BSCI 201 Exam 4 Lecture Notes 11 26 14 Lecture The Nervous System The Nervous System 2 divisions of the Nervous System o Central Nervous System CNS Consists of the brain and the 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 nerves spinal nerves and sensory receptors o Composition of Nervous Tissue o Nervous system is composed mainly of Nervous Tissue Connective tissue and blood vessels are also present o Nervous tissue is composed of 2 types of cells Neurons and Supporting Cells o Neurons nerve cells are conducting cells o Supporting cells are non conducting cells Structure of a Neuron o o 3 regions cell body 2 types of processes Cell body Soma Perikaryon Contains the nucleus and all other cytoplasmic organelles EXCEPT CENTRIOLES hence neurons are generally AMITOTIC 1 Contains well developed rough ER called Nissl Body or Chromatophilic substance Contains intermediate filaments called neurofilaments BIOSYNTHETIC region a neuron Dendrites Axon Tapering processes that act as the RECEPTIVE regions of a neuron Receive and convey electrical signals toward the cell body A single process extending from the cell body each neuron can have only one axon body Generates and transmits action potentials AWAY from the cell CONDUCTING region of a neuron Branches at the end into telodendria which end in bulbous ends called axon terminals synaptic knobs buttons Classification of Neurons o 2 types Structural and Functional classification o 3 Structural Classifications of neurons Multipolar neuron has at least 3 processes one axon and at least 2 dendrites most abundant neuron in the human body body that bifurcates into a central process and a peripheral process has 2 processes one axon and one dendrites has one short process extending from the cell Bipolar neuron Pseudounipolar neuron o 3 Functional Classifications of neurons 1 Motor or Efferent neuron transmits impulses AWAY from the CNS to effector organs glands organs 2 Sensory or Afferent neuron transmits impulses from sensory receptors TOWARD the CNS 3 Association neurons or interneurons located in the CNS between the sensory neurons and the motor neurons Most of the neurons 99 in the body are association neurons Definitions o Tract a bundle of axons in the CNS o Nerve a bundle of axons in the PNS o Nucleus a cluster of neuron cell bodies in the CNS o Ganglion a cluster of neuron cell bodies in the PNS Structure of a Nerve or a Tract o The plasma membrane of an axon is called an axolemma 2 o Each axon is wrapped in a delicate connective tissue membrane called endoneurium perineurium o A bundle of endoneurium covered axons is called a fascicle o Each fascicle is covered by the coarse connective tissue membrane called the o A bundle of perineurium covered fascicles form the nerve or a tract which is covered in a tough connective tissue membrane called the epineurium 6 Types of Supporting Cells Neuroglia o 4 Supporting cells are located in the CNS Astrocytes Most abundant Numerous extensions that wrap around neurons Involved in forming the BLOOD BRAIN BARRIER a selective barrier that regulate the chemicals environment of the brain Regulate brain function Microglia Ependymal cells Since the specific immune system does not have access to the CNS the microglia act as macrophages to engulf destroy pathogens and cell debris Ciliated columnar cells that line the ventricles cavities in the brain that contain cerebrospinal fluid CSF Currents created by beating of cilia to circulate the CSF Oligodendrocytes Their extensions myelinate axons of neurons in the CNS Are analogous to schwann cells both cause myelination of axons but the process of myelination is different o Only axons can be myelinated o 2 Supporting cells are located in the PNS Schwann cells neurolemmocytes Myelinate axons of neurons in the PNS Satellite cells Surround cell bodies of neurons and control their chemical environment o 12 1 14 Myelination of axons in the PNS by Schwann cells 3 o Each Schwann cell wraps around a segment of an axon and the plasma membrane of the Schwann cell forms concentric rings external to the axolemma of the axon called the myelin sheath o The rest of the Schwann cell the nucleus cytoplasm form a neurilemma external to the concentric rings the myelin sheath o Myelination of axons in the CNS by oligodendrocytes o One oligodendrocyte sends several extensions to myelinate segments of several axons Hence a myelinated axon in the CNS has extensions from several oligodendrocytes with the nuclei of the oligodendrocytes located away from the sites of myelination o In the CNS myelinated axons have a myelin sheath concentric rings of extensions from oligodendrocytes hence neurilemma is absent Myelin Sheath o Myelinated axons appear whitish white fibers In the CNS these fibers are called myelinated tracts In the PNS these fibers are called myelinated nerves o Function of myelination 1 Protection protect the axon from trauma 2 Electrical Insulation prevents interference of electrical current in neighboring axons in the same nerve or tract 3 Increase in the rate of impulse transmission the axons generate and transmit action potentials electrical currents that are transmitted over long distances without loosing intensity A transmitted or conducted action potential is referred to as an impulse In myelinated axons areas without myelination in between myelinated segments are called Nodes of Ranvier o The myelinated segments of an axon do not generate action potentials because the lipid bilayer of the plasma membrane of the Schwann cells or the oligodendrocytes is a poor conductor of electrical currents Hence the action potentials are generated only by the nodes of Ranvier o The action potential appears to jump from node to node this type of conduction is referred to as salatory 4 o 12 3 14 Neurophysiology conduction and is faster than in continuous conduction in an unmyelinated axon 4 Regeneration of a severed myelinated axon is possible in myelinated axons in the PNS with a neurilemma because the neurilemma forms a regeneration tube to guide the severed axon to grow back to almost it original location o Generation of action potentials by the axons of neurons axons are the only regions of neurons that can generate action potentials each neuron can have only one axon o Resting Neuron the axolemma will exhibit separation of charges called the
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