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General principle of nervous system • Nervous system – Multi-tasking unit in the body • Thinking • Regulation of actions General design • Central nervous system neuron – Basic functional unit – 100 billion units – Signals received by synapses • Located in neural dentrites and cell bodies • Few hundreds to 200,000 synaptic connection General design • Central nervous system neuron – Outgoing signals • Axon (branched) – Special feature • Signals move forward only • Signals travel in a specific direction for performing specific function Sensory receptors • Initiation of nervous system activity – Sensory experience – Immediate reaction – Generation of memory • Somatic sensory system – Sensory information from the entire body surface and deep tissues – Information enters CNS via the peripheral nervous system • Somatic sensory system – Information conducted into multiple sensory areas • The spinal cord (all level) • The reticular substance of the lower brain • Thalamus • Celebellum • Cerebral cortex Motor parts of nervous system • Motor functions of the nervous system – Skeletal muscle contraction – Smooth muscle contraction – Activity of endocrine and exocrine glands • Effectors – Anatomical structures whose functions are under neural control • Skeletal motor system • Autonomic motor system – Smooth muscles – Glands • Level of control – The spinal cord (all level) – The reticular substance of the lower brain – Thalamus – Celebellum – Cerebral cortex • Specific role for specific level of control – Lower level (lower brain and spinal cord) • Autonomic – Higher level • Thought process • Deliberate movement Integrative function • Processing incoming information – Generation of appropriate mental and motor responses • Brain – Discards > 99% of all sensory information • Some relegated to subconscious – Channels information to proper integrative and motor region of the brain • Generation of appropriate responses • Role of synapses – Direct pathway of nerve impulses • Spreading of the signals throughout the nervous system – Very complex • Degree of difficulty in passing the impulses • Input from other areas in the nervous system – Facilitatory – Inhibitory • Selective blocking and passing of impulses • Selective amplification and channeling of the signals Memory • Very small amount of input – Immediate responses • Larger portion – Stored for future motor activity and thought process – Majority in cerebral cortex but some in the lower brain • Role of synapses – Increased ability to pass the particular type of signal • Facilitation – Subsequent adaptation of brain to the particular signal • Repeated exposure and passing through the synapses • Generation of the signal within the brain and movement of the signal through synapses in the absence of input • Importance of memory – Reference point for the action – Comparison of new input with previously stored information – Generation of new memory or appropriate reaction Major level of CNS function • Spinal cord • Lower brain/subcortical • Higher brain/cortical Spinal cord • Neural circuits in the cord – Walking movements – Withdrawal reflex – Support reflex against gravity – Reflex regulating local blood vessel, digestive tract movement, or urinary excretion • Information from upper level of control – Commanding the cord center to perform specific function Lower brain/subcortical • Primitive parts of the brain • Subconscious activities – Arterial pressure – Respiration – Equilibrium – Feeding reflex – Many emotional responses Higher brain/cortical • Memory storehouse • Function in association with lower centers – Precise regulation of functions dictated by the lower center – Thought process • Use of information from lower centers to trigger activation of stored information CNS synapses • Information – Action potential/nerve impulses • Each impulse – Blocked in its transmission from one neuron to the next – Changed from a single impulse to multiple impulses – Combined with other impulses to become more complex/intricate impulses Types of synapses • Two types – Chemical – Electrical Types of synapses • Electrical synapses – Conduct electricity from one cell to the next • Direct open fluid channels • Gap junctions – Very few in the CNS – Transmission of information • Either direction Types of synapses • Chemical synapses – The majority of synapses used in signal transmission within the CNS • Neurotransmitters • One-way conduct – Chemical synapses – Signals transmission in only one direction • Neuron that secretes transmitter (presynaptic) • Neuron that