Anatomy ch 12

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Anatomy ch 12
2010-11-08 23:15:16
Anatomy Nervous system

Lecture notes over Ch 12 Nervous system
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  1. Parts of PNS
    • 1. Somatic nervous system
    • 2. Autonomic nervous system
    • 3. Enteric Nervous system
  2. Somatic Nervous System SNS
    • part of PNS
    • Consists of sensory neurons that carry into form the head, body wall, limbs, and special senses (vision, hearing, taste, smell) from the CNS
    • Consists of motor neurons that carry info from the CNS to the skeletal muscle only
    • Voluntary
  3. Autonomic Nervous System ANS
    • Part of PNS
    • Consists of sensory neurons that carry info from visceral organs to the CNS
    • Consists of motor neurons that carry info from the CNS to smooth muscle, cardiac muscle, and glands
    • involuntary
    • ANS is divided into :
    • 1. Sympathetic division - Mobilizes the body during emergencies - Fight or flight
    • 2. Parasympathetic division - Provides non-emergency functions - Rest and Digest
  4. Enteric Nervous System - ENS
    • Part of PNS
    • consists of sensory and motor neurons that serve the gastrointestinal tract only
    • involuntary
  5. Two major types of cells that make up nervous tissue
    • 1. Neurons
    • 2. Neuroglia
  6. Neuron
    • Excitable cells
    • Highly specialized to conduct action potential all over the body
    • Do not multiply or divide - not very repairable
  7. Parts of Neuron
    • 1. Cell body -
    • a. Contains nucleus -
    • b. cytoplasm contains typical organelles
    • c. nissl bodies - site of protein synthesis in neurons

    2. Dentrites - slender, highly branched extensions; receiving region of a neuron

    3. Axon - single, ling extension through which action potentials travel, the transmitting region of a nueron
  8. Components of an axon
    • 1. Axon hillock - cone-shaped elevation where the axon joins the cell body
    • 2. Initial segment - part of the axon closest to the axon hillock
    • 3. Trigger zone - junction between the axon hillock and initial segment where nerve impulses arise
    • 4. Axoplasm - cytoplasm of the axon
    • 5. Axolemma - the plasma membrane of the axon
    • 6. Axon collaterals - side branches off the main axon - usually at right angles
    • 7. Axon terminal - an axon ends by dividing into many fine processes - some have synaptic endbulbs
    • 8. Myelin sheath - insulates, holds axon together
    • 9. Internode - section of axon covered by myelin sheath
    • 10. Node of Ranvier - spaces between the internodes
  9. synapse
    • site where one neuron communicates with another neuron or an effector (such as glandular cells or muscle cells)
    • synaptic end bulbs - enlarged tips of axon terminals - release synaptic vesicles (neurotransmitters (chemicals) packaged inside vesicles)
    • Varicosities - string of swollen bumps on the axon terminal filled with synaptic vesicles
  10. Structural classification of neurons
    • 1. Multipolar neurons -many dendrites
    • a. Have two or more dendrites and one axon
    • b. Most common type of neuron in CNS
    • 2. Bipolar Neuron
    • a. Have a long dendrite and an axon
    • b. Rare, except for special sense organs
    • 3. Unipolar Neuron
    • a. Dendrite and axon fuse into single structure
    • b. Cell body is located off to one side
    • c. Found in most sensory neurons of PNS
  11. Neuroglia
    • "glue" that holds nervous tissue together - support, nourish, protect neurons
    • make up about half the volume of the CNS
    • Smaller than neurons but 5 to 50 times more numerous
    • NOT excitable and do not conduct action potentials
    • ABLE to multiply and divide - when neurons are damaged neuroglia fill in spaces formerly occupied by neurons
  12. Neuroglia of the CNS
    • classified on the basis of size, cytoplasmic process, and intracellular organization
    • 1. Astrocytes
    • 2. Oligodendrocytes
    • 3. Microglia
    • 4. Ependymal cells
  13. Astrocytes
    • Neuroglia of the CNS
    • star-shaped, largest, most numerous of the neuroglia
    • Help maintains the BBB blood-brain barrier - restricts the movement of substances between the blood and interstitial fluid of the CNS
  14. Oligodendrocytes
    • neuroglia of the CNS
    • resemble astrocytes but are smaller and contain fewer processes
    • "tie" axons together and increase structural organization
    • Form and maintain the Myelin sheath - a multilayered lipid and protein covering around some axons that insulates them and increases the speed of nerve impulse conduction - such axons are called myelinated
    • One oligodendrocyte can produce myelin sheaths for several axons
  15. Microglia
    • neuroglia of the CNS
    • small cells with slender processes that give off numerous thorny-looking projections
    • Phagocytes - patrol nervous tissue to remove debris
  16. Ependymal cells
    • neuroglia of the CNS
    • specialized cuboidal or columnar cells (with microvilli) that line the central canal of the spinal cord and the ventricle of the brain - produce cerebrospinal fluid
  17. Neuroglia of the PNS
    • completely surround axons and cell bodies
    • 1. Schwann cells
    • 2. Satellitel cells
  18. Schwann cells
    • neuroglia of the PNS
    • Form myelin sheath around axons in the PNS - can only myelinate a singe axon or group of bundled unmylinated axons
    • participate in axon regeneration - more easily accomplished in the PNS than the CNS
  19. Satellite cells
    • surround neuron cell bodies of PNA ganglia (clusters of neuron cell bodies)
    • monitor and regulate the environment around neurons - similar in function to astrocytes
  20. neurolemma
    • the outer nucleated cytoplasmic layer of he schwann cell which encloses the myelin sheath (sheath of schwanns)
    • found only around axons in the PNS
  21. ganglion
    • ganglion - cluster of neuronal cell bodies located in the PNS
    • nucleus - cluster of neuronal cell bodies located in the CNS
  22. White matter
    Grey matter
    • In a freshly dissected brain or spinal cord, some regions look white and others grey
    • White matter - composed of mostly myelinated axons - myelin give the white color
    • Grey matter - contains unmyelinated axons, neuronal cell bodies, dendrites, axon terminals, and neuroglia - grey becuase nissl bodies look grey
    • Form outer layer of brain and butterfly shaped portion of spinal cord
  23. membrane potential
    • membrane potential - difference in charge across the membrane
    • Flow of ions through specific ion channels can chnage the resting membrane potential

