Physiology_الفصل الثاني

Nervous System

2 types of cells in the nervous system: Neurons. Supporting cells. Nervous system is divided into: Central nervous system (CNS): Brain. Spinal cord. Peripheral nervous system (PNS): Cranial nerves. Spinal nerves.
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Neurons

Basic structural and functional units of the nervous system. Cannot divide by mitosis. Respond to physical and chemical stimuli. Produce and conduct electrochemical impulses. Release chemical regulators. Nerve: Bundle of axons located outside CNS. Most composed of both motor and sensory fibers.
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Neurons (continued)

Cell body (perikaryon):“Nutrition center.”Cell bodies within CNS clustered into nuclei, and in PNS in ganglia.Dendrites:Provide receptive area.Transmit electrical impulses to cell body.Axon:Conducts impulses away from cell body.Axoplasmic flow:Proteins and other molecules are transported by rhythmic contractions to nerve endings.Axonal transport:Employs microtubules for transport.May occur in orthograde or retrograde direction. www.freelivedoctor

Neurons (continued)

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Functional Classification of Neurons

Based upon direction impulses conducted. Sensory or afferent: Conduct impulses from sensory receptors into CNS. Motor or efferent: Conduct impulses out of CNS to effector organs. Association or interneurons: Located entirely within the CNS. Serve an integrative function.
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Structural Classification of Neurons

Based on the # of processes that extend from cell body. Pseudounipolar: Short single process that branches like a T. Sensory neurons. Bipolar neurons: Have 2 processes. Retina of the eye. Multipolar: Have several dendrites and 1 axon. Motor neuron.
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Ion Gating in Axons

Changes in membrane potential caused by ion flow through ion channels. Voltage gated (VG) channels open in response to change in membrane potential. Gated channels are part of proteins that comprise the channel. Can be open or closed in response to change. 2 types of channels for K+: 1 always open. 1 closed in resting cell. Channel for Na+: Always closed in resting cells. Some Na+ does leak into the cells.
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Ion Gating in Axons (continued)

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Action Potentials (APs)

Stimulus causes depolarization to threshold.Voltage Gated Na+ channels open.Electrochemical gradient inward. + feedback loop.Rapid reversal in membrane potential from –70 to + 30 mV.VG Na+ channels become inactivated.Voltage Gated K+ channels open.Electrochemical gradient outward. - feedback loop.Restore original Resting membrane potential. www.freelivedoctor

Action Potentials (APs) (continued)

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Membrane Permeabilites

Action Potential is produced by an increase in Na+ permeability. After short delay, increase in K+ permeability.
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Action Potentials (APs) (continued)

Depolarization and repolarization occur via diffusion, do not require active transport. Once AP completed, Na+/K+ ATPase pump extrudes Na+, and recovers K+. All or none: When threshold reached, maximum potential change occurs. Amplitude does not normally become more positive than + 30 mV because VG Na+ channels close quickly and VG K+ channels open. Duration is the same, only open for a fixed period of time. Coding for Stimulus Intensity: Increased frequency of AP indicates greater stimulus strength. Recruitment: Stronger stimuli can activate more axons with a higher threshold.
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Conduction in an Unmyelinated Axon

Cable spread of depolarization with influx of Na+ depolarizes the adjacent region membrane, propagating the AP. Conduction rate is slow. AP must be produced at every fraction of micrometer. Occurs in 1 direction; previous region is in its refractory period.
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Conduction in Myelinated Axon

Myelin prevents movement of Na+ and K+ through the membrane. Interruption in myelin (Nodes of Ranvier) contain VG Na+ and K+ channels. AP occurs only at the nodes. AP at 1 node depolarizes membrane to reach threshold at next node. Saltatory conduction (leaps). Fast rate of conduction.
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Synapse
Functional connection between a neuron and another neuron or effector cell. Transmission in one direction only. Axon of first (presynaptic) to second (postsynaptic) neuron. Synaptic transmission is through a chemical gated channel. Presynaptic terminal (bouton) releases a neurotransmitter (NT).
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Electrical Synapse

Impulses can be regenerated without interruption in adjacent cells. Gap junctions: Adjacent cells electrically coupled through a channel. Each gap junction is composed of 12 connexin proteins. Examples: Smooth and cardiac muscles, brain, and glial cells.
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Chemical Synapse

