Chapter 12 - Section 12.3 - Study Guide - Assess Your Learning Outcomes - Page 471: 3

The diameter of a nerve fiber and the presence or absence of a myelin sheath significantly influence the conduction speed of nerve impulses (action potentials) along the nerve fiber. These factors are crucial for determining the efficiency and speed of electrical signal transmission in the nervous system. Here's how they affect conduction speed: 1. **Fiber Diameter:** - **Larger Diameter:** Nerve fibers with larger diameters conduct nerve impulses more rapidly than those with smaller diameters. This is because a larger diameter provides more space for ions to move and decreases the resistance to the flow of ions along the axon. As a result, larger-diameter fibers allow for faster conduction of action potentials. - **Smaller Diameter:** Conversely, nerve fibers with smaller diameters have higher resistance to ion flow due to the limited space, leading to slower conduction of action potentials. 2. **Presence or Absence of Myelin:** - **Myelinated Nerve Fibers:** Myelin acts as an insulating layer around nerve fibers and enhances the speed of action potential conduction. In myelinated nerve fibers, the myelin sheath covers portions of the axon with small gaps called Nodes of Ranvier. Action potentials "jump" from one node to another, which is known as saltatory conduction. This type of conduction significantly accelerates the propagation of nerve impulses. - **Unmyelinated Nerve Fibers:** Nerve fibers without myelin sheaths conduct action potentials more slowly than myelinated fibers. In unmyelinated fibers, the action potential travels continuously along the entire length of the axon membrane, which requires more time and energy for ion flow. In summary, the conduction speed of a nerve fiber is determined by its diameter and the presence or absence of myelin: - Larger-diameter fibers conduct action potentials more rapidly. - Myelinated fibers conduct action potentials faster, primarily due to saltatory conduction. - Smaller-diameter fibers and unmyelinated fibers conduct action potentials more slowly. The combination of fiber diameter and myelination is critical for the efficient transmission of information in the nervous system. Motor neurons, for example, often have larger diameters and myelin sheaths to enable rapid muscle contractions, while sensory neurons vary in diameter and myelination depending on the specific sensory modality and function they serve.

The diameter of a nerve fiber and the presence or absence of a myelin sheath significantly influence the conduction speed of nerve impulses (action potentials) along the nerve fiber. These factors are crucial for determining the efficiency and speed of electrical signal transmission in the nervous system. Here's how they affect conduction speed: 1. **Fiber Diameter:** - **Larger Diameter:** Nerve fibers with larger diameters conduct nerve impulses more rapidly than those with smaller diameters. This is because a larger diameter provides more space for ions to move and decreases the resistance to the flow of ions along the axon. As a result, larger-diameter fibers allow for faster conduction of action potentials. - **Smaller Diameter:** Conversely, nerve fibers with smaller diameters have higher resistance to ion flow due to the limited space, leading to slower conduction of action potentials. 2. **Presence or Absence of Myelin:** - **Myelinated Nerve Fibers:** Myelin acts as an insulating layer around nerve fibers and enhances the speed of action potential conduction. In myelinated nerve fibers, the myelin sheath covers portions of the axon with small gaps called Nodes of Ranvier. Action potentials "jump" from one node to another, which is known as saltatory conduction. This type of conduction significantly accