CHAPTER 12 (REV 13) Flashcards
BIOL 235 (142 cards)
1
Q
What is the Central Nervous System?
A
Consists of the brain and spinal cord. The brain is the part of the CNS that is located in the skull. Spinal cord is connected to the brain through the foremen magnum of the occipital bone.
2
Q
What is the peripheral nervous system?
A
Consists of all nervous tissue outside the CNS. Includes nerves and sensory receptors.
3
Q
A bundle of hundreds to thousands of axons plus associated connective tissue and blood vessels that lie outside the brain and spinal cord.
A
A nerve
4
Q
12 pairs of these emerge from the brain
A
Cranial nerves
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Q
31 pairs emerge from the spinal cord
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Spinal nerves
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Q
A structure of the nervous system that monitors changes in the external or internal environment.
A
Sensory Receptor.
7
Q
Information from the PNS input into the CNS from the sensory receptors in the body. This division provides the CNS with sensory information about the somatic senses and special senses.
A
Sensory or Afferent Division
8
Q
Information from the PNS conveys output from the CNS to effectors (muscles and glands). This division is further subdivided into a somatic nervous system and autonomic nervous system.
A
Motor or efferent division
9
Q
Conveys information (output) from the CNS to skeletal muscles only. Motor responses can only be consciously controlled, the action of this art of the PNS is voluntary.
A
Somatic nervous system (SNS)
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Q
A process that conveys output from the CNS to smooth muscle, cardiac muscle and glands. Because its motor responses are not normally under conscious control the action is involuntary.
A
Autonomic Nervous System
11
Q
The two branches of the AN
A
Sympathetic nervous system: fight or flight responses
Parasympathetic nervous system: rest and digest activities
12
Q
What is the third branch of the autonomic nervous system?
A
Enteric Nervous System (ENS)
Extensive network of over 100 million neurons confirmed to the wall of the GI tract.
13
Q
What are the three basic functions of the nervous system?
A
Sensory Function: Sensory receptors detect internal stimuli, such as an increase in blood pressure or external stimuli. This information is than carried into the brain and spinal cord through the cranial and spinal nerves
Integrative Function: The nervous system processes sensory information by analyzing it and making decisions for appropriate responses - an activity known as integration
Motor function: Once sensory information is integrated, the nervous system may elicit an appropriate motor response by activating effectors through cranial and spinal nerves.
14
Q
The ability to respond to a stimulus and convert it into an action potential
A
Electrical Excitability
15
Q
Any change in the environment that is strong enough to initiate an action potential
A
Stimulus
16
Q
An electrical signal that propagates (travels) along the surface of the membrane of a neuron.
A
Action potential
17
Q
Three parts of the neurons
A
The cell body - (soma) contains a nucleus surrounded by cytoplasm that includes typical cellular organelles such as lysosomes, mitochondria and Golgi complex.
Dendrites: receiving or input portions of a neuron
Axon: propagates nerve impulses toward another neuron, a muscle fibre, or a gland cell. Long and thin, cylindrical projection that often joins to the cell body at a cone-shaped elevation called the axon hillock.
18
Q
Neuronal bodies also contain free ribosomes and prominent clusters of rough endoplasmic reticulum.
A
Nissl bodies
19
Q
The cytoskeleton contains what supportive components?
A
Neurofibrils, microtubules.
20
Q
A collection of neuron cell bodies outside the CNS is called a what?
A
Ganglion
21
Q
A general term for any neuronal process that emerges from the cell body of the neuron.
A
Nerve Fiber
22
Q
Neurons have two types of processes
A
Multiple dendrites
Single Axon
23
Q
The part of the axon closest to the axon hillock is called the?
A
Initial Segment
24
Q
In most neurons, nerve impulses arise at the junction of the axon hillock and the initial segment - an area called the?
A
Trigger Zone
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The cytoplasm of an axon
Axoplasm
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The plasma membrane that surrounded the axoplasm
Axolemma
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Along the length of an axon the side branches take off at a right angle to the axon.
Axon collaterals
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The axon and its collaterals end by dividing into many fine processes called
Axon terminals
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Site of communication between two neurons or between a neuron and effector cell is called a what?
