BSCI201 Final Exam Study Guide The Nervous System 2 Divisions o Central Nervous System CNS consists of the brain and spinal cord located in the dorsal body cavity surrounded by meninges o Peripheral Nervous System PNS consists of all the neural structures outside of the CNS including the cranial nerve spinal nerves and sensory receptors Nervous System composed of nervous tissue connective tissue wrappings and blood vessels Composition of Nervous Tissue o Neurons nerve cells conducting cells o Supporting cells non conducting cells neuroglia Structure of a neuron o 3 regions of a neuron cell body 2 types of processes Cell Body soma perikaryon Contains the nucleus and all other cytoplasmic organelles except centrosomes contains 2 centroiles at right angles o Neurons are AMITOTIC Contains well developed rough ER called Nissil Bodies or Chromatophilic Substance o Indicates neurons secrete proteins Contains intermediate filaments called neurofilaments Cell body biosynthetic region of the neuron Dendrite Tapering processes that act as the receptive rgions of a neuron Receive and convey electrical signals towards the cell body Axon single process extending from the cell body with uniform diameter Each neuron has ONE axon o The single axon can give off side branches called axon collaterals Extends from a hill like region of the cell body called the AXON HILLOCK Axon gives off end branches called TELODENDRIA ends form bulbous structures called buttons AXON TERMINALS or synaptic knobs o Store and release neurotransmitters Generates and transmits action potentials away from the cell body Axon lacks the ability to synthesize proteins and relies on the cell Conducting region of the neuron body o Transport of substances from the cell body down the axon ANTEROGRADE TRANSPORT neurotranmitter o Transport of substances up the axon to the cell body RETROGRADE transport metabolic wastes broken down neurotransmitters Classification of Neurons o Structural Classification Multipolar neuron has at least 3 processes one axon and at least 2 dendrites most abundant in the human body Bipolar neuron has 2 processes one axon and one dendrite Pseudounipolar neuron has one short process from the cell body and it bifurcates into a central process and a peripheral process o Functional Classification Motor Efferent Neurons transmits impulses away from CNS to effector organ glands organ Sensory Afferent Neurons transmits impulses from sensory receptors toward the CNS Association Neurons Interneurons located in the CNS between the sensory neurons and the motor neurons 99 of neurons in the body are association neurons Supporting Cells Neuroglia o CNS Astrocytes Most abundant Numerous extensions that wrap around the neurons Involved in forming the blood brain barrier a selective barrier that regulates the chemical environment of the brain Regulates brain function Microglia acts as a microphage to engulf destroy pathogens and cell debris b c immune system does not have access to CNS Ependymal Cells ciliated columnar cells that line the ventricles cavities in the brain that contain cerebrospinal fluid Oligodendrocytes extensions myelinate axons of neurons in CNS Schwann Cells neuroloemmocytes Myelinate axons of neurons in the PNS Satellite Cells surround cell bodies of neurons and control the chemical environment Myelination of Axons o Myelination of Axons in the PNS by Schwann Cells Entire Schwann cell wraps around a segment of the axon and forms concentric rings around the segment of axon external to the axolemma plasma membrane of axon Myelin Sheath concentric rings of plasma membrane of the Schwann Cell o PNS Each Schwann cell wraps around segment of an axon external to the axolemma Neurolema Schwann cell cytoplasm and nucleus located external outside of myelin sheath Nodes of Ranvier gaps between myelin sheath Several Schwann Cells myelinate an axon in PNS o Myelination of Axons in CNS by Oligodendrocytes Each oligodendrocyte sends cytoplasmic extensions to several axons to myelintae the axons Hence the nucleus and the cytoplasm and the oligodendrocytes are located away from the sites of myelination Neurolemma is absent o Advantages of Myelination 1 Protective Function composed of fatty protect that protect the axon against trauma acts as cushioning 2 Insulates the axons to prevent interference of electrical activity of neighboring axons 3 Because myelin sheath acts as an insulator it s a poor conductor of electrical current hence action potentials are generated only at the nodes of ranvier and the impulse jumps form node to node Type of conduction of action potential is referred to as SALUTATORY CONDUCTION faster than continuous conduction which occurs at unmyelinated axons Severed Axons in the PNS can regenerate but severed axons I CNS cannot o In PNS can regenerate because When axon is severed in the PNS cells of the immune system clean up the damaged area of cell debris process known as debridement which sets the stage for regeneration Neurilemma of Schwann cell forms a regeneration tube that guides regeneration of severed axon o In CNS fail to regenerate because Microglia poorly clean up area of damage debridement is not complete No neurilemma to guide growth of severed axon Presence of growth inhibiting proteins in CNS inhibit regeneration of a severed axon 3 types of Nerve Fiber based on diameter and degree of myelination o Group A Fibers largest diameter and heavily myelinated Least resistance to current flow conduction velocity is high Use salutatory conduction Transmits impulse at rate of 150 m s o Group B Fibers intermediate diameter and lightly myelinated Uses salutatory conduction Transmits impulse at a rate of 15 m s o Group C Fibers smallest diameters and unmyelinated Most resistance to current flow Uses continous conduction Transmits impulse at a rate of 1 m s Factors Affecting the Rate of Impulse of Transmission o Diameter of Axons larger axons transmit impulses at a faster rate than smaller axons because the larger axon has a larger diameter and therefore presents with less resistance impulse transmission Resistance in smaller axons is higher which impedes impulse transmission o Degree of Myelination myelinated axons transmit impulses at a faster rate than unmyelinated axons Myelinated axons use salutatory conduction where action potentials are generated only at the Nodes of Ranvier hence impulse jumps from node to node down the axon Unmyelinated axons use continuous conduction where action
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