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Chapter 8: Nervous System (8-3 Events involved in the generation &…

Chapter 8: Nervous System

8-1: Two major anatomical divisions of the nervous system

  A. The organs of the nervous system are divided into 2 major groups.
        1. Central Nervous System (CNS) = brain & spinal cord
        2. Peripheral Nervous System (PNS) = nerves that extend from the brain (Cranial nerves) & spinal cord (Spinal nerves)

  B. General Functions of Nervous System (3 Fold Function)
        1. Sensory Input Function
              a. PNS
              b. Sensory receptors (Located at the ends of peripheral neurons) detect changes (i.e. are stimulated) occurring in their surroundings
              c. Once stimulated, sensory receptors transmit a sensory impulse to the CNS
              d. A sensory impulse is carried on a sensory neuron.
        2. Integrative Function
              a. CNS (Brain and/or spinal cord)
              b. Involves interpretation of an incoming sensory impulse (i.e. decision is made concerning what's going to happen next, based on sensory impulse)
              c. Integration occurs in interneurons
              d. A motor impulse begins
        3. Motor Function
              a. PNS
              b. Involves the response of a body part
              c. Motor impulses are carried from CNS to responsive body parts called effectors
              d. A motor impulse is carried on a motor neuron
              e. Effectors = 2 types
                    -Muscles (That contract)
                    -Glands (That secrete a hormone)

8-2 Distinguish b/w neurons & neuroglia on the basis of structure & function

  A. Neuron = the structural & functional unit of the nervous system; a nerve cell
        - Neuron Structure - Each neuron is composed of a cell body and many extensions from the cell body called neuron processes or nerve fibers. 
              a. Cell Body = central portion of neuron; contains usual organelles, except centrioles
                    -Identify: nucleus, prominent nucleolus, & many Nissl bodies = RER
              b. Neuron Processes/Nerve Fibers = extensions from cell body; two types:
                    -Dendrites
                          1. Many per neuron
                          2. Short & branched
                          3. Receptive portion of a neuron
                          4. Carry impulses toward cell body
                    -Axons
                          1. One per neuron
                          2. Long, thin process
                          3. Carry impulses away from cell body
                          4. Note terminations of axon branch = axonal terminals; synaptic knobs

        5. Axons in PNS
              a. Large axons are surrounded by a myelin sheath produced by many layers of Schwann Cells (Neuroglial cell)
                    -"Myelinated nerve fiber"
                    -Myelin = Lipoprotein
                    -Interruptions in the myelin sheath b/w Schwann cells = Nodes of Ranvier
              b. Small axons do not have a myelin sheath
                    -"Unmyelinated nerve fibers"
                    -However all axons (In PNS) are associated with Schwann cells.

        6. Axons in CNS (i.e. in Brain & Spinal Cord)
              a. Myelin is produced by an oligodendrocyte rather than Schwann cells
              b. A bundle of myelinated nerve fibers = "White Matter"
                    1. PNS = Nerve
                    2. CNS = Tract/ Column
              c. This is in contrast to CNS "Gray Matter" = A bundle of cells bodies (Or umyelinated nerve fibers)
                    1. PNS = Ganglia
                    2. CNS = Nucleus

  -Regeneration of Nerve Fibers
        1. Cell body injury = Death of neuron
        2. Damage to an axon may allow for regeneration

  B. Neuroglial Cells = accessory cells of nervous system from supporting network for neurons, "nerve glue"
        1. PNS = Schwann cells produces myelin & Satellite cells nourish neurons
        2. CNS = 4 types; provide bulk of brain & spinal cord tissue:
              a. Oligodendrocyte
                    -Looks like an eyeball
                    -Function: Produces myelin
              b. Astrocyte
                    -Star-shaped
                    -Function: Nourishes neurons
              c. Microglia
                    -Looks like a spider
                    -Function: Phagocytosis
              d. Ependymall Cells
                    -Epithelial-like layer
                    -Function: Lines spaces in CNS
                    -1. Brain = Ventricles
                    -2. Spinal Cord = Central canal