Synapse
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The tips of some axon terminals swell into bulb shaped structures called
Synaptic end bulbs
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String of swollen bulbs - tips of the axon terminals
Varicosities
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Both synaptic end bulbs and varicosities contain many tine membrane enclosed saves called
Synapatic vesicles that store a chemical called a neurotransmitter
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There are two types of transport systems that carry materials from the cell body to the axon terminals and back. Slower and Faster
Slow axonal transport - moves about 1-5 mm per day. Conveys axoplasm in one direction only from the cell body toward the axon terminals. Supplies new axoplasm to developing or regenerating axons and replenishes axoplasm in growing and mature axons.
Fast axonal transport - capable of moving materials at a distance of 200-400 mm per day. Uses proteins that function as motors to move materials away from and toward the cell body.
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Fast axonal transport that occurs in a forward direction (term?) moves organelles and synaptic vesicles from he cell body to the axon terminals.
Anterograde
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Fast axonal transport that occurs in a backward direction moves membrane vesicles and other cellular materials from the axon terminals to the cell body to be degraded or recycled.
Retrograde
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Structural Classification of Neurons are done according to the number of processes extending from the cell body - describe the three.
1. Multipolar Neurons
Have several dendrites and one axon. Most neurons in the brain and spinal cord are of this type as well as all motor neurons
2. Bipolar neurons
Have one main dendrite and one axon. Found in the retina of the eye, inner ear and the olfactory area of the brain.
3. Unipolar Nuerons
Have dendrites and one axon that are fused together to form a continuous process that emerges from the cell body. More appropriately called pseudounipolar neurons.
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Some neurons are named for the histologist who first described them
Purkinje cells
Pyramidal Cells
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Functional Classification of neurons - according to the direction in which th nerve impulse is conveyed with respect to the CNS
1. Sensory neurons (afferent) Contain sensory receptors at their distal ends or are located just after sensory recents that are separate cells.
2. Motor Neurons (efferent)
Convey action potentials away from eh CNS to effectors (muscles and glands) in the periphery (PNS) through cranial or spinal nerves.
3. Interneurons (association neurons)
Mainly located within the CNS between sensory and motor neurons. Interneurons integrate incoming sensory information.
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Smaller than neurons. They are 5-25 times more numerous. Make up about half the volume of the CNS. They actively participate in the activities of the nervous tissue.
Neuroglia
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Brain tumors derived from glia
gliomas
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Star-shaped cells have many processes and are the largest/most numerous.
Astrocytes
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What are the two types of astrocytes?
Protoplasmic astrocytes: short branching processes and are found in gray matter
Fibrous astrocytes: have long unbranched processes are located mainly in white matter.
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Functions of Astrocytes
1. Contain microfilaments that give them considerable strength - enables them to support neurons
2. Processes of astrocytes wrapped around blood capillaries isolate neurons of the CNS from various potentially harmful substances in blood by secreting chemicals that maintain the unique selective permeability characteristics of the endothelial cells of the capillaries.
3. In the embryo, astrocytes secrete chemicals that appear to regulate the growth, migration and interconnection among neurons in the brain.
4. Astrocytes help to maintain the appropriate chemical environment for he generation of nerve impulses.
5. Astrocytes may also play a role in learning and memory by influencing the formation of neural synapses.
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Resemble astrocytes but are smaller and contain fewer processes.
Oligodendrocytes
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A multilayered lipid and protein covering around some axons that insulates them and increases the speed of nerve impulse conduction.
Myelin sheath
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These are small cells with slender processes that give off numerous spikelike projections.
Microglial cells or Microglia
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Cuboidal to columnar cells arranged in a single layer that possess microvilli and cilia. These cells line the ventricles of the brain and central canal of the spinal cord.
Ependymal Cells
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These cells encircle PNS axons. Like oligodendrocytes, they form the myeline sheath around axons. They can encircle/enclose as many as 20 or more unmyelinated axons
Schwann Cells
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These flat cells surround the cell bodies of neurons of PNS ganglia. Provide structural support, regulate the exchanges of materials between neuronal cell bodies and interstitial fluid
Satellite Cells
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What two types of neuroglia produce myeline sheaths?
Schwann cells and Oligodendrocytes.
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What is the outer nucleated cytoplasmic layer of the Schwann cells which encloses the myeline sheath?
Neurolemma
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Gaps in the myelin sheath, appear at intervals along the axon.
Nodes of Ranvier
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A bundle of axons that is located in the CNS. Interconnect neurons in the spinal cord and brain
Tract
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Composed primarily of myelinated axons. Found in the brain or spinal cord.
