What I have been working on....16:43 May 01 2007
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Chapter 11
ANS
-The autonomic nervous system consists of visceral motor nerve fibers that regulate the activity of smooth muscles, cardiac muscles, and glands. Autonomic means “a law unto itself,” and because we generally cannot control such activities as the pumping of our heart or the movement of food through our digestive tract, the ANS is also referred to as the involuntary nervous system. ANS has two functional subdivisions, the sympathetic and the parasympathetic, which typically work in opposition to each other-what one subdivision stimulates, the other inhibits.
Neuroglia
-Neurons associate closely with much smaller cells called neuroglia or simpl glial cells. There are six types of neuroglia-four in the CNS and two in the PNS. Each type has a unique function, but in general, these cells provide a supportive scaffolding for neurons. Some produce chemicals that guide young neurons to the proper connections, and promote neuron health and growth. Others wrap around and insulate neuronal processes to speed up action potential conduction.
-those that lie along the nerves in the PNS are called ganglia “Knot on a string,” “swelling.”
Classification
-Multipolar neurons have three or more processes. They are the most common neuron type in humans (more than 99% of neurons belong to this class) and the major neuron type in the CNS.
-Bipolar nurons have two process-an axon and a dendrite-that extend from opposite sides of the cell body. These rare neurons are found in some of the special sense organs. Exmaples include some neurons in the retina of the eye and in the olfactory mucosa.
-Unipolar neurons have a single short process that emerges from the cell body and dives T-like into proximal and distal branches. The more distal process, which is often associated with a sensory receptor, is the peripheral process, whereas that entering the CNS is the central process. Unipolar neurons are more accurately called psuedounipolar neurons because they originate as bipolar neurons. Then, during early embryonic development, the two processes converge and partially fuse to form the short single process that issues from the cell body. Unipolar neurons are found chiefly in ganglia in the PNS, where they function as sensory neurons.
Functional Classification
-this scheme groups neurons according the direction in which the nerve impulse travels relative to the central nervous system. Based on the criterion, there are sensory neurons, motor neurons, and interneurons.
Sensory or afferent neurons transmit impulses from sensory receptors in the skin or internal organs, toward or into the central nervous system. Except for certain neurons found in some special sense organs, virtually all sensory neurons are unipolar, and their cell bodies are located in sensory ganglia outside the CNS. Only the most distal parts of these unipolar neurons act as impulse receptor sites, and the peripheral processes are often very long.
Although the receptive endings of some sensory neurons are naked, in which case those terminals themselves function as sensory receptors, many bear receptors that include other cell types. The various types of general sensory receptor end oranges, such as those of the skin.
Motor or efferent neurons carry impulses away from the CNS to the effector organs of the body periphery. Motor neurons are multipolar, and except for some neurons of the autonomic nervous system, their cell bodies are located in the CNS.
Interneurons or association neurons, lie between motor and sensory neurons in neural pathways and shuttle signals through CNS pathways where integration occurs. Most internurons are confined within the CNS. They make up over 99% of the neurons of the body, including most of those int eh CNS. Almost all interneurons are multipolar, but there is considerable diversity in both size and fiber-branching patterns.
Resting Membrane potential
When one microelectrode of the voltmeter is inserted into the neuron and the other is in the extracellular fluid, a voltage across the membrane of approximately -70 mV is recorded. The minus sign indicates that the cytoplasmic side of the membrane is negatively charged relative to the outside. This potential difference in a resting neuron is called the resting membrane poteial, and the membrane is said to be polarized. The value of the resting membrane potential varies in different types of neurons.
Graded Potentials
Graded potentials are short-lived, localized changes in membrane potential that cane be either depolarizations or hyperpolarizations. These changes cause current flows that decrease in magnitude with distance. Graded potentials are called “graded” because their magnitude varies directly with stimulus strength. The stronger the stimulus, the more the voltage changes and the farther the current flows.
Action Potentials
The principal way neurons send signals over long distances is by generating and propagating action potentials, and for the most part, only cells with excitable membranes-neurons and muscle cells-can generate action potentials. And action potential is a brief reversal of membrane potential with a total ampliture of about 100 mV. A depolarization phase is followed by a repolarization phase and often a short period of hyperpolarization. The whole event is over in a few milliseconds. Unlike graded potentials, action potentials do not decrease in strength with distance.
