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The Nervous System Ch. 12

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Title: The Nervous System Ch. 12


1
The Nervous SystemCh. 12 13
  • Lindsey Bily
  • Austin High School
  • Anatomy Physiology

2
The Nervous System
  • Made up of the brain, spinal cord and nerves.
  • The purpose of the nervous system is to detect
    changes in internal and external environment,
    evaluate the information, and possibly respond by
    causing changes in the muscles or glands.
  • Divided into the Central and Peripheral Nervous
    System.

3
Divisions of the Nervous System
  • Central Nervous System brain and spinal cord and
    nerves that lie completely within the brain and
    spinal cord.
  • Peripheral Nervous System nerve tissues that lie
    in the outer regions of the nervous system.
  • Cranial Nerves nerves that originate in the
    brain.
  • Spinal Nerves nerves that originate in the
    spinal cord.

4
Central and Peripheral Nervous System
  • ?CNS
  • PNS ?

5
Afferent and Efferent Divisions
  • Obviously, signals go to the brain and back out
    of it, so we need nerves to send messages both
    ways.
  • Afferent Division incoming or sensory pathways
    (usually blue in diagrams).
  • Efferent Division outgoing or motor pathways
    (usually red in diagrams).

6
Somatic and Autonomic Nervous System
  • Somatic Nervous System carry information to the
    skeletal muscle cells. Voluntary.
  • Autonomic Nervous System carry information to
    the smooth and cardiac muscles, and glands.
    Involuntary.
  • Sympathetic fight or flight
  • Parasympathetic normal resting activities rest
    and repair.

7
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8
Cells of the Nervous System
  • Glia or Glial cells do not conduct information
    but support the function of neurons.
  • Neurons excitable cells that conduct nerve
    impulses.

?Neurons Glial Cells (gray) ?
9
Glial Cells
  • Glia literally means glue.
  • There are about 900 billion glial cells in the
    body. 9 times the number of stars in the Milky
    Way.
  • Unlike neurons, glial cells can divide throughout
    their life.
  • Susceptible to cancer due to their ability to
    divide. Most brain cancers are due to glial
    cells.

10
Types of Glial Cells
  • Astrocytes Stars of the Nervous System They
    get glucose from the blood and feed it to the
    neurons. Help to form the Blood Brain Barrier
    (BBB).
  • Microglia In CNS. They are usually small and
    stationary, but enlarge and move around when they
    are needed to eat (phagocytosis) microorganisms
    and cellular debris during inflamed or
    degenerating nerve tissue.
  • Ependymal cells form thin sheets that line fluid
    filled cavities in the brain and spinal cord.
    Similar to epithelial cells.
  • Oligodendrocytes similar to astrocytes but have
    fewer branches. Help to hold nerve fibers
    together and produce the fatty myelin sheath
    around the nerve fibers in the CNS.
  • Schwann Cells found only in the PNS. Serve the
    same role as oligodendrocytes.

11
Types of Glial Cells
  • ?Motor neuron (red) astrocytes (green)
  • Microglia (green) ?

Ependymal Cells
Oligodendrocytes Schwann
Cells
12
Schwann Cells
  • Many Schwann Cells wrap themselves around a
    single neuron.
  • Myelin is a white fatty substance that insulates
    the neuron like plastic on a wire.
  • The microscopic gaps between Schwann Cells are
    called Nodes of Ranvier.
  • Very important for nerve impulse conduction.
  • Cells with myelin are called white fibers and
    gray fibers when they are non-myelinated.

13
Blood Brain Barrier
  • Formed by the astrocytes that wrap their feet
    around the capillaries in the brain.
  • Regulates the passage of ions and molecules into
    and out of the brain.
  • Water, oxygen, carbon dioxide, glucose and small
    lipid soluble molecules such as alcohol can cross
    the barrier easily.
  • Ions (Na and K) are regulated because they
    could disrupt nerve impulses.
  • Must be taken into consideration when developing
    drug treatments for brain disorders.
  • Ex. Parkinsons need dopamine but it cannot pass
    BBB. They are given L-dopa which can pass and is
    made into dopamine by the brain cells.

