BSCI201 Final Exam Study Guide Dr Opoku Spring 2014 THE NERVOUS SYSTEM 2 divisions of the NS 1 Central Nervous System CNS consists of the brain and the spinal cord located in the dorsal body cavity surrounded by meninges 2 Peripheral Nervous System PNS consists of the all neural structures outside of the CNS including the cranial nerves spinal nerves and sensory receptors Composition of Nervous Tissue The NS is composed mainly of nervous tissue connective tissue and blood vessels are also present Nervous tissue is composed of 2 types of cells neurons and supporting cells Neurons nerve cells that are conducting cells Supporting cells non conducting cells Structure of a Neuron 3 regions of a neuron cell body 2 types of process 1 Cell body Soma Perikaryon a Contains the nucleus and all other cytoplasmic organelles EXCEPT CENTRIOLES neurons are AMITOTIC b Contains well developed rough ER called Nissl Body or Chromatophilic substance c Contains intermediate filaments called neurofilaments d Biosynthetic region of a neuron 2 Dendrites a Tapering processes that act as the receptive regions of a neuron b Receive and convey electrical signals toward the cell body generate graded potentials that dissipate over a distance toward the cell body c At least one per neuron 3 Axon axon a A single process extending from the cell body each neuron can only have one b Generates and transmits action potentials AWAY from the cell body conducting region of a neuron only region that can generate transmit action potentials results in the neurotransmitters c Branches at the end into telodendria which end in bulbous ends called axon terminals synaptic knobs buttons Classification of Neurons 2 types Structural and Functional classification 1 Structural classification of neurons 3 types based on number of processes extending from cell body a Multipolar neuron has at least 3 processes one axons and at least 2 dendrites most abundant neuron in the human body b Bipolar neuron has 2 processes one axon and one dendrite sensory pathways c Pseudounipolar neuron has one short process extending from the cell body that bifurcates into a central process and a peripheral process not considered dendrites or axons rare in human body i The two processes have a uniform diameter myelinated transmission of action potentials toward and away cell body 2 Functional classification of neurons 3 types based on the direction of impulse traveling axon potentials transmissions organs glands organs TOWARD the CNS a Motor efferent neuron transmits impulses AWAY from the CNS to effector b Sensory afferent neuron transmits impulses from sensory receptors c Association interneurons neurons located in the CNS between sensory neurons and the motor neurons most of the neurons 99 in the body are association neurons hence the association neurons are multipolar neurons Definitions Tracts a bundle of axons in the CNS Nerve a bundle of axons in the PNS Nucleus a cluster of neuron cell bodies in the CNS Ganglion a cluster of neuron cell bodies in the PNS Structure of a Nerve or Tract The plasma membrane of an axon is called an axolemma Each axon is wrapped in a delicate connective tissue membrane called endoneurium A bundle of endoneurium covered axons is called a fascicle Each fascicle is covered by the coarse connective tissue membrane called the perineurium A bundle of perineurium covered fascicles form the nerve or a tract which is covered in a tough connective tissue membrane called the epineurium Supporting Cells 6 Supporting cells Neuroglia CNS has 4 types of supporting cells o Astrocytes Most abundant Numerous extensions that wrap around neurons Involved in forming the blood brain barrier a selective barrier that regulate the chemical environment of the brain Regulate brain function NO direct contact between the blood and the neurons in the brain o Microglia Since there is no direct contact between the blood and the neurons in the brain the immune cells reside in blood do not have access to these neurons to engulf and destroy pathogens and cell debris Microglia act as macrophages to engulf destroy pathogens and cell debris o Ependymal cells Ciliated columnar cells that line the ventricles cavities in the brain that contain cerebrospinal fluid CSF which is filtered and drained continuously Provide nutrients O2 to neurons o Oligodendrocytes Their extensions myelinate axons of neurons in the CNS each can myelinate segments of several axons which means each axon is myelinated by cellular extensions of several oligodendrocytes PNS has 2 types of supporting cells o Schwann cells neurolemmocytes Myelinate axons of neurons in the PNS each Schwann cell wraps around a segment of the axon to form the myelin sheath whitish fatty protein Analogous to oligodendrocytes in the CNS o Satellite cells Surround cell bodies of neurons and control their chemical environment Myelination of Axons Myelination of axons in the CNS Oligodendrocytes neurilemma is absent Myelination of axons in PNS Schwann cells Axons in the CNS are myelinated by extensions from the oligodendrocytes hence Each Schwann cell wraps around a segment of an exon external to the axolemma Schwann cell squeezes around the segment of axon wrapping concentric rings of its plasma membrane called myelin sheath around the axon o Function of myelin sheath is protection electrical insulation and increase rate of The cytoplasm and the nucleus of the Schwann cell squeezed outside the myelin sheath is impulse transmission the neurilemma The spaces between adjacent myelin sheaths are called Nodes of Ranvier Severed axons in the PNS and CNS Severed axons in the CNS fail to regenerate because The microglia poorly clean up area of damage debridement is not complete No neurilemma to form a regeneration tube to guide growth of severed axon Presence of growth inhibiting proteins in the CNS inhibit regeneration of severed axon Severed axons in the PNS can regenerate because Cells of the immune system clean up the damaged area of cell debris debridement which sets the stage for regeneration The neurilemma of the Schwann cell forms a regeneration tube that guides regeneration of the severed axon Neurophysiology Generation of Action Potentials A Depolarization entry of Na sodium influx into axoplasm cytoplasm of axon Makes membrane potential less negative Threshold potential action potential develops an all or none phenomenon Upshoot or spike due to an explosive entry of sodium ions a positive membrane potential is
View Full Document
Unlocking...