Nervous System Friday November 16 2012 12 40 PM Divisions of the Nervous System has the spinal cord and the brain 1 CNS 2 PNS has the nerves and sense organs General Characteristics of a nerve cell dendrites receptive region a multipolar neuron b bipolar neuron c pseudounipolar neuron 2 functional classification direction of impulse conduction a sensory afferent neurons b motor efferent neurons c association interneurons MULTIPOLAR NEURONS most abundant structural class axon conducting region a neuron can have only one axon axon is the only region that can generate and transmit conduct action potentials which travel away from the cell body to the axon terminal axon terminals contain vesicles which have neurotransmitters neurotransmitters are released when action potentials arrive at the axon terminals CLASSIFICATION OF NEURONS 1 structural classification number of processes extending from the cell body there are always one axon in perspective of the number of processes extending from the cell body cell body with 1001 processes one axon 1000 dendrites BIPOLAR NEURONS has one axon and one dendrite PSEUDOUNIPOLAR NEURON a short stalk extends from the cell body and bifurcates into 2 structures one is myelinated like axon only axons can be myelinated have uniform diameter like axons dendrites are tapered one of the structures conducts impulses impulses traveling action potential away from the cell body like axons the other structure conduct impulses towards the cell body like dendrite based on this conundrum the 2 structures are referred to as peripheral process and central process the peripheral process will conduct impulses towards cell body and central will take impulses away FUNCTIONAL CLASSIFICATION defines a simple reflex arc a sensory afferent neurons axons generate and conduct transmit action potentials toward the CNS and the sensory neurons synapse the neurons in the CNS called b association neurons interneurons integrate interpret the information and and provide command through c the motor efferent neurons for action to be taken in humans most of the neurons are association neurons and since the most abundant structural class is the multipolar neurons association neurons are of the structural class multipolar neurons multipolar neurons are mostly of the functional class association neurons AXONS do not exist in isolation they associate with other axons to form bundles called tracts in the CNS nerves in the PNS STRUCTURAL ORGANIZATION OF TRACT OR NERVE considered organs Nervous tissue CT blood vessels is composed of 2 types of cells Neurons supporting cells Tract bundle of axons in CNS Nerve bundle of axons in PNS Nucleus cluster of neuron cell bodies in CNS Ganglion cluster of neuron cell bodies in PNS Structure of Nerve Endoneurium wraps an axon Perineurium CT wraps a fascicle Epineurium CT wraps a bundle of perineurium covered fascicles forms nerve tract Supporting Cells in CNS 1 Astrocytes Blood Brain barrier 2 Microglia macrophages 3 Ependymal cells ciliated columnar cells 4 Oligodendrocytes myelinated axons Supporting Cells in PNS 1 Schwann Cells myelinated axons 2 Satellite Cells Myelination only axons can be myelinated Axons in the PNS are myelinated by Schwann Cells each Schwann cell wraps around the axon concentric rings of plasma membrane of schwann cell MYELIN SHEATH whitish fatty protein The rest of the schwann cell cytoplasm nucleus form the NEURILEMMA Schwann cells myelination of the axon is not complete gaps in between myelin sheath called NODES OF RANVIER axolemma is exposed myelin sheath located outside of axolemma In the tract nerve the endoneurium is outside the myelin sheath One schwann cell can myelinate a segment of only one axon Axons in the CNS by the Oligodendrocytes oligodendrocytes myelinated axons use extensions to wrap around segments of axons Oligodendrocytes can myelinate segments of several axons Cytoplasm nucleus of oligodendrocytes are located a distance from the site of myelination neurolemma is absent Function of Myelin sheath myelination the axolemma of the axon Prevents interference of electrical activity from neighboring axons in the Protects Electrical Insulation tract nerve acts as insulator bad conductors of electrical current electrical activity action potential can only be generated at the Nodes of Ranvier Conduction of action potentials down a myelinated axon appears to jump from node to node towards the axon terminals this type of conduction is SALTATORY conduction Faster than Continuous conductors which occurs in unmyelinated axon where entire surface of axolemma must be depolarized Increase in rate of impulse transmission Severed Axons in PNS CAN REGENERATE Severed Axons in CNS CANNOT REGENERATE Phases of Action Potential Generated transmitted only by axons Resting Membrane Potential 1 Depolarization Phase due to SODIUM INFLUX Na goes down concentration gradient by moving into the axon since Na is a cation the RMP becomes less negative the RMP potential is destroyed and as the potential gets to a critical potential called THRESHOLD potential explosive entry of Na that drives the potential to 0 30mV increase in the potential between threshold potential and 30mV is called the UPSTROKE or spike The spike for action potential is the same for a weaker stimulus as the a stronger stimulus 30mV 2 Repolarization Phase due to closing of Na channels which halt sodium influx due to opening of K channels which cause K EFFLUX At 30mV K channels open and K goes down concentration gradient K potassium leaves the axon for the external environment termed K Efflux Hence potential reverses towards the RPM 3 Hyperpolarization Phase when the potential reaches the RMP since K channels remain open more K leaves the axon than is required to establish RMP potential becomes more negative than the RMP this more neg potential is referred to as UNDERSHOOT or hyperpolarization Na K pumps actively transport 3Na out of the axon actively transport 2K into the axon to re establish RMP Resting Membrane Potential Amplitude of an action potential is the same for all stimuli that generate action potential Refractory Periods during Action Potential 1 Absolute Refractory Period during depolarization phase all Na channels are opened and sodium influx is occurring another action potential cannot be generated at that site of the axolemma 2 Relative Refractory Period during repolarization phase when Na channels are closed but K channels are open and potassium efflux is
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