White matter
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Contains neuronal cell bodies, dendrites, unmyelinated axons, axon terminals and neuroglia. Appears grayish because the Nissl bodies impart a gray color and there is little or no myeline in these areas.
Gray Matter
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What two ways do neurons communicate using electrical signals?
Graded Potentials (short-distance communication)
Action potentials (long distance)
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Understand the functions of graded potentials and action potentials.
See table
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The plasma membrane of excitable cells exhibits an electrical potential difference (voltage) across the membrane.
Membrane potential
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In excitable cells - the voltage is termed -
Resting Membrane potential
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A flow of charged particles
Current
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A concentration (chemical) difference plus an electrical difference.
Electrochemical gradient
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The electrical signals produced by neurons and muscle fibers rely on four types of ion channels
1. Leak channels
2. Ligand-gated channels
3. Mechanically gated channels
4. Voltage-gated channels
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How can the resting membrane potential of a cell be measured?
The tip of a recording micro electrode is inserted inside the cell, and a reference electrode is placed outside the cell in the extracellular fluid. In neurons, the resting membrane potential ranges from -40 to -90 mV. Typical value is -70 mV.
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A cell that exhibits a membrane potential is considered to be?
Polarized. Most body cells are polarized. The membrane potential varies from +5 mV to -100 mV in different types of cells.
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The resting membrane potential arises from three major factors
1. Unequal distribution of ions in the ECF and cytosol
2. Inability of most anions to leave the cell
3. Eletrogenic nature of the Na K ATPases.
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A small deviation from eh resting membrane potential that makes the membrane either rmore polarized (inside more negative) or less polarized (inside less negative).
Graded potential
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When the response makes the membrane more polarized (inside more negative)
Hyperpolarizing graded potential
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When the response makes the membrane less polarized (inside less negative)
Depolarizing graded potential.
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The mode of travel by which graded potentials die out as they spread along the membrane is called?
Decremental condduction
Because they die out within a few millimetres of their point of origin - graded potentials are useful for short - distance communication only.
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The process by which graded potentials add together
Summation
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A sequence of rapidly occurring events that decrease and reverse the membrane potential and then eventually restore it to the resting state
Action Potential (Impulse)
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The negative membrane potential becomes less negative, reaches zero and the becomes positive
Depolarizing Phase
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The membrane potential is restored to the resting state of -70 mV.
Repolarizing phase
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The membrane potential temporarily becomes more negative than the resting level.
After-hyperpolarizing phase
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An action potential occurs in the membrane of the axon of a neuron when depolarization reaches a certain level termed
Threshold (about -55 mV in many neurons)
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A weak depolarization that cannot bring the membrane potential to threshold
Subthreshold stimulus (an action potential will not occur in response)
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A stimulus that is just strong enough to depolarize the membrane to threshold
Threshold Stimulus (an action potential will occur in response)
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Several action potentials will form in response to a ____ - a stimulus that is strong enough to depolarize the membrane above threshold
Suprathreshold Stimulus
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An action potential is generated in response to a threshold stimulus but does not form when there is a sub threshold stimulus. An action potential either occurs completely or not at all
All or None Principle.
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Depolarizing Phase
Each voltage gated Na channel has two separate gates - activation gate and an inactivation gate.
When a depolarizing graded potential or some other stimulus causes
the membrane of the axon to depolarize to threshold, voltage-gated
Na+ channels open rapidly. Both the electrical and the chemical gra-
dients favor inward movement of Na+, and the resulting inrush of Na+
causes the depolarizing phase of the action potential
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Repolarizing Phase
Shortly after the activation gates of the voltage-gated Na+ channels
open, the inactivation gates close. Now the
voltage-gated Na+ channel is in an inactivated state. In addition to
opening voltage-gated Na+ channels, a threshold-level depolariza-
tion also opens voltage-gated K+ channels. Because the voltage-gated K+ channels open more slowly,
their opening occurs at about the same time the voltage-gated Na+
channels are closing.
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After-Hyperpolarizing Phase
While the voltage-gated K+ channels are open, outflow of K+ may be
large enough to cause an aft er-hyperpolarizing phase of the action
potential. During this phase, the voltage-gated K+
channels remain open and the membrane potential becomes even
more negative (about −90 mV). As the voltage-gated K+ channels
close, the membrane potential returns to the resting level of −70 mV.
