Ch. 4: The Nervous System Flashcards
defn: neurons
specialized cells capable of transmitting electrical impulses and then translating those electrical impulses into chemical signals
how is a neuron’s shape determiend?
matches its functions as dictated by the other cells with which that neuron interacts
defn + aka: cell body
aka: soma
contains the nucleus, the endoplasmic reticulum, and ribosomes
defn + func: dendrites
appendages emanating directly from the soma
receive incoming messages from other cells
the information received from the dendrites is transmitted through the cell body
defn + func: axon hillock
integrates the incoming signals from the cell body (sums up the signals from the dendrites)
important role in action potentials
defn: action potential
the transmission of electrical impulses down the axon to the synaptic bouton
all-or-nothing messages
ultimately cause the release of neurotransmitters into the synaptic cleft
defn: axon
a long appendage that terminates in close proximity to a target structure (a muscle, gland, or other neuron)
defn + func: myelin
a fatty membrane that insulates most mammalian nerve fibers to prevent signal loss or crossing and increases the speed of conduction in the axon
func: myelin sheath
maintains the electrical signal within one neuron
what two types of cells are myelin produced by + where are these cells located?
- oligodendrocytes (in the CNS)
- Schwann cells (in the peripheral NS)
defn + func: nodes of Ranvier
why are nodes of Ranvier critical for rapid signal conduction?
small breaks in the myelin sheath with exposed areas of axon membrane at certain intervals along the axon
critical for rapid signal conduction –> the insulation of myelin is so effective that the membrane is only permeable to ion movement at the nodes of Ranvier
defn + aka: nerve terminal
aka: synaptic bouton (knob)
enlarged and flattened structure at the end of the axon to maximize transmission of the signal to the next neuron and ensure proper release of neurotransmitters
defn: neurotransmitters
chemicals that transmit info between neurons
defn + func: synaptic cleft
a small space into which the terminal portion of the axon release neurotransmitters which bind to the dendrites of the adjacent neurons (the postsynaptic neuron)
NEURONS ARE NOT PHYSICALLY CONNECTED TO EACH OTHER
defn: synapse
the nerve terminal + synaptic clef + postsynaptic membrane
defn: nerve
multiple neurons may be bundled together to form a nerve in the peripheral nervous system
what are the three categories of nerves?
- sensory
- motor
- mixed (carry both sensory and motor info)
defn: ganglia
cell bodies of neurons of the same type are clustered together into ganglia
defn + func: tracts
axons may be bundled together to form tracts in the CNS
only carry one type of info
defn: nuclei
the cell bodies of neurons in the same tract are grouped into nuclei
defn + aka: glial cells
aka: neuroglia
other cells in the nervous system that support and myelinate neurons and play structural and supportive roles
what are the 5 types of nueroglia/glial cells?
- astrocytes
- ependymal cells
- microglia
- oligodendrocytes
- Schwann cells
func: astrocytes (glial cells)
nourish neurons and form the blood-brain barrier which controls the transmission of solutes from the bloodstream into nervous tissue
func: ependymal cells
line the ventricles of the brain and cerebrospinal fluid which physically supports the brain and serves as a shock absorber
func: microglia
phagocytic cells that ingest and break down waste products and pathogens in the CNS
func: oligodendrocytes and Schwann cells
produce myelin around axons
oligodendrocytes: CNS, Schwann cells: PNS
defn: resting membrane potential
the net electric potential difference that exists across the cell membrane, created by movement of charge molecules across that membrane
about - 70 mV with the inside neuron being negative relative to the outside
what is the overarching role of potassium and sodium with the resting potential?
the two most important ions involved in generating and maintaining the resting potential
defn + steps (3) + effect: potassium leak channels
transmembrane, allow the slow leak of potassium out of the cell to facilitate the outward movement of potassium
- as potassium continually leaks out of the cell, the cell loses a small amount of positive charge
- leaving behind a small amount of negative charge
- making the outside of the cell slightly positively charged
defn + steps (4): equilibrium potential of potassium
- 90 mV (negative: potassium is leaving the cell)
- as negative charge build up inside the cell, some K will be drawn back into the cell due to attraction between the positive K ions and the negative potential building inside the cell
- as the potential difference continues to grow, potassium will be more strongly draw back into the cell
- at a certain potential, each K cation that is pushed out due to the concentration gradient will be matched by another K cation pulled back in due to the electrical potential
- now there is no net movement of the ion as the cell is in equilibrium with respect to potassium
defn + func: sodium leak channels
facilitates movement of sodium INTO the cell
defn + steps: equilibrium potential of sodium
60 mV (positive: sodium is moving into the cell)
the slow leak of sodium into the cell causes a buildup of electric potential
why is the resting potential a tug of war?