Polarizing
Depolarizing phase: Increase in Na+ permeability and reversal of membrane potential. As the axon membrane is depolarized by local currents, the sodium channels open and Na+ rushes into the cell. This influx of positive charge depolarizes that local “patch” of membrane further, opening more Na_ channels so that the cell interior becomes progressively less negative. When depolarization at the stimulation site reaches a certain critical level called threshold, depolarization becomes self-generating, urged on by positive feedback. That is, after being initiated by the stimulus, depolarization is driven by the ionic currents created by Na_ influz. As more Na+ enters, the membrane depolarizes further and opens still more channels until all Na+ channels are open. At this point, Na+ permeability is about 1000 times greater than in a resting neuron. As a result, the membrane potential becomes less and less negative and then overshoots to about +30mV as Na+ rushes in along its electrochemical gradient. This rapid depolarization and polarity reversal produces the sharply upward spike of the action potential.
Repolarizing phase: Decrease in Na+ permeability. The explosively rising pahse of the action potential persists for only about 1 ms. It is self-limiting because the slow inactivation gates of the Na+ channels begin to close at this point. As a result, the membrane permeability to Na+ declines to resting levels, and the net influx of Na+ stops completely. Consequently , the AP spike stops rising and reverses direction. As Na+ entry declines, the slow voltage-gated K+ channels open and K+ rushes out of the cell, following its electrochemical gradient. Consequently, internal negativity of the resting neuron is restored, an event called repolarization. Both the abrupt decline in Na+ permeability and the increased permeability to K+ contribute to repolarization.
Hyperpolarization: K+ permeability continues. The period of increased K+ permeability typically lasts longer than needed to restore the resting state. As a result of the excessive K+ efflux, an afterhyperpolarization, also called the undershoot, is seen on the AP curve as a slight dip following the spike. Also as this point, the Na+ channels begin to reset back to their original position by changing shape to reopen their inactivation gates and close their activation gates.
Although repolarization restores resting electrical conditions, it does not restore resting ionic conditions. The ion redistribution is accomplished by the sodium-potassium pump following repolarization. While it might appear that tremendous numbers of Na+ and K_ ions change places during action potential generation, this is not the case. Only small amounts of sodium and potassium cross the membrane. Because an axon membrane has thousands of Na+-K+ pumps, these small ionic changes are quickly corrected.
Threshold
Not all local depolarization events produce Aps. The depolarization must reach threshold values if an axon is to “fire.” What determines the threshold point? One explanation is that threshold is the membrane potential at which the outward current created by K+ movement is exactly equal to the inward current created by Na+ movement. Threshold is typically reached when the membrane has been depolarized by 15 to 20 mV from the resting value. This depolarization status seems to represent an unstable equilibrium state at which one of two things can happen. If one more Na+ enters, further depolarization occurs, opening more Na+ channels and allowing more Na+ entry. If, on the other hand, oine more K+ leaves, the membrane potential is driven away from threshold, Na+ channels close and K+ continues to diffuse outward until the potential returns to resting value.
Mylination
On unmylinated axons, Aps are generated at sites immediately adjacent to each other and conduction is relatively slow, a type o f AP propagation called continuous conduction. The presence of myelin sheath dramatically increases the rate of AP propagation because myelin acts as an insulator, both preventing almost all leakage of charge from axon and allowing the membrane voltage to change more rapidly. Current can pass through the membrane of a myelinated axon only at the nodes of Ranvier, where the myelin sheath is interrupted and the axon is bare, and essentially all the voltage-gated Na+ channels are concentrated at the nodes. Thus, when an Ap is generated in a myelinated fiber, the local depolarizing current does not dissipate through the adjacent membrane regions but instead is maintained and move rapidly to the next node, a distance of approximately 1mm, where it triggers another AP.
SP and SI
Many receptors present on postsynaptic membranes at chemical synapses are specialized to open ion channels, thereby converting chemical signals to electrical signals. Unlike the coltage-gated ion channels responsible for APs, however, these chemically gated channels are relatively insensitive to changes in membrane potential. Consequently, channel opening at postsynaptic membranes cannot possibly become self-amplifying or sel-generating. Instead, neurotransmitter receptors mediate graded potentials-local changes in membrane potential that are graded according to the amount of neurotransmitter released and the time it remains in the area.