Unlike voltage-gated Na+ channels, most voltage-gated K+ channels
do not exhibit an inactivated state. Instead, they alternate between
closed (resting) and open (activated) states.
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Refractory Period
The period of time aft er an action potential begins during which an
excitable cell cannot generate another action potential in response to
a normal threshold stimulus
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The period of time during which a second action potential can be initiated but only by a larger-than-normal stimulus. Coincides with the period when the voltage-gated K+ channels are open after inactivated Na channels have returned to their resting state.
Relative Refractory Period
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To communicate information from one part of the body to another, action potentials in a neuron must travel from where they arise at the trigger zone of the axon to the axon terminals. An action potential keeps its strength as it spreads along the membrane. Depends on positive feedback.
Propagation
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Two types of propagation
Continuous Conduction: Involves step-by-step depolarization and depolarization of each adjacent segment of the plasma membrane. Ions flow through their voltage-gated channels in each adjacent segment of the membrane.
Saltatory Conduction: Special mode of action potential propagation that occurs along myelinated axons, occurs because of the uneven distribution of voltage gated channels.
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Factors That Affect the Speed of Propagation
1. Amount of myelination
2. Axon diameter
3. Temperature
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Classification of Nerve Fibers
A fibers: largest in diameter and are myelinated. Brief absolute refractory period and conduct nerve impulses.
B Fibers: Axons with diameters of 2-3 um. Myelinated and exhibit saltatory conduction. Conduct sensory nerve impulses from the viscera to the brain and spinal cord.
C Fibers: Smallest diameter axons and all are unmyelinated. C fibers exhibit the longest absolute refractory periods. These unmyelinated axons conduct some sensory impulses for pain, touch, pressure, heat.
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A nerve cell that carries a nerve impulse toward a synapse
Presynaptic neuron
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The cell that receives a signal
Postsynaptic cell
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Carries a nerve impulse away from a synapse or effector cell
Postsynaptic neuron
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A cell that responds to the impulse at the synapse
Effector Cell
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Most synapses between neurons are - from axon to dendrite
Axodendritic
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From axon to cell body
Axosomatic
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From axon to axon
Axoaxonic
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Action potentials conduct directly between the plasma membranes of adjacent neurons.
Electrical Synapse
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Structures that impulses conduction through. Contains a hundred or so tubular connexions which act like tunnels to connect the cytosol of the two cells directly
Gap Junctions
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Electrical Synapses have two main advantages
1. Faster Communication
Action potentials conduct directly through gap junctions, electrical synapses are faster than chemical synapses. The action potential passes directly from the presynaptic cell to the postsynaptic cell.
2. Synchronization
Electrical synapses can synchronic the activity of a group of neurons or muscle fibers
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A space of 20-50 nm that is filled with interstitial fluid. (Chemical Synapses)
Synaptic Cleft
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Post synaptic neuron receives the chemical signal in produces a type of graded potential
Postsynaptic potential
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Time required for these processes at a chemical synapse
Synaptic delay (o.5 msec).
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Signal Transmission at a chemical Synapse
1. A nerve impulse arrives at a synaptic end bulb of a presynaptic axon
2. The depolarizing phase of the nerve impulse opens voltage gated Ca channels which are present in the membrane. Ca flows inward through the opened channels
3. An increase in the concentration of Ca inside the presynaptic neuron serves as a signal that triggers exocytosis of the synaptic vesicles.
4. The neurotransmitter molecules diffuse across the synaptic cleft and bind to neurotransmitter receipts in the postsynaptic neuron's plasma membrane.
5. Binding of neurotransmitter molecules to their receptors on ligand-gated channels opens the channels and allows particular ions to flow across the membrane.
6. As ions flow through the channels - the voltage across the membrane changes. Postsynaptic potential. Depending on which ions the channels admit, the post synaptic potential may be a depolarization or hyper polarization.
7. When a depolarizing postsynaptic potential reaches threshold, triggers an action potential in the axon of the postsynaptic neuron.
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At most chemical synapses...
....Only one-way information transfer
can occur—from a presynaptic neuron to a postsynaptic neuron or an
eff ector, such as a muscle fiber or a gland cell.
