Chapter 2 Nerve Cells And Nerve Impulses

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The nervous system consists of two cell types:

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neurons and glia

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Role of neurons:

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receive information and transmit it to other cells
(human brain contains ~100 bil neurons)

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membrane:

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surface of a cell, a structure that separates the inside of the cell from the outside environment.
composed of 2 layers of fat molcules, free to flow around each other
protein channels permit water, oxygen, sodium, potassium flow

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nucleus:

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animal cells have one, contains chromosomes

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mitochondrion:

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performs metabolic activities, provide energy to the cell, require fuel and oxygen

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ribosomes:

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sites where cells synthesize new protein molecules, proteins provide building materials, some ribosomes float freely, some are attached to the endoplasmic reticulum

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Large neurons have:

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dendrites, a soma (cell body), an axon, and presynaptic terminals

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Tiny neurons lack:

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axons and developed dendrites

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Motor neurons:

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have soma in the spinal cord, receives excitation from other neurons through its dendrites and conducts impulses along its axon to a muscle

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Sensory neurons:

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specialized at one end to be highly sensitive to a particular type of stimulation (light, touch, sound)

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Dendrites:

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branching fibers that get narrower near their ends, surface is lined with specialized synaptic receptors where dendrites receive information from other neurons, more surface area = more information can be received. Dendrites vary from one neuron to another, has to do with how the dendrites combine different kinds of input

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Dendritic spines:

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the short outgrowths that increase surface area available for synapses

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Soma:

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cell body (contains nucleus, ribosomes, mitochondria, etc.), covered with synapses on the surface

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Axon:

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thin fiber of constant diameter, longer than dendrites, the information sender of the neuron, sends impulses toward other neurons, organs, or muscles

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Myelin sheath:

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an insulating material that covers vertebrate axons, with interruptions known as nodes of Ranvier

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Nodes of Ranvier:

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interruptions in the myelin sheath

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Presynaptic terminal:

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axon branches swell at the tip, end bulb, or bouton; the point from which the axon releases chemicals that cross through the junction between one neuron and the next

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Afferent vs Efferent axon:

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afferent: brings information in
efferent: carries information away

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Intrinsic neuron:

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a cell's dendrites and axon are contained within a single structure

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Glia:

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the 2nd type of nervous system cell; does exchange chemicals with adjacent neurons; smaller, more numerous than neurons; occupy more volume

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Astrocytes:

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star-shaped cells, wrap around the presynaptic terminals of a group of functionally related axons; help synchronize activity of the axons--send messages in waves; remove waste, control amount of blood flow to each brain area

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Microglia:

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very small cells; remove wastes, viruses, fungi, etc.

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Oligodendrocytes and Schwann cells:

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O: in brain and spinal cord, S: in periphery; they are specialized types of glia that build myelin sheaths that surround & insulate certain vertebrate axons

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Radial glia:

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guide the migration of neurons and the growth of their axons and dendrites during embryonic development.

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The blood-brain barrier:

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mechanism; keeps most chemicals out of the vertebrate brain; depends on the arrangement of the endothelial cells that form the walls of the capillaries (so tight that nothing passes through); small, uncharged molecules pass freely (oxygen, CO2) so do molecules that dissolve in the fats of the membranes (vitamins A & D)

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Active transport:

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a protein-mediated process -> expends energy and pumps chemicals from blood into the brain (chemicals like glucose, amino acids, purines, etc.)

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Why we need the blood-brain barrier:

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the vertebrate brain doesn't normally replace damaged neurons (that are virus-infected, for example) so the body builds a wall along the sides of the brain's blood vessels that keeps out most viruses, bacteria, and harmful chemicals, but also most nutrients

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Role of glucose:

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simple sugar; vertebrate neurons depend on it almost entirely; the metabolic pathway that uses glucose requires a lot of oxygen; glucose is practically the only nutrient that passes the blood-brain barrier in adults

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Thiamine:

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vitamin B1; many chronic alcoholics are deficient; a chemical necessary for the use of glucose; prolonged deficiency can lead to death f neurons and Korsakoff's syndrome

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The nerve impulse:

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the electrical message that is transmitted down the axon of a neuron.
The impulse does not travel directly down the axon but is regenerated at points along the axon..."travels" without weakening
The speed of nerve impulses ranges from approximately 1 m/s to 100 m/s.

