Lecture Quiz 6 Flashcards
What does the nervous system provide for the body?
coordination and communication
What three things does the nervous system include?
sensory input
integration
motor output
What is the central nervous system defined as?
center of integration and communication
consists of the brain and spinal cord
What is the peripheral nervous system defined as?
serves as communication lines between the CNS and the rest of the body
What does afferent PNS do?
carries impulses from receptors to CNS
contains somatic and visceral branches
What does efferent PNS do?
carries impulses from CNS to effector organs
What are the two branches of efferent PNS?
somatic/voluntary branch - to skeletal muscles
autonomic/involuntary - to internal organs: smooth and cardiac muscles and glands
What are the two branches of autonomic PNS?
sympathetic - nervous system prepares the body for the emergency situations
parasympathetic - nervous system regulates all non-emergency bodily functions (rest and digest) and energy storage
What is contained in the cell body of a neuron?
contains nucleus and organelles
Describe dendrites
short multiple cylindrical processes that carry impulses towards the body
Describe axons
a long process that carries impulses away from the body of the neuron
arises from the body of a neuron at the axon hillock, which is where action potential is generated
end of axon widens forming synaptic knob, which is where neurotransmitter is stored in vesicles
Describe unipolar neurons
have only one process extending from the body and divides into two axons - peripheral and central
Where are unipolar neurons found?
these are sensory neurons found in dorsal root ganglia and cranial nerve ganglia
Describe bipolar neurons?
Have a long dendrite and axon and a centrally located body
Where are bipolar neurons found?
rare
only found in special senses
eye, olfactory
Describe multipolar neurons
have multiple short dendrites and a single long axon
most common type of neuron in CNS
What is the functional classification of neurons?
sensory - afferent neurons
motor - efferent neurons
interneurons - associated neurons
What happens for every 1 neuron traveling toward the CNS?
10 neurons traveling away from CNS
200,000 traveling within the CNS
What is a reflex and what does it involve?
the hard-wired, unconscious rapid response to external stimulus
involves spinal nerves and effector cell electrical impulses
When does a reflex occur?
when an immediate response is required
What are the components of a reflex arc?
receptor sensory neuron integration center motor neuron effector organ
Briefly describe an electric current
a flow of electrons
flows from region of negative charge to a region of positive charge
What is it called when two regions possess a charge difference relative to each other?
they have a potential difference
units = volts
What is resting potential
the potential difference across the cell membrane at rest
the inner surface of the membrane is negatively charged
outer surface is positively charged
this is actively maintained by the cell
Describe how resting potential is maintained by the cell
ion concentration inside and outside the cell is not the same
cells are relatively low on Na and high on K and anions
Na and K diffuse across the membrane in hopes to equalize their concentrations - anions are too big to diffuse
ATP dependent Na/K pump returns diffused ions back, thus maintaining their gradient
loss of K>gain of Na - inner surface is negatively charged and outer surface is positively charged
potential difference is generated
K ions are responsible for the resting potential
What is it called when a system contains two regions which possess a potential difference?
System is said to be polarized
What is resting membrane potential?
-70mVolts
polarized
What happens if the membrane potential decreases and the inner surface becomes less negative?
this is called depolarization
ex: Na+ enters the cell
What happens if membrane potential increases?
the inner surface becomes even more negative
hyperpolarized
ex: K+ leaves the cell
What is action potential?
Nerve impulse
a brief, transient reversal of membrane polarity from -70mV to +30mV that propagates along the cell membrane
this is the principle way neurons communicate
What cells are able to generate action potential?
neurons and muscle cells
Where is action potential generated?
axon hillock
What does change in permeability of a membrane cause and why does this occur?
