Neurons neuroglia and organization of the nervous system
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Neurons, neuroglia and organization of the nervous system. Overview of the nervous system. General definitions. Central nervous system (CNS)- brain and spinal cord Peripheral nervous system (PNS)- links the CNS to target systems, including sensory organs

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Neurons, neuroglia and organization of the nervous system

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Neurons, neuroglia and organization of the nervous system


Overview of the nervous system


General definitions

  • Central nervous system (CNS)- brain and spinal cord

  • Peripheral nervous system (PNS)- links the CNS to target systems, including sensory organs

  • Autonomic nervous system (ANS)- spans CNS and PNS; controls visceral functions


Functions of the nervous system

  • Sensory- detection of internal and external stimuli (sensory neurons)

  • Integrative- processing of sensory information (interneurons)

  • Motor- response to integrated “decision” (motor neurons)


Organization of the nervous system


Structure and function of neurons

  • Act through propagation of action potentials

  • Vary considerably in size

  • Dendrites, cell body and axons

  • May be myelinated

  • Synapse with other neuron or muscle cell; release neurotransmitters


Neurotransmitter action

  • About 100 neurotransmitters known

  • Amino acids, polypeptides, gases

  • Released into synaptic cleft, bind to membrane of target cell


Structural diversity in neurons

  • Multipolar- many dendrites, one axon

    • Most neurons in CNS

  • Bipolar- one dendrite, one axon

    • Sensory organs

  • Unipolar- sensory

    • Axon termini extend into CNS


Neuroglial cells

  • Much smaller than neurons and more numerous

  • Do not propagate action potentials

  • Can replace themselves


Four types of neuroglia in CNS

  • Oligodendrocytes

    • Myelinating cells

  • Astrocytes

    • Blood-brain barrier (BBB)

  • Microglia

    • Phagocytes (from bone marrow)

  • Ependymal cells

    • Line ventricles of brain; produce cerebrospinal fluid (CSF)


Neuroglia of the CNS


Neuroglia of the PNS

  • Schwann cells

    • Myelinating cells

    • Help direct axon regeneration

  • Satellite cells

    • Support, protection, regulation of molecular exchange

    • “Filter out” other stimuli


Why myelin?

  • Increases speed of nerve impulse transmission

  • Myelinating cells leave gaps on axons (nodes of Ranvier)

  • Schwann cells can help regenerate axons

  • Demyelination can lead to loss of function (MS)


Classification of neurons

  • Exteroreceptors- deal with “external environment”

  • Interoreceptors- deal with internal environment

  • Proprioreceptors- position and movement


Neuronal circuits


Neural circuits in CNS vary in direction and complexity

  • One presynaptic→one postsynaptic is simplest

  • One presynaptic→ many postsynaptic (divergence) amplifies a sensory signal

  • Many presynaptic →one postsynaptic (convergence) brings information from many sources into one pathway

  • May move repeatedly through circuit (reverberating)- repeated or learned activities

  • Parallel-discharge- pathways diverge and then converge- complex activities requiring concentration?


Repair and regeneration of neurons

  • Neurons can grow new dendrites or axon termini

  • Repair is limited to certain neurons in the PNS

  • Growth factors promote formation of new neurons; limited to specific regions of brain (hippocampus) in human adults

  • Growth does occur in fetal and juvenile stages- what turns it off?


Synapses (chemical)


Electrical synapses

  • Membranes of presynaptic and postsynaptic cells are fused

  • Transmission is faster

  • Can be bidirectional

  • Generally associated with defensive reflexes


Gray matter and white matter in the nervous system


Medically significant conditions

  • Multiple sclerosis (demyelination)

  • Epilepsy (uncontrolled electrical discharges)

  • Tumors

  • Neuropathies- affect specific nerves

  • Infectious disease


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