Cns 2


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Cns 2

  1. 1. The Nervous System
  2. 2. 2 division of the Nervous System <ul><li>Central Nervous System (CNS) - consists of the brain and the spinal cord; located in the dorsal body cavity surrounded by meninges. </li></ul><ul><li>Peripheral Nervous System (PNS) – consists of the all neural structures outside of the CNS including the cranial nerves, spinal nerves and sensory receptors </li></ul>
  3. 3. Figure 13.2
  4. 4. Figure 13.1
  5. 5. Composition of Nervous Tissue <ul><li>The Nervous System is composed mainly of Nervous Tissue; connective tissue and blood vessels are also present. </li></ul><ul><li>Nervous tissue is composed of 2 types of cells: Neurons and Supporting Cells </li></ul><ul><li>Neurons = nerve cells are conducting cells </li></ul><ul><li>Supporting cells are non-conducting cells </li></ul>
  6. 6. Structure of a neuron <ul><li>3 regions of a neuron: cell body + 2 types of processes </li></ul><ul><li>Cell body = Soma = Perikaryon </li></ul><ul><ul><li>Contains the nucleus and all other cytoplasmic organelles EXCEPT CENTRIOLES hence, neurons are generally AMITOTIC </li></ul></ul><ul><ul><li>Contains well-developed rough ER called Nissl Body or Chromatophilic </li></ul></ul><ul><ul><li>substance </li></ul></ul><ul><ul><li>_ Contains intermediate filaments called neurofibrils </li></ul></ul><ul><ul><li>_ BIOSYNTHETIC region a neuron </li></ul></ul><ul><li>Dendrites </li></ul><ul><ul><li>Tapering processes that act as the RECEPTIVE regions of a neuron </li></ul></ul><ul><ul><li>Receive and convey electrical signals toward the cell body </li></ul></ul><ul><li>Axon </li></ul><ul><ul><li>A single process extending from the cell body – each neuron has only one axon </li></ul></ul><ul><ul><li>Generates and transmits action potentials= CONDUCTING region of a neuron </li></ul></ul><ul><ul><li>Branches at the end to form terminal branches which end in bulbous ends called axon terminals=synaptic knobs=boutons </li></ul></ul>
  7. 7. Classification of Neurons <ul><li>2 types: </li></ul><ul><li>3 Structural Classification of neurons: </li></ul><ul><ul><li>Multipolar neuron has at least 3 processes – one axon and at least 2 dendrites; most abundant neuron in the human body </li></ul></ul><ul><ul><li>Bipolar neuron has 2 processes – one axon and one dendrites </li></ul></ul><ul><ul><li>Unipolar neuron has one short process from the cell body and it bifurcates into a central process and a peripheral process </li></ul></ul><ul><li>3 Functional classification of neurons: </li></ul><ul><ul><li>Motor or Efferent neuron transmits impulses AWAY from the CNS to effector organs = glands, organs </li></ul></ul><ul><ul><li>Sensory or Afferent neuron transmits impulses from sensory receptors TOWARD the CNS </li></ul></ul><ul><ul><li>Association neurons or interneuron located in the CNS between the sensory neurons and the motor neurons </li></ul></ul><ul><ul><li>Most of the neurons (99%) in the body are associated neurons </li></ul></ul>
  8. 8. Definitions <ul><li>Tract = a bundle of axons in the CNS </li></ul><ul><li>Nerve = a bundle of axons in the PNS </li></ul><ul><li>Nucleus – a cluster of neuron cell bodies in the CNS </li></ul><ul><li>Ganglion = a cluster of neuron cell bodies in the PNS </li></ul>
  9. 9. Figure 13.3
  10. 10. Structure of a Nerve ( or a Tract) <ul><li>The plasma membrane of an axon is called an axolemma </li></ul><ul><li>Each axon is wrapped in a delicate connective tissue membrane called ENDONEURIUM </li></ul><ul><li>A bundle of endoneurium-covered axons is called a fascicle </li></ul><ul><li>Each fascicle is covered by the coarse connective tissue membrane called the PERINEURIUM </li></ul><ul><li>A bundle of perineurium-covered fascicles form the nerve or a tract which is covered in a tough connective tissue membrane called the EPINEURIUM </li></ul>
  11. 11. 6 Types of Supporting cells <ul><li>Supporting cells = Neuroglia </li></ul><ul><li>4 Supporting cells are located in the CNS </li></ul><ul><ul><li>Astrocytes </li></ul></ul><ul><ul><li>Microglia </li></ul></ul><ul><ul><li>Ependymal </li></ul></ul><ul><ul><li>Oligodendrocytes </li></ul></ul><ul><li>2 Supporting cells are located in the PNS </li></ul><ul><ul><li>Schwann cells </li></ul></ul><ul><ul><li>Satellite cells </li></ul></ul>
  12. 12. 4 Types of supporting Cells in the CNS <ul><li>Astrocytes </li></ul><ul><ul><li>Most abundant </li></ul></ul><ul><ul><li>Numerous extensions that wrap around neurons </li></ul></ul><ul><ul><li>Involved in forming the BLOOD-BRAIN BARRIER, a selective barrier that regulate the chemicals environment of the brain </li></ul></ul><ul><ul><li>Regulate brain function </li></ul></ul><ul><li>Microglia </li></ul><ul><ul><li>Since the specific immune system does not have access to the CNS; the microglia act as macrophages to engulf/destroy pathogens and cell debris. </li></ul></ul><ul><li>Ependymal cells </li></ul><ul><ul><li>Ciliated columnar cells that line the ventricles – cavities in the brain that contain cerebrospinal fluid (CSF) </li></ul></ul><ul><ul><li>Currents created by beating of cilia circulate the CSF </li></ul></ul><ul><li>Oligodendrocytes </li></ul><ul><ul><li>Their extensions myelinate axons of neurons in the CNS </li></ul></ul>
