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Flashcards in Nerve and Synapse Deck (75):
1

Communications and control network that allows an organism to interact in appropriate ways with its environment

Nervous System

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o Central control hub of the nervous system
o Includes the brain and spinal cord

Central Nervous System

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o Provides an interface between the environment and the central nervous system
o Includes sensory neurons, somatic motor neurons, and autonomic motor neurons

Peripheral Nervous System

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Includes the following:
o Transmission of information via neural networks
o Transformation of information by recombination with other information (neural integration)
o Perception of sensory information
o Storage and retrieval of information (memory)
o Planning and implementation of motor commands
o Thought processes and conscious awareness
o Learning
o Emotion and motivation

Information Processing

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• Totality of an organism's responses to its environment
• May be covertly or overtly expressed

Expression Of Behavior

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Anatomically and physiologically specialized for communication and signaling

Neurons (Nerve Cells)

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o Characterized as supportive cells that sustain neurons both metabolically and physically
o Maintain internal milieu of the nervous system

Neuroglia (Nerve Glue)

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Types of Neuroglia

1. MICROGLIA (Latent Macrophages)
- Scavenger cells that resemble macrophages
- Remove debris resulting from injury or disease
2. MACROGLIA
- Supportive matrix of the central nervous system
- Protects neurons from extreme variations in their extracellular environment
3. EPENDYMAL CELLS
- Epithelial lining the ventricular spaces of the brain
- CSF is secreted in large part by specialized ependymal cells of the choroid plexus

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Types of Macroglia

1. Central Nervous System
O Astrocytes
O Oligodendroglia

2. Peripheral Nervous System
O Schwann Cells
O Satellite Cells

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• Star-shaped
• Help regulate the CNS microenvironment
• Foot processes form the blood-brain barrier

Astrocytes

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Types of Astrocytes

1. FIBROUS ASTROCYTES
o Contain many intermediate filaments
o Found in white matter
2. PROTOPLASMIC ASTROCYTES
o Found in gray matter
o Granular cytoplasm

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• Involved in myelin formation around axons in the central nervous system
• many central axons can be myelinated by a single oligodendrocyte

Oligodendrocytes

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• Involved in myelin formation around axons in the peripheral nervous system
• Each Schwann cell myelinates only one axon

Schwann Cells

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Importance of Myelination

• Increases speed of action potential conduction
• Restricts flow of ionic current to small unmyelinated portions of the axon between adjacent sheath cells (nodes of Rangier)
• This process is called SALTATORY conduction

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• Encapsulate dorsal root and cranial nerve ganglion cells
• Regulate their microenvironment in a fashion similar to that used by astrocytes

Satellite Cells

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• Contains the nucleus of the nerve cell
• Metabolic and integrative center of the neuron

Cell Body (Soma)

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• Processes that extend outward from the cell body and arborize extensively (arbor vitae)
• Small knobby projections (dendritic spines)

Dendrites

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• Long fibrous process that originates from a thickened area of the cell body (axon hillock)
• Divides into presynaptic terminals, ending in synaptic knobs (aka terminal buttons or buttons)
• Contain granules or vesicles which contain neurotransmitters

Axon

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one process, with different segments serving as receptive surfaces and releasing terminals

Unipolar

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have two specialized processes:
- dendrite that carries information to the cell
- axon that transmits information from the cell

Bipolar

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a single process splits into two, both of which function as axons—one going to skin or muscle and another to the spinal cord

Pseudo-Unipolar

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more than two specialized processes

Multipolar

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• Ability to respond to environmental changes called stimuli
• Possessed by all cells
• Highest degree of development of this property is seen in neurons

Excitability

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• Any change in the environment that will influence an organism and cause a response

Stimulus

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• Type of energy transmitted by the stimulus
• adequate stimulus is the particular form of energy to which a receptor is most sensitive
• EXAMPLE: light for the rods and cones of the eye