    • Excitable cells have a polarized resting membrane (+ out / -in)
    • Intracellular fluid is negative
    • Extracellular fluid is postive

  24. Ion Channels
    • Establishes membrane potential across the cells (sodium out, potassium in)
    • Ions move down their electrochemial gradient (chemical gradient plus electrical gradient)
    • - areas of high conentration to areas of low concentration
    • - toward area of opposite charge
    • 1. Leakage Gates
    • 2. Voltage gated channels
    • 3. Ligand-gated channel
    • 4. Mechanically gated channel
  25. Leakage channels
    • gates randomly alternate between open and closed
    • found all over the neuron (cell body, dentrites, axon)
    • Many more K+ leakage channels than Na+ (membrane much more permiable to K+)

    as K+ leaks out a negative ions build up inside - electrical gradient will bring some back in
  26. Voltage gated channels
    • open or close in response to a change in membrane potential (voltage)
    • participate in the generation and conduction of action potentials
    • Found mostly along axons (where action potential is generated)
  27. Ligand-gated channels
    • Open or close when a chemical (ligand) binds to the membrane
    • ligands - neurotransmitters, hormones, and particular ions

    ex AChL opens cation channels that allow Na+ and Ca2+ to diffuse inward and K+ to diffuse outward
  28. Mechanically gated channels
    • Open or close when the membrane is physically distorted - vibration, touch, pressure, or tissue streatching
    • Found in membranes of sensory receptors - eg auditory receptors in the ears
  29. Resting Membrane Potential
    • a form of potential energy
    • exists because of polarized plasma membrane - separation of negative and positive electrical charges across the plasma membrane
    • Maintained by three conditions:
    • 1. Unequal distribution of ions across the plasma membrane
    • 2. Inability of most anions to leave the cell (Phosphate in ATP and amino acids)
    • 3. Leakage channels countered by sodium-potassium gates (Na+/K+ ATPases) expel 3 Na+ for every 2 K+ imported - electrogenic - more positive out than in- contribute to negativity
    • Typically measures -70mV

    • When the membrane potential changes two types of signals are produced:
    • 1. Graded potential
    • 2. Action potential
  30. Graded Potential - local potential
    • Always the first potential produced - signals over a short distance only
    • Occurs at dendrites and in cell body and rarely in axons
    • called graded because they vary in size depending on the stimulus

    • Hyperpolarization - when the response makes the membrane more polarized (inside more negative)
    • Depolarization - when the response makes the membrane less polarized (inside less negative)

    localized - signal only travels a short distance - to transfer the signal to other neurons -action potential is needed to link graded potentials at the dendrites or cell body to the cell membrane at the synaptic terminals
  31. Action Potential
    All or none principle - action potential either occurs completely or it does not occur at all

    • A stimulus that is strong enough to reach threshold levels will produce an action potential
    • sub-threshold stimulus - a stimulus that is not strong enough to reach threshold levels will not produce and action potential

    An action potential starts near the axon hillock and moves along the entire length of an axon to the synaptic terminals