  13. 13. 2 Types of Supporting Cells in the PNS <ul><li>Schwann cells = neurolemmocytes </li></ul><ul><ul><li>Myelinate axons of neurons in the PNS </li></ul></ul><ul><li>Satellite cells </li></ul><ul><ul><li>Surround cell bodies of neurons and control their chemical environment </li></ul></ul>
  14. 14. Myelination of axons <ul><li>Myelination of axons in the PNS by Schwann cells : </li></ul><ul><li>Each Schwann cell wraps around a segment of an axon ( external to the axolemma) </li></ul><ul><li>Schwann cell squeezes around the segment of axon wrapping concentric rings of its plasma membrane called MYELIN SHEATH around the axon </li></ul><ul><li>The cytoplasm and the nucleus of the Schwann cell squeezed outside the myelin sheath is called the NEURILEMMA </li></ul><ul><li>The spaces between adjacent myelin sheaths are called NODES OF RANVIER </li></ul><ul><li>Myelination of axons in the CNS by oligodendrocytes </li></ul><ul><li>The axons in the CNS are myelinated by extensions from the oligodendrocytes hence, neurilemma is absent </li></ul>
  15. 15. Severed axons in the PNS can regenerate but severed axons in the CNS cannot <ul><li>Severed axons in the PNS regenerate because </li></ul><ul><ul><li>When the axon is severed in the PNS, cells of the immune system clean up the damaged area of cell debris, a process known as debridement, which sets the stage for regeneration </li></ul></ul><ul><ul><li>The neurilemma of the Schwann cell forms a REGENERATION TUBE that guides regeneration of the severed axon </li></ul></ul><ul><li>Severed axons in the CNS fails to regenerate because : </li></ul><ul><ul><li>The microglia poorly clean up area of damage – debridement is not complete </li></ul></ul><ul><ul><li>No neurilemma to guide growth of severed axon </li></ul></ul><ul><ul><li>Presence of growth-inhibiting proteins in the CNS inhibit regeneration of a severed axon </li></ul></ul>
  16. 16. Figure 13.4
  17. 17. Neurophysiology – Generation of Action Potential <ul><li>Resting membrane potential RMP) is -70mV </li></ul><ul><ul><ul><li>. Depolarization phase – entry of sodium ions = sodium influx, makes membrane potential less and less negative </li></ul></ul></ul><ul><ul><ul><li>Threshold Potential - action potential develops = an all-or-none phenomenon </li></ul></ul></ul><ul><ul><ul><li>Upshoot or spike due to an explosive entry of Sodium ions = a positive membrane potential reached </li></ul></ul></ul><ul><li>Repolarization phase – sodium channels close and potassium channels open and potassium ions rush out = potassium efflux, and reversal of membrane potential toward a negative membrane potential </li></ul><ul><li>Hyperpolarization phase – more potassiom ions leave the cell driving the membrane potential below the RMP </li></ul>
  18. 18. Characteristics of Action Potentials <ul><li>All-or-none phenomenon – an action potential will be generated if depolarization reaches a threshold potential </li></ul><ul><li>Self propagating – once generated by the axon, it is propagated down the axon to the axonal terminals; a propagated or transmitted action potential is called a IMPULSE </li></ul><ul><li>Since all action potentials appear the same have the same shape and amplitude irrespective of stimulus strength. Thus, the difference between a stronger stimulus that causes the generation of an action potential and a weaker stimulus that causes the generation of an action potential is that the stronger stimulus causes the impulse to be generated at a higher frequency than the weaker stimulus </li></ul>
  19. 19. 2 Refractory periods during an action potential <ul><li>Absolute Refractory Period – the depolarization phase of the action potential when sodium channels are opened, another action potential can not be generated </li></ul><ul><li>Relative Refractory Period – the repolarization phase of the action potential when the sodium channels are closed ( potassium channels are open), an exceptionally strong stimulus can cause sodium channels leading to depolarization and the generation of another action potential </li></ul>
  20. 20. Factors affecting the rate of impulse transmission = Conduction Velocity <ul><li>Diameter of the axon – larger axons transmit impulses at a faster rate than smaller axons because the larger axon have larger diameter and therefore presents with less resistance impulse transmission; the resistance in the smaller axons is higher which impedes impulse transmission </li></ul><ul><li>Degree of myelination – myelinated axons transmit impulses at a faster rate than unmyelinated axons. </li></ul><ul><li>Myelinated axons use SALTATORY conduction where action potentials are generated only at the nodes of Ranvier hence, the impulse “jumps from node to node down the axon </li></ul><ul><li>Unmyelinated axons use CONTINUOUS conduction where action potentials developed stepwise across the entire axolemma </li></ul>
  21. 21. 3 Types of Nerve Fibers <ul><li>Based on their diameter and degree of myelination </li></ul><ul><li>Group A fibers – have the largest diameter and heavily myelinated; transmit impulse at the rate of 150 m/s (=300 miles per hour) </li></ul><ul><li>Group B fibers – intermediate diameter and lightly myelinated; </li></ul><ul><li>transmit impulses at a rate of 15 m/s </li></ul><ul><li>Group C fibers – smallest diameters and unmyelinated; transmit impulses at a rate of 1 m/s </li></ul>