ns lecture

• Nervous system


  Body's control center and communication network, directs the function of the different organs and systems of the body
• Endocrine system


  Second important regulating system, controls rapid electrical nerve impulses while the endocrine system produces hormones that are released in blood which brings about its effect
• Functions of the nervous system

  • Sensory input - uses sensory receptors to monitor changes
  • Integration - processes & interprets input & decide
  • Output - effects response (EFFECTORS)
• Classification of cells in the nervous system

  • Glial cells/Supporting cells
  • Neurons/Nerve cells
• Glial cells


  • Support, insulate & protect delicate neurons, do not conduct impulses, never lose their ability to divide, most brain tumors are glial in nature
• Types of glial cells

  • Astrocytes
  • Microglia
  • Ependymal cells
  • Oligodendrocytes
• Astrocytes


  • Star-shaped cells that wrap around nerve cells to form supporting network in the brain & spinal cord, attach neurons to blood vessels & tissues helping regulate nutrients & ions needed by the body, cover nonmyelin portion of neurons & blood vessels in CNS forming a blood-brain barrier (BBB) to prevent toxic substances from entering the brain, most numerous
• Microglia


  • Spider-like phagocytes that dispose debris, including brain cells & bacteria, act in response to inflammation & injury
• Ependymal cells


  • Line central cavities of the brain & spinal cord, their cilia helps circulate the CSF that fills cavities & forms protective cushion around CNS
• Oligodendrocytes


  • Look like small astrocytes, wrap their flat extensions tightly around the nerve fibers, producing fatty (lipid) insulating covering called MYELIN SHEATH in CNS
• Schwann cells


  • Form myelin sheaths around nerve fibers in the PNS, surround the axon
• Satellite cells


  • Provide protection, act in cushioning cells, help regulate chemical environment of neurons
• Neuron


  Highly specialized to transmit message (nerve impulse) from 1 part of the body to another, the structural and functional unit of the nervous system, amitotic (do not divide)
• Common features of a neuron

  • Cell body
  • Processes or fibers (dendrites, axons)
• Cell body


  • Part of the neuron with nucleus & organelles for protein synthesis, metabolic center of neuron, transparent nucleus contains nucleolus, cytoplasm contains the usual organelles except centrioles, mitochondria, golgi bodies, lysosomes, network of threads (neurofibrils) important in maintaining cell shape, extensive rough ER which has granular structure called NISSL BODIES also referred to as CHROMATOPHILIC substances where protein synthesis occurs, neurotransmitters are made in the cell body of neurons through protein synthesis, packaged by the golgi complex, synaptic vesicles carry them down trough the axon until it reaches the synaptic knob (axonal transport)
• Dendrites


  • Small extensions projecting from the cell body, receptive areas of the neuron, neuron process that convey incoming messages (electrical signals) towards the cell body, the more dendrites a neuron has, the more information it can process
• Axons


  • Single, long extension of the cell body, conducting region of the neuron, generates electrical signals for communication & conducts them away from the cell body, impulse starts from axon hillock (trigger zone) to terminal branches (secretory regions)
• Axonal terminals (terminal arborization)

  • Contain hundred of tiny vesicles or membranous sacs that contain chemicals called NEUROTRANSMITTERS, when impulse reach axonal terminal, they stimulate release of neurotransmitters into extracellular space
• Axoplasm


  Cytoplasm within the axon
• Synaptic cleft


  A tiny gap that separates axial terminal from the next neuron
• Synapse


  Functional junction, areas where terminal branches of axon are anchored but not touching the end of dendrites of the next neuron, allows neurons to communicate with the other cell
• Myelin sheath


  • Whitish, fatty material which has a waxy appearance that covers nerve fibers, protects and insulates the fibers & increases transmission rate of nerve impulses, axons outside CNS are myelinated by Schwann Cells, these cells wrap around in a jelly-roll fashion which increases the speed at which impulses are transmitted, has gaps or indentations called NODES of RANVIER which allow ions to flow freely from ECF to axons, assisting in developing action potentials for nerve transmission, myelinated axons conduct action potentials more quickly (3-15 meters/sec) than unmyelinated due to Nodes of Ranvier, myelinated fibers are also found in the CNS, where it is the OLIGODENDROCYTES that form CNS myelin sheaths, in contrast to Schwann cells, each of which deposits myelin around a small segment of one nerve fiber, the oligodendrocytes with their many flat extensions can coil around as many as 60 different fibers at the same time, although myelin sheaths formed by oligodendrocytes and Schwann cells are similar, the CNS sheaths lack neurolemma (thin sheath around the axon containing most of cytoplasm/ cytoplasm outside the myelin sheath)
• Classifications of neurons


