of total brian mass iv. Lying in the posterior cranial fossa of the skull, on the basilar part of the occipital bone, the brain stem has four general functions: 1. It acts as a passageway for all the fiber tracts running between the cerebrum and the spinal cord. 2. It is heavily involved with the innervation of the face and head; 10 of the 12 pairs of cranial nerves attach to it. 3. It produces the rigidly programmed, automatic behaviors necessary for survival. 4. It integrates auditory reflexes and visual reflexes. v. Brain stem has same structural plan as psinal cord, outer white matter surrounding inner region of gray matter vi. Brain nuceli or gray matter are also located in white matter of brain stem vii. The Medulla Oblangata
- Most caudal part of brain stem
- Contirnuous with spinal cord at level of foramen magnum of skull
- White matter fiber tracts connecting more rostal regions of brain with spinal cord must pass through medulla oblangata a. Externally, two longitudinal ridges called pyramids flank the ventral midline of the medulla. i. Ridges are formed by the pyramidal tracts, large fiber
muscles. The hypoglossal nucleus, a motor nucleus, is located dorsomedially, just deep to the fourth ventricle. 7. Running through core of brain stem is loose cluster of brain nuclei, reitucla rfomraiton 8. Brian nuclei in reticular formation form three colums on each side that extend length of brain stem a. Midline raphe nuclei, which are flanked by b. Medial nuclear group c. Lateral nuclear group 9. Reticular formaiton nuclei that are clustered around motor nuclei of cranial nerves function to coordinate reflexes and autonomic bheabiours involving cranial nerves 10. Most important nuclei in medulla’s reticular formation involved w/ viscerl activaties: a. The cardiac center adjusts the force and rate of the heartbeat. b. The vasomotor center regulates blood pressure by stimulating or inhibiting the contraction of smooth muscle in the walls of blood vessels, thereby constricting or dilating the vessels. Constriction of arteries throughout the body causes blood pressure to rise, whereas dilation reduces blood pressure. c. The medullary respiratory center controls the basic rhythm and rate of breathing. viii. The Pons
- Bulge wedged between midbrain and medulla oblongata
- It is separated from cerebrellum by fourth ventircle
- Pons forms ventral bridge between brain stem and cerebellum
- Thick pyramidal motor tracts descending from cerbral cortex pass through pons ventrally
- Interspersed among fibres of these tracts are numerous pontine nuclei, which are relay brain nuclei in path that connects portion of cerebral cortext w/ cerebllum a. Involved w. Cooridantion of coluntary movements
- Ponting nuclei send axons to cerebellum in thick middle cerebella peduncles
- Several cranial attach to pons a. Tirgeminal innervates skin of face and cheiwing muscles b. Abducens innervates eye-moving muscle c. Facial supplies muscles of facial expression, among other functions
- Reticular formation nuclei in pona lie ventral to cranial nerve nuclei and function in control of autonomic behaviours ix. The Midbrain
- Lies between diencephalon and pons
- Central cabotu os cerebral aqueduct, which divides midbrain to tectum dorsally and paired cerebral peduncles ventrally
- White fiber tracts and relay nuclei in midbrain connect regions of CNS
- Cerebral peduncles on vbentral surface of brain form vertical pillars that appear to hold up cerebrum a. Peduncles contain pyramidal motor tracts descending from
cerebrum toward spinal cord b. Ventral part of each peduncle that contains this tracts is crus cerebri 5. Superior cerebellar peducnles, lcoated dorsally, contain fiber tracts that connect midbrian to cerebellum 6. In cross section, 2 pigmeneted brain nuclei embedded in white matter of midbrain are viewable a. Subtantia nigra whose neural cell bodies contain dark melanin pigment, os deep to pyramidal tracts in cerebral prduncle b. Brain nucleus is funcitonally linked to deep gray matter of cerebrum, basal nuclei and is onvolved in controlling voluntary movement c. Degeneration of neurons in substantia nigra is cause of PArkingon’s disease 7. Oval red nucleus lies deep to substantia nigra a. Reddish hue is due to rich blood supply and to presence of iron pigment in cell bodies of it's neurons b. Minor motorfunction: helping to bring about flexion movements of limbs c. Associated w/ cerebellum x. Cell bdoies of motor neurons that contibute to oculomotor and trochlear nuclei are locsated in ventral part of gray matter surrounding cerebral aqueduct, periaqueductal gray matter
