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  • 1. The Nervous System
  • 2. 2 division of the Nervous System
    • Central Nervous System (CNS) - consists of the brain and the spinal cord; located in the dorsal body cavity surrounded by meninges.
    • Peripheral Nervous System (PNS) – consists of the all neural structures outside of the CNS including the cranial nerves, spinal nerves and sensory receptors
  • 3. Figure 13.2
  • 4. Figure 13.1
  • 5. Composition of Nervous Tissue
    • The Nervous System 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 are conducting cells
    • Supporting cells are non-conducting cells
  • 6. Structure of a neuron
    • 3 regions of a neuron: 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
      • Contains well-developed rough ER called Nissl Body or Chromatophilic
      • substance
      • _ Contains intermediate filaments called neurofibrils
      • _ BIOSYNTHETIC region a neuron
    • Dendrites
      • Tapering processes that act as the RECEPTIVE regions of a neuron
      • Receive and convey electrical signals toward the cell body
    • Axon
      • A single process extending from the cell body – each neuron has only one axon
      • Generates and transmits action potentials= CONDUCTING region of a neuron
      • Branches at the end to form terminal branches which end in bulbous ends called axon terminals=synaptic knobs=boutons
  • 7. Classification of Neurons
    • 2 types:
    • 3 Structural Classification of neurons:
      • Multipolar neuron has at least 3 processes – one axon and at least 2 dendrites; most abundant neuron in the human body
      • Bipolar neuron has 2 processes – one axon and one dendrites
      • Unipolar neuron has one short process from the cell body and it bifurcates into a central process and a peripheral process
    • 3 Functional classification of neurons:
      • Motor or Efferent neuron transmits impulses AWAY from the CNS to effector organs = glands, organs
      • Sensory or Afferent neuron transmits impulses from sensory receptors TOWARD the CNS
      • Association neurons or interneuron located in the CNS between the sensory neurons and the motor neurons
      • Most of the neurons (99%) in the body are associated neurons
  • 8. Definitions
    • Tract = 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
  • 9. Figure 13.3
  • 10. Structure of a Nerve ( or a 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
  • 11. 6 Types of Supporting cells
    • Supporting cells = Neuroglia
    • 4 Supporting cells are located in the CNS
      • Astrocytes
      • Microglia
      • Ependymal
      • Oligodendrocytes
    • 2 Supporting cells are located in the PNS
      • Schwann cells
      • Satellite cells
  • 12. 4 Types of supporting Cells 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
      • Since the specific immune system does not have access to the CNS; the microglia act as macrophages to engulf/destroy pathogens and cell debris.
    • Ependymal cells
      • Ciliated columnar cells that line the ventricles – cavities in the brain that contain cerebrospinal fluid (CSF)
      • Currents created by beating of cilia circulate the CSF
    • Oligodendrocytes
      • Their extensions myelinate axons of neurons in the CNS
  • 13. 2 Types of Supporting Cells 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
  • 14. Myelination of axons
    • Myelination of axons in the PNS by Schwann cells :
    • Each Schwann cell wraps around a segment of an axon ( 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
    • The cytoplasm and the nucleus of the Schwann cell squeezed outside the myelin sheath is called the NEURILEMMA
    • The spaces between adjacent myelin sheaths are called NODES OF RANVIER
    • Myelination of axons in the CNS by oligodendrocytes
    • The axons in the CNS are myelinated by extensions from the oligodendrocytes hence, neurilemma is absent
  • 15. Severed axons in the PNS can regenerate but severed axons in the CNS cannot
    • Severed axons in the PNS regenerate because
      • 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
      • The neurilemma of the Schwann cell forms a REGENERATION TUBE that guides regeneration of the severed axon
    • Severed axons in the CNS fails to regenerate because :
      • The 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 the CNS inhibit regeneration of a severed axon
  • 16. Figure 13.4
  • 17. Neurophysiology – Generation of Action Potential
    • Resting membrane potential RMP) is -70mV
        • . Depolarization phase – entry of sodium ions = sodium influx, makes membrane potential less and 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 reached
    • 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
    • Hyperpolarization phase – more potassiom ions leave the cell driving the membrane potential below the RMP
  • 18. Characteristics of Action Potentials
    • All-or-none phenomenon – an action potential will be generated if depolarization reaches a threshold potential
    • 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
    • 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
  • 19. 2 Refractory periods during an action potential
    • Absolute Refractory Period – the depolarization phase of the action potential when sodium channels are opened, another action potential can not be generated
    • 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
  • 20. Factors affecting the rate of impulse transmission = Conduction Velocity
    • 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
    • Degree of myelination – myelinated axons transmit impulses at a faster rate than unmyelinated axons.
    • 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
    • Unmyelinated axons use CONTINUOUS conduction where action potentials developed stepwise across the entire axolemma
  • 21. 3 Types of Nerve Fibers
    • Based on their diameter and degree of myelination
    • Group A fibers – have the largest diameter and heavily myelinated; transmit impulse at the rate of 150 m/s (=300 miles per hour)
    • Group B fibers – intermediate diameter and lightly myelinated;
    • transmit impulses at a rate of 15 m/s
    • Group C fibers – smallest diameters and unmyelinated; transmit impulses at a rate of 1 m/s