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Neuron
One nerve cell, includes dendrites, cell body (soma), axon to conduct nerve impulses
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Nerve Fibers
Axons, peripheral and central processes
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Nerve
A bundle of nerve fibers
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Ganglia
A collection of neuron cell bodies located outside the CNS
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CNS
Central Nervous System
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PNS
Peripheral Nervous System
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Nuclei
Clusters of neuron cell bodies in CNS
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Gray matter
Neuron cell bodies
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White matter
myelinated nerve fibers, form tracts in CNS
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Neural networks
Interconnecting circuits of neurons
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CNS Organs
Brain and Spinal Cord
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PNS Organs
Nerves and Ganglia
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Divisions of Motor
- Somatic: supplies motor impulses to skeletal muscles (voluntary)
- Autonomic (visceral) system: supplies motor impulses to cardiac, smooth muscle, glands (involuntary)
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Divisions of Autonomic
Sympathetic and Parasympathetic-dual innervation of effectors.
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Division of Sensory
- General (body wide) and special (localized)
- Somatic (body wall, cutaneous) and autonomic (visceral)
- Visceral-pertaining to internal organs
- Somatic-pertaining to the body's framework-skin, muscle, tendon, joints
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Neuroglia
- Do not conduct impulses, are supporting cells.
- Serve to nourish and protect neurons.
- Some form blood-brain barrier with capillary cell walls.
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Schwann Cells
- Form myelin sheath around axons in PNS
- Myelin acts as insulator and speeds nerve impulse conduction
- Breaks between Schwann cells are Nodes of Ranvier-where ions can cross membranes in APs
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Afferent or sensory (Classes of Neurons)
Transmit information from receptors to CNS
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Interneurons (Classes of Neurons)
Connecting, located entirely in CNS
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Efferent or motor (Classes of Neurons)
Transmit info from CNS to effector cells
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Graded Potentials
Dendrites and soma
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Action Potentials
Axon, peripheral process (nerve impulse-a propagated action potential)
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Synaptic Transmission
Neurotransmitters cross a synapse; may include signal transduction
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Sensory Transduction
Sensory recpetors
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Potential
- A voltage difference across a cell membrane
- The adjectives membrane, resting, graded, action, and post synaptic all define conditions under which a potential is measured or the way it develops in cells; all are used tp define potentials
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Polarize
- Another series of terms is used to describe the direction of membrane potential changes relative to the resting state (-70mV)
- -Hyperpolarize-inside of cell is more negative than usual
- -Depolarize-positive charge is added to inside of cell, inside of cell becomes positive
- -Repolarize-membrane potential returns to resting value after a change
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Gating
Ion channels are gated 3 ways: voltage (electrical), chemical (receptor), mechanically (stretch)
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Resting Membrane Potential
- Neurons are polarized, Na outside, K and protein inside, inside is negative (-70mV)
- (Neurons are polarized, can be depolarized, repolarized, hyperpolarized)
- All cells have resting membrane potential, only muscle and nerve cells are excitable:can change membrane potential in response to a stimulus
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Action Potential
- An action potential is a transient, all or none reversal of membrane polarity
- They constitute an electrical signal which can be propagated by nerve and muscle cells.
- APs are initiated by graded potentials. If the incoming stimulus is strong enough (reaches threshold)voltage gated Na channels will open-membrane becomes highly permeable to Na
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Voltage gated Na channels
- Are opened by Na from graded potential, as more Na enters cell, more channels open=a positive feedback cycle which causes the cell to depolarize (+50 mV)
- This localized reversal of membrane polarity is an action potential.
- The voltage gated channels are found initially at axon hillock, and all along axon.
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Conduction
- Along a neuron
- Action potentials=brief reversal of membrane polarity in one place; how is message conducted?
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Propagation
- One AP results in Na entry, which is the trigger for voltage gated channels on adjacent membrane to open
- A series of APs os omitiated down the axon membrane=propagation
- The propagated action potential is called a nerve impulse
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Refractory period
- time during which neuron cannot respond to a second stimulus.
