Nervous System Part 2
Cytoplasmic anions cannot escape due to _____ or _____.
Cytoplasmic anions cannot escape due to size or charge (phosphates, sulfates, small organic acids, proteins, ATP, and RNA)
True/False: René Descartes in the 17th century supported Galen's theory
True
What are Local potentials?
changes in membrane potential of a neuron occurring at and nearby the part of the cell that is stimulated
Electrophysiology
-study of cellular mechanisms for producing electrical potentials and currents -Basis for neural communication and muscle contraction
Process of local potential
-A chemical stimulant binds to a receptor on the neuron -Opens Na^+ gates and allows Na^+ to enter cell -Entry of a positive ion makes the cell less negative; this is a depolarization: a change in membrane potential toward zero mV -Na^+ entry results in a current that travels toward the cell's trigger zone; this short-range change in voltage is called a local potential
Action potential
-Action potential—dramatic change in membrane polarity produced by voltage-gated ion channels -Only occurs where there is a high enough density of voltage-regulated gates -Neurosoma (50 to 75 gates per 〖"μm" 〗^2 ); cannot generate an action potential -Trigger zone (350 to 500 gates per 〖"μm" 〗^2 ); where action potential is generated --If excitatory local potential reaches trigger zone and is still strong enough, it can open these gates and generate an action potential
Action potential is a rapid up-and-down shift in the membrane voltage involving a sequence of steps:
-Arrival of current at axon hillock depolarizes membrane -Depolarization must reach threshold: critical voltage (about -55 mV) required to open voltage-regulated gates -Voltage-gated Na^+ channels open, Na^+ enters and depolarizes cell, which opens more channels resulting in a rapid positive feedback cycle as voltage rises
§Steps in action potential shift in membrane voltage, Cont'd:
-As membrane potential rises above 0 mV, Na^+ channels are inactivated and close; voltage peaks at about +35 mV -Slow K^+ channels open and outflow of K^+ repolarizes the cell -K^+ channels remain open for a time so that membrane is briefly hyperpolarized (more negative than RMP) -RMP is restored as Na^+ leaks in and extracellular K^+ is removed by astrocytes
The Refractory Period
-During an action potential and for a few milliseconds after, it is difficult or impossible to stimulate that region of a neuron to fire again -Refractory period—the period of resistance to stimulation -Two phases: 1. Absolute refractory period -No stimulus of any strength will trigger AP -Lasts as long as Na^+ gates are open, then inactivated 2. Relative refractory period -Only especially strong stimulus will trigger new AP -K^+ gates are still open and any effect of incoming Na^+ is opposed by the outgoing K^+ Generally lasts until hyperpolarization ends -Only a small patch of neuron's membrane is refractory at one time (other parts of the cell can be stimulated)
Steps of Nerve Regeneration Part 1
-Fiber distal to the injury cannot survive and degenerates --Macrophages clean up tissue debris at point of injury and beyond -Neurosoma swells, ER breaks up, and nucleus moves off center --Due to loss of nerve growth factors from neuron's target cell -Axon stump sprouts multiple growth processes as severed distal end continues to degenerate -Schwann cells, basal lamina and neurilemma form a regeneration tube --Enables neuron to regrow to original destination and reestablish synaptic contact
Characteristics of action potential (unlike local potential)
-Follows an all-or-none law --If threshold is reached, neuron fires at its maximum voltage --If threshold is not reached, it does not fire -Nondecremental: do not get weaker with distance -Irreversible: once started, goes to completion and cannot be stopped
Gliomas grow rapidly and are highly ________.
