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Neurology NERVOUS SYSTEM INTRODUCTION TO NERVOUS SYSTEM Histology of Nervous Tissue • 2 types of cells • Neurons − Structural & functional part of nervous system − Specialized functions • Neuroglia (glial cells) − Support & protection of nervous system Neurons • Function • Conduct electrical impulses • Structure • Cell body − Nucleus with nucleolus − Cytoplasm • Cytoplasmic processes − Dendrites − Axon Basic functional unit of N.S. Specialized cell All cells have same basic properties information processing 1 Neurology Transmits Integrates Stores Regulation of behavior ~ Nervous Tissue: Neurons Neurons = nerve cells Cells specialized to transmit messages Major regions of neurons Cell body – nucleus and metabolic center of the cell Processes – fibers that extend from the cell body (dendrites and axons) Neuron Cell Body Location Most are found in the central nervous system Gray matter – cell bodies and unmylenated fibers Nuclei – clusters of cell bodies within the white matter of the central nervous system Ganglia – collections of cell bodies outside the central nervous system Neuron Anatomy Cell body Nucleus Large nucleolus 2 Neurology Extensions outside the cell body Dendrites – conduct impulses toward the cell body Axons – conduct impulses away from the cell body (only 1!) Axons and Nerve Impulses Axons end in axonal terminals Axonal terminals contain vesicles with neurotransmitters Axonal terminals are separated from the next neuron by a gap Synaptic cleft – gap between adjacent neurons Synapse – junction between nerves Chemical Synapses Why are neurons connected? 3 Neurology Are all neurons equal in size? Brain vs spinal cord vs peripheral nerves? About how many neurons are in the human brain? 100 billions About how many neurons are in the spinal cord? 1 billion How long do you think the longest axon in the world is? around 15 feet Nerve Fiber Coverings Schwann cells – produce myelin sheaths in jelly-roll like fashion Nodes of Ranvier – gaps in myelin sheath along the axon Functional Classification of Neurons Sensory (afferent) neurons Carry impulses from the sensory receptors Cutaneous sense organs Proprioceptors – detect stretch or tension Motor (efferent) neurons Carry impulses from the central nervous system Neuroglia Neuroglia of CNS ○ Astrocytes Form the blood-brain barrier Form a structural framework for the CNS 4 Neurology Repair damaged neural tissue Control the interstitial environment of the CNS ○ Oligodendrocytes CNS Form myelin sheaths ○ Microglia Phagocytose foreign microbes, etc. ○ Ependymal Line ventricles of the brain, secrete cerebrospinal fluid Neuroglia of PNS ○ Schwann cells Form myelin sheaths of PNS ○ Satellite cells Nerve Fibers of the PNS ◦ An axon and its sheaths Myelinated axon Axon is surrounded by a myelin sheath Unmyelinated axon Axon has no myelin sheath Structural Classification Neurons 5 Neurology Nervous Tissue: Support Cells (Neuroglia or Glia) Astrocytes Abundant, star-shaped cells Brace neurons Form barrier between capillaries and neurons Control the chemical environment of the brain (CNS) Nutrition. Microglia (CNS) Spider-like phagocytes Dispose of debris Ependymal cells (CNS) Line cavities of the brain and spinal cord Circulate cerebrospinal fluid Oligodendrocytes(CNS) Produce myelin sheath around nerve fibers in the central nervous system 6 Neurology Blood Brain Barrier : Function Maintains stable brain environment large fluctuations in periphery Barrier to poisons Retains neurotransmtters & other chemicals Regulates nutrient supplies glucose levels active transport ~ Structural Classification of the Nervous System Central nervous system (CNS) Brain Spinal cord Peripheral nervous system (PNS) Nerve outside the brain and spinal cord Central Nervous System: Consists of Brain Located in cranial vault of skull And Spinal cord Located in vertebral canal . Brain and spinal cord Continuous with each other at foramen magnum 7 Neurology Peripheral Nervous System: comprises peripheral nerves (cranial & spinal ) & ganglia Two subcategories Sensory or afferent (input) Motor or efferent (output) • Divisions – Somatic nervous system – Autonomic nervous system (ANS) » Sympathetic » Parasympathetic » Enteric PNS (Cranial 8 Nerves ) Neurology Functions of the Nervous System 1. Sensory input – gathering information To monitor changes occurring inside and outside the body (changes = stimuli) 2. Integration – to process and interpret sensory input and decide if action is needed. 3. Motor output A response to integrated stimuli The response activates muscles or glands Functional Classification of the Peripheral Nervous System Sensory (afferent) division Nerve fibers that carry information to the central nervous system Motor (efferent) division Nerve fibers that carry impulses away from the central nervous system Motor (efferent) division Two subdivisions Somatic nervous system = voluntary Autonomic nervous system = involuntary 9 Neurology Anatomical classification • Cerebral hemispheres • Diencephalon • Thalamus • Hypothalamus • Brain stem • Midbrain • Pons • Medulla • Cerebellum • Spinal cord Parts of Brain Cerebrum Diencephalon Brainstem Cerebellum Usual pattern of gray/white in CNS • White exterior to gray • Gray surrounds hollow central cavity • Two regions with additional gray called “cortex” • Cerebrum: “cerebral cortex” • Cerebellum: “cerebellar cortex” 10 Neurology 11 Neurology Spinal cord anatomy Gray commissure with central canal Columns of gray running the length of the spinal cord Posterior (dorsal) horns (cell bodies of interneurons) Anterior (ventral) horns (cell bodies of motor neurons) Lateral horns in thoracic and superior lumbar cord White matter of the spinal cord (myelinated and unmyelinated axons) Ascending fibers: sensory information from sensory neurons of body up to brain Descending fibers: motor instructions from brain to spinal cord Stimulates contraction of body’s muscles Stimumulates secretion from body’s glands Commissural fibers: white-matter fibers crossing from one side of cord to the other Most pathways cross (or decussate) at some point Most synapse two or three times along the way, e.g. in brain stem, thalamus or other 12 Neurology Anatomical Tracts 1- Motor system 2- Somatic –sensory system 13 Neurology younis alomary 14