- It is due to nature of gated channel, ensures one way conduction of impulses
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All-or-none Principle
- If threshold is reached, an AP is stimulated
- APs are all the same size=all or none
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Saltatory Conduction
- AP jumps from one node to next in myelinated fibers
- Increases speed of conduction
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Coding
- The information that is transmitted is in the frequency of APs generated
- APs are frequency modulated; graded potentials are amplitude modulated
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General Properties of APs
- 1. occur only in excitable membranes (they are only ones with voltage gated channels)
- 2. requires a minimal (threshold) potential for initiation
- 3. upstroke of AP is result of a positive feedback loop, is explosive, self-perpetuating
- 4. APs are all-or-none phenomena; amplitude of response is independent of stimulus intensity
- 5. APs are propagated without a decline in amplitude
- 6. Refractory period- membrane has diminished excitability immediately after an AP; this prevents fushion or impulses, permits propagation of discrete impulses
- 7. APs rapidly conduct information over long distances
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Graded Potential
- A stimulus is a physical, chemical or electrical event that alters neuron membrane permeability, makes neuron permeable to ions which diffuse in (depolarization) or out (hyperpolarization)
- The event that alters the membrane permeability is gating of an ion channel (stretch, chemical, voltage)
- This event occurs in sensory receptors, dendrites, soma
- The change in membrane permeability is a graded potential
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Graded Potential
- Changes in membrane potential confined to a small region of the membrane.
- They are graded in magnitude, the size of the graded potential reflects size of stimulus
- Gated channels open to initiate GPs, the resulting current flow spreads passively
- GPs are important in signaling over short distances
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Receptor Potentials (GP)
Generated by sensory stimuli
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Postsynaptic Potentials (GP)
Generated in dendrites and soma by neurotransmitters; can be excitatory (EPSP); depolaraization or inhibitory (IPSP); hyperpolarization
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Pacemaker potentials (GP)
Spontaneous oscillations in membrane potentials that trigger APs (heart)
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Temporal summation
repeated stimuli over time=larger graded potentials
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Spatial summation
GPs initiated at many sites in a neuron=larger graded potential
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GPs are used in these situations
- Initiate info flow in sensory cells
- Conduct info from dendrites to axons
- Represent the entire information transfer in neurons with very short axons (CNS)
- Initiate action potentials in cardiac muscle cells
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Synaptic Transmission
Conduction across a synapse
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Synapse
- Region of communication between two neurons
- Consists of synaptic knob, synaptic cleft, postsynaptic membrane
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Electrical Synapse
- Ions pass through gap junction between cells
- Useful to synchronize electrical events rapidly, found in heart and smooth muscle
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Chemical Synapse
A chemical called a neurotransmitter is released from presynaptic neuron, influences postsynaptic neuron, initiates a GP via chemical gating of ion channels
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Presynaptic release of neurtransmitter
- Propagated APs open Ca voltage gated channels at axon terminals
- Ca enters axon and effects exocytotic release of neurotransmitter from synaptic vesicles
- Release is quantal-number of vesicles released relates to frequency of APs
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Postsynaptic receptors
- Neurotransmitters bind to specific receptors in post-synaptic membranes; there are 2 possible effects:
- 1. the binding gates (opens or closes) an ion channel which is an integral part of receptor protein, ions move driven by concentration gradients, either into or out of cells
- 2. the binding of NT to receptor initiates a complex mechanism called signal transduction; the off switch is removal of neurotransmitter molecules-by enzymatic degradation or reuptake
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PSPs (postsynaptic potentials, graded potentials)
- Can be excitatory or inhibitory
- Depends on nature of ion channel, not neurotransmitter
- EPSP-excitatory= Na/K channels open, Na ions will enter cell and depolarize it
- IPSP-inhibitory=Cl or K channels will hyperpolarize cell (Cl- flows in, K+ flows out)
- A particular synapse is always either excitatory or inhibitory
- Each presynaptic neuron makes only one kind of neurotransmitter
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Neurotransmitters
- Chemicals released from presynaptic membrane
- They bind to postsynaptic membrane receptors and usually gate ion channels
- Quickly inactivated by enzymes or removed by presynaptic reuptake pumps
- Acetylcholine and norepinephrine are usual PNS neurotransmitters
- Others in CNS include dopamine, seratonin, endorphins, GABA, glutamate, glycine
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The binding of neurotransmitters to postsynaptic membranes can cause the following..
- Block Na voltage gated channels in axons-tetrodotoxin
- Stimulate release of NT-amphetamine, black widow venom
- Prevent NT release-botulism toxin prevents ACH release, tetanus toxin blocks release of inhib NT
- Block enzymatic degradation-malathion, sarin, flea collar, MAO inhibitors
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Reflex
- An automatic, involuntary repsonse to a stimulus; it is mediated by a reflex arc
- Reflexes include knee jerks, withdrawal, homeostatic adjustments, postural adjustments
- (The reflex is the observed action; the reflex arc is the wiring that accomplishes the reflex)
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Reflex Arc
- A conduction pathway, the most basic neural pathway
- The neuron is the structural unit of the NS, the reflex arc is the functional unit
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5 basic components of a reflex arc
- Sensor, sensory neuron, interneuron, motor neuron, effector
- Note that in some relfex arcs there is no interneuron, in some there are more than one
- Information flow has a specific directionality in reflex arcs
- This is ensured by refractory period in axons and postsynaptic receptors in synapses
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