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Nervous System (8-3 descibe the events involved in the generation and…

- - - - threshold stimulus cannot produce another NI
      - the RMP has to be restored before it can be depolarized again
    - - subthreshold stimulus (5mV) = no AP; no NI
      - threshold stimuli (15mV) = yes AP; yes NI
      - threshold stimulus (20mV) = yes AP; yes NI, but no greater intensity than above
    - - +15Mv = threshold = AP = NI
      - +5, +5, +5, = +15mV = threshold = AP = NI
    - - unmyelinated nerve fibers: NI must travel the length of the nerve fiber; slow
      - myelinated nerve fiber: "saltatory conduction"
        
        very fast transmission
        
        Ni jumps from node of Ranvier to node of Ranvier
  - - - occurs between the axon of one neuron and dendrite or cell body of a second neuron
      - note that the two neurons do not touch. there is a gap between them = synaptic cleft
  - - - the NI is an electrical impulse
      - a NI is similar to a row of dominos falling
      - a nerve impulse begins on a dendrite (or cell body of a neuron), runs toward the cell body, through the cell body, and then down the axon
  - - - (threshold stimulus)
      - +15mV stimulus
    - - Na+ channels open (rapid depolarization)
      - action potential (nerve impulse) is produced
    - - K+ channels open
      - (repolarization)
      - Na+ channels close
  - - - ACh is released by
        
        all motor neurons (stimulate skeletal muscles)
        
        some CNS neurons
    - - monoamines (modified amino acids)
        
        are widely distributed in the brain where they play a role in:
        
        circadian rhythm
        
        emotional behavior
        
        are present in some motor neurons of the ANS
        
        include
        
        epinephrine
        
        norepinephrine
        
        dopamine
        
        serotonin
        
        histamine
      - unmodified amino acids
        
        glutamate
        
        aspartate
        
        GABA (gamma aminobutyric acid)
        
        glycine
    - - destruction of neurotransmitter
        
        enzymes that are present in the synaptic cleft destroy NT
        
        for example, acetylcholinesterase
      - reuptake of neurotransmitter
        
        NT is transported back into pre-synaptic knob
      - both of the above processes prevent continual stimulation of the post-synaptic membrane
  - - - Therefore, a threshold stimulus = +15mV
      - the threshold potential for a neuron is -55mV
    - - this event is called the action potential
      - the action potential represents the start of the nerve impulse on a neuron
  - - - alter a neurons response to a NT
      - block the release of a NT
    - - synthesis is increased during painful stress
      - bind to the same receptors in the brain as the narcotic morphine
      - relieve pain
    - - same as above, but with a more potent and longer lasting effect
  - - - the cell membrane of a neuron must be depolarized (to approximately -55mV) in order for a certain ion channels to open and therefore start a nerve impulse
  - - - K+ = high outside
      - Na+ = high outside
      - Cl = high outside
      - negatively charged proteins or Anions = high outside
- - - - lateral horns
      - anterior (ventral) horns
      - posterior (dorsal) horns
    - - central canal (lined by ependymal cells)
      - gray commissure
      - anterior median fissure
      - posterior median fissure
    - - Locations
        
        lateral funiculi or white column
        
        anterior (ventral) funiculi or white column
        
        posterior (dorsal) funiculi or white column
      - the white matter of the spinal cord represents the location of our major nerve pathways called "nerve tracts"
        
        provide a 2-way system of communication
        
        in general, ascending tract are located in the posterior (dorsal) columns and conduct sensory (afferent) impulses from body parts to brain
        
        in general, descending tracts are located in the anterior (ventral) columns and conduct motor (efferent) impulses from brain to effectors
        
        general characteristics of nerve tracts
        
        most consist of 2-3 successive neurons
        
        most exhibit somatotropy (i.e. tracts from/to upper body are located on outside, tracts from/to lower body on inside)
        
        most cross over
        
        all pathways are paired (right and left)
  - - - dorsal root ganglion
        
        ganglion = aq bundle of cell bodies outside the CNS
        
        DRG contains the cells bodies of sensory (afferent) neurons bringing impulses to the CNS
  - - - start = foramen magnum
      - end = tapers to point (conus medullaris) and terminates near the intervertebral disc that separates the 1st - 2nd lumbar vertebra
- - - - astrocyte
        
        star-shaped
        
        function - nourishes neurons
      - microglia
        
        looks like spider
        
        function - phagocytosis
      - oligodendrocyte
        
        looks like eyeball
        
        function - produces myelin
      - ependymal cells
        
        epithelial-like layer
        
        function - lines spaces in CNS
        
        brain - ventricles
        
        spinal cord - central canal
  - - - Multipolar neurons (most common neuron in CNS)
        
        many extensions
        
        many dendrites lead toward cell body, one axon leads away from cell body
      - Bipolar neurons (rare, sight, smell, and hearing)
        
        two extensions
        
        one fused dendrite leads toward cell body, one axon leads away from cell body
      - Unipolar neurons (most PNS sensory neurons)
        
        one process from cell body
        
        forms central and peripheral processes
        
        only distal ends are dendrites
    - - Sensory neurons
        
        Afferent neurons
        
        carry sensory impulses from sensory receptors to CNS
        
        input information to CNS
        
        location of receptors = skin and sense organs
        
        somatic receptors
        
        external receptors (touch, pressure, temp, sight, smell, hearing, and touch)
        
        proprioceptors (position and movement)
        
        PNS
        
        visceral receptors (digestive, respiratory, cardiovascular, urinary, reproductive, taste, deep pressure, pain)
      - Interneurons (association)
        
        link other neurons together (i.e. sensory neuron to interneuron to motor neuron)
        