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Electrical gradient:

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neuron membranes maintain an electrical gradient; a difference in electrical charge between the inside and outside of the cell

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Neuron membranes:

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maintain electrical gradient; has 2 layers of phospholipid molecules (containing chains of fatty acids and a phosphate group); membranes maintain an electrical polarization

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Polarization:

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a difference in electrical charge between two locations; inside membrane -> slightly negative (due to negatively charged proteins)

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Resting potential:

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the state of the neuron prior to the sending of a nerve impulse; the resting potential is due to the negatively charged proteins inside the cell

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Selectively permeable membrane:

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the membrane is selectively permeable: electrically charged ions and molecules can't pass through; but some biologically important molecules can cross membrane channels that are sometimes open and sometimes closed (sodium, potassium, calcium)

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Potassium channels:

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are almost closed with the membrane is at rest, potassium ions (K+) flow slowly
sodium channels are closed at rest

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Sodium-potassium pump:

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protein complex, repeatedly transports 3 sodium ions out of the cell while drawing 2 potassium ions in; through active transport -> requiring energy; sodium stays out, but potassium leaks out carrying the positive charge out -> increases electrical gradient

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When the neuron is at rest, 2 forces tend to push sodium into the cell:

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the electrical gradient and the concentration gradient; the electrical gradient tends to pull potassium in, but concentration gradient tends to push it out because its more concentrated inside the cell; almost balanced but the S-P pump keeps pulling in potassium so not quite; negative proteins inside and chloride ions (-) outside the cell

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Resting potential:

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the difference in the voltage between the inside and outside of the neuron; the body invests energy to operate the S-P pump to maintain resting potential; the resting potential prepares the neuron to respond rapidly; neuron excitation opens sodium channels -> sodium enters cell explosively -> can respond strongly and rapidly to stimulus

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Neuron stays rested until stimulation occurs; stimulation takes place at synapses

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When axon's membrane is at rest there is negative potential inside the axon

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Hyperpolarization:

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increased charge, increased polarization; can apply negative charge using electrode

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Depolarization:

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reduced polarization

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Threshold of excitation:

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a levels above which any stimulation produces a massive depolarization; stimulation beyond a certain level; massive depolarization of the membrane; when reached, the membrane opens sodium channels and permits rapid flow -> potential shoots up far beyond strength of the stimulus

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Action potential:

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occurs when a neuron sends information down an axon, away from the cell body; rapid depolarization and slight reversal of the usual polarization caused by the rapid flow of sodium ions; the action potential varies among axons, but each axon stays consistent

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Voltage-gated channels:

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membrane channels whose permeability depends on the voltage difference across the membrane (ie sodium channels); as membrane becomes depolarized, channels open and sodium flows; if depolarization is less than the threshold, sodium crosses only a little more than usual; channels open wide when potential across the membrane reaches threshold

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The all-or-none principle:

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There are no big or small action potentials in one nerve cell - all action potentials are the same size. Therefore, the neuron either does not reach the threshold or a full action potential is fired

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Threshold:

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When the depolarization reaches about -55 mV a neuron will fire an action potential. If the neuron does not reach this critical threshold level, then no action potential will fire.

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Action potential process:

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Action potentials are caused by an exchange of ions across the neuron membrane. A stimulus first causes sodium channels to open. Because there are many more sodium ions on the outside, and the inside of the neuron is negative relative to the outside, sodium ions rush into the neuron. Remember, sodium has a positive charge, so the neuron becomes more positive and becomes depolarized. It takes longer for potassium channels to open. When they do open, potassium rushes out of the cell, reversing the depolarization. Also at about this time, sodium channels start to close. This causes the action potential to go back toward repolarization

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Local anesthetic drugs:

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block sodium channels and therefore prevent action potentials from occurring ie novocaine and xylocaine

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Refractory period:

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After an action potential, a neuron has a refractory period during which time the neuron resists the production of another action potential

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Absolute refractory period:

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the first part of the period in which the membrane can not produce an action potential

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Relative refractory period:

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the second part in which it take a stronger than usual stimulus to trigger an action potential

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Axon hillock:

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a swelling where the axon exits the soma; where action potential begins

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Propagation of the action potential:

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the term used to describe the transmission of the action potential down the axon

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Myelin:

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insulating material composed of fats and proteins

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Myelinated axons:

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those covered with a myelin sheath

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Nodes of Ranvier:

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short sections of axon that interrupts the myelin sheath periodically; each node is ~ 1 micrometer wide

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Saltatory conduction:

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the jumping of action potentials from node to node; conserves energy because instead of admitting sodium ions at every point along the axon and then having to pump them out via the S-P pump, a myelinated axon admits sodium only at its nodes

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Local neurons:

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smaller, only exchange information with closest neighbors; receives information from other neurons and produces graded potentials

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Graded potentials:

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membrane potentials that vary in magnitude without following the all-or-none law; when a local neuron is stimulated, it depolarizes or hyperpolarizes in proportion to the intensity of the stimulus