14
Blood Brain Barrier
15
Multiple Sclerosis
  • Myelin disorder of the oligodendrocytes.
  • Loss of myelin and destruction of the
    oligodendrocytes.
  • Hard plaquelike lesions replace the myelin and
    causes inflammation.
  • Impaired nerve conduction, loss of coordination,
    visual impairment and speech disturbances.
  • Most common in women 20-40.
  • Caused by autoimmunity or a viral infection.

16
Neurons
  • We have about 100 billion neurons. This is only
    about 10 of all the nervous system cells in the
    brain.
  • Neurons are also called nerve fibers.
  • Parts of the neuron
  • Cell body
  • Dendrites branch off from the cell body. Means
    tree. They receive stimuli and conduct
    electrical signals towards the cell body and/or
    axon.
  • Axon a single process that comes off the cell
    body via the axon hillock. They conduct impulses
    away from the cell.

17
Neurons
18
Classification of Neurons
  • Multipolar one axon, several dendrites. Most
    neurons in the brain and spinal cord.
  • Bipolar one axon and one highly branched
    dendrite. Least common type of neuron, found in
    retina, inner ear, and olfactory pathway (nasal).
  • Unipolar or Psuedounipolar single process
    extending from the cell body. Always sensory
    neurons that conduct information to the CNS.

19
Classification of Neurons
  • Multipolar
  • Bipolar
  • Unipolar

20
Classification of Neurons
  • Afferent (sensory) neurons Transmit impulses to
    the CNS.
  • Efferent (motor) neurons transmit impulses away
    from CNS towards or to muscles or glands.
  • Interneurons Transmit impulses from afferent
    neurons to efferent neurons. Lie completely
    within the CNS.

21
Reflex Arc
  • Neurons are often arranged in a pattern called a
    reflex arc. Its a signal conduction route.
  • Most common form is a 3-neuron arc (sensory?
    interneuron? motor)
  • 2-neuron arc (sensory ? motor)
  • Synapse place where nerve information is passed
    from one neuron to another. Passed from the
    synaptic knobs of one neuron to the dendrites of
    the other.

22
Reflex Arc
23
Nerves and Tracts
  • Nerves bundles of nerve fibers in the Peripheral
    nervous system held together by several layers of
    connective tissue. (ex. Sciatic nerve)
  • Tracts bundles of nerve fibers in the central
    nervous system. (ex. Corticospinal tract)
  • White matter myelinated nerve fibers
  • Gray matter unmyelinated nerve fibers and cell
    bodies

24
Repair of Nerve Fibers
  • Mature neurons cannot divide, damaged neurons
    cannot be replaced.
  • They can sometimes repair themselves in the PNS
    if the damage is not too severe.
  • 1. After the damage has occurred, the distal
    portion of the axon degenerates.
  • 2. macrophages move in and remove the debris.
  • 3. the neurolemma (nerve sheath formed by Schwann
    Cells) forms a tunnel from the point of injury to
    the effector.
  • 4. new Schwann cells grow within the tunnel to
    support axon growth.

25
Repair of Nerve Fibers
  • The skeletal muscle that is innervated to the
    damaged nerve atrophies as it is not being
    stimulated.
  • If the damaged axon doesnt repair itself,
    sometimes a nearby healthy neuron will establish
    a connection with the muscle.
  • One damaged axon in a single neuron can shut down
    an entire nerve pathway if not repaired.

26
CNS Repair
  • Cells in the CNS hardly ever repair themselves.
    They lack a neurolemma to build a tunnel and
    astrocytes fill in damaged areas and form scar
    tissue.
  • Most spinal cord injuries involve crushing or
    bruising of the nerves.
  • Inflammation after the accident causes more
    damage to surrounding nerves.
  • Early treatment of the antiinflammatory drug,
    methylprednisolone is prescribed within 8 hours
    of injury to reduce the swelling.

27
Nerve Impulses
  • Neurons exhibit excitability and conductivity.
  • A nerve impulse is a wave of electrical
    fluctuation that travels along the plasma
    membrane.
  • Membrane potential Cells have slightly more (-)
    charges inside the cell than on the outside
    (extracellular fluid is more ).
  • This difference in ion concentration across the
    plasma membrane has potential energy.