Modality

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• Site on the body where the stimulus originated
• A single sensory axon and all its peripheral branches is called a sensory unit

Location

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Is the spatial distribution from which a stimulus produces a response

Receptive Field

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Sensation from receptors at the peripheral edge of the stimulus is inhibited compared to sensation from central receptors

Lateral Inhibition

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Signaled by the response amplitude or frequency of action potential generation

Intensity

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Magnitude of the sensation felt is proportional to the log of the intensity of the stimulus

Weber-Fechner Law

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- Time from start to end of a response in receptors
- Receptors can be classified as:
o Rapidly adapting (phasic) receptors
o Slowly adapting (tonic) receptors

Duration

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At constant stimulation, frequency of the action potentials in sensory nerves decline over time

Adaptation or Desensitization

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• For neurons, the curve clearly flattens out with long stimulus durations, reaching an asymptote
• When the stimulus strength is below the rheobase, stimulation is ineffective even when stimulus duration is very long

Rheobase

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• Stimulus duration equal to twice the rheobase

Chronaxie

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The ____ the chronaxie, the more excitable a nerve is

Lower

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• Once threshold intensity is reached, a full-fledged action potential is produced
• Further increases in the intensity of a stimulus produce no increment or other change
• Action potential fails to occur if the stimulus is subthreshold in magnitude

All-Or-None Principle

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• Junction that permits neurons to pass electrical or chemical signals to other cells
• Essential component to neuronal communication
o with other neurons
o with muscles
o with glands

Synapse

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• Low-resistance pathway between cells
• Direct flow of currents from one cell to another
• Neurotransmission is multidirectional

Electrical Synapse

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• Comprise almost all the synapses in the CNS used for signal transmission
• Presynaptic neuron secretes neurotransmitters that act on postsynaptic cell
• Neurotransmission is unidirectional

Chemical Synapse

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is the time required for the multiple steps in chemical neurotransmission

Synaptic Delay

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• Action potential in one presynaptic neuron produces an action potential in one postsynaptic cell
EXAMPLE: neuromuscular junction

One-To-One Synapse

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• Action potential in one presynaptic neuron produces an action potential in many postsynaptic cell
• Causes amplification of synaptic activity
EXAMPLE: motoneurons on Renshaw Cells

One-To-Many Synapse

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• Multiple presynaptic neurons are required to depolarize one postsynaptic cell and produce an action potential
EXAMPLE: widespread in CNS/PNS

Many-To-One Synapse

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Small-molecule endogenous chemicals that transmit signals from a neuron to a target cell across a synapse

Neurotransmitter

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Essential Characteristics of Neurotransmitters

• Have precursors and/or synthesis enzymes located in the presynaptic neuron
• Present in the presynaptic neuron
• Available in sufficient quantity affect postsynaptic cells
• Capable of binding to postsynaptic receptors
• Biochemical mechanism for inactivation

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• Secreted by neurons in many areas:
o Large pyramidal cells in motor cortex
o Basal ganglia (nucleus basalis)
o Skeletal muscles
o All preganglionic neurons of ANS
o Postganglionic neurons of parasympathetic NS
o Some postganglionic neurons of sympathetic NS
• May be excitatory or inhibitory
• Action terminated by metabolism (enzymatic degradation) by acetylcholinesterase

Acetylcholine

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• Secreted by many neurons:
o Brain stem
o Hypothalamus
o locus ceruleus in the pons
o Postganglionic neurons of sympathetic nervous system
• Control overall activity and mood of the mind, such as increasing the level of wakefulness
• May be excitatory or inhibitory
• Action terminated by repute (NET) and metabolism (monoamine oxidase, catechol-O-methyltransferase)

Norepinephrine and Epinephrine

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• Secreted by neurons in the substantial nigra
• May be excitatory or inhibitory
• Action terminated by reuptake (DAT) and metabolism (monoamine oxidase, catechol-O-methyltransferase)

