BSCI201: Final ExamStudy GuideThe Nervous System- 2 Divisions:o Central Nervous System (CNS) consists of the brain and spinal cord; located in thedorsal body cavity surrounded by meningeso 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 Tissueo Neurons = nerve cells/conducting cellso Supporting cells = non-conducting cells (neuroglia)- Structure of a neurono 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 Substanceo 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 axono 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 knobso Store and release neurotransmitters- Generates and transmits action potentials away from the cell body- Conducting region of the neuron- Axon lacks the ability to synthesize proteins and relies on the cell bodyo 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 Neuronso 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 processo 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 = Neurogliao 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 debrisb/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 CNSo PNS Schwann Cells = neuroloemmocytes- Myelinate axons of neurons in the PNS Satellite Cells surround cell bodies of neurons and control the chemical environment- Myelination of Axonso 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 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 PNSo 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 absento 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 nodesof 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 cannoto 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 axono 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 myelinationo 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/so Group B Fibers – intermediate diameter and lightly myelinated Uses salutatory conduction Transmits impulse at a rate of 15 m/so 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 Transmissiono 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 transmissiono Degree of Myelination
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