- Cranial nerves contorl most of muscles that move eyes xi. Nuclei that contribute to autonomic behaviours are viewable in corss section through midbrain xii. Around cerebral aqueduct is periqueducatl gray matter
- Incolved in fight or flight reaction, consitutiong midbrain link between part of cerebrum that perceoves fear and autonomic pathway that produces physiological reactions associated w/ fear a. Increased in hear rate, skyrocketing blood pressure, wild fleeing or defensive fressing and suppression of pain when person is injrued
- Seemes to mediate response to visceral pain dirong which it decreases heart rate and blood pressure, rpduces cold sweat and discourages movement xiii. Auditory and visual reflexes are integrated by brain nuclei in corpora quadrigemina which make up tectum xiv. Bria nuclei form four bumos on dorsal surface of midbrain
- Superior and inferior colliculi
- 2 superior colliculi act in visual refleces
- 2 inferior colliculi belong to auditory ssytem
- Act in reflexive repsonse to sounds g. Clinical Application: Localisation of a Brain Stem Injury i. Location of brain stem injury is determined by assessing both cranial nerve function and peripheral sensory and motor fucntion ii. Cranial nerves exit from brain stem in groups:
- III and IV from midbrain
- V and VI from junction to pons and medulla
- IX, X and XII from medulla iii. Specific nerve deficits can indicate region of injury or lesion
intervertebral disc between lumbar vertebrae L1 and L2. iii. This is merely the average level; the location of the end of the spinal cord varies from one person to another, from the inferior margin of thoracic vertebra T12 to the superior margin of lumbar vertebra L3. r. Cord forms from neuroectoderm and it's segmented appearance reflects pattern of adjacent comites s. Spinal cord segment is used clinically to indicate region of spinal cord from which axonal processes that form given spinal nerve emerge t. Each spinal cord segment is designated by spinal nerves that arises from it i. First throacid cord segment (spinal cord segment T1) is hwere first throacic nerve (spinal nerve t1) emerges from spinal cord, 5th thoracic cord segment (spinal cord segment T5) is where 5th spinal nerve (spinal nerve T5) emerges, and so forth u. Since spinal cord does not extend to end of spinal column, cord segments are located superior to where corresponding spinal nerbes emerge through intervertebral fromaina i. Spinal cord segment T5 is located at the level of vertebra T4. This discrepancy is most pronounced in the lumbar and sacral regions of the cord: The lumbar cord segment L1 is located at vertebra T11, and the sacral cord segment S1 is at vertebra L1. v. Cord is wider laterally than anteroposteriorlu i. 2 grooves - dorsal (posterior) median sulcius and wider ventral (anterior) median fissue - run length of cord and partly divide it into right and halves w. White Matter of the Spinal Cord i. Cord consists of outer region of white matter and inner region of gray matter ii. White matter of cord is composed of myelinated and unmyelinates axons that allow communciation between different parts of spinal cord and vetween cord and brain iii. Fibres are classified as being one of three types
- Ascending a. Most in spinal cord carry sensory informtaion from sneosry neurons of body up to the brain
- Descending a. Most carry mtoor instructiuons from brain to spinal cord, to stimulate contraction of body’s muscles and secretion from it's glands
- Commissural a. White-matter fibres that carry information from one side of psinal cord to the other iv. White matter on each side of spinal cord is divided into 3 funiculi
- Dorsal (posterior) funiculus
- Ventral (anterior) funiculus
- Lateral funiculus v. VEntral and lateral are continuous w/ each other and are divided by imaginary line that extends out from ventral horn of gray matter