    • The generation of an action potential takes place in two phases
    • 1. Depolarizing phase
    • 2. Repolarizing phase
  32. Phases of action potential
    • 1. Depolarizing phase
    • a. stimulus causes depolarization to threshold (-55mV)
    • b. voltage gated Na+ channels open
    • c. inrush of Na+ causes rapid depolarization (+30mV)
    • d. Na+ channels inactivate (resting, activated, inactivated)
    • e. Voltage gated K+ channels open slowly as Na+ channels close
    • 2. Repolarizing phase
    • a. Na+ voltage gated channel inactivated
    • b. K+ channels are open - return potential to -70mV
    • 3. Refractory period
    • The time during which an excitable cell cannot generate another action potential
    • a. Absolute refractory period - no action potential possible, no matter how strong the stimulus
    • b. Relative refractory period - a stronger than normal stimulus may generate a second action potential
  33. Propagation (conduction) of Nerve Impulses
    Defined as the traveling of a nerve impulse from one part of the body to another

    • 1. Continuous conduction - Occurs in unmyelinated axons
    • -action potentials trigger action potentials in adjacent region
    • -Initial segment goes into refractory period while adjacent segment depolarizes

    • 2. Saltatory propagation - occurs in myelinated axons
    • - action potential "leaps" from node to node
    • - much faster than continuous propagation and more efficient
    • -- requires fewer ion channels

    • propagation speed depends on axon diameter and myelination
    • wider (larger) diameter = less resistance - faster flow rate
  34. Three types of axons
    • 1. Type A fibers
    • - Largest diameter axons
    • - Myelinated
    • - Propagation speed: up to 300mph
    • - Sensory neurons that control position, balance, delicate touch and pressure
    • - Motor neurons that control skeletal muscles

    • 2. Type B fibers
    • - smaller diameter axons
    • - myelinated
    • - propagation speed - up to 38 mph
    • - some sensory neurons from viscera and some autonomic motor neurons

    • 3. Type C fibers
    • - smallest diameter axons
    • - unmyelinated
    • - propagation speed - up to 4 mph
    • - some sensory neurons from viscera and some autonomic motor neurons

    Type B & C carry info on temp, pain, general touch - also carry info to smooth muscle, cardiac, muscles, and glands
  35. Signal transmission at synapses
    Action potentials are passed on electrically or chemically

    • Presynaptic neuron sends the signal
    • Postsynaptic neuron receives the signal
  36. Electric synapses
    • Rare
    • -Gap junctions connect presynaptic membranes with postsynaptic membranes
    • - Gap junctions have pores that allow ions to flow between the two cells
    • - Very efficient propagation of action potentials
    • -- faster communication
    • -- synchronization
    • -found in cardiac muscle, visceral smooth muscle, and the developing embryo
  37. Chemical synpses
    • - Most abundant type of synapse
    • - Involves chemical messengers called neurotransmitters
    • -- Neurotransmitters carry impulse across the synapse
    • - Two types of neurotransmitters
    • -- Excitatory neurotransmitters
    • -- Inhibitory neurotransmitters
  38. Nerve

    nerve - bundle of axons in the PNS

    tract- bundle of axons in the CNS
  39. synaptic cleft
    steps in signal transmission at a synaptic cleft
    • a synaptic cleft is a space between two neurons filled with intersitital fluid
    • a chemical synapse is an electrical signal converted into a chemical signal and then converted into another electric signal

    • 1. Nerve impulse arrives at synaptic end bulb of presynaptic neruon
    • 2. Depolarixation triggers the opening of voltage gated calcium channels and calcium enters the synaptic end bulb
    • 3. Calcium triggers the exocytosis of synaptic vessicles holding neurotransmitters
    • 4. Neurotransmitters diffuse across the synaptic cleft and bind to neurotransmitter receptors on the postsynaptic neuron
    • 5. receptors trigger opening of ion channels in postsynaptic membrane and a postsynaptic potential (change in membrane voltage)occurs
    • 6. Depending in the type of ion involved, the postsynaptic potential leads to either
    • -Hyperpolarazation: triggers no aciton potential
    • - Depolarization: triggers one or more action potentials
  40. Excitatory postsynaptic potential - EPSP
    Inhibitory postsynaptic potential - IPSP
    in the postsynaptic neuron, an excitatory neurotransmitter will depolarize the membrane and bring it closer to threshold (or push over threshold to action potential)

    a neurotransmitter that is inhibitory causes hyperpolarization - the inside of the membrane becomes even more negative - farther from threshold - results form opening Cl- or K+ channels
  41. Removal of Neurotransmitters from a synaptic cleft
    removal of the neurotransmitter form the synaptic cleft is essential to normal synaptic funcitons. If the neurotransmitter remains in the cleft it will cause indefinite stimulation of the postsyaptic structure - remove it so gates dont stay open

