Hw 13 A&P – Flashcards

initial segment Graded potentials created in the dendrites and soma will, if sufficiently depolarizing, generate an action potential in the initial segment of the axon. The action potential will then propagate away from this region, down the axon.

at every segment of the axon In unmyelinated axons, the action potential is regenerated continuously along every segment of the axon (continuous propagation). In humans, only small diameter axons (for example, type C fibers) are unmyelinated. These neurons carry low-priority information, such as smell (olfaction) and temperature sensations.

sodium (Na+) Sodium ions enter the cell during the beginning of an action potential. Not only does this (further) depolarize the membrane where those channels are located, but it also sets up local currents that depolarize nearby membrane segments. In the case of myelinated axons, these local currents depolarize the next node, 1-2 mm away.

The previous axonal segment is refractory. A propagating action potential always leaves a trail of refractory membrane in its wake. The trailing membrane takes some time to recover from the action potential it just experienced, largely because the membrane's voltage-gated sodium channels are inactivated. By the time this membrane segment is ready to (re)generate another action potential, the first propagating action potential is long gone.

1 meter per second While 1 m per second (2 mph) seems slow, most axons are short, and these speeds are fast enough for low-priority information such as smell (olfaction), temperature, and general touch sensations. Unmyelinated type C fibers have propagation speeds in this range.

have lower membrane resistance to ion movement In a myelinated axon, action potential regeneration occurs at the nodes where myelin is absent. Here, the ion channels associated with the action potential provide a low resistance pathway for ions to cross the axon membrane. In contrast, the myelin surrounding the internode regions makes it difficult for ions to cross the membrane. Therefore, membrane resistance at the internodes is higher than membrane resistance at the nodes. Conversely, membrane resistance at the nodes is lower than membrane resistance at the internodes.

at the nodes In myelinated axons, voltage-gated sodium channels are largely restricted to the nodes between myelinated internodes. Therefore, action potentials only regenerate at the nodes. The high membrane resistance of the internodes ensures that local currents generated at one node will quickly bring the next node to threshold, even though it is 1-2 mm away.

saltatory propagation Saltatory propagation is derived from the Latin word saltare, which means leaping.

Propagation is faster in myelinated axons. The internode segments of myelinated axons allow local currents to travel quickly between nodes where the action potential is regenerated. This leaping of action potentials from node to node is several times faster than the continuous propagation found in unmyelinated axons. Myelinated axons also tend to have larger diameters, which enhances propagation speed.

Without myelin, the internode membrane resistance decreases, preventing local currents from reaching adjacent nodes. Myelin increases the membrane resistance of the axon section it surrounds, allowing local currents to travel between nodes, even though they are 1-2 mm apart. Removing myelin decreases the membrane resistance of internode regions. This shortens the distance that local currents travel because more charge now exits at the internode regions before it reaches the next node.

Continuous conduction Yes! An action potential is conducted continuously along an unmyelinated axon from its initial segment to the axon terminals. The term continuous refers to the fact that the action potential is regenerated when voltage-gated Na+‎ channels open in every consecutive segment of the axon, not at nodes of Ranvier.

depolarizing currents established by the influx of Na+‎ flow down the axon and trigger an action potential at the next segment Yes! The Na+‎ diffusing into the axon during the first phase of the action potential creates a depolarizing current that brings the next segment, or node, of the axon to threshold.

The inactivation gates of voltage-gated Na+‎ channels close in the node, or segment, that has just fired an action potential. Yes! At the peak of the depolarization phase of the action potential, the inactivation gates close. Thus, the voltage-gated Na+‎ channels become absolutely refractory to another depolarizing stimulus.

The myelin sheath increases the speed of action potential conduction from the initial segment to the axon terminals. Yes! The myelin sheath increases the velocity of conduction by two mechanisms. First, myelin insulates the axon, reducing the loss of depolarizing current across the plasma membrane. Second, the myelin insulation allows the voltage across the membrane to change much faster. Because of these two mechanisms, regeneration only needs to happen at the widely spaced nodes of Ranvier, so the action potential appears to jump.

Inactivation gates of voltage-gated Na+‎ channels close, while activation gates of voltage-gated K+‎ channels open. Yes! Closing of voltage-gated channels is time dependent. Typically, the inactivation gates of voltage-gated Na+‎ channels close about a millisecond after the activation gates open. At the same time, the activation gates of voltage-gated K+‎ channels open.

Myelinated axons with the largest diameter Yes! The large diameter facilitates the flow of depolarizing current through the cytoplasm. The myelin sheath insulates the axons and prevents current from leaking across the plasma membrane.

hypoglossal

weakness of the sternocleidomastoid muscle

oculomotor

X

trigeminal

smell his food.

hearing and balance

visceral sensation and motor control

trigeminal

eye movement

olfaction

oculomotor

spastic, uncontrolled muscle contractions.

motor commands to skeletal muscles

decussation

primary motor cortex

ventral root

upper motor neuron