8-2 Cont... Classification of Neurons

  A. Functional Classification:
        1. Sensory Neurons
              a. PNS
              b. Afferent neurons
              c. Carry sensory impulses from sensory receptors to CNS
              d. Input information to CNS
              e. Location of receptors = skin & sense organs
                    -Somatic Receptors
                          1. External Receptors (Touch, pressure, temp, sight, smell, & hearing)
                          2. Proprioceptors (Position & movement)
                    -Visceral Receptors (Digestive, respiratory, cardiovascular, urinary, reproductive, taste, deep pressure & pain)
        2. Interneurons (Association)
              a. CNS
              b. Link other neurons together (i.e. sensory neuron to interneuron to motor neuron)
        3. Motor Neurons
              a. PNS
              b. Efferent neurons
              c. Carry motor impulses away from CNS & to effector
              d. Output information from CNS
              e. Effectors = Muscles & glands
                    -Somatic Motor Neurons (Skeletal muscle)
                    -Visceral Motor Neurons (Muscles & Glands)
                          1. Sympathetic (Adrenergic)
                          2. Parasympathetic (Cholinergic)

        B. Structural Classification:
              1. Multipolar Neurons (Most common neuron in CNS)
                    a. Many extensions
                    b. Many dendrites lead toward cell body, one axon leads away from cell body. 
              2. Bipolar Neurons (Rare, sight, smell, & hearing)
                    a. Two extensions
                    b. One fused dendrite leads toward cell body, one axon leads away from cell body. 
              3. Unipolar Neurons 
                    a. One process from cell body
                    b. Forms central & peripheral processes
                    c. Only distal ends are dendrites

8-3 Events involved in the generation & propagation of an action potential.

  A. Resting Nerve Cells
        1. A resting neuron's cell membrane is said to be polarized = electrically charged. (The charge inside the cell is different than the charge outside)

        Consequently, a potential difference (PD) exists across this resting cell membrane. 

        2. DEF: Potential Difference (PD) = the difference in electrical charge bw 2 points (Across a cell membrane)

        3. The resting membrane potential (RMP) of a neuron is results from the distribution of ions across the cell membrane. 
              a. K+ = High Inside
              b. Na+ = High Outside
              c. Cl- = High Outside
              d. Negatively charged proteins or Anions- ; high inside. 

              Recall that these ions concentrations are maintained by active transport mechanisms.

        4. The RMP of a nerve cell is measured to be -70 mV or millivolts (inside/ outside)

        5. As long as the RMP in a nerve cell is undisturbed, it remains polarized. However, in order for a nerve impulse to be started or propagated in a nerve cell, this resting potential must be disturbed.

  B. Membrane Potentials
        The RMP of -70mV can be disrupted or changed in one or two directions: 

        1. More Negative = "Hyperpolarization"
        2. Less Negative (Toward Zero) = "Depolarization" 
              a, 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.

  C. Action Potential:
        1. When the resting membrane potential of a neuron is depolarized to -55mV, threshold potential is reached;
              a. The threshold potential for a neuron is -55mV 
              b. Therfore; a threshold stimulus = +15mV
        2. When threshold potential is reached, the rapid opening of Na+ channels results in rapid depolarization (& even reversal of the membrane potential [MP] to +30mV)
              a. This event is called the action potential 
              b. The action potential represents the start of the nerve impulse on a neuron
        3. Then K+ channels open, (While Na+ channels close), & repolarization occurs = recovery of the RMP to -70mV.
        4. This all occurs very quickly = 1/1000 sec.

  D. Nerve Impulse Transmission
        A. DEF: Nerve Impulse (NI) = the propagation of action potentials (AP) along a nerve fiber; (The entire length of the neuron)
              1. The NI is an electrical impulse
              2. An NI is similar to a row of dominos falling (Once the first domino falls, the entire row will fall)
              3. A nerve impulse begins on a dendrite (Or cell body of a neuron), runs toward the cell body, through the cell body, & then down the axon. 

