Nerve, Muscle, and Synapse Flashcards
What are the different components of the CNS used in NMS?
- Cerebral Cortex
- Cerebellum
- Brainstem
- Spinal Cord
What are the different components of the PNS used in NMS?
Peripheral nerves
What are the different cells in the nervous system?
Neurons and glia
What are the three types of neurons?
- Afferent
- Efferent
- Interneurons
What is the function of afferent neurons?
To carry information from periphery to the spinal cord via the dorsal roots
What is the function of efferent nurons?
To carry information from the spinal cord to the periphery via the ventral roots
What is the function of interneurons?
To carry information between neurons
What is the difference between excitatory and inhibitory neurons?
- Excitatory currents prompt one neuron to share info with the next through action potential
- Inhibitory currents reduce the probability that such a transfer will take place
What are the functions of glia? (3)
- Provide structure/support isolating neurons from one another
- Produce myelin
- Guide migrating neurons and direct axonal outgrowth during development
What are the components of a relfex loop?
- Receptor
- Afferent Neuron
- Interneuron
- Efferent neuron
- Effector
What is the function of the receptor in the reflex loop?
Receives information and generates impulses
What is the function of the afferent neuron in the reflex loop?
Carry information from the receptor to interneurons in the spinal cord
What is the function of the interneuron in the reflex loop?
Process information and generate a response
What is the function of the efferent neuron in the reflex loop?
Carry information from the spinal cord to the efferent organ
What is the function of the effector in the reflex loop?
Receive information from efferent neuron and show the appropriate response
How does the afferent neuron enter the spinal cord?
Via the dorsal horn
How does the efferent neuron ‘leave’ the spinal cord?
Via the ventral horn
What types of cells is white matter composed of?
Nerve fibres, and glia
What types of cells is grey matter composed of?
Neurons, glia, and synapses
What are the structures within a neuron? (5)
- Dendrites
- Cell body
- Junction of axon hillock and initial segment of axon
- Axon
- Axon terminals and synaptic end bulbs
What is the function of the dendrites?
To receive and transmit electrical impulses from other cells towards the cell body
What is the function of the cell body?
Integrate incomaing signals and generate outgoing signal to axon
What is the function of the axon?
Pass messages away from the cell body to other neurons, muscles, and glands
What is the function of the terminal branches of axon?
Form junctions with other cells
What are the types of neurons and where are they found? (3)
- Bipolar cell (retina)
- Pseudo-unipolar cell (ganglion cell of dorsal root)
- Multipolar cells (motor neuron of spinal cord, pyramidal cell of hippocampus, and Purkinje cell of cerebellum)
What is the direction of flow in neurons?
Dendrites to cell body to axon hillock to axon to synaptic terminals
What is the function of the axon hillock?
Acts as an administrator, sums up the total signals received, both inhibitory and excitatory signals. If the sums exceed sthe limiting threshold, the action potential is triggered.
Which structures in the neuron have plasma membranes which include chemically gated channels?
Dendrites and cell body
Which structures in the neuron have plasma membranes which include coltage-gated Na+ and K+ channels?
Axon hillock and axon
Which structures in the neuron have plasma membranes which include voltage-gated Ca 2+ channels?
Axon terminals and synaptic end bulbs
How do dendrites receive stimuli?
Activation of chemically or mechanically gated ion channels
What do dendrites produce in afferent neurons?
Produce generator or receptor potentials
What do dendrites produce in efferent neurons?
Excitatory and inhibitory postsynaptic potentials
What structures surround the axon? (3)
- Schwann cell
- Myelin sheath
- Nodes of Ranvier
What is the function of Schwann cells?
Wrap around PNS axons to form a myelin sheath
What is the function of the myelin sheath?
Protective membrane which allows electrical impulses to transmit quickly and effeciently along the nerve cell
What is the function of Nodes of Ranvier?
Act as repeaters to regenerate the action potential as it propagates in a saltatory manner (leaping) along the axon to the nerve terminal
What are the components of the neuronal cell membrane?
- Phospholipid Bilayer
- Protein pumps and channels
What types of channels are in the neuronal cell membrane?
- Passive ion channels (leak channels)
- Ligand-gated ion channels
- Voltage-gated ion channels
What is Resting Membrane Potential (Em)?