        CNS
      - Motor Neurons
        
        PNS
        
        efferent neurons
        
        carry motor impulses away from CNS and to effectors
        
        output information from CNS
        
        effectors - muscles and glands
        
        somatic motor neurons (skeletal muscles)
        
        visceral motor neurons (muscle and glands)
        
        sympathetic (adrenergic)
        
        parasympathetic (cholinergic)
  - - - cell body injury - death of neuron
      - damage to an axon may allow for regeneration
    - - each neuron is composed of a cell body and many extensions from the cells body called neuron processes or nerve fibers
        
        cell body- central portion of a neuron; contains usual organelles, except centrioles
        
        nucleus, prominent nucleolus, and many Nissl bodies = RER
        
        Neuron processes/ nerve fibers- extensions from cell body
        
        Dendrites
        
        many per neuron
        
        short and branched
        
        receptive portion of a neuron
        
        carry impulses toward cell body
        
        Axons
        
        long, thin processes
        
        carry impulses away from cell body
        
        axon branch - axonal terminals, synaptic knobs
        
        one per neuron
        
        Axons in PNS
        
        Large axons are surrounded by a myelin sheath produced by many layers of Schwann Cells (neuroganglial cells)
        
        myelin - lipoprotein
        
        interruptions in the myelin sheath between Schwann cells - Nodes of Ranvier
        
        "myelinated nerve fiber)
        
        Small axons do not have a myelin sheath
        
        "unmyelinated nerve fibers"
        
        however all axons (in PNS) are associated with Schwann cells
        
        Axons in CNS
        
        myelin is produced by an oligodendrocyte rather than Schwann cells
        
        a bundle of myelinated nerve fibers = "white matter"
        
        PNS = nerve
        
        CNS = tract/column
        
        this is in contrast to CNS "gray matter" = a bundle of cell bodies (or unmyelinated nerve fibers)
        
        PNS = ganglia
        
        CNS = nucleus
- - - - Dura mater
        
        tough, white fibrous CT
        
        contains many blood vessels and nerves
        
        note: DM splits into two layers where it encloses the dural sinuses (that collect venous blood from the brain)
        
        outermost membrane that is attached to the inner periosteum of the skull
      - Arachnoid mater
        
        middle layer
        
        thin net-like membrane
        
        beneath the arachnoid mater lies a wide space called the sub-arachnoid space
        this space is filled with cerebrospinal fluid (CSF) and serves as a cushion for the brain
      - Pia mater
        
        very thin delicate CT
        
        many nerves and blood vessels = nourishment
        
        dips into grooves and contours
        
        inner layer that clings to brain surface
    - - CSF fills the subarachnoid space and central canal
      - the space between the dura mater and the bone is called the epidural (subdural) space and is filled with loose CT and fat
      - note that the dura mater is not attached to bone of the vertebrae (as in the brain where it is attached to the skull)
    - - in addition to filling the subarachnoid space, the CSF fills the ventricles (interconnected cavities) within the cerebral hemispheres and brain stem
      - ventricles
        
        are continuous with central canal of spinal cord
        
        are filled with cerebrospinal fluid
        
        are lined by ependymal cells
      - Secretion and circulation of CSF
        
        CSF is secreted by specialized capillaries in choroid plexuses into the lateral ventricles (1 & 2)
        
        CSF circulates down into the 3rd and 4th ventricle and then into either:
        
        the central canal of spinal cord
        
        the subarachnoid space of meninges
        
        CSF is reabsorbed back into the bloodstream through arachnoid granulations that project into dural sinuses
        
        CSF movement is aided by cilia of ependymal cells
      - CSF
        
        Total volume in above spaces = 150 mL
        
        about 1 liter is secreted daily to replenish the circulating 150 mL every 3-4 hours
        
        Functions
        
        mechanical protection (cushion)
        
        chemical protection (ions, hormones)
- - - - Sensory Input Function
        
        PNS
        
        Sensory receptors (located at the ends of peripheral neurons) detect changes (i.e. are stimulated) occurring in their surroundings
        
        Once stimulated, sensory receptors transmit a sensory impulse to the CNS
        
        a sensory impulse is carried on a sensory neuron
      - Integrative Function
        
        involves interpretation of an incoming sensory impulse (i.e. decision is made concerning what is going to happen next, based on sensory impulse)
        
        integration occurs in interneurons
        
        a motor impulse beings
        
        CNS (brain and/or spinal cord)
      - Motor Function
        
        involves the response of a body part
        
        motor impulses are carried from CNS to responsive body parts called effectors
        
        a motor impulse is carried on a motor neuron
        
        PNS
        
        effectors
        
        muscles (that contract)
        
        glands (that secrete a hormone)
    - - Central Nervous System (brain and spinal cord)(interneurons)
      - Peripheral Nervous System (cranial nerves and spinal nerves)
        
        Sensory (input into CNS)(Afferent neurons)
        
        Motor (output from CNS)(efferent neurons)
        
        Somatic (effectors: skeletal muscle)(conscious control)
        
        Autonomic (effectors: smooth muscle; cardiac muscle; glands)(unconscious control)
        
        Parasympathetic (homeostasis)(neurotransmitter: acetylcholine)
        
        Sympathetic (fight-or-flight)(neurotransmitter: norepinephrine)
    - - Central Nervous System
        
        brain and spinal cord
      - Peripheral Nervous System
        
        nerves that extend from the brain (cranial nerves) and spinal cord (spinal nerves)