Terminal bouton is separated from postsynaptic cell by synaptic cleft. Neurotransmitters(NT) are released from synaptic vesicles. Vesicles fuse with axon membrane and NT released by exocytosis. Amount of NTs released depends upon frequency of AP(action potential).
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Synaptic Transmission

Neurotransmitter release is rapid because many vesicles form fusion-complexes at “docking site.”Action Potential travels down axon to bouton.Voltage Gated Ca2+ channels open.Ca2+ enters bouton down concentration gradient.Inward diffusion triggers rapid fusion of synaptic vesicles and release of NTs.Ca2+ activates calmodulin, which activates protein kinase.Protein kinase phosphorylates synapsins.Synapsins aid in the fusion of synaptic vesicles. www.freelivedoctor

Synaptic Transmission (continued)

NTs are released and diffuse across synaptic cleft. NT (ligand) binds to specific receptor proteins in postsynaptic cell membrane. Chemically-regulated gated ion channels open. EPSP(excitatory post synapti: depolarization. IPSP(inhibitory post synaptic potential): hyperpolarization. Neurotransmitter inactivated to end transmission.
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Chemical Synapses

EPSP (excitatory postsynaptic potential): Depolarization. IPSP (inhibitory postsynaptic potential): Hyperpolarization
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Acetylcholine (ACh) as NT(neurotransmitter)

ACh is both an excitatory and inhibitory NT, depending on organ involved. Causes the opening of chemical gated ion channels. Nicotinic ACh receptors: Found in autonomic ganglia and skeletal muscle fibers. Muscarinic ACh receptors: Found in the plasma membrane of smooth and cardiac muscle cells, and in cells of particular glands.
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Ligand-Operated ACh Channels

Most direct mechanism. Ion channel runs through receptor. Receptor has 5 polypeptide subunits that enclose ion channel. 2 subunits contain ACh binding sites. Channel opens when both sites bind to ACh. Permits diffusion of Na+ into and K+ out of postsynaptic cell. Inward flow of Na+ dominates. Produces EPSPs.
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G Protein-Operated ACh Channel

Only 1 subunit. Ion channels are separate proteins located away from the receptors. Binding of ACh activates alpha G-protein subunit. Alpha subunit dissociates. Alpha subunit or the beta-gamma complex diffuses through membrane until it binds to ion channel, opening it.
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Acetylcholinesterase (AChE)
Enzyme that inactivates ACh. Present on postsynaptic membrane or immediately outside the membrane. Prevents continued stimulation.
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ACh in CNS

Cholinergic neurons: Use ACh as NT. Axon bouton synapses with dendrites or cell body of another neuron. First VG channels are located at axon hillock. EPSPs spread by cable properties to initial segment of axon. Gradations in strength of EPSPs above threshold determine frequency of APs produced at axon hillock.
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ACh in PNS

Somatic motor neurons synapse with skeletal muscle fibers. Release ACh from boutons. Produces end-plate potential (EPSPs). Depolarization opens VG channels adjacent to end plate.
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Monoamines as NT(neurotransmitter)

Monoamine NTs: Epinephrine. Norepinephrine. Serotonin. Dopamine. Released by exocytosis from presynaptic vesicles. Diffuse across the synaptic cleft. Interact with specific receptors in postsynaptic membrane.
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Inhibition of Monoamines as NT

Reuptake of monoamines into presynaptic membrane. Enzymatic degradation of monoamines in presynaptic membrane by MAO. Enzymatic degradation of catecholamines in postsynaptic membrane by COMT.
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Mechanism of Action

Monoamine NT do not directly open ion channels. Act through second messenger, such as cAMP. Binding of norepinephrine stimulates dissociation of G-protein alpha subunit. Alpha subunit binds to adenylate cyclase, converting ATP to cAMP. cAMP activates protein kinase, phosphorylating other proteins. Open ion channels.
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Norepinephrine (NE) as NT

NT in both PNS and CNS. PNS(peripheral nervous system): Smooth muscles, cardiac muscle and glands. Increase in blood pressure, constriction of arteries. CNS: General behavior.
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Polypeptides as NT

Endogenous opiods: Brain produces its own analgesic endogenous morphine-like compounds, blocking the release of substance P. Beta-endorphin, enkephalins, dynorphin. Neuropeptide Y: Most abundant neuropeptide in brain. Inhibits glutamate in hippocampus. Powerful stimulator of appetite. NO: Exerts its effects by stimulation of cGMP. Macrophages release NO helps kill bacteria. Involved in memory and learning. Smooth muscle relaxation.
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