    • 1. Diffusion - cleft is interstitial space - some will diffuse away
    • 2. Enzymatic degradation - enzymes break apart neurotransmitters
    • 3. Uptake by cells - many neurotransmitters are actively transported back into the neuron that released them (reuptake). glial cells take the neurotransmitters into themselves (from the cleft)
  42. Summation

    Two types of summation
    summation is the process by which graded potentials add together - One ESPS is usually not enough to bring about an action potential - several ESPS have to occur to reach threshold

    1. Spatial summation - space - two or more stimuli arrive simultaneously at seperate synapses on the same neuron - currents overlap and threshold is reached

    2. Temporal summation -time - two or more stimuli arrive rapidly at the same synapse - effects build on each other and together they reach threshold

    a single postsynaptic neuron recieves signals from many presynaptic neurons - ESPS and ISPS - the SUM of these signals determines the effect on the postsynaptic neuron
  43. neurotransmitters
    neurosecretory cells
    two classes of neurotransmitters
    • about 100 known neurotransmitters
    • neurosecretory cells - neurons in the brain that secrete neurotransmitter and hormones

    • neurotransmitters are divided into two classes based on size
    • 1. Small- molecule neurotransmitters
    • 2. neuropeptides
  44. small-molecule neurotransmitters
    • 1. Acetylcholine - ACh - present in both CNS and PNS
    • - can be excitatory (NMJ) or inhibitory (slowing heart rate)
    • - Acetylcholinesterase inactivates ACh in the synapse by breaking it down

    • 2. Amino Acids - Present only in CNS
    • - Excitatory amino acid neurotransmitters include
    • ----glutamate- most excitatory neurons in the CNS communicate via glutamate
    • ----aspartate
    • - Inhibitory amino acid neurotransmitters are
    • ----GABA - only in CNS - most common inhibitory neurotransmitter
    • ---- glycine

    • 3. Biogenic Amines - amino acids that are modified and decarboxylated (carboxyl group removed)
    • - Excitiatory or Inhibitory depending on the type of receptors
    • ---Norepinephrine and epinephrine - released by adrenal gland - used in awakening, dreaming, and regulating mood
    • ---Dopamine - used in emotional response, addictive behaviors and pleasurable experiences

    CATECHOLAMINES - norepinephrine, epinephrine, and dopamine - chemically all similar - inactivated by reuptake into synaptic end bulbs - then either recycled back into the synaptic vesicles or destroyed by an enzyme (COMT or MAO)

    4. ATP - excitatory neurotransmitter in both CNS and PNS

    5. Nitric Oxide - forms on demand (not stored in synaptic vesicles) - highly lipid soluable, can leak out of cells - acts as a "relaxing factor"and brings on VASODILATION (muscles relax and blood vessels dialate)
  45. Neuropeptides
    • neurotransmitters that are made of 3 to 40 linked amino acids- CNS and PNS
    • Excitatory and Inhibitory
    • - Enkephalins - potent natural pain-relievers
    • - Endorphins - runners high - feel good
    • - Substance P - pain transmitters - keep up from permanently hurting ourselves
  46. neural circuits
    5 types of neural circuits
    neural circuits are funcitonal groups of neurons that process specific types of information

    • 1. Simple series circuit - christmas lights - one presynaptic neuron stimulates one postsynaptic neuron which stimulates another neuron...
    • -rare except for reflexes

    2. Divergent circuit - pyramid shceme - one presynaptic neuron stimulates several postsynaptic neurons which each stimulate several more neurons

    3. Convergent circuit - several presynaptic neurons synapse with a single postsynaptic neurons - arrow- many converge to one

    4, Reverberating circuit - a form of positive feedback - collateral branches of postsynaptic neurons reach back to the presynaptic nuerons and further stimulate it - breathing

    5, Parallel after-discharge processing - both temporal and spacial summation - a single presynaptic cell stimulates a group of neurons which all synapse with a common postsynaptic cell - the last neuron sums the ESPS and IPSP
  47. Regeneration and repair of nervous tissue
    nervous system exhibits plasticity - the capability to change based on experience

    despite plasticity, neural regeneration (the ability to replicate or repair themselves) is very limited - In the CNS little or no repair occurs - In the PNS dendrite and axons may be repairs if the cell body and Schwanns cell is in tact

    Regeneration of an axon in the PNS:

    chromatolysis - the nissl bodies break up into fine granular masses

    Wallerian degeneration - degeneration of the distal portion of the axon and myelin sheath

    regeneration tube - the schwanns cells on either side of the injured site multiply by mitosis, grow toward each other and form a regeneration tube that guides growth of the new axon form the proximal area across the injured area into the distal area previously occupied by the original axon