BMS 302 Exam #1

Pathway of Signals in the Nervous System

Stimulus sensed by Sensor → afferent pathway to Integrator —> integrates outputs from sensors into system —> efferent pathway to Effector —> response initiated

Brownian Motion

random motion of particles due to inherent vibration (factor of diffusion rate)

Saltatory conduction

type of conduction where action potential skips from node to node down the axon (myelinated axon)

Continuous conduction

action potential travels continuously down axon (unmyelinated axon)

Resting Membrane Potential

• measure of potential energy difference between inside/outside of cell

• inside is 70mV more negative than outside

• [K+] inside > [K+] outside

• [Na+] inside < [Na+] outside

Leakage Channels

selective based on size & charge, but ions can diffuse freely

Chemically (Ligand) Gated Channels

neurotransmitter attaches to membrane, causing gate to open

Voltage-gated Channels

Charges inside/outside of cell change, causing gate to open

Driving Force

• net movement of ions across the membrane

• dependent on electrical and concentration gradients

Graded Potential

• caused by sub-threshold stimulus

• localized

• chemical gates open (NOT voltage gates)

Action Potential

• threshold stimulus reached

• electrical change in membrane potential that is propagated along an excitable cell

• Na+ voltage-gated channels open

• depolarization occurs

Absolute Refractory Period

Na+ voltage-gated channels inactivated, another action potential CANNOT be generated

Relative Refractory Period

repolarization —> Na+ voltage-gated channels close and can be reopened by a stronger stimulus to generate another action potential

Motor Unit

A single motor neuron and all of the muscle fibers it innervates

Excitation-Contraction Coupling

1. Nerve Impulse/Electrical event

2. Voltage-gated Ca2+ channel opens and floods sarcoplasm

3. Vesicles with ACh fuse with plasma membrane and release ACh into synapse

4. ACh binds to receptor on muscle (typically Na+ channel)

5. Chemically-gated Na+ channels open

6. change in sarcolemma potential (membrane potential)

7. If threshold is reached, Na+ voltage-gated channels open and action potential is generated

Propagation into Myofibrils

• Muscle action potential propagated into T-tubules

• DHP receptor gets activated (modified voltage-sensitive Ca2+ channel on T-tubule)

• interacts with Ryanodine receptor on sarcoplasmic reticulum (SR)

• Ca2+ efflux from SR

Actin

• myofilament

• at rest, myosin binding sites are blocked by tropomyosin

• troponin complex —> Ca2+ binding site

Myosin

• myofilament

• heads bind to binding sites on actin

Role of ATP

1. Relaxation: Ca2+ ions re-sequestered back into SR via active transport; Release myosin heads from actin binding sites

2. Contraction: energize myosin head

Motor Unit Summation

strength/voltage of stimulus

Wave Summation

frequency of stimulus