potassium’s movement pulls the cell potential toward - 90 mV
sodium’s movement pulls the cell potential toward 60 mV
but neither ion ever wins, instead a balance of -70 mV is reached
= the resting membrane potential = the net effect of sodium and potassium’s equilibrium potentials
why is the resting membrane potential closer to potassium’s equilibrium potential than sodium’s?
because the cell is much more permeable to potassium
func: Na+/K+ ATPase
continually pumps sodium and potassium back to where they started (potassium into the cell, sodium out of the cell) to maintain their respective gradients and restore the resting potential
mnemonic: direction of ion movement by Na+/K+ ATPase
pumpKin (K in) –> Na out
can neurons receive excitatory or inhibitory input?
both!
defn + cause + effect: depolarization
defn: raising the membrane potential from its resting potential
cause: excitatory input
effect: makes the neuron more likely to fire an action potential
defn + cause + effect: hyperpolarization
defn: lowering the membrane potential from its resting potential
cause: inhibitory input
effect: makes the neuron less likely to fire an action potential
when is an action potential triggered?
if the axon hillock receives enough excitatory input to be depolarized to the threshold value
value: threshold value
- 55 mV to - 40 mV
does every stimulus generate a response?
no! for example, a small excitatory signal may not be sufficient to bring the axon hillock to threshold
defn: summation
the additive effect of multiple singals
defn + ex: temporal vs. spatial summation
TEMPORAL SUMMATION = multiple signals are integrated during a relatively short period of time (i.e. many small excitatory signals)
SPATIAL SUMMATION = the additive effects are based on the number and location of the incoming signals (i.e. a large number of inhibitory signals firing directly on the soma will cause more profound hyperpolarization of the axon hillock than the depolarization caused by a few excitatory signals firing on the dendrites of a neuron)
what are the steps of an action potential? (3)
- Sufficient depolarization across the cell membrane to threshold leads to the generation of an action potential
- followed by repolarization
- and hyperpolarization before returning to the resting membrane potential
what happens if the cell is brought to threshold? (2)
- voltage-gated sodium channels open in the membrane
- these channels permit passage of sodium ions
- as sodium passes through these ion channels the cell rapidly depolarizes (the membrane potential becomes more positive)
defn + func: electrochemical gradient
promotes the migration of sodium into the cell
the interior of cell is more negative than the exterior of the cell which favors the movement of positively charged sodium cations into the cell
there is a higher concentration of sodium outside the cell than inside which also favors the movement of sodium into the cell
are sodium channels opened or inactivated in response to changes in resting potential?
both!
defn: inactivated vs. deinactivated sodium channels
INACTIVATED: When Vm approaches 35 mV
DEINACTIVATED: Vm has to be brought back near resting potential
defn: closed vs. open vs. inactive sodium channel states
CLOSED = before the cell reaches threshold and after inactivation has been reversed
OPEN = from threshold to approximately 35 mV
INACTIVE = from approx. 35 mV to the resting potential
the positive potential inside the cell triggers the sodium channels to inactivate, what happens to the potassium channels at the same time?
they are triggered to open!
when does an electrochemical gradient that favors the efflux of potassium arise?
once sodium as depolarized the cell
defn: repolarization
as positively charged potassium cations are driven out of the cell, there will be a restoration of the negative membrane potential (repolarization)
how does hyperpolarization happen and what is the important function of hyperpolarization?
HOW: the efflux of K+ causes an overshoot of the resting membrane potential
FUNCTION: it makes the neuron refractory to further action potentials
defn: absolute vs. relative refractory period
ABSOLUTE = no amount of stimulation can cause another action potential to occur
RELATIVE = there must be greater than normal stimulation to cause an action potential because the membrane is starting from a potential that is more negative than its resting value
defn: impulse propagation
for a signal to be conveyed to another neuron, the action potential must travel down the axon and initiate neurotransmitter release
what are the steps of impulse propagation? (4)
- as sodium rushes into one segment of the axon, it will cause depolarization in the surrounding regions of the axon
- this will bring subsequent segments of the axon to threshold, opening the sodium channels in those segments
- each of these segments then continues through the rest of the action potential in a wave-like fashion until the action potential reaches the nerve terminal
- after the action potential has fired in one segment of the axon, that segment becomes momentarily refractory
what is the functional consequence of momentarily refractory segments of the axon?
that information can only flow in one direction
what 2 factors impact the speed of action potentials?
- the length of the axon
- the cross-sectional area of the axon
what is the impact of increased axon length?
- higher resistance
- slower conduction
what is the impact of greater axonal cross-sectional area?