Measure of electrical potential difference between intracellular environment and extracellular environment.
What value is the resting membrane potential?
-70mV
What ions are primarily involved in setting resting membrane potential?
K+ (Potassium) and Na+ (Sodium)
What pumps and channels are primarily involved in setting resting membrane potential?
Na+/K+ exchanged, Na+ leak channels, K+ leak channels
What sets the net charge of the neuronal cell membrane?
Na+/K+ pump
How does the Na+/K+ obtain energy to operate?
From the hydrolysis of ATP to ADP + P
What is the flow of the Na+/K+ pump?
3 Na+ molecules move out of the cell, and 2 K+ molecules move into the cell
What is the resulting charge inside the cell from the Na+/K+ pump?
Negative; it loses a positive charge at every cycle of the pump
The Na+/K+ pump creates _____
An electrical gradient across the cell membrane
What ion is the cell membrane more permeable to: K+ or Na+?
K+; far more than Na+ which results in the overall negative resting charge of the neuron
What proteins affect the resting potential of the membrane?
The leak channels; they are always open and allow for the passive flow of ions
What are the forces acting on each ion that moves through the leak channels?
- The chemical gradient
- The electrical gradient
What is the equillibrium potential for K+?
-90mV
What is the equillibrium potential for Na+?
+60mV
How is Em -70mV if K+ alone would make it -90mV and Na+ would make it +55mV?
The permeant the ion, the greater its ability to force Em towards its own equillibrium potential; permeability is 50-100x greater to K+ than Na+
How does the production of action potentials begin?
The cell is at threshold, -55mV, from RMP due to stimulus causing a production of graded potentials
What is an action potential?
An electrical signal generated due to the activity of voltage-gated Na+ and voltage-gated K+ channels.
What is an action potential in terms of voltage change?
Going from -70mV to +30mV and back to resting over a period of a few ms
How does stimulus to afferent neurons create action potentials?
Generator Potential:
Mechanical deformation leads to the generation of a generator potential in afferent neurons.
Action Potential Initiation:
If the generator potential reaches the threshold, it triggers the initiation of an action potential.
Transmission to CNS:
Action potential travels along the afferent neuron, conveying information about the stretch to the central nervous system.
What occurs in depolarization?
The cell is taken from -55mV to +30mV; the initial increase removes activation gates, allowing Na+ to flow in from voltage-gated channels
Why does the voltage change in depolarization?
The influx of Na+ into the cells bring the membrane closer to its equilibrium potential (+55mV)
How long does depolarization/open confirmation last?
Only a few ms; as it occurs, voltage-gated K+ channels open and repolarization occurs
Summarize this graph
Resting Phase: The membrane potential is at resting potential, typically around -70 mV.
Depolarization: Stimulus triggers the opening of voltage-gated sodium channels, allowing Na+ ions to rush into the cell. Membrane potential becomes more positive (depolarizes).
Threshold: The critical point at which depolarization is sufficient to open voltage-gated sodium channels, initiating an action potential.
Rising Phase: Rapid influx of sodium ions causes a sharp increase in membrane potential, reaching around +40 mV.
Repolarization: Voltage-gated potassium channels open, allowing K+ ions to exit the cell. Membrane potential returns toward resting level.
Hyperpolarization: The efflux of K+ ions may temporarily overshoot the resting potential, creating a hyperpolarized state.
Restoration to Resting State: Sodium-potassium pumps actively transport ions, restoring the original ion concentrations. The membrane potential returns to resting potential.
What is the reason for change in polarity during the depolarization stage?
Voltage-gated Na+ channels are open
What is the reason for change in polarity during the repolarization stage?
Voltage-gated K+ channels are open, while Na+ channels are inactivated
What is the reason for change in polarity during the hyperpolarizing stage?
Voltage-gated Na+ channels are at rest, while voltage-gated K+ channels are still open
Why is the respolarization stage essential?
It inactivates the calcium channels, which ensures the neurons does not continue to release neurotransmitters
Why do action potentials only travel in one direction?
The refractory period; the channels cannot open again after closing for a short period
What causes electrotonic conduction within the axon?
Spread of current within the axon; this is the changes in the charge we see across the membrane
How can the speed of action potential propagation be increased?
Myelination; it as an electrical insulator and adds electrical resistance. This ensures the ions stay within the cell and move faster, rather than the channels constantly opening and closing.
