Question

Conduction velocity is increased when the time constant is decreased. How is this accomplished?

1) By myelination of axons
2) By demyelination in MS
3) By increasing the diameter of axons
4) By decreasing the diameter of axons

Answer 1

The speed of conduction in axons is influenced by myelination, axon diameter, and the presence of nodes of Ranvier. Myelin acts as an insulator and wider axons have less resistance to ion diffusion, both of which contribute to faster conduction. Demyelination, such as in multiple sclerosis, slows down conduction due to current leakage.

Step-by-step explanation:

The speed of conduction in axons is influenced by the myelination of axons, the diameter of the axons, and the presence of nodes of Ranvier. Myelin acts as an insulator, preventing current from leaving the axon, and thus increasing the speed of action potential conduction. Additionally, wider axons have less resistance to ion diffusion, allowing for faster propagation of the action potential. In contrast, demyelination, as seen in multiple sclerosis, slows down conduction due to current leakage from previously insulated areas of the axon.

Conduction velocity refers to the speed at which nerve impulses or action potentials travel along a nerve fiber. It is a critical parameter in understanding the function of the nervous system and is often used to assess the integrity of nerve pathways. The conduction velocity of nerve impulses can vary depending on several factors:

Myelination: Nerve fibers can be myelinated or unmyelinated. Myelin is a fatty substance that wraps around the nerve fiber, insulating it and allowing for faster conduction of electrical impulses. Myelinated fibers generally have higher conduction velocities than unmyelinated fibers.

Axon Diameter: Larger-diameter nerve fibers tend to conduct impulses more quickly than smaller-diameter fibers. This is because larger fibers offer less resistance to the flow of electrical current.

Temperature: Conduction velocity is influenced by temperature. In general, nerve impulses travel faster at higher temperatures and slower at lower temperatures. This is due to the effect temperature has on the rate of ion movement across cell membranes.

Pathological Conditions: Certain diseases or conditions, such as demyelinating diseases (e.g., multiple sclerosis) or nerve damage, can affect conduction velocity. For example, in demyelinating diseases, the loss of myelin can significantly slow down the conduction of nerve impulses.

The measurement of conduction velocity is often used in clinical neurophysiology to assess the health of nerves. It is commonly measured by recording the time it takes for an action potential to travel between two points along a nerve. This can be done using techniques such as nerve conduction studies (NCS) or electromyography (EMG).

Conduction velocity is expressed in units of distance per unit time, such as meters per second (m/s) or millimeters per millisecond (mm/ms). Different types of nerve fibers, such as sensory and motor fibers, can have different conduction velocities. For example, motor neurons that control muscle movements generally have higher conduction velocities than sensory neurons.

Answer 2

Conduction velocity can be increased by myelination of axons and by increasing their diameter, which leads to faster signal propagation by reducing the time constant. Demyelination and decreasing diameter would have the opposite effect.(option 1)

Step-by-step explanation:

To increase conduction velocity, the time constant must be decreased. This can be achieved in several ways:

1. By myelination of axons - Myelination provides an insulating layer around axons, leading to saltatory conduction where action potentials 'jump' from one node of Ranvier to the next. This significantly increases the speed of conduction velocity.
2. By increasing the diameter of axons - A larger diameter reduces internal resistance, allowing depolarization to spread more quickly, thus increasing conduction velocity.

However, options 2) By demyelination in MS and 4) By decreasing the diameter of axons would result in a slower conduction velocity, not faster. Demyelination causes a loss of the insulating myelin sheath, which leads to current leak and slower signal propagation. Similarly, a smaller diameter increases resistance and slows down the transmission of the action potential.

Hence, the answer is option 1