Very phys Chapter 7 The Nervous System 1 Organization of the nervous system most cells cannot divide but be repaired a Divided into two parts i Central nervous system CNS brain and spinal cord 1 Tract Collection of axons in the CNS 2 Nuclei group of cell bodies inside the CNS ii Peripheral nervous system PNS cranial and spinal nerves 1 Nerve collection of axons in the PNS 2 Ganglion group cell bodies inside the PNS b Tissues is composed of two types of cells i Neurons conduct impulses ii Glial cells neuroglia that support the neurons 1 Two types found in the PNS a Schwann cells form myelin sheaths around peripheral axons i Myelinated axons conduct impulses more rapidly even unmyelinated axons in the PNS have a neurilemma b Satellite cells support cell bodies within ganglia of the PNS 2 Four types found in the CNS a Oligodendrocytes form myelin sheaths around axons in CNS i Myelin gives tissues a white color white matter b Microglia phagocytize foreign material clean up crew i Originate from bone marrow c Astrocytes regulate the external environment of neurons most abundant d Ependymal cells line the ventricles and secrete CSF 2 Classification of neurons via structure a Dendrites receive impulses and conduct a graded impulse toward a cell body b Axons conducts action potentials AP away from the cell body i Vary in length from few mm to m ii Connected to the cell body via the axon hillock where AP s are generated iii Can form many branches called axon collaterals iv Covered in myelin w open spots called nodes of Ranvier c Three different types of structural forms i Pseudounipolar neurons single short process that branches like a T to form a pair of longer processes Ex sensory neurons ii Bipolar neurons have two processes one on either end Ex found in the retina of the eye iii Multipolar neurons most common have several dendrites extending from one axon Ex motor neurons 3 Classification of neurons via function based on direction impulses are conducted a Sensory neurons conduct from sensory receptors to the CNS b Motor neurons conduct impulses from the CNS to target muscles or glands Somatic reflexes of the body voluntary control of skeletal muscles i ii Autonomic innervate involuntary targets 1 Sympathetic fight or flight Parasympathetic rest and digest c Association interneurons located completely within the CNS integrate functions of the nervous system 4 Function of the nervous system a Respond to chemical and physical stimuli b Conduct electrochemical impulses Axonal transport i Anterograde transport from cell body to dendrites and axon uses kinesin motors 1 Kinesin proteins move synaptic vesicles and mitochondria ii Retrograde transport from dendrites and axon to the cell body uses dynein motors 1 Dynein proteins move membranes and vesicles Ex tetanus rabies herpes viruses c Release chemical regulators d Enable perception of sensory stimuli learning memory and control of muscles and glands 5 Regeneration of a cut neuron a In the PNS i b The cut piece of the neuron is degenerated and phagocytosed by Schwann cells then the cell forms a regeneration tube ii Growth hormones are released and growth of axon sprouts occur iii New axon eventually connects to the undamaged axon In the CNS not as able to regenerate i ii Death receptors are released and promote apoptosis cell suicide Inhibitory proteins in the myelin sheath and glial scarring from astrocytes occur and prevent regeneration 1 Ex glycoproteins inhibit the growth by binding to a Nogo receptor on the axon Injury in the CNS stimulates growth of axon collaterals but not central neurons iii c Neurotrophins promote neuronal growth in the fetal brain Four types 1 Nerve growth factor NGF 2 Brain derived neurotrophic factor BDNF 3 Glial derived neurotrophic factor GDNF a Needed in the adult brain to maintain spinal motor neurons to sustain neurons in the brain that use dopamine ii 4 Neurotrophin 3 Neurotrophin 4 5 In adults neurotrophins aid in the maintenance of sympathetic ganglia and the regeneration of sensory neurons 6 Astrocytes the most abundant glial cell Have long processes with end feet that associate with blood capillaries and axon terminals a Functions i Take up K from the extracellular environment to maintain ionic gradient of neurons ii Take up extra neurotransmitter NT released from axon terminals and recycles them 1 Mainly recycles and converts glutamate major excitatory NT glutamine 2 Glutamine can be used to reform glutamate and or form GABA major inhibitory NT iii End feet around capillaries take up glucose from the blood for use by neurons to make ATP 1 Converted first to lactic acid which neurons use as energy to metabolize it into CO2 and H2O for the production of ATP 2 MRI and PET scans which visualize brain locations by their metabolic activities are based on the functions of astrocytes and neurons iv Make lactate which active neurons rely on for aerobic respiration Astrocytes can store glycogen and make lactate from it 1 Lactate has been shown to help consolidate long term memories in the hippocampus v Needed for the formation of synapses in the CNS few form in the absence of astrocytes vi Regulate neurogenesis in the adult brain differentiation of glial cells neurons vii Forms the blood brain barrier viii Release gliotransmitters that can stimulate or inhibit neurons 1 Glutamate ATP adenosine and D serine b Astrocytes are considered excitable because they respond to stimulation by vhanges in their Ca2 concentration AP s in neurons can produce a change in Ca2 which stimulates the astrocytes to release ATP and prostaglandin E2 from the end feet causing vasodilation or an increase in blood flow 7 The Blood Brain Barrier a Capillaries in the brain do not have pores b t adjacent cells like the rest of the body They are Substances can therefore only be moved via bulk or active transport making it very selective joined by tight junctions i ii Movement is transcellular through the epithelial cells iii Astrocytes influence the production of enzymes which produce GDNF which can destroy toxic substances iv The tightness of the BBB presents problems when using chemotherapy drugs because they do not penetrate the brain as easily as other organs 1 Ex Parkinson s patients that need dopamine are given a precursor L dopa instead because it can cross the BBB but dopamine cannot 8 Electrical Activity in Axons a Resting membrane potential of neurons is 70mV High K inside High Na outside at rest Established by negative
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