        B. Characteristics of a Nerve Impulse (NI)
              1. Refractory Period = the period following the NI when a threshold stimulus cannot produce another NI
                    a. The RMP has to be restored before it can be depolarized again; (Dominos must set up in order to be knocked down again)
              2. All or Nothing Response = if a nerve cell responds at all, it responds completely.
                    a. Subthreshold stimulus (5mV) = no AP; no NI
                    b. Threshold stimulus (15mV) = yes AP; yes NI
                    c. > Threshold stimulus (20mV) = yes AP; yes NI, but no greater intensity then above.
              3. Summation = many subthreshold stimuli received one after another may allow threshold potential to be reached, trigger an AP & begin a NI on a neuron.
                    a. +15 mV = threshold = AP = NI
                    b. +5, +5, +5 = +15mV = threshold = AP = NI
              4. Conduction = the manner in which the NI runs down the neuron/ nerve fiber
                    a. Unmyelinated nerve fibers: NI must travel the length of the nerve fiber; slow. 
                    b. Myelinated nerve fiber: Saltatory Conduction (ESP: Jump/Leap)
                          -NI jumps from node of Ranvier to node of Ranvier
                          -Very fast transmission

  Synaptic (Chemical) Transmission
        Nerve Impulses are transferred from one neuron to the next through synaptic transmission. 

        A. Synapse = the junction bw 2 neurons where a nerve impulse is transmitted. 
              1. Occurs bw the axon of one neuron & dendrite or cell body of a second neuron
              2. Note that the 2 neurons do not touch. There is a gap bw them = synaptic cleft. 
        B. Scheme of Synaptic Transmission
              1. NI reaches axonal terminal of pre-synaptic neuron causing depolarization of synaptic knob
              2. Ca++ channels open and calcium ions rush into axonal terminal causing
              3. Synaptic vesicles (filled with neurotransmitter/NT) to release NT via exocytosis into the synaptic cleft
              4. NT diffuses across synaptic cleft and depolarizes the post-synaptic neuron's membrane
              5. An action potential (AP) is triggered and a NI begins in the post-synaptic neuron.

        C. Neurotransmitters (NT)
              1. At least 30 different produced by CNS
              2. Some neurons produce/ release only one while release many 
              3. Most typical NT is acetycholine (ACh)
                    a. ACh is released by
                          -All motor neurons (Those that stimulate skeletal muscle)
                          -Some CNS neurons
              4. Others NTs include:
                    a. Monoamines (Modified amino acids) 
                          -Are widely distributed in the brain where they play a role in:
                                1. Emotional behavior and
                                2. Circadian rhythm
                          -Are present in some motor neurons of the ANS
                          -Include = epinephrine, norephinephrine, dopamine, serotonin, histamine
                          -Unmodified amino acids = glutamate, aspartate, GABA (Gamma aminobutyric acid), glycine
              5. Fate of Neurotransmitter in Synaptic Cleft:
                    a. Destruction of Neurotransmitter: Enzymes that are present in the synaptic cleft destroy NT. For example, acetycholinesterase destroys ACh.
                    b. Reuptake of Neurotransmitter: NT is transported back into pre-synaptic knob.

Both of the above processes prevent continual stimulation of the post-synaptic membrane.