-Gliomas grow rapidly and are highly malignant -Blood-brain barrier decreases effectiveness of chemotherapy -Treatment consists of radiation or surgery
Properties of local potentials (unlike action potentials)
-Graded: vary in magnitude with stimulus strength --Stronger stimuli open more Na^+ gates -Decremental: get weaker the farther they spread from the point of stimulation --Voltage shift caused by Na^+ inflow diminishes with distance -Reversible: if stimulation ceases, the cell quickly returns to its normal resting potential -Either excitatory or inhibitory: some neurotransmitters make the membrane potential more negative—hyperpolarize it—so it becomes less likely to produce an action potential
In CNS—an oligodendrocyte _____ several nerve fibers in its immediate vicinity
-In CNS—an oligodendrocyte myelinates several nerve fibers in its immediate vicinity -Anchored to multiple nerve fibers -Cannot migrate around any one of them like Schwann cells -Must push newer layers of myelin under the older ones; so myelination spirals inward toward nerve fiber -Nerve fibers in CNS have no neurilemma or endoneurium
Schwann cells
-In PNS, Schwann cell spirals repeatedly around a single nerve fiber -Lays down as many as one hundred layers of membrane -No cytoplasm between the membranes -Neurilemma: thick, outermost coil of myelin sheath --Contains nucleus and most of its cytoplasm --External to neurilemma is basal lamina and a thin layer of fibrous connective tissue—endoneurium
Regeneration of damaged peripheral nerve fiber can occur if _______.
-Its neurosoma is intact -At least some neurilemma remains
Unmyelinated nerve fibers
-Many CNS and PNS fibers are unmyelinated -In PNS, Schwann cells hold 1 to 12 small nerve fibers in surface grooves -Membrane folds once around each fiber
What is myelination?
-production of the myelin sheath -Begins at week 14 of fetal development -Proceeds rapidly during infancy -Completed in late adolescence -Dietary fat is important to CNS development
Membrane is not very permeable to ______ but RMP is slightly influenced by it.
-Membrane is not very permeable to sodium (Na^+ )but RMP is slightly influenced by it -Na+ is about 12 times as concentrated in the ECF as in the ICF -Some Na^+ leaks into the cell, diffusing down its concentration and electrical gradients -This Na^+ leakage makes RMP slightly less negative than it would be if RMP were determined solely by K^+
Brain tumors arise from:
-Meninges (protective membranes of CNS) -Metastasis from nonneuronal tumors in other organs -Often glial cells that are mitotically active throughout life
Myelin sheath is ________.
-Myelin sheath is segmented -Nodes of Ranvier: gap between segments -Internodes: myelin-covered segments from one gap to the next -Initial segment: short section of nerve fiber between the axon hillock and the first glial cell -Trigger zone: the axon hillock and the initial segment --Play an important role in initiating a nerve signal
-Na^+/ K^+ pump moves 3 _____ out for every 2 _____ it brings in.
-Na^+/ K^+ pump moves 3 Na^+ out for every 2 K^+ it brings in -Works continuously to compensate for Na^+ and K^+ leakage, and requires great deal of ATP (1 ATP per exchange) --70% of the energy requirement of the nervous system -Necessitates glucose and oxygen be supplied to nerve tissue (energy needed to create the resting potential) -The exchange of 3 positive charges for only 2 positive charges contributes about −3 mV to the cell's resting membrane potential of −70 mV
Nerve Growth Factor
-Nerve growth factor (NGF)—protein secreted by a gland, muscle, or glial cells and picked up by the axon terminals of neurons --Prevents apoptosis (programmed cell death) in growing neurons --Enables growing neurons to make contact with their targets -Isolated by Rita Levi-Montalcini in 1950s -Won Nobel prize in 1986 with Stanley Cohen -Use of growth factors is now a vibrant field of research
What is Multiple sclerosis? (disease of the myelin sheath)
-Oligodendrocytes and myelin sheaths in the CNS deteriorate -Myelin replaced by hardened scar tissue -Nerve conduction disrupted (double vision, tremors, numbness, speech defects) -Onset between 20 and 40 and fatal from 25 to 30 years after diagnosis -Cause may be autoimmune triggered by virus
Steps of Nerve Regeneration Part 2
-Once contact is reestablished with original target, the neurosoma shrinks and returns to its original appearance --Nucleus returns to normal shape --Atrophied muscle fibers regrow -But regeneration is not fast, perfect, or always possible -Slow regrowth means process may take 2 years -Some nerve fibers connect with the wrong muscle fibers; some die -Regeneration of damaged nerve fibers in the CNS cannot occur at all
During action potential shift, only a thin layer of the _____ next to the cell membrane is affected.