the concentration of ions is changed, therefore membrane polarity is changed
this occurs due to voltage-gated channels
Describe m-gates
activation gates
closed at membrane potential less than -55mV
open at membrane potential more than -55mV
Describe h-gates
inactivation gates
opened if membrane potential is less than +30mV
closed if membrane is above 30mV
upon closure, remain insensitive to voltage changes for 2-5 msec
Describe sodium voltage-gated channels briefly
Has two gates - m gates (activation) and h gates (inactivation)
Describe voltage-gated sodium channels at rest
-70mV h-gates are open m-gates are closed Na+ is impermeable to sodium ions closed
Describe voltage-gated sodium channels when the membrane depolarizes to -55mV
m-gates open
h-gate remain open
Na+ channel is active or open
allows Na influx
Describe voltage-gated sodium channels when the membrane depolarizes to +30mV
h-gates close and remain closed regardless of the voltage across the membrane
Na+ channel becomes inactive even though m-gates are still open
when membrane hits -55mV, m-gates close
h-gate remains closed for 2-5 msec
membrane potential returns to its resting state at -70mV
Describe K+ voltage-gated channels
closed at rest (-70mV)
slowly open as depolarization progresses
Describe the resting state of action potential
voltage-gated Na+ channels closed
voltage-gated K+ channels closed
resting potential is maintained by K+ leakage through the non-voltage-gated channels and Na+/K+ pump
Describe the generator or graded potential of action potential
initial event (graded potential) causes change in membrane potential to a threshold of -55mV Na+ voltage-gated channels open
Describe the depolarization stage of action potential
Na+ ions rush inward down the concentration and electrical gradient
Na+ ions accumulate on the inner surface of the membrane
membrane reverses polarity - positive on inside and negative on outside
Inflow of sodium gradually decreases
membrane reaches 0mV potential, electrical gradient disappears and concentration gradient decreases
membrane potential reaches +30mV and Na+ inactivation gates close
K+ channels have been slowly opening
Describe the repolarization phase of action potential
K+ channels are completely opened when membrane is depolarized
K+ ions rapidly exit the cell down the concentration and electrical gradient
When the positive charge dissipates, the membrane is said to be repolarized because it reverts to its original state of negative on the inside, positive on the outside
Describe the hyperpolarization phase of action potential
K+ channels are slow to react to the changes in membrane potential
when membrane potential is restored, K+ channels don’t close right away
this allows excessive amounts of K+ to leave the cell
Membrane potential is less than -70mV
What happens during the resting state of action potential?
Na+/K+ pumps bring ion concentrations back to normal
resting potential is restored
What is the all-or-none phenomenon in action potential?
if neuron stimulation does not cause the rise of membrane potential to -55mV, Na+ channels do not open and action potential is not generated
this kind of stimulus is called a subthreshold stimulus
threshold stimulus causes action potential even if it barely reaches the threshold limit
What is the relationship between the strength of the initial impulse and action potential?
The strength of the initial impulse beyond the threshold does not affect the strength or duration of action potential
What is the coding for stimulus intensity?
the rate of action potentials generated per unit of time (frequency) is proportional to the strength of the original stimulus
weak stimulus = few APs
strong stimulus = many APs
What is the refractory period of action potentials?
time during which cell is not sensitive to a new stimulus
What happens during the absolute refractory period of action potentials?
no stimulus of any strength can generate a new action potential
When does absolute refractory occur?
during depolarization (Na+ channels are opened and may not open any wider) during repolarization (Na+ channels are completely closed for 2-5msec due to h-gates regardless of stimulus)
What happens during a relative refractory period and when does this occur?
Only a strong stimulus can cause new depolarization
this happens during hyperpolarization
h-gates of Na+ channels become sensitive to voltage channels, but a bigger change in voltage is needed to reach the threshold of -55mV
Describe polarity changes between membrane segments along the axon
during polarization, only a segment of cell membrane reverses polarity
neighboring segment maintains polarity
What happens with current along the membrane of an axon?
There are positive and negative areas on surface of membrane
therefore, current flows between them
this causes new sodium channels to open
action potential is said to be self propagating
What direction does current flow along the axon?
current flow only occurs along the membrane of an axon in the forward direction, away from the body
flow cannot occur in retrograde due to the refractory period
What happens with the strength of action potential?
It is non-decremental
action potential does not decrease in strength regardless of distance traveled
Describe the myelin sheath
Schwann cells in PNS
oligodendrocytes in CNS
these cells wrap themselves around axons in concentric layers forming the myelin sheath
Adjacent Schwann cells do not touch each other
Where do you find myelin sheath on a cell?
Only along axons
never along dendrites
What is between the Schwann cells and what occurs there?
These gaps are called nodes of Ranvier
axons are bare here
ion channels are only functioning here
How does the myelin sheath affect action potential?
acts as an insulator
AP is only propagated at nodes of Ranvier
this significantly speeds up the rate of conduction
in unmyelinated axons, AP must be generated over and over along the entire length of the membrane
What is the speed of conduction dependent on in action potentials?
depends on the size of the neuron and the degree of myelination
Describe A fibers
large, heavily myelinated
conduct impulse up to 150 m/s
motor neurons are A fibers
Describe B fibers
smaller but myelinated
conduct at 10 m/s
autonomic neurons are B fibers
Describe C fibers
small, non myelinated
conduct at .5 m/s
pain neurons are type C fibers
What are the two ways to initiate an action potential?
spontaneously - caused by ionic channels in some muscle and nerve cells
due to a stimulus - neurons change their membrane potential in response to a stimulus from the environment
What is synaptic stimulation?
stimulus that causes AP
postsynaptic potentials are caused by conformational changes of the receptor protein when neurotransmitter stimulates it
What is receptor stimulation?
generator potentials result from receptor stimulation by light, heat, stretch, etc
What are receptors?
sensory nerve fibers or independent cells that are specialized to respond to a stimulus from the environment
respond only to a particular stimulus
What are exteroceptors?
sensitive to outside stimuli
includes receptors for general senses (throughout the body) and receptors for special senses (vision, hearing, equilibrium, taste, smell)
What are interoceptors?
aka visceroceptors
sensitive to stimuli within the body - barorecptors, hunger, thirst, osmoreceptors)
What are proprioceptors?
sensitive to stimuli from stretch of muscle, joint capsules, and ligaments
How do receptors react to a particular type of stimulus (broadly and typically)?
they increase the permeability of Na+ channels causing depolarization
sodium inflow is responsible for generator potential
GPs are usually depolarizing
How do receptors react to a particular type of stimulus (rarely but possible)?
K+ or Cl- channels may be activated
this would result in hyperpolarization
What type of channels are Na+ channels at receptors?
they are NOT voltage-gated, therefore no h- or m-gates
no threshold, no refractory period
Describe the strength of generator potentials
vary in strength
the stronger the stimulus, the stronger and longer the depolarization
not every GP results in AP
threshold and refractory periods are not observed in GP
Decremental - decrease ins trength along the membrane of a dendrite
What happens if a generator potential reaches the axon?
If membrane is depolarized to -55mV, an AP results
if GP deteriorates or reaches subthreshold depolarization, no AP is generated
What are tonic receptors?
generate impulses at a constant rate unless inhibited
increases or decreases rate of GPs based on amount of stimulus
show little adaptation
What are phasic receptors?
normally “off” and can be activated by a stimulus
adapt quickly
ex - pressure, touch, smell
What is sensory adaptation?
a decrease in the magnitude of grading potential
arises from the depletion of the concentration gradient at the receptor
How is information from sensory receptors to the brain carried?
as action potentials
all identical in strength and duration
How are receptors categorized into modality?
Ask - What type of sensation?
each receptor is sensitive to one type of stimulus
related stimuli to the area in the brain designated to limited area of cerebral cortex called “primary sensory area”
all action potential received from these neurons are interpreted as sensation associated with modality
How are receptors categorized into location?
Ask - where in the body does the stimulus originate?
each sensory cell has a receptive field or the area from which it receives stimuli
since the brain “knows” the receptive field of each fiber reaching it, it knows the origin of the stimulus
How are receptors categorized into intensity?
Ask - how much stimulus is present?
intensity of graded potentials varies with the strength of the stimulus
intensity of APs always the same
rate of APs relates to the strength of the stimulus
Give a brief overview of generator potential
has no threshold
no refractory period
decremental
varies in amplitude and duration
Give a brief overview of action potential
has a threshold (+30mV)
refractory period
non-decremental
the same in amplitude and duration