ES
At excitatory synapses, neurotransmitter binding causes depolarization of the postsynaptic membrane. However, in contrast to what happens on axon membranes, a single type of chemically gated ion channel opens on postsynaptic membranes. This channel allows Na+ and K+ to diffuse simultaneously through the membrane in opposite directions. Although this two-way cation flow may appear to be self-defeating when depolarization is the goal, remember that the electrochemical gradient for sodium is much steeper than that for potassium. Hence Na+ influx is greater than K+ efflux, and net depolarization occurs.
IS
Binding of neurotransmitters at inhibitory synapses reduces a postsynaptic neuron’s ability to generate an AP. Most inhibitory neurotransmitters induve hyperpolarization of the postsynaptic membrane by making the membrane more permeable to K+ or Cl-. Sodium ion permeability is not affected. If K+ channels are opened, K+ moves out of the cell; if Cl- channels are open, Cl- moves in. In either case, the charge on the inner face of the membrane becomes more negative. As the membrane potential increases and is driven farther from the axon’s threshold, the postsynaptic neuron becomes less and less likely to “fire” and larger depolarizing currents are required to induce an AP. Such changes in potential are called inhibitory postsynaptic potentials.
SPN
Summation by the Postsynaptic Neuron: A single EPSP cannot induce an AP in the postsynaptic neuron. But if thousands of excitatory axon terminals are firing on the same postsynaptic membrane, or if smaller number of terminals are delivering impulses rapidly, the probability of reaching threshold depolarization increases greatly. Thus, EPSPs can add together, or summate, to influence the activity of a postsynaptic neuron. Nerve impulses would never be initiated if this were not so.
Saptial Summation occurs when the postsynaptic neuron is stimulated at the same time by a large number of terminals from the same or, more commonly, different neurons. Huge numbers of its receptors bind neurotransmitter and simultaneously initiate EPSPs, which summate and dramatically enhance depolarization.
E vs. I
We can summarize this classification scheme by saying that some neurotransimitters are excitatory, some inhibitory and others exert both effects, depending on the specific receptor types with which they interact. For example, the amino acids GABA and glycine are usually inhibitory, whereas glutamate is typically excitatory. On the other hand, ACh and NE each bind to atleast two receptor types that cause opposite effects. For example, acetylcholine is excitatory at neuromuscular junctions in skeletal muscle and inhibitory in cardiac muscle.
Types of circuits
In diverging circuits, one incoming fiber triggers responses in ever-increasing numbers of neurons farther and farther along in the circuit. Thus, diverging circuits are often amplifying circuits. Divergence can occur along a single pathway or along several. These circuits are common in both sensory and motor systems.
The pattern of converging circuits is opposite that of diverging circuits, but they too are common in both sensory and motor pathways. In a converging circuit, the pool receives inputs from several presynaptic neurons, and the circuit has a funneling, or concentrating, effect. Incoming stimuli may converge from one area or from many different areas, which results in strong stimulation or inhibition. The former condition helps explain how different types of sensory stimuli can have the same ultimate effect.
In reverberating or oscillating circuits, the incoming signal travels through a chain of neurons, each of which makes collateral synapses with neurons in a previous part of the pathway. As a result of the positive feedback , the impulses reverberate, giving a continuous output signal until one neuron in the circuit fails to fire. Reverberating circuits are involved in control of rhythmic activities, such as the sleep-wake cycle, breathing, and certain motor activities. Some researchers believe that such circuits underlie short-term memory. Depending on the specific circuit, reverberating circuits may continue to oscillate for seconds, hours or a lifetime.
In parallel after-discharge circuits, the incoming fiber stimulates several neurons arranged in parallel arrays that eventually stimulate a common output cell. Impulses reach the output cell at different times, creating a burst of impulses called an after-discharge that lasts 15 ms or more after the initial input has ended. This type of circuit has no positive feedback, and once all the neurons have fired, circuit activity ends. Parallel after-discharge circuits may be involved in complex, exacting types of mental processing.
COMMENTS
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