receives transmitter (postsynaptic) • Importance of one-way conduction – Movement of signal toward specific goal • Focused transmission of signals from peripheral neurons toward the specific areas of the CNS Synapses anatomy • Components of anterior motor neuron – Soma • Main body – Axon – Dendrites • Projections of soma – Presynaptic terminals • Cover surface of dendrites • End of nerve fibrils • Neurons in the brain – Different from anterior motor neuron • • • • Size of the cell Dendrite length, number, and size Length and size of axon Number of presynaptic terminals Presynaptic terminal • Excitatory/inhibitory • Structures – Synaptic cleft – Transmitter vesicles • Neurotransmitters – Mitochondria • ATP production – Receptor proteins Role of Ca ion • Presynaptic membrane – Large concentrations of voltage-gated Ca channels • Arrival of action potential – Large influx of Ca ions via opening of Ca channels – Release of neurotransmitters • Unclear mechanism Receptor proteins • Surface of the postsynaptic neuron membrane – Binding site (extracellular) – Ionophore component (intracellular) • Ion channel • Secondary messenger activator • Ion channels – Cation channels • Activated by excitatory neurotransmitters • Prevents influx of anions (Cl) – Negatively charged canal – Anion channels • Activated by inhibitory neurotransmitters • Prevent influx of cations – Smaller diameter • Secondary messenger system – Prolonged excitation/inhibition • Changes in long-term response characteristics of the neuron – G-protein coupled receptors • Most common – Opening of the ion channels – Activation of cAMP/cGMP – Activation of intracellular enzymes – Activation of gene transcription Excitation vs. inhibition • Additional dimension to nervous function • Excitation – Opening of Na channels • Increased membrane potential – Depressed conduction • K channels and/or Cl channels • Decreased diffusion of cations/anions • Increased positive charge – Various changes in the intracellular metabolism Excitation vs. inhibition • Inhibition – Opening of Cl channels – Increased K conductance out of the neuron – Inhibition of cellular metabolism Neurotransmitters • Two classes – Small-molecule, rapidly acting transmitters • Cause most acute responses • Acetylcholine, norepinephrine, dopamine, glycine, GABA, glutamine, serotonin, nitric oxide – Neuropeptides • Slow acting molecules • Larger in size • Prolonged action that results in long-term changes of neurons Electrical events • Resting potential – Around -65 mV – Important for positive and negative control • Sensitive to changes • Charges evenly distributed throughout soma – Highly conductive intracellular fluid – Large diameter Excitation • Transmitters – Increased Na permeabiity • Large influx of Na ions – Increased membrane potential • Excitatory post synaptic potential (EPSP) • -65 mV to -45 mV – Generation of action potential • Excitation requires simultaneous discharge of many terminals (Summation) Inhibition • Inhibitory transmitters – Opening of chloride channels • Influx of Cl ions into the inside – Opening of K channels • Removal of K from inside – Hyperpolarization (-70 mV) • Inhibitory postsynaptic potential (IPSP) Inhibition • Presynaptic inhibition – Release of inhibitory neurotransmitters • Opening of anion channels on the terminal fibril • Inhibition of synaptic transmission Summation • Spatial summation – Generation of ESPS • Cannot be done by a single presynaptic terminal (0.5 to 1 mV ESPS) – Simultaneous stimulation of many terminals • Summation of ESPS to reach threshold of firing • Spatial Summation Summation • Temporal summation – Release of transmitters • Opening of postsynaptic channels for a very short period – A second opening of the channels • Increased ESPS – Addition of successive rapid discharge • Summation of successive ESPS Facilitation of neurons • Summated postsynaptic potential – Not enough for threshold – Neurons become facilitated • Membrane potential reaches near the threshold but not enough for firing • Quicker response to the stimuli Role of dendrites • Signal reception – Summation of signals from many separate presynaptic nerve fibers – Excitation triggered by the signals from dendrites • Electronic conduction – No action potential conduction because of low voltage-gated Na channels and high threshold • Decremental conduction – Location of presynaptic terminals • Closer to the soma, greater the negative membrane potential becomes • Loss of ESPS from dendrites before reaching the soma – Permeable to K and Cl • Further the excitatory synapses are, greater the amount of decrease in ESPS