NERVOUS SYSTEM 3

NERVOUS SYSTEM 3

1. FUNCTIONS

   • Sensory Input
     • Info gathered by sensory receptors regarding external & internal changes
       • E.G. see a glass of H20
   • Integration
     • Processing & interpretation sensory input
       • E.G. decide its safe to drink
   • Motor Output
     • Activation & effector organs (muscles & glands) produces response
       • E.G. signal sent to arm to pick up glass of H20

2. COMPONENTS

   • Central Nervous System (CNS)
     • Brain
       • Enclosed & protected by skull
     • Spinal Cord
       • Enclosed & protected by vertebral column
     • Main integration & control center
     • Responsible for decision making, thinking process, memories, & emotions
   • Peripheral Nervous System (PNS)
     • Spinal Nerves
       • 31 pairs
     • Cranial Nerves
       • 12 pairs
     • Ganglia
     • Functional Divisions
       • Sensory (afferent)
         • Somatic sensory fibers
           • Transmit impulses from skin, skeletal muscles, bones & joints to CNS
             • Aware of this sensation
         • Visceral sensory fibers
           • Transmit impulses from visceral organs to CNS
             • Unaware of this sensation
               • Only aware if abnormality b.c pain present
             • E.G. enzyme secretion
       • Motor (efferent)
         • Somatic nervous system
           • Voluntary
         • Autonomic nervous system
           • Involuntary
           • Regulates smooth muscle, cardiac muscle, glands
           • Functional subdivision (oppositional forces)
             • Sympathetic
               • STRESS emotion
                 • Fight or Flight Mode
                   • ^ HR, ^ RR, sweaty palms, ^ temp
                   • Excitement or danger
             • Parasympathetic
               • PEACE emotion
                 • Rest & Digest Mode
           • E.G. heart rate, digestion
         • Transmits signals away from CNS & towards effector organs
           • E.G. muscles & glands

3. TISSUES

   • Neuroglia (glial cells)
     • Small supporting cells, surrounding neurons
     • In CNS (4 types)
       • Astrocytes
         • Most abundant. star shaped, cling to neurons, synaptic endings, & capillaries
         • Functions: support & guide neurons, manage chemical environment, information processing, nerve impulses & neurotransmitters
       • Microglial
         • Thorny extensions
         • Functions: monitor neurons, migrate towards injured neurons, phagocytize microorganisms & neuronal debris (transforms into macrophage & removes dead neurons)
       • Ependymal
         • Squamous or columnar
         • Functions: ciliated (circulates CSF), line cerebrospinal fluid-filled cavities, forms permeable barrier
       • Oligodendrocytes
         • Branched
         • Functions: form insulated myelin sheaths in nerve fibers
     • In PNS (2 types)
       • Satellite
         • Surround neuron cell body
         • Function: similarly to astrocytes
       • Schwann
         • Surround peripheral nerve fibers, myelination of nerve fiber (axon)
         • Function: form myelin sheath, regenerate damaged nerve fibers
   • Neurons (nerve cells)
     • Excitable cells that transmit electrical signals
     • Structural units, conduct impulses, amitotic, high metabolic rate (02 & glucose supply)
     • Plasma membrane site of electrical signalling
     • STRUCTURE
       • Cell body
         • Synthesizes proteins, membranes, chemicals
         • Location: nuclei (CNS) & ganglia (PNS)
       • Dendrites
         • Branched processes, input region, convey info as graded potentials, dendritic spines
       • Axon
         • Structure: short, long (a.k.a nerve fibers), axon hillock, axon collaterals, terminal branches, axon terminals
           • E.G. motor neurons controlling skeletal muscle of great toe (nerve fibers)
         • Function: conducting region, generates nerve impulses, transmits impulses via axolemma to axon terminal, terminal secretes neurotransmitters released in EC.space, excites or inhibits, rapidly decay if damaged, possess internal transport mechanisms, movement
           • Anterograde: away from cell body
             • E.G. mitochondria, cytoskeletal elements, membrane components, enzymes
           • Retrograde: toward cell body
             • E.G. organelles to be degraded, signal molecules, viruses, bacterial toxins
             • Clinical homeostatic imbalance
               • Specific viruses & bacteria can damage neural tissue by axonal transport
                 • E.G. polio, rabies, herpes simplex virus, tatanus
       • Myelin sheath
         • Structure: white protein lipid substance
         • Function: protects & electrically insults axon, increases speed of nerve impulse transmission
         • E.G. myelinated fibers
         • In PNS formed by Schwann cells
           • Plasma membranes less protein
             • No channels or carriers thus good electrical insulators
               • Interlocking proteins bind adjacent myelin membranes
           • 1. Schwann cell envelopes axon
             • 2. Schwann cell rotates around axon, wrapping plasma membrane around in successive layers
               • 3. Schwann cell cytoplasm forced from b/w membranes, & tight wrappings left form myelin sheath
           • Myelin sheath gap a.k.a Nodes of ranvier
         • In CNS formed by Oligodendrocytes
           • Can wrap 60 axons
           • Myelin sheath gap a.k.a Nodes of ranvier
           • White matter: area of brain & spinal cord with dense collections of myelinated fibers
             • Primarily fiber tracts
           • Gray matter: neuron cell bodies
       • Structural Classification (3 types)
         • Multipolar
           • 3 or more processes (1 axon, multiple dendrites)
             • E.G. purkinje cell of cerebellum & pyramidal cell
         • Bipolar
           • 2 processes (1 axon, 1 dendrite)
             • E.G. retina, olfactory mucosa, ear
         • Unipolar (a.k.a pseudounipolar)
           • Peripheral (distal) process
             • Sensory receptor
           • Proximal (central) process
             • E.G. dorsal root ganglion cell in spinal cord & sensory ganglia in cranial nerves
           • 2 axons
       • Functional Classification (3 types)
         • Sensory (afferent)
           • Transmit impulses from sensory receptors toward CNS, unipolar, cell bodies located in ganglia of PNS
         • Motor (efferent)
           • Carry impulses from CNS to effectors, multipolar, cell bodies located in CNS
         • Interneurons
           • Shuttle signals though CNS pathways, located within CNS, composes 99% of bodes neurons

