Chapter 12 Flashcards
Everything done in NS involves (3) fundamental steps
1) sensory function detects internal & external stimuii
2) interpretation is made (analysis)
3) motor response occurs (reaction)
Divisions of the Nervous System (2)
CNS → brain & spinal cord
PNS → all nervous tissue outside CNS (includes nerves, ganglia, enteric plexuses & sensory receptors)
Most signals that __ muscles to ___ and __ to ___ originate in CNS
stimulate muscles to contract and glands to secrete originate in CNS
PNS is further divided into (3) ?
1) somatic sensory system (SNS)
2) autonomic nervous system (ANS)
3) enteric nervous system (ENS)
**SNS** consists of (3)?
1) somatic sensory (afferent) neurons
2) Somatic motor (efferent) neurons
3) Interneurons
SNS
1) somatic sensory (afferent) neurons
convey info from sensory receptors in head, body wall & limbs toward CNS
SNS
2) Somatic motor (efferent) neurons
conduct impulses from CNS to skeletal muscle under voluntary control in periphery
SNS
3) Interneurons
any neurons that conduct impulses between afferent and efferent neurons within the CNS
PNS → 2) ANS
consists of? (2)
1) Sensory Neurons
2) Motor Neurons
- sympathetic division
- parasympathetic division
*
- parasympathetic division
ANS → sensory neurons
convey info from :
autonomic sensory receptors mostly in visceral organs (stomach/lungs) → CNS
ANS → motor neurons
under involuntary control
conduct nerve impulses from CNS → smooth muscle, cardiac muscle & glands
ANS → motor neurons
- (2) branches
Sympathetic division
Parasympathetic division
Neurons
- gather info where?
- process info where?
- transmit info where?
1) gather info at dendrites
2) process info in dendritic tree & cell body
3) transmit info down axon to axon terminals
Synapse
site of communication between two neurons OR between a neuron & another effector cell
Synaptic Cleft
gap between pre & postsynaptic cells
Synaptic end bulbs & other varicosities on axon terminals of presynaptic neurons contain many?
tiny membrane-enclosed sacs (synaptic vesicles) that store packets of neurotransmitter chemicals.
→ many neurons even contain 2 or 3 types of neurotransmitters, each with different effects on postsynaptic cell
Like muscle fibers, neurons are electrically excitable
- communicate with each other using (2) types of electrical signals
1) Graded Potential
2) Action Potential
1) Graded Potential
used for short-distance communication only
only in dendrites & cell bodies
2) Action Potential
allow long-distance communication within body
at axon hillock
Producing electrical signals in neurons depends on ?
existance of resting membrane potential (RMP)
How is a cell’s RMP created?
created using ion gradients & variety of ion channels that open/close in response to specific stimuli
Prevalence of Ion Channels in Body
present in plasma membrane of all body cells but especially prominant part of NS
Steps of touching a pen → to CNS
touch → GP in sensory receptor of skin → triggers axon to form AP → along axon into CNS
→ causes release of neurotransmitters at synapse with interneuron → stimulates interneuron to form GP in dendrites & cell body (process repeats as interneurons in higher parts of brain [thalamus/cerebral cortex] are activated)
→ perception occurs when interneurons in cerebral cortex are activated
Steps of using a pen →
stimulus in brain → GP in dendrites/cell body of upper motor neuron → causes AP in axon → neurotransmitter release →GP in lower motor neuron → AP → neurotransmitter release at NMJ → stimulate muscle fibers → AP → contraction
When Ion channels are open, this allows..?
specific ions to move across plasma membrane down their electrochemical gradient
Electrochemical Gradient
concentration (chemical) difference plus an electrical difference
Chemical (concentration) part → ions move from areas of [high] to [low]
Electrical aspect → cations move toward negatively-charged area & anions move towards positively-charged area
Active channels open in response to?