- faster propagation due to
- decreased resistance
is the effect of the axon’s cross-sectional area or length more impactful?
cross-sectional area
why does myelin maximize the speed of transmission?
it is a fabulous insulator which prevents the dissipation of the electric signal
defn: saltatory conduction
the signal “hops” from node to node (of Ranvier)
does increased intensity of a stimulus increase the potential difference of an action potential or the frequency of firing?
the frequency of firing
NOT the potential difference
defn: presynaptic neuron vs. postsynaptic neuron
PRESYNAPTIC: the neuron preceding the synaptic cleft
POSTSYNAPTIC: the neuron after the synaptic cleft
defn: effector
if a neuron signals to a gland or muscle, rather than another neuron, the postsynaptic cell is called an effector
where are neurotransmitters stored prior to release?
in membrane-bound vesicles in the nerve terminal
what happens when the action potential reaches the nerve terminal/how do neurotransmitters move (steps)? (5)
- voltage-gated calcium channels open (allowing calcium to flow into the cell)
- this sudden increase in intracellular calcium triggers fusion of the membrane-bound vesicles with the cell membrane at the synapse
- this causes exocytosis of the neurotransmitter
- once released into the synapse, the neurotransmitter molecules diffuse across the cleft and bind to receptors on the postsynaptic membrane
- this allows the message to be passed from one neuron to the next
what happens if the neurotransmitter receptor is a ligand-gated ion channel?
the postsynaptic cell will either be depolarized or hyperpolarized
what happens if the neurotransmitter receptor is a G protein-coupled receptor?
it will cause either changes in the levels of cyclic AMP or an influx of calcium
what are the three main mechanisms of neurotransmission regulation (removing the neurotransmitter from the synaptic cleft)? provide typical examples of each.
- neurotransmitters can be broken down by enzymatic reactions (the breakdown of acetylcholine (ACh) by acetylcholinesterase)
- neurotransmitters can be brought back into the presynaptic neuron using reuptake carriers (used by serotonin (5-HT), dopamine (DA), norepinephrine (NE))
- neurotransmitters may diffuse out of the synaptic cleft (nitric oxide (NO))
defn: supraspinal circuits
scenarios that require input from the brain or brainstem (not just the spinal cord)
defn: white vs. grey matter
which is deeper in the brain?
which is deeper in the spinal cord?
WHITE MATTER = consists of axons encased in myelin sheaths
GREY MATTER = consists of unmyelinated cell bodies and dendrites
the white matter lies deeper than the grey matter in the brain
in the cord: white matter lies on the outside, grey matter is deep within
what are the four regions of the spinal cord?
- cervical
- thoracic
- lumbar
- sacral
defn + func: vertebral column
protects the spinal cord
transmits nerves at the space between adjacent vertebrae
defn: dorsal root ganglia
the cell bodies of sensory neurons that bring information in from the periphery and enter on the dorsal side of the spinal cord
on what side of the spinal cord do motor neurons exit?
ventrally (the side closest to the front of the body)
what is one primary difference between the somatic and autonomic nervous systems?
the peripheral component of the autonomic nervous system contains two neurons
a motor neuron in the somatic nervous system goes directly from the spinal cord to the muscle without synapsing
what are the two neurons in the ANS that work in series to transmit messages from the spinal cord? how do these work (steps)? (3)
- preganglionic neuron
- postganglionic neuron
- preganglionic neuron soma is in the CNS
- its axon travels to a ganglion in the PNS
- it synapses on the cell body of the postganglionic neuron which then stimulates the target tissue
what is the role of the vagus nerve in the PNS? + aka for vagus nerve
aka: cranial nerve X
responsible for much of the parasympathetic innervation of the thoracic and abdominal cavity
defn + example + steps (2): monosynaptic reflex arc
there is a single synapse between the sensory neuron that receives the stimulus and the motor neuron that responds to it
example: knee-jerk reflex
1. when the patellar tendon is stretched, info travels up the sensory (afferent, presynaptic) neuron to the spinal cord where it
2. interfaces with the motor (efferent, postsynaptic) neuron that causes contraction of the quad
reflex serves to protect the muscles
defn + example + steps (4): polysynaptic reflex arc
there is at least one interneuron between the sensory and motor neurons
example: withdrawal reflex (stepping on a nail)
1. the extremity you step on the nail with will be stimulated to flex which is monosynaptic BUT
2. if the person is to maintain balance, the other food must be planted firmly on the ground
3. so the motor neuron that controls the quad in the opposite limb must be stimulated to extend it
4. interneurons in the spinal cord provide connections from the incoming sensory info the motor neurons of the supporting limb