28
Membrane Potentials
  • A membrane is polarized if it has a membrane
    potential.
  • We can measure the potential difference between
    the two sides of the polarized membrane in
    (Vvolts or mV millivolts).
  • -70 mV tells us that the difference in charge is
    70 mV and that the inside of the cell is negative
    (-).
  • 30 mV tells us that the difference in charge is
    30 mV and the inside of the cell is positive ().

29
Membrane Potential
30
Resting Potential
  • A neuron is resting when it is not conducting
    nerve impulses.
  • Stays about -70mV (RMP-resting membrane
    potential)
  • There are no gates or they are closed to not
    allow anions (-) in or out of the cell.
  • Cations () Na and Kcan move in and out of the
    cell through gates.

31
Resting Potential
  • K gates are usually open and Na gates are
    usually closed.
  • There are also Na and K pumps that are active
    transport mechanisms.
  • Pumps 3 Na out for every 2 K in and at
    different rates.
  • If 100 K are pumped inside the cell, 150 Na are
    pumped out.
  • This maintains a difference in charges inside
    and out of the cell. Slightly more positive
    outside the cell.

32
Resting Potential
  • Very little Na diffuses through the membrane.
    The pump maintains a imbalance of ions inside and
    outside of the cell.

33
Local Potentials
  • The resting membrane potentials (RMP) of neurons
    can fluctuate due to certain stimuli.
  • Local Potential slight change in the RMP.
  • Stimulus-gated Na channels open in response to a
    sensory stimulus or stimulus from another neuron
    (excitation)
  • When they open, more Na rushes into the cell,
    causing it to become more . Depolarization
    (movement of the membrane potential to zero mV).

34
Local Potentials
  • Stimulus-gated K channels open during
    inhibition. Causing the outside of the cell to
    become more . Hyperpolarization (movement of
    the membrane potential away from zero mV.) Now we
    are below the RMP.
  • Local potentials are graded potentials meaning
    they can be large or small depending on the
    strength of the stimulus. They are also isolated
    to a particular location on the plasma membrane
    and do not travel down the axon.

35
Local Potentials
  • Small depolarizations or hyperpolarizations
    applied to certain dendrites on a neuron.

36
Action Potentials
  • Action Potential is the membrane potential of a
    neuron that is conducting an impulse. Also called
    a nerve impulse.
  • There are 6 steps in conducting an action
    potential.
  • http//www.metope.org/neuron/

37
Action Potentials
  • An adequate stimulus must be applied and the
    stimulus-gated Na channels will open to allow
    Na in (depolarization).
  • If the level of depolarization surpasses the
    threshold potential (usually -59 mV)
    voltage-gated Na channels will open allowing
    MORE Na in the cell.
  • As more Na comes inside, the voltage inside the
    cell gets closer and closer to 0 mV and will
    continue to 30 mV. Means we now have more ions
    in the cell than outside of the cell.
  • Voltage-gated Na channels only stay open for
    about 1 millisecond before they close. Action
    potentials are all-or-none, either they will
    occur or not at all.
  • Once the peak of the action potential is reached
    , it starts to move back to -70 mV (resting
    potential). This is called repolarization. The
    reaching of the threshold potential causes
    voltage-gated K channels to open as well, but
    they are slow to respond. So they dont open
    until the 30 mV potential is reached. Then K
    pours out and Na goes back out.
  • Because so much K pours out of the cell, the
    voltage goes past -70 mV for a brief period of
    hyperpolarization, but then it gets back to the
    resting state.

38
Refractory Period
  • Brief period during which a local area on the
    axons membrane can not be restimulated.
  • Absolute refractory period ½ a millisecond after
    the threshold potential is surpassed. Axon will
    not respond to any stimulus.
  • Relative refractory period few milliseconds
    after the absolute refractory period. Can only
    be stimulated if the stimulus is really strong.
  • A strong stimulus causes more action potentials
    vs. a weak stimulus. However, the strength of
    each action potential is the same.