Why is the speed of action potential conduction important?
The rate limits the flow of information within the nervous system; if it is slower, less info can be shared in a given time
What forms myelination in the PNS?
Schwann cells
What forms myelination in the CNS?
Oligodendrocytes
Why is myelination discontinuous?
Nodes of Ranvier; these are what allows the current to jump between axons
How many axons does one oligodendrocyte ensheath?
Many
How many axons does one schwann cell ensheath?
One; however, it takes many Schwann cells to ensheath one axon
What is saltatory conduction?
Myelination speeds up conduction by enhancing electrotonic efficiency.
Action potentials (AP) aren’t regenerated along the entire axonal membrane. Instead, regeneration occurs at nodes of Ranvier, and electrotonic conduction takes place between nodes
What are the types of afferent fibres?
- Group 1
- Group 2
- Group 3
- Group 4
Which of the afferent fibre types is the fastest?
Group 1; it has the largest diameter from myelination
What type of sensory receptors use group 1 afferent fibres?
Skeletal muscle proprioreceptors
What type of sensory receptors use group 2 fibres?
Skin mechanoreceptors
What type of sensory receptors use group 3 fibres?
Pain/temperature receptors
What type of sensory receptors used group 4 fibres?
Pain/itch/temperature receptors
What is the slowest afferent fibre type and why?
Group 4; they are not myelinated
What is different in speed of AP conduction between myelinated axons and unmyelinated axobns?
- Myelinated: 12-130 m/sec
- Unmyelinated: 0.5-2 m/sec
How long is the absolute refactory period?
Almost 2 ms
By the time the absolute refractory period is over, how far along is the AP?
2 - 20cm down the axon (myelinated)
Where does the action potential travel to and how?
- Travels to synaptic terminals in CNS
- Via electrotonic or saltatory conduction
What is the functional significance of electrical synapses?
- Fast
- Bidirectional
- Communication via cytoplasm for sharing regulatory signals
What is the difference between a chemical and electrical synpase?
- In chemical, info is transferred via the release of a neurotransmitter from one cell that is detected by an adjacent cell
- In electrica, the cytoplasm of adjacent cells are directly connect by clusters of gap junctions
What are the steps in chemical synpases?
- Action potential reaches the axon terminal and depolarizes the membrane
- Voltage-gated Ca2+ channels open and flows in
- This Ca2+ influx triggers synaptic vesicles to release neurotransmitters by binding with the end of the synpase
- Neurotransmitters bind to receptors on target cell
What are excitatory postsynaptic potential (EPSP)?
A change in membrane potential that makes the target more likely to fire its own action potential
What are inhibitory postsynaptic potential (IPSP)?
A change in membrane potential that makes the target cell less likely to fire its own action potential
How do EPSPs work?
They depolarize the cell, which helps bring the potential to threshold
How do IPSPs work?
They keep the membrane potential below threshold to prevent the firing of an action potential. They also counteract the excitatory effect of EPSPs.
How do EPSPs and IPSPs interact?
A postsynaptic neuron integrates the inputs it receives and “decides” whether to fire an action potential
What is spatial summation?
The integration of postsynaptic potentials that occur in diferent location at the same time
What is temporal summation?
The integration of postsynaptic potentials that occur in the same place but at slightly different times
What is an example of spatial summation?
if an IPSP occurred together with the two EPSPs, fired at the same time, it might prevent the membrane potential from reaching threshold and keep the neuron from firing an action potential.
What is an example of temporal summation?
If a presynaptic neuron fires quickly twice in row, causing two EPSPs, the second EPSP may arrive before the first one has dissipated, bumping the membrane potential above threshold.
A synapse can only function effectively if?
If there is some way to “turn off” the signal once it’s been sent; this allows the cell to return to resting potential, ready for new signals
What are ways a synaptic cleft can be cleared of neurotransmitter? (4)
- Broken down by an enzyme
- Reuptake by presynaptic neuron
- Diffuse away
- “Mopped up” by nearby glial cells
What is a key difference between action potential and synaptic signaling?
- Action potential is all-or-nothing
- Synaptic signaling is more flexible; the amount of neurotransmitter released, number of receptors, and how readily a cell responds to activation of receptors can all be altered
What is synaptic plasticity?