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neurotransmitter causes either an excitatory or an inhibitory graded
potential. A neurotransmitter that causes depolarization of the post-
synaptic membrane is excitatory because it brings the membrane
closer to threshold. A depolarizing postsynaptic
potential is called an
Excitatory Postsynaptic Potential (EPSP)
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A neurotransmitter that causes hyperpolarization of the postsynaptic membrane is inhibitory. During hyperpolarization, generation of an action potential is more difficult than
usual because the membrane potential becomes inside more negative and thus even farther from threshold than in its resting state. A
hyperpolarizing postsynaptic potential is termed a what?
Inhibitory post-
synaptic potential (IPSP).
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A type of neurotransmitter receptor that contains a neurotransmitter binding site and an ion channel
Ionotropic Receptor
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A type of neurotransmitter receptor that contains a neurotransmitter binding site but lacks an ion channel as part of its structure
Metabotropic Receptor
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Removal of the neurotransmitter from synaptic cleft is essential for normal synaptic function. If a neurotransmitter could linger in the synaptic cleft it would influence three things
Postsynaptic neuron, muscle fiber, gland cell
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How is a neurotransmitter removed?
1. Diffusion - some of the released neurotransmitter molecules diffuse away from the synaptic cleft. Once a neurotransmitter molecule is out of reach of its receptors, it can no longer exert an effect.
2. Enzymatic Degradation - certain neurotransmitter are inactivated through enzymatic degradation.
3. Uptake by Cells - many neurotransmitters are actively transported back into the neuron that released them (reuptake). Others are transported into neighbouring neuroglia (uptake). The neurons that release norepinephrine - rapidly take up the norepinephrine and recycle it into new synaptic vesicles.
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What are the two types of summation?
Spatial summation - summation of postsynaptic potentials in response to stimuli that occur at different locations in the membranes of postsynaptic cells at the same time.
Temporal summation - summation in response to stimuli that occur at the same location in the membrane of the postsynaptic cell but at different time.
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The sum of all excitatory and inhibitory effects at any given time determines the effect on the postsynaptic neuron. They respond how?
1. EPSP: if the total excitatory effects are greater than the total inhibitory effects but less than the threshold level of stimulation - the result is an EPSP that does not reach threshold.
2. NERVE IMPULSE: If the total excitatory effects are greater than the total inhibitory effects and threshold is reached, one or more nerve impulses (action potentials) will be triggered. Impulses continue to be generated as long s the EPSP is at or above the threshold level
3. IPSP: If the total inhibitory effects are greater than the excitatory effects, the membrane hyper polarizes. The result is inhibition of the postsynaptic neuron and an inability to generate a nerve impulse.
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In the brain, certain neurons secrete hormones.
Neurosecretory cells
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Release by many PNS neurons and by some CNS neurons. An excitatory neurotransmitter at some synapses. The binding of this neurotransmitter to inotropic receptors opens cation channels. It is also an inhibitory neurotransmitter at other synapses. Binds to metabotropic receptors coupled to G proteins that open K channels
Acetylcholine
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There are some amino acids that are neurotransmitters. There are two that have powerful excitatory effects. At some glutamate synapses,
binding of the neurotransmitter to ionotropic receptors opens cation channels. The consequent inflow of cations (mainly Na+ ions)produces an EPSP. Inactivation of glutamate occurs via reuptake.
Glutamate transporters actively transport glutamate back into the synaptic end bulbs and neighboring neuroglia.
Glutamate and Aspartate
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Two important inhibitory neurotransmitters (AA). At many synapses, the binding of GABA to ionotropic receptors opens Cl− channels. GABA is found only in the CNS where it is the most common inhibitory neurotransmitter.
Gamma-aminobutyric acid (GABA) and Glycine
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Certain amino acids are modified and decarboxylated to produce biogenic amines. Those that are prevalent in the nervous system include>
Biogenic Amines
Norepinephrine - plays roles in
arousal (awakening from deep sleep), dreaming, and regulating mood.
Epinephrine - A smaller number of neurons in the brain use epinephrine as a neurotransmitter. Both epinephrine and norepinephrine also serve as hormones. Cells of the adrenal medulla, the inner portion of the adrenal gland, release them into the blood.
