Nerve and Synapse Flashcards
Communications and control network that allows an organism to interact in appropriate ways with its environment
Nervous System
o Central control hub of the nervous system
o Includes the brain and spinal cord
Central Nervous System
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
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
- Totality of an organism’s responses to its environment
* May be covertly or overtly expressed
Expression Of Behavior
Anatomically and physiologically specialized for communication and signaling
Neurons (Nerve Cells)
o Characterized as supportive cells that sustain neurons both metabolically and physically
o Maintain internal milieu of the nervous system
Neuroglia (Nerve Glue)
Types of Neuroglia
- MICROGLIA (Latent Macrophages)
- Scavenger cells that resemble macrophages
- Remove debris resulting from injury or disease - MACROGLIA
- Supportive matrix of the central nervous system
- Protects neurons from extreme variations in their extracellular environment - EPENDYMAL CELLS
- Epithelial lining the ventricular spaces of the brain
- CSF is secreted in large part by specialized ependymal cells of the choroid plexus
Types of Macroglia
- Central Nervous System
O Astrocytes
O Oligodendroglia - Peripheral Nervous System
O Schwann Cells
O Satellite Cells
- Star-shaped
- Help regulate the CNS microenvironment
- Foot processes form the blood-brain barrier
Astrocytes
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
- Involved in myelin formation around axons in the central nervous system
- many central axons can be myelinated by a single oligodendrocyte
Oligodendrocytes
- Involved in myelin formation around axons in the peripheral nervous system
- Each Schwann cell myelinates only one axon
Schwann Cells
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
- Encapsulate dorsal root and cranial nerve ganglion cells
* Regulate their microenvironment in a fashion similar to that used by astrocytes
Satellite Cells
- Contains the nucleus of the nerve cell
* Metabolic and integrative center of the neuron
Cell Body (Soma)
- Processes that extend outward from the cell body and arborize extensively (arbor vitae)
- Small knobby projections (dendritic spines)
Dendrites
- 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
one process, with different segments serving as receptive surfaces and releasing terminals
Unipolar
have two specialized processes:
- dendrite that carries information to the cell
- axon that transmits information from the cell
Bipolar
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
more than two specialized processes
Multipolar
- Ability to respond to environmental changes called stimuli
- Possessed by all cells
- Highest degree of development of this property is seen in neurons
Excitability
• Any change in the environment that will influence an organism and cause a response
Stimulus
- 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
- Site on the body where the stimulus originated
* A single sensory axon and all its peripheral branches is called a sensory unit
Location
Is the spatial distribution from which a stimulus produces a response
Receptive Field
Sensation from receptors at the peripheral edge of the stimulus is inhibited compared to sensation from central receptors
Lateral Inhibition
Signaled by the response amplitude or frequency of action potential generation
Intensity
Magnitude of the sensation felt is proportional to the log of the intensity of the stimulus
Weber-Fechner Law
- 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
At constant stimulation, frequency of the action potentials in sensory nerves decline over time
Adaptation or Desensitization
- 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
• Stimulus duration equal to twice the rheobase
Chronaxie
The ____ the chronaxie, the more excitable a nerve is
Lower
- 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
• 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
- Low-resistance pathway between cells
- Direct flow of currents from one cell to another
- Neurotransmission is multidirectional
Electrical Synapse
- 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
is the time required for the multiple steps in chemical neurotransmission
Synaptic Delay
• Action potential in one presynaptic neuron produces an action potential in one postsynaptic cell
EXAMPLE: neuromuscular junction
One-To-One Synapse
• 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
• Multiple presynaptic neurons are required to depolarize one postsynaptic cell and produce an action potential
EXAMPLE: widespread in CNS/PNS
Many-To-One Synapse
Small-molecule endogenous chemicals that transmit signals from a neuron to a target cell across a synapse
Neurotransmitter
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
• 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
• 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
- 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
- Secreted mainly at synapses in the spinal cord (Renshaw cells)
- Always acts as an inhibitory transmitter
Glycine
• 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)
• 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
- Secreted by nuclei from median raphe of brain stem
- Inhibitory neurotransmitter
- Controls mood and sleep
Serotonin
- Secreted in areas of the brain responsible for long-term behavior and memory
- Short-acting inhibitory neurotransmitter
Nitric Oxide
Which neurotransmitter is purely excitatory?
Glutamate
Which neurotransmitters are purely inhibitory?
Serotonin, Glycine, GABA, Nitric Oxide
Which neurotransmitters are may be excitatory or inhibitory?
Acetylcholine, Norepinephrine, Epinephrine, Dopamine
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)
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)
Process of adding up postsynaptic potentials and responding to their net effect
Summation
♣ Occurs when two EPSPs arrive at a postsynaptic neuron simultaneously
♣ Together, they produce greater depolarization
Spatial Summation
♣ Occurs when EPSPs arrive at a postsynaptic neuron in rapid succession
♣ EPSPs overlap in time, adding in stepwise fashion
Temporal Summation
- 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
- Opposite of facilitation
- Mechanism in which one presynaptic neuron suppresses another one
- IPSPs bring membrane potential farther from threshold potential
Presynaptic Inhibition
- 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
- 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
• Within the discharge zone of an input neuron, an input neuron acting alone can make the postsynaptic cells fire
Discharge Zone
- 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
• 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
- Decrease in expected response
* Occurs due to presynaptic fibers sharing postsynaptic neurons
Occlusion
• Signals from multiple inputs unite to excite a single neuron
Convergence
• Input from just one neuron may produce output through dozens of neurons
Divergence
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
o Signal is transmitted in two directions from the neuronal pool
o EXAMPLE]: dorsal columns, thalamic pathways
Divergence Into Multiple Tracts
• Also known as close-chain or oscillatory circuit
Reverberation
- 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