The ability of synpases to strengthen or weaken over time, in response to increases or decreases in their activity
What does synaptic plasicity play a role in?
- Learning
- Memory
- Addiction
What are gap junctions?
A form of channel that allows current/ions to flow directly from one cell into another
Which synapse is faster: electrical or chemical?
Electrical; in synapses with both, the electrical response occurs earlier than the chemical response
What are the benefits of electrical synapses?
- They’re fast
- Allow for the synchronized activity of groups of cells
- They can carry current in both directions
What are the downsides of electrical synpases?
- Cannot turn an excitatory signal in one neuron into an inhibitory signal in another
- Lack versatility, flexibility, and capacity for signal modulation
What is a directly-gated chemical synapse?
- Neurotransmitter receptors are also ion channels
- When a neurotransmitter binds with the receptor, the channels open, and the ions flow through the membrane
- Causes rapid change in the membrane potential
What are indirectly gated chemical synapses?
- Neurotransmitter receptors are coupled to intracellular signaling pathways through G proteins
- When a neurotransmitter binds with these receptors, it activates G proteins, which initiate a cascade of intracellular events
- This cascade can lead to the opening or closing of ion channels
What ions passing through the receptor channel in a directly-gated synapse results in a EPSP?
K+ and Na+ or only Na+
What ions passing through the receptor channel in a directly-ated synapse results in a IPSP?
Cl- or only K+
Which type of chemical synapse has shorter lasting effects?
Directly-gated; in turn, they effects are faster in onset
What are the intracellular events that occur after an indirectly-gated synapse transmitter is binded?
- Activates the 2nd messenger system (GTP activates adenyly cyclase which converts ATP to cAMP)
- CAMP activates protein kinases which phosphorylate channels and cause it to open or close, causing change in membrane permeability
What are the steps in synaptic transmission? (8)
How can presynaptic neurons be excitatory?
- Glutamate binds to a receptor and opens ligand-gated Na+ chanels
- Na+ enters postsynaptic cells and result in small depolarization, EPSP.
Is one EPSP enough to hit threshold?
No; you need multiple EPSPs not counteracted by IPSPs to hit threshold
How can presynaptic neurons be inhibitory?
- Inhibitory transmitters (GABA, glycine) bind to receptors and open ligand-gated Cl- channels
- Cl- enters the postsynatpic cells and result in smal hyperpolarization, IPSP, which prevent generation of APs
Synaptic potentials ____ with distance
Decay
What does temporal summation look like?
What does spatial summation look like?
What does spatial summation of EPSP and IPSP look like?
For spatial summation to occur, what must happen?
PSPs from different regions must also overlap in time
Define integration
process of summing together all the inputs into a pattern of action potential output in the postsynaptic cell.
Neurons receive average inputs from how many other neurons?
10000-40000
A neuron sits at -70Mv and has a threshold of -50mV. It then simultaneously receives 10 IPSPs of 0.5mV each and 20 EPSPs of 1mV each.
Does the cell fire an AP?
No; the cell would be at -55mV which is below threshold
Where do PSPs occur in the neuron?
Mostly in dendrites and soma
Where do APs occur in the neuron?
Initiated at axon hillcock; transmit to synaptic terminal
What is the duration of PSPs?
msec to sec
What is the difference of conduction in PSPs and APs?
- PSP: passive over short distances
- AP: active with long distance transmission
What are the three types of muscle?
- Smooth
- Cardiac
- Skeletal
What is smooth muscle?
- Found in the walls of hollow organs
- Not generally under voluntary control
What is cardiac muscle?
- Striated muscle found in the walls of the heart
- Not under voluntary control
What is skeletal muscle?
- Striated muscle attached to the skeleton
- Under voluntary control
What is the role of motor neurons?
- Stimulate skeletal muscle cells to contract
- Functional unit of the motor system; it represents the smallest increment in force that can be generated
What is endomysium?
The innermost sheath surround individual muscle fibres; ensures each skeletal muscle cell is electrically insulated from each other
What are the differences between synaptic transmission at neuromuscular junction and a central synapse? (3)
- One AP in motoneuron generates one AP in muscle cell (summation is required in CNS)
- Each muscle fiber (cell) is only innervated by one presynaptic axon
- no inhibitory transmitters released at N-M junction
What molecules are involved in the sliding filament theory? (6)
- Myosin
- Actin
- Tropomyosin
- Troponin
- ATP
- Calcium ions
In skeletal muscle cells, what does myosin look like?