Dopamine - Brain neurons containing the neurotransmitter dopamine (DA) (DŌ-pa-mēn) are active during emotional responses, addictive behaviors, and pleasurable experiences
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Norepinephrine, dopamine and epinephrine are classified chemically as what?
catecholamines
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Two enzymes that break down catecholamines are
Catechol-O-methytransferase (COMT)
Monoamine oxidase (MAO)
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Also known as 5-hydroxy-tryptamine. Concentrated in the neurons in a part of the brain called the raphe nucleus. Involved in sensory perception, temperature regulation, control of mood, appetite and sleep.
Serotonin
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The characteristic ring structure of the adenosine portion of ATP is called a what?
Purine ring
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True of False!
Adenosine itself, as well as its triphosphate, diphosphate, and monophosphate derivatives (ATP , ADP , and AMP), is an excitatory neurotransmitter in both the CNS and the PNS.
True
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An important excitatory neurotransmitter secreted in the brain, spinal cord, adrenal glands and nerves to the penis and has widespread effects throughout the body. NO contains a single nitrogen atom, in contrastto nitrous oxide (N2O), or laughing gas, which has two nitrogen atoms.
Nitric Oxide
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This enzyme catalyzes formation of NO from the amino acid arginine. Based on the present of NOS, it is estimated that more than 2% of the neurons in the brain produce NO.
Nitric Oxide Synthase (NOS)
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This is like NO - not produced in advance and packaged into synaptic vesicles. It is formed as NEEDED and diffuses out of cells that produce it into adjacent cells. This is an excitatory neurotransmitter produced in the brain. Might protect against excess neuronal activity and might be related to dilation of blood vessels, memory, olfaction(sense of smell), vision, thermoregulation, insulin release, and anti-
inflammatory activity.
Carbon Monoxide
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Consist of 3-40 amino acids linked by peptide bonds called what?
Neuropeptides
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What are neuropeptides?
Numerous and widespread in both the CNS/PNS. Bind to metabotropic receptors and have excitatory or inhibitory actions. Formed in the neuron cell body and packaged into vesicles and transported to axon terminals.
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The quest to find the naturally occurring substances that use these receptors brought to light the first neuropeptides: two molecules, each a chain of five amino acids
Enkephalins
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Opioid Peptides - pain relieving effect.
Endorphins
Dynorphins
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Released by neurons that transmit pain related input from peripheral pain receptors into the central nervous system. Enhancing the perception of pain.
Substance P
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Functional groups of neurons that process specific types of information
Neural circuits
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Simple series circuit
A presynaptic neuron stimulates a single postsynaptic neuron. The second neuron then stimulates another, and so on. However, most neural
circuits are more complex.
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A single presynaptic neuron may synapse with several postsynaptic neurons. Such an arrangement, permits one presynaptic neuron to influence several postsynaptic neurons(or several muscle fibers or gland cells) at the same time.
Divergence
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The nerve impulse from a single presynaptic neuron causes the stimulation of increasing numbers of cells along the circuit.
Diverging Circuit
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Several presynaptic neurons synapse with a single postsynaptic neuron. This arrangement permits more effective stimulation or inhibition of the postsynaptic neuron
Convergence
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The postsynaptic neuron receives nerve impulses from several different sources.
Converging Circuit
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The incoming impulse stimulates the first neuron which stimulates the second, third and so on. Branches from later neurons synapse with earlier ones. This arrangement sends impulses back through the circuit, again and again. The output signal may last from a few seconds to many hours.
Reverberating Circuit
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A single presynaptic cell stimulates a
group of neurons, each of which synapses with a common postsynaptic cell. A differing number of synapses between the first and last neurons imposes varying synaptic delays, so that the last neuron exhibits multiple EPSPs or IPSPs. If the input is excitatory, the postsynaptic neuron then can send out a stream of impulses in quick succession.
Parallel after discharge circuits may be involved in precise activities such as mathematical calculations.
Parallel After-Discharge Circuit
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The nervous system exhibits the capability to change based on experience
Plasticity
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The capability to replicate or repair themselves - Neurons
Regeneration
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The birth of new neurons from undifferentiated stem cells - occurs regularly in some animals
Neurogenesis.
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Stimulated cells taken from the brains of adult mice to proliferate into both neurons and astrocytes.
Epidermal Growth Factor EGF
Discovered in 1992 - Canadian
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Axons and dendrites that are associated with a neurolemma may undergo repair if the cell body is intact. If the Schwann cells are functional and if scar tissue formation dose not occur to rapidly