39
Conduction of the Action Potential
  • The action potential causes an electrical current
    to flow down segments of the axons membrane.
  • It will never move backward due to the refractory
    period of the membrane before the AP.
  • In myelinated fibers, the myelin sheath prevents
    ion movement, so electrical changes only occur in
    the gaps between myelin (Nodes of Ranvier).
  • The AP seems to leap from node to node. This is
    called saltatory conduction. (Latin-saltare-to
    leap)

40
Conduction of the Action Potential
  • The larger the diameter of the fiber, the faster
    it conducts impulses.
  • Myelinated fibers conduct impulses faster than
    unmyelinated fibers.
  • Fastest fibers innervate skeletal muscles and can
    fire impulses close to 300 mph.
  • Slowest fibers, such as sensory receptors in the
    skin, conduct impulses at less than 1 mph.

41
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42
Saltatory Conduction
43
Anesthetics
  • Block pain.
  • Inhibit the opening of Na channels so the nerve
    cannot conduct impulses.
  • Bupivacaine (Marcaine) used in dental
    procedures.
  • Procaine used to block signals in sensory
    pathways of the spinal cord.
  • Benzocaine and phenol found in over the counter
    products that release pain associated with
    teething, sore throat pain, and other ailments.

44
Synaptic Transmission
  • Synapse is where signals are sent from one neuron
    to another (presynaptic neuron to the
    postsynaptic neuron)
  • Types of Synapses
  • Electrical occur where two cells are joined at
    gap junctions (cardiac muscle, some smooth
    muscle). The impulse goes from one plasma
    membrane to the other.
  • Chemical Use chemicals (neurotransmitters) to
    send a signal from the pre- to the postsynaptic
    cell.

45
Chemical Synapse
  • Structures of the chemical synapse.
  • 1. Synaptic knob- tiny bulge at the end of the
    presynaptic neurons axon. Contains numerous
    small sacs or vesicles that contain
    neurotransmitter.
  • 2. synaptic cleft- space between the synaptic
    knob and the plasma membrane of the postsynaptic
    neuron, 1 millionth of an inch wide!
  • 3. the plasma membrane of the postsynaptic
    neuron- has protein receptors embedded in which
    neurotransmitters bind.

46
Types of Synapses
Electrical Synapse Chemical Synapse
47
How Synaptic Transmission Occurs
  • Action potentials cannot cross synaptic clefts
    even though the spaces are so tiny.
  • Instead, neurotransmitters (NT) are released in
    cause a response in the postsynaptic neuron.
  • Excitatory NT cause depolarization and inhibitory
    NT cause hyperpolarization.

48
How Synaptic Transmission Occurs
  1. Action potential (AP) reaches a synaptic knob,
    causing voltage-gated Ca 2 channels to open and
    allow Ca 2 to diffuse into the knob rapidly.
  2. Increase in Ca2 causes NT to be released into
    the synaptic cleft.
  3. The NT binds to receptors on the postsynaptic
    membrane which causes the ion gates to open.
  4. The NT will either cause an excitatory
    postsynaptic potential (EPSP) or an inhibitory
    postsynaptic potential (IPSP).
  5. Once the NT binds to the receptor its action is
    terminated.

49
Neurotransmitters
  • Neurotransmitters are how neurons talk to one
    another.
  • Can be excitatory or inhibitory.
  • Their affect is determined by the receptor, not
    the actual NT.

50
Types of Neurotransmitters
  • Acetylcholine- excites skeletal muscles but
    inhibits cardiac muscles.
  • Amines- (seratonin, histamine, dopamine,
    epinephrine and norepinephrine). Affect learning,
    motor control, emotions, etc.
  • Amino Acids- (glutamate, GABA, glycine). Some are
    excitatory and some are inhibitory in the CNS.
  • Other small molecule transmitters- (nitric oxide
    NO and carbon monoxide CO).
  • Neuropeptides- short strands of amino acids.
    Include enkephalins and endorphins. Inhibitory
    and block pain.

51
Antidepressants
  • Severe psychic depression occurs when there is a
    lack of norepinephrine, dopamine, serotonin and
    other amines.
  • Antidepressants work several different ways.
  • They may inhibit enzymes that are used to
    inactivate the NT.
  • They may block the reuptake of the NT by the
    neuron, keeping them in the synapse longer.
  • Cocaine blocks the reuptake of dopamine, giving a
    temporary feeling of well-being.
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