vi. 3 funiculi contain many fiber tracts vii. Each fiber tracts is composed of axons thta all have similar destinations and functions x. Gray Matter of the Spinal Cord and Spinal Roots i. Gray matter of cord consists of mixture of neuron cell bodies; short, nonmyelinated neurons and neuroglia
- Spinal cord is shaped like letter H ii. Crossbar of H is called gray commisure and is composed of unmyeliated axons that cross from one side of the CNS to the other iii. Within gray commissure is narrow central cavity of spinal cord, the central canal iv. 2 posterior arms of H are dorsal (posterior) horns, and 2 anterior arms are ventral (Anterior) horns v. In 3D, arms form columns of gray matter that run entire length of spinal cord
- In throcic and superior lumbar segments of spinal cord, small lateral gray matter colums, lateral horns, are present vi. Neurons in the spinal gray matter are organised functionally vii. Dorsal horns consist entireenitrely of interneurons
- Receive info from sensory neurons whose cell bodies lie outside spinal cord in dorsal root ganglia and whose axons reach spinal cord via dorsal roots
- Ventral horns contain cell bodies of motor neurons that send axons out of spinal cord via ventral roots to supply mscules and glands
- Present in ventral hrons a. Size varies along length of spinal cord, refleting amount of skeletal musculature innervated at each lvel i. Hroms are alrgest in cervical and lumbar segments of cord, whoch innervate upper and lower limbs ii. Segments form previosuly described cervical and lumbar enlargement sof spinal cord viii. Spinal gray matter cna be further classigied according to onnervation of somatic and visceral regions of the body ix. Scheme recognises 4 zones of spinal cord gray matter: somatic sensory (SS), visceral sensory (VS), visceral motor (VM), andsomatic motor (SM). These zones are equivalent to the functional divisions of the PNS y. Clinical Application: Flaccid And Soastic Paralysis i. Any localised damage to spinal cord/spinal roots leads to some fucntional loss, either parlaysis or paresthesia ii. Severe damage to ventral horn or to ventral motor roots destorys somatic motor neurons in region of injury and results in complete, or flaccid, paralysis of skeletal muscles served iii. Since muscles are no longer stimualted by neurons, they shrink away and spinal reflexes are absent iv. Damage to only descending fiber tracts in white matter of spinal cprd leaves spinal cord motor neurons and spinal reflexes intaces
supply i. In one ascending tact, axons transmitting impulses from superior parts of body lie lateral to axons carrying impulses from inferior body part g. All pathways are bilaterally symetrical, occring on both right and elft side of brai/spinal cord h. Segments of these pathways that travel thorugh spinal cord are ascending and descending spinal tracts i. Ascending tracts carry snesory infor to brain ii. Descending tracts carry motor instructions to effectors of body i. Prinicpal ascending and descending tracts of spinal cord are mapped in cross section through cervical spinal cord j. Spinal tracts are named according to their origina and destination k. Ascending Pathways i. Onduct general somatic sensory impulses superiorly through chains of 2 or 3 neruons to various regions of the brain ii. First neuron in pathway, firt order neuorn, is sneosry neuron which extends from sensory receptor into spinal cord
- Sunapses in CNS with another neuron in pathwat, second-roder neuron iii. In some ascending apthway, second-order neuron synapses w/ third-order neuron l. Descending Pathwyas i. Descending spinal tracts deliver motor (output) insturctions from briant o spinal crod ii. Each pathway contains upper motor neurons and lower motor neurons iii. Upper motor neurons originate in gray matter of brain and send long axons down descending tract of spinal cord iv. Axons synapse with lower motor neurons in ventral horm of spinal v. Axons of lower mtoor neurons exit ventral root of spinal cord to inervate muscles or glands of body
Chapter 13 - Central Nervous System (CNS)
Course: Appl Human Anat W/Lab (APK 2100C)
University: University of Florida
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