        D. Neuropeptides
              1. Synthesized by CNS neurons
              2. Act as NTs or neuromodulators that either;
                    a. Alter a neuron's response to a NT
                    b. Block the release of a NT
              3. Include enkephalins
                    a. Synthesis is increased during painful stress
                    b. Bind to the same receptors in the brain as the narcotic morphine
                    c. Relieve pain
              4. Include endorphines
                    a. Same as above, but w a more potent and longer lasting effect

  Synaptic (Chemical Transmission)
  E. Disorders Associated with Neurotransmitter Imbalances
  -Alzheimer's= Deficient ACh
  -Clinical Depression= Deficient norephinephrine/ serotonin
  -Epilepsy= Excess GABA leads to excess norephinephrine & dopamine

8-5 Three meningeal layers that surround the central nervous system

  B. Meninges
        The membranes around the brain and spinal cord are called "meninges"; three distinct layers
  1. Brain: 
        a. Dura Mater
        -Outermost membrane that is attached to the inner periosteum of the skull
        -Tough, white fibrous CT
        -Contains many blood vessels & nerves
        -Note: DM splits into two layers where it encloses the dural sinuses (The collect venous blood from the brain)
        b. Arachnoid Mater
        -Middle layer
        -Thin net-like membrane
        -Beneath the arachnoid mater lies a wide space called teh sub-arachnoid space
              This space is filled with cerebrospinal fluid (CSF) & serves as a cushion for the brain. 
        c. Pia Mater
        -Inner layer that clings to brain surface
        -Very thin delicate CT
        -Many nerves & blood vessels = nourishment
        -Dips into grooves & contours
              **Subdural hematoma & meningitis
  2. Spinal Cord: 
        a. Note that the dura mater is not attached to bone of the vertebra (as in the brain where it is attached to the skull)
        b. The space bw the dura mater & the bone is called the epidural (subdural) space and is filled with loose CT & fat 
        c. CSF fills the subarachnoid space & central canal

  C. Ventricles & Cerebrospinal Fluid (CSF)
        1. In addition to filling the subarachnoid space, CSF fills the ventricles (Interconnected cavities) w/in the cerebral hemispheres and brain stem
        2. THE VENTRICLES
              a. Continuous with central canal of spinal cord
              b. Filled with CSF
              c. Lined by ependymal cells (Remember this neuroglial cell in CNS?)
        3. Secretion & Circulation of CSF
              a.

8-6 Role of gray matter & white matter in the spinal cord

  The spinal cord is a nerve column that passes downward from brain into the vertebral canal. Recall that it is part of the CNS. Spinal nerves extend to & from the spinal cord & are part of the PNS.

  A. Gross Structure  of Spinal Cord:
        1. Length = ab 17 inches
              a. Start = Foramen magnum (BIG HOLE)
              b. End = tapers to point (Conus Medurallis) & terminates near the intervertebral disc that separates the 1st - 2nd lumbar vertebra.
        2. Contains 31 segments (& therefore gives rise to 31 pairs of spinal nerves)
        3. Note cervical & lumbar enlargements
        4. Note cauda equina ("horse's tail") in which the lower lumbar & sarcal nerves travel downward (Lower spinal nerves must "chase' their points of exit. 
        5. Note filum terminale that represents distal portion of the tail (Pia mater)
  B. Cross-Sectional Anatomy of Spinal Cord
        1. Gray Matter or "butterfly" = bundles of (Interneuron) cell bodies:
              a, Posterior (Dorsal) horns, Lateral horns, and Anterior (Ventral) horns
        2. Note Location of:
              a. Central canal (Lined by epithelial cells), Gray commissure, Anterior median fissure, & Posterior median sulcus. 
        3. White Matter = myelinated (interneuron) axons: 
              a. Location
                    -Posterior (Dorsal) funiculi or white column, Lateral funiculi or white column, & Anterior (Ventral)
              b. 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
                    1. In general, ascending tracts are located in the posterior (dorsal) columns and conduct sensory (Afferent) impulses from body parts to brain.
                    2. In general, descending tracts are located in the anterior (Ventral) columns & conduct motor (Efferent) impulses from brain to effectors.
                          a. General characteristics of nerve tracts:
                                -Most cross over
                                -Most consist of 2-3 successive neurons
                                -Most exhibit somatotropy (Tracts from/to upper body are located on outside, tracts from/to lower body on inside)
                                -All pathways are paired (R&L)
        4. Other Important Features;
              a. Ventral & Dorsal Root