  • Structural (multipolar, bipolar, unipolar)
  • Functional (sensory, association, motor)
• Multipolar neurons


  • Neurons that have several dendrites & 1 axon, most neurons in the brain & spinal cord are of this type
• Bipolar neurons


  • Has 1 dendrite & 1 axon, they function as receptor cells in special sense organs, rare in adults & found only in special senses
• Unipolar neurons


  • Have 1 process extending from cell body, this process branches into a central branch (function as axon) & a peripheral branch (function as dendrite), the branch that functions as axon enters the brain and spinal cord, the branch that function as dendrites connect to peripheral part of the body
• Types of neurons according to function

  • Sensory/afferent neuron
  • Interneurons/association neurons
  • Motor/efferent neurons
• Sensory/afferent neurons


  • Neurons carrying impulses from sensory receptors to CNS, cell bodies of sensory neurons are always found in ganglion outside the CNS, keeps us informed about what is happening both inside & outside of the body, are of UNIPOLAR type, ensuring a 1-way transmission of impulse, dendrites of sensory neurons are usually associated with specialized receptors that are activated by specific changes occurring nearby, simple types of sensory receptors are seen in skin (e.g. cutaneous sense organs/pain receptors) & in muscles or tendons (e.g. proprioceptors)
• Interneurons/association neurons


  • Found in brain & spinal cord, they are MULTIPOLAR type, transmit sensory impulse to appropriate part of the brain or spinal cord for interpretation and processing, connects motor and sensory neurons in neural pathways, like motor neurons, their cell bodies are always located in the CNS
• Motor/efferent neurons


  • Nerves carrying impulses from CNS to viscera , muscles or glands, cell bodies of motor neurons are always found in CNS, are MULTIPOLAR in type, brings about reaction to the original stimulus (usually muscular or glandular)
• White matter


  Group of myelinated axons from many neurons supported by neuroglia, forms nerve tract in CNS, most nerves are white matter, tracts are made of white matter
• Gray matter


  Group of unmyelinated axons & their neuroglia, gray matter at the surface of the brain is called CORTEX, ganglia are masses of gray matter, areas of gray matter in the spinal cord is called HORN
• Functions of CNS and PNS

  • CNS produces myelin sheath (oligodendrocytes), collection of axons/nerve fibers (nerve tracts/tracts), collection of cell bodies (nuclei)
  • PNS produces myelin sheath (Schwann cells), collection of axons/nerve fibers (nerves), collection of cell bodies (ganglia)
• Most cell bodies are found in CNS (nuclei) & are well protected by skull or vertebral column, they carry out metabolic functions of neurons, ganglia are collection of cell bodies outside of the CNS</b>
• Types of tracts


  • Ascending tract (conduct impulse up the cord & concerned with sensation)
  • Descending tract (conduct impulse down the cord & concerned with motor function)
• Synaptic transmission


  When an impulse reaches the axon terminal, it triggers the release of neurotransmitters like acetylcholine into the synaptic cleft, which allows impulse to travel across the synapse, other neurotransmitters in the body are epinephrine or adrenaline, norepinephrine, serotonin, dopamine and endorphins
• Reflex


  Rapid, predictable, and involuntary response to a stimuli, allows us to respond more quickly than if we had to consciously think about what to do
• Types of reflexes


  • Somatic (include all reflexes that stimulate the skeletal system, e.g. when pulling away the hand from a hot object)
  • Autonomic (regulates the activity of smooth muscles, the heart and glands, e.g. secretion of saliva (salivary reflex), changes in size of pupil (pupillary reflex))
• Reflex arc


  Pathway that results in a reflex, basic unit of the nervous system and the smallest and simplest pathway able to receive a stimulus, enter the CNS for interpretation, and produce a response
• Components of the reflex arc

  • Sensory receptor in the skin
  • Sensory or afferent neuron
  • Association or internuncial neuron within the spinal cord
  • Motor or efferent neuron
  • Effector organ