It is bunded together to form thick filaments
What is the shape and components of myosin molecules?
- Golf club with two heads
- The head (cross bridge) has the ability to move back and forth
- The flexing movement of the head provides the “power stroke” for muscle contraction
What is the significance of the myosin cross bridge?
How do they allow for contraction? How do they obtain energy?
- It has two important bind sites
- One binds ATP, where energy is transfered to the myosin as ATP is hydrolyzed
- The second site has a strong attraction for binding to actin
What is actin in the skeletal muscle cell?
- Major component of the thin filament
- Composed of two actin subunits twisted into a double helical chain
- Each has a specific bind sit for the mysoin crossbridge
What is tropomyosin in the skeletal muscle cells?
- Part of the thin filament
- Entwines around the actin
- In the unstimulated muscle, is covers the binding sites of the actin and prevent myosin cross bridge binding
How are tropomyosin molecules moved to allow the binding with myosin?
The presence of troponin, which is attached and spaced periodically along the tropomyosin strand
How does the troponin move?
- After an AP, calcium ions are relased from the terminal cisternae and bind to troponin
- This causes a conformational change, “dragging” the tropomyosin strands off the binding sites
What are the six steps of cross bridge cycling?
- The influx of calcium, triggering the exposure of binding sites on actin
- The binding of myosin to actin
- The power stroke of the cross bridge that causes the sliding of the thin filaments
- The binding of ATP to the cross bridge, which results in the corss bridge disconnecting from actin
- The hydrolysis of ATP, which leads to the re-energizing and repositioning of the cross bridge
- The transport of calcium ions back into the sacroplasmic reticulum
How are the binding sites on actin exposed?
- AP brings the release of calcium ions
- Calcium ions flood the cytosol, and bring to the troponin
- This causes a change in confirmation of the troponin-tropomyosin complex
- This exposes the binding sites of actin
How is the cross bridge re-energized and repositioned?
- The release of the myosin cross bridge from actin triggers the hydrolysis of the ATP into ADP and Pi
- Energy is transffered from ATP to the cross bridge, which brings it back to its high-energy conformation
In order for the cross bridge to disconnect from actin, what must happen?
An ATP molecule must bidn to its site on the myosin cross bridge
What is the power stroke?
When the cross bridge flexes, pullin the thin filament inward toward the center of the sacromere; the energy from ATP is transformed into mechanical energy for the contraction
How does the multiple cross bridge cycle work?
- There are four which cycle in a coordinated manner
- During a contraction, all cross bridges are neither bound nore disconnected at the same time
How do the calcium ions return to the sacroplasmic reticulum?
Through active transport via specialized ion pumps in the membrane; the pumps are energized by ATP
Who are the three roles of ATP in muscle cells?
- Energizing the power stroke of the myosin cross bridge
- Disconnected the cross bridge from the binding site on actin at the conclusion of a power stroke
- Pumping Ca2+ back into the sacroplasmic reticulum
What are the two types of muscle fibres?
- White
- Red
Features of white muscle fibres (5)
- Large in diameter
- Light in colour due to reduced myoglobin
- Surrounded by few capillaries
- Relatively few mitochondria
- High glycogen content
How do white muscle fibres produce ATP?
- Glycolysis
- Little myoglobin and few capillaires = less oxygen
- High glycogen = glucose available for glycolysis
How do red muscle fibres produce ATP?
- TCA cycle and oxidative phosphorylation (which require mitochondria and oxygen)
- Metabolize fatty acids which turn into Acetyl CoA
Features of red muscle fibres (5)
- Half the size of white muscle fibres
- Dark red due to large quantity of myoglobin
- Surrounded by many capillaries
- Numerous mitochondria
- Low glycogen content
What type of activities are white muscle fibres well suited for?
Activities that require power and speed for a short duration
What type of activities are red muscle fibres well suited for?
Activities that require endurance and continuous contraction
Why are white muscle fibers better for quick activity?
Glycolysis synthesizes ATP quickly which results in rapid cross bridge cycling
Why do white muscle fibres fatigue quickly?
Build-up of lactic acid and depletion of glycogen