Dopamine

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• Secreted mainly at synapses in the spinal cord (Renshaw cells)
• Always acts as an inhibitory transmitter

Glycine

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• Secreted by nerve terminals:
o Spinal cord
o Cerebellum
o Basal ganglia
• Many areas of the cerebral cortex
• Always acts as an inhibitory transmitter

Gamma-AminoButyric Acid (GABA)

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• Secreted by:
o Presynaptic terminals in many of the sensory pathways
o Many areas of the cerebral cortex
o Always an excitatory neurotransmitter
• Most prevalent excitatory neurotransmitter in brain

Glutamate

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• Secreted by nuclei from median raphe of brain stem
• Inhibitory neurotransmitter
• Controls mood and sleep

Serotonin

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• Secreted in areas of the brain responsible for long-term behavior and memory
• Short-acting inhibitory neurotransmitter

Nitric Oxide

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Which neurotransmitter is purely excitatory?

Glutamate

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Which neurotransmitters are purely inhibitory?

Serotonin, Glycine, GABA, Nitric Oxide

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Which neurotransmitters are may be excitatory or inhibitory?

Acetylcholine, Norepinephrine, Epinephrine, Dopamine

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o Inputs that depolarize the postsynaptic cell, bringing it closer to threshold and closer to firing an action potential
o Caused by opening of Na+ and K+ channels

Excitatory Postsynaptic Potentials (EPSP)

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o Inputs that hyperpolarize the postsynaptic cell, moving it away from threshold and farther from firing
o Caused by opening Ca+ channels

Inhibitory Postsynaptic Potentials (IPSP)

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Process of adding up postsynaptic potentials and responding to their net effect

Summation

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♣ Occurs when two EPSPs arrive at a postsynaptic neuron simultaneously
♣ Together, they produce greater depolarization

Spatial Summation

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♣ Occurs when EPSPs arrive at a postsynaptic neuron in rapid succession
♣ EPSPs overlap in time, adding in stepwise fashion

Temporal Summation

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• Process where one neuron enhances the effect of another neuron
• EPSPs bring membrane potential nearer threshold potential, but not yet at firing level

Presynaptic Facilitation

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• Opposite of facilitation
• Mechanism in which one presynaptic neuron suppresses another one
• IPSPs bring membrane potential farther from threshold potential

Presynaptic Inhibition

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• Bringing additional neurons into play as the stimulus becomes stronger
• Enables the nervous system to judge stimulus strength by which neurons, and how many of them, are firing

Recruitment

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• Neurons may function in larger ensembles called neuronal pools or neural networks
• Thousands to millions of interneurons concerned with a particular body function
• EXAMPLES: neuronal pools for rhythm of your breathing

Neuronal Pools

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• Within the discharge zone of an input neuron, an input neuron acting alone can make the postsynaptic cells fire

Discharge Zone

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• Within the broad facilitated zone, a presynaptic input neuron synapses with other neurons in the pool
• Can stimulate neurons to fire only with the assistance of other input neurons

Facilitated Zone

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• Neurons are in the subliminal fringe if:
o they are not discharged by an afferent volley (not in the discharge zone)
o they have their excitability increased

Subliminal Fringe

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• Decrease in expected response
• Occurs due to presynaptic fibers sharing postsynaptic neurons

Occlusion

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• Signals from multiple inputs unite to excite a single neuron

Convergence

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• Input from just one neuron may produce output through dozens of neurons

Divergence

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o An input signal spreads to an increasing number of neurons as it passes through successive orders of neurons in its path
o EXAMPLE: corticospinal tract

Amplifying Divergence

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o Signal is transmitted in two directions from the neuronal pool
o EXAMPLE]: dorsal columns, thalamic pathways

Divergence Into Multiple Tracts

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• Also known as close-chain or oscillatory circuit

Reverberation

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• One input neuron diverges to stimulate several chains of neurons
• Each chain has a different synapses but eventually reconverges on one output neuron

Parallel-After-Discharge