4. MEMBRANE POTENTIALS

   • Resting Membrane Potential
     • -70mV
     • Cytoplasmic side of membrane negatively charged
     • Voltage difference: -40mV to -90mV
     • Membrane polarized
     • Dependant on
       • Differences in plasma membrane permeability
         • Impermeable to anionic proteins
         • Semi-permeable to Na+ through leakage channels
           • Na+ travels down concentration gradient into cell
         • 75-100x more permeable to K+
           • K+ travels down concentration gradient out of cell
         • Permeable to Cl-
         • Na+/K+ ATPase pump stabilizes RMP
           • Maintains concentration gradients for Na+ & K+
           • 3 Na+ pumped out of cell & 2 K+ pumped into cell
       • Differences in ionic composition of ICF/ECF
         • ECF higher concentration of Na+ than ICF (balanced by Cl- ions)
         • ICF higher concentration of K+ ions than ECF (important role in membrane potential)
     • Changes when
       • Concentrations of ions across membrane change & membrane permeability to ions shift
         • Produces (2 signal types)
           • 1. Graded Potentials
             • Incoming signals operating over short distances
           • 2. Action Potentials
             • Long-distance signals of axons
           • Signals receive, integrate, send info
       • Hyperpolarization: increase in membrane potential (away from 0)
         • Probability of impulse decreases
       • Depolarization: decrease in membrane potential (moves toward 0 & above)
         • Probability of impulse increases
   • Graded Potentials
     • Short-lived, localized change
       • Stronger the stimulus, the more voltage changes & farther the current flows
       • Decay with distance: current lost via plasma membrane, voltage declines with distance from stimulus
         • Site of initial depolarization
           • Resting potential
     • Triggered by stimulus that opens gated ion channels
       • Causes depolarization or hyperpolarization
     • Named according to location & function
       • Receptor potential (generator potential): in receptors of sensory neurons
         • Stimulus: energy (heat or light)
       • Postsynaptic potential: neuron graded potential
         • Stimulus: neurotransmitter released by another neuron
           • Neurotransmitter releases into synapse
   • Action Potentials (a.k.a Nerve Impulse)
     • Principle way neurons send signals
       • Long distance neural communication
     • Occur in muscle cells & axons of neurons
     • Brief reversal in membrane potential (voltage ~100mV)
     • Don't decay
       • Depolarization > Repolarization > Hyperpolarization
     • Opens voltage-gated channels
     • Generated (by 4 steps)
       • 1. Resting state: All gated Na+ & K+ channels are closed
         • Only Na+ & K+ leakage channels open
           • Maintains RMP
         • Each Na+ channel has 2 voltage-sensitive gates
           • Activation Gate: closed at rest, opens at depolarization, allows Na+ to enter cell
           • Inactivation Gate: open at rest, block channel once open to prevent more Na+ from entering cell
         • Each K+ channel has 1 voltage-sensitive gate
           • Closed at rest, opens with depolarization
       • 2. Depolarization: Na+ channels open
         • Local currents open voltage-gated Na+ channels, Na+ rushes into cell
           • Na+ activation & inactivation gates open
             • Na+ influx causes more depolarization thus opening more Na+ channels
               • Threshold (-55 to -50mV), positive feedback causes all Na+ channels to open
                 • Spike in action potential
                 • Membrane polarity +30mV
               • ICF less negative
       • 3. Repolarization: Na+ channels are inactivating, & K+ channels open
         • Na+ channel inactivation gates close
           • Membrane permeability to Na+ declines to resting state
           • AP spike ceases to rise
           • Voltage-gated K+ channels open
             • K+ exits cell down electrochemical gradient
             • Repolarization (membrane returns to RMP)
       • 4. Hyperpolarization: some K+ channels remain open, & Na+ channels reset
         • Some K+ channels remain open, allowing excessive K+ efflux
           • Inside of membrane more - than its resting state
         • Na+ channels rest
     • All or none phenomenon
       • AP either happens completely or not at all
     • Propagation: allows AP to transmit from origin down entire axon length toward terminals
     • Refractory Period: time in which neuron can't trigger another AP
       • Voltage-gated Na+ channels are open
       • Absolute Refractory Period: Na+ channels open until reset, ensures AP all or none event, ensures one-way transmission of nerve impulses
         • Relative Refractory Period: most Na+ channels at resting state, some K+ channels open, repolarization occurs, threshold for AP generation elevated
   • Chemically gated ion channels
     • Open in response to binding of proper neurotransmitter
       • E.G. K+ moving out
   • Voltage-gated ion channels
     • Open in response to changes in membrane potential
       • E.G. Na+ enters

5. SYNAPTIC NEUROTRANSMISSION

   • Synapse: when neurons in close enough proximity to eachother that they're capable of transmitting chemical signals from one cell to another
     • Synaptic Cleft: small space between synapses
       • Presynaptic Neuron: synapse where signal initiates
         • Neurotransmitters
           • Packaged into vesicle sacs
         • Excited by AP
           • Contents into synaptic cleft
       • Postsynaptic Neuron: synapse where signal is received
         • Neurotransmitters interact with receptors on postsynaptic membrane
           • Residue neurotransmitter molecules removed by process of diffusion or reabsorbed into presynaptic neuron for recycling
             • Can also be broken down by enzymes

6. ANESTHETIC

   • Definition: "a reversible condition induced by drugs, intended to result in pain relief, memory loss, reducing anxiety, & paralysis of muscles"
   • 1. Local
     • Method: shot, spray, ointment
     • Numbs small area for minor procedures
       • E.G. dentist
     • Reversibly bind to voltage-gated sodium channels
       • Blocks Na+ movement through the channels
         • Blocks AP & neural conduction
         • Preferentially block small nerve fibers
     • Small unmyelinated C fibers (pain signal) and small myelinated Aδ fibers (pain and temperature) are blocked before larger myelinated Aγ, Aβ and Aα fibers (postural, touch, pressure and motor signals)
     • Nerve bundle fibers located circumferentially
       • Thus may be affected before sensory fibers
         • Extremities: proximal sensory fibers located circumferentially to distal sensory fibers
           • Loss of sensation spreads from proximal to distal part of limb
     • CNS: following absorption, local anesthetics cause stimulation, producing restlessness & tremors that may proceed to convulsion
       • Stimulation is caused by inhibition of inhibitory neuronal activity
     • PNS: affect transmission at the neuromuscular junction and ganglionic synapse
   • 2. Regional
     • Method: injected near cluster of nerves
     • Numbs large area for mediocre procedures
       • E.G. epidurals during labour
         • Bupivacaine hydrochoride
           • Stabilizes neuronal membrane & prevents initiation & transmission of nerve impulses
         • Ropivacine hydrochloride
           • High dose: blocks motor neuron function
           • Low Dose: blocks sensory neuron function
         • Morphine
           • Acts as agonist by activating opioid receptors in CNS & PNS
             • Pain threshold elevated
   • 3. General
     • Completely unconscious, no awareness, no memory
       • E.G. isoflurane, enflurane, somnothane, suprane
     • Method: intravenous, inhaling gasses
     • During procedure: vital functions monitored, breathing assessed & controlled