-Only a thin layer of the cytoplasm next to the cell membrane is affected In reality, very few ions are involved
What ion has greatest influence on RMP?
-Potassium (K^+ ) has greatest influence on RMP -Plasma membrane is more permeable to K^+ than any other ion -Leaks out until electrical charge of cytoplasmic anions attracts it back in and equilibrium is reached (no more net movement of K^+) -K+ is about 40 times as concentrated in the ICF as in the ECF
RMP results from____?
-RMP results from the combined effect of three factors -Ions diffuse down their concentration gradient through the membrane -Plasma membrane is selectively permeable and allows some ions to pass easier than others -Electrical attraction of cations and anions to each other
Conduction speed
-Small, unmyelinated fibers: 0.5 to 2.0 m/s -Small, myelinated fibers: 3 to 15.0 m/s -Large, myelinated fibers: up to 120 m/s -Slow signals sent to the gastrointestinal tract where speed is less of an issue -Fast signals sent to skeletal muscles where speed improves balance and coordinated body movement
Electrical potential
-a difference in concentration of charged particles between one point and another -Living cells are polarized and have a resting membrane potential -Cells have more negative particles on inside of membrane than outside -Neurons have about −70 mV resting membrane potential
Electrical current
-a flow of charged particles from one point to another -In the body, currents are movements of ions, such as Na^+ or K^+, through channels in the plasma membrane -Gated channels are opened or closed by various stimuli -Enables cell to turn electrical currents on and off
What is Tay-Sachs disease? (disease of the myelin sheath)
-a hereditary disorder of infants of Eastern European Jewish ancestry -Abnormal accumulation of glycolipid called GM2 in the myelin sheath --Normally decomposed by lysosomal enzyme --Enzyme missing in individuals homozygous for Tay-Sachs allele --Accumulation of ganglioside (GM_2 ) disrupts conduction of nerve signals --Blindness, loss of coordination, and dementia -Fatal before age 4
What is the Myelin sheath?
-insulation around a nerve fiber -Formed by oligodendrocytes in CNS and Schwann cells in PNS Consists of the plasma membrane of glial cells 20% protein and 80% lipid
Tumors
-masses of rapidly dividing cells -Mature neurons have little or no capacity for mitosis and seldom form tumors
What did Santiago Ramón y Cajal do?
-used stains to trace neural pathways -He showed that pathways were made of distinct neurons (not continuous tubes) -He demonstrated how separate neurons were connected by synapses
Speed at which a nerve signal travels along surface of nerve fiber depends on what two factors?
1. Diameter of fiber -Larger fibers have more surface area and conduct signals more rapidly 2. Presence or absence of myelin -Myelin further speeds signal conduction
Different neurons can be stimulated by what?
Different neurons can be stimulated by chemicals, light, heat, or mechanical disturbance
What did Galen think in relationship to nerves?
Galen (Roman physician) thought brain pumped a vapor called psychic pneuma through hollow nerves and into muscles to make them contract
Many Schwann cells or oligodendrocytes are needed to cover ___ nerve fiber.
Many Schwann cells or oligodendrocytes are needed to cover one nerve fiber.
Action potential is often called a _____, as it happens so fast
Spike
What did Camillo Golgi develop?
an important method for staining neurons with silver in the 19th century
What did Luigi Galvani discover?
the role of electricity in muscle contraction in the 18th century
Resting membrane potential exists because ________.
§Resting membrane potential (RMP) exists because of unequal electrolyte distribution between extracellular fluid (ECF) and intracellular fluid (ICF)