stimulus (they are “gated”)
(3) types of active, gated channels
1) ligand-gated
2) voltage-gated
3) mechanically-gated
Ligand-gated Channels
respond to a neurotransmitter or hormone and are mainly concentrated at synapse
Voltage-gated Channels
respond to changes in transmembrane electrical potential & mainly located along neuronal axon
Mechanically-gated Channels
respond to mechanical deformation (applying pressure to receptor due to vibration, touch, stretch)
Leakage channels
gated but NOT active
- open & close randomly
RMP exists because of?
small buildup of negative ions in cytosol inside membrane & positive ions in ECF on surface
→ occurs very close to membrane, cytosol elsewhere is electrically neutral
Why is RMP slightly negative?
leakage channels favor gradient where more K+ leaks out than Na+ leaks in
(more K+ channels than Na+ channels)
- also large negatively charged proteins that always remain in cytosol
What would eventually destroy RMP if left unchecked?
small inward leakage of Na+
What offsets inward Na+ leak & outward K+ leak?
NA+/K+ ATPases (sodium-potassium pump)
- pumps out Na+ as fast as it leaks in
Polarized cell
A cell that exhibits an RMP
Cell is “primed”
Cell in polarized state (exhibiting an RMP) → ready to produce an AP
To produce an AP, what needs to happen first?
Graded potential must first be produced in order to depolarize cell to threshold
- A graded potential occurs whenever …?
ion flow in mechanically/ligand-gated channels produce current that is localized
- it spreads to adjacent regions for short distance & then dies out within few millimeters of its point of origin.*
(2) types of Graded Potentials
1) Depolarizing graded potential
2) Hyperpolarizing graded potential
1) Depolarizing graded potential
From RMP, stimulus that causes cell to be less negatively charged than ECF
2) Hyperpolarizing graded potential
stimulus that causes cell to be more negatively charged
Graded potentials have dif names depending on (2)
type of stimulus
where they occur
Graded Potentials occur mainly in? (2)
dendrites & cell body
do NOT travel down axon
(2) main phases of AP
1) depolarizing
2) repolarizing
AP steps
GP → depolarization of neuron from -70mV to threshold (-55mV)
→ Na+ channels open → Na+ inflow → inside becomes more positive
repolarization → Na+ channels inactivating, K+ channels **open → ** K+ outflow
after-hyperpolarizing phase → K+ channels still open, membrane potential becomes even more negative (-90mV)
As K+ channels close, MP returns to resting level (-70mV)
Absolute refractory period
period of time during which cell can’t generate another AP, no matter how strong stimulus is
coincides with period of Na+ channel activation & inactivation (inactivated Na+ channels must 1st return to resting state.)
This places an upper limit of 10–1000 nerve impulses/second, depending on neuron
Relative refractory period
period of time during which 2nd AP can be initiated, but only by larger-than-normal stimulus
- coincides with when voltage-gated K+ channels are still open after inactivated Na+ channels have returned to resting state.
Do graded potentials exhibit refractory periods?
NO
In contrast to action potentials, graded potentials do not exhibit refractory period
Propagation of AP down length of axon begins at?
trigger zone near axon hillock by passive spread
Propagation of AP down length of axon begins at trigger zone near axon hillock by passive spread, the current proceeds by (2)
1) continuous conduction
2) saltatory conduction
1) continuous conduction
slower, in unmyelinated axons
2) saltatory conduction
much faster process in myelinated axons
AP jumps from one node to next
In addition to nodes of Ranvier that allow saltatory conduction, speed of an AP is also affected by? (3)
1) axon diameter
2) amount of myelination
3) temperature
(2) factors that play role in determining perception of stimulus or extent of response
1) Frequency of AP
2) # of neurons recruited (activated)
(3) Fiber Types of Neuronal Axons
1) A fibers
2) B fibers
3) C fibers
1) A fibers
large, fast (12-130 m/sec) → (290 mph)
myelinated neurons
carry touch & pressure sensations
many motor neurons are this type
2) B fibers
medium size & speed (15 m/sec) → 34 mph
comprise myelinated visceral sensory
3) C fibers
smallest & slowest (2 m/sec)
comprise unmyelinated sensory & autonomic motor neurons
Signal transmission at the synapse is ..
one-way transfer from presynaptic neuron to postsynaptic neuron
When AP reaches end bulb of axon terminals…
voltage-gated Ca2+ channels open → Ca2+ flows inward from ECF → triggering release of neurotransmitter
→ crosses synaptic cleft & binds to ligand-gated receptors on post-s membrane
Conversion of signal from pre to post-synaptic neuron
presynaptic neuron converts electrical signal (nerve impulse) into chemical signal (released neurotransmitter)
postsynaptic neuron recieves this chemical signal → generates electrical signal (postsynaptic potential)
Synaptic Delay
time required for these processes at chemical synapse (conversion of electrical signal → chemical → electrical)
about 0.5 msec
Neurotransmitter effects can be modified by in many ways (4)
1) synthesis → stimulated/blocked
2) release → blocked/enhanced
3) removal → stimulated/blocked
4) receptor site → blocked/activated
Agonist
any chemical that enhances or stimulates the effects at a given receptor
Antagonist
any chemical that blocks or diminishes the effects at a given receptor
A neurotransmitter causes either an …(2)
excitatory or an inhibitory graded potential
Excitatory postsynaptic potential (EPSP)
- causes depolarization of postsynaptic cell, bringing it closer to threshold
- Although single EPSP normally does not initiate a nerve impulse, postsynaptic cell does become more excitable.*
Inhibitory postsynaptic potential (IPSP)
- hyperpolarizes postsynaptic cell taking it farther from threshold.
Spatial Summation
summation of postsynaptic potentials in response to stimuli that occur at different locations in the membrane of a postsynaptic cell at the same time
involves input from dif neurons stimulating post-s neuron in dif parts of membrane at same time
Temporal Summation
summation of postsynaptic potentials in response to stimuli that occur at same locations in the membrane of a postsynaptic cell but at different times
Whether or not postsynaptic cell reaching threshold depends on ?
net effect after summation of all postsynaptic potentials.
If neurotransmitter could linger in synaptic cleft, it would influence ?
postsynaptic neuron/muscle fiber/gland cell indefinitely
Therefore, removal of neurotransmitter is essential for normal function
Removal of neurotransmitters is accomplished by ? (3)
1) Diffusion
2) Enzymatic Degredation
3) Re-uptake by cells
1) Diffusion
Some of the released neurotransmitter molecules diffuse away from synaptic cleft.
Once a neurotransmitter molecule is out of reach of its receptors, it can no longer exert an effect.
2) Enzymatic Degredation
Certain neurotransmitters are inactivated through enzymatic degradation.
For example, the enzyme acetylcholinesterase breaks down acetylcholine in the synaptic cleft.
3) Re-uptake by cells
Many neurotransmitters are actively transported back into neuron that released them (reuptake).
Others are transported into neighboring neuroglia (uptake).
- The neurons that release norepinephrine, for example, rapidly take up norepinephrine & recycle it into new synaptic vesicles. *
- →membrane proteins that accomplish such uptake are called neurotransmitter transporters.*
Integration
process by post-synaptic neuron that combines all excitatory & inhibitory inputs & responds accordingly
→occurs over & over as interneurons are activated in higher parts of the brain (such as the thalamus & cerebral cortex).
Neuronal Circuit
many neurons organized into complex networks
Types of Circuits (4)
1) Diverging
2) Converging
3) Reverbrating
4) Parallel after-discharge
1) Diverging
small # of neurons in brain stimulate much larger # of neurons in spinal cord.
2) Converging
opposite of diverging circuit
large # of neurons stimulate smaller # of neurons in spinal cord
3) Reverbrating
- used in?
impulses sengt back through circuit time & time again
used in breathing, coordinated muscular activies, waking up & short-term memory
4) Parallel after-discharge
used in?
involve single presynaptic cell that stimulates group of neurons, which then synapse with common postsynaptic cell
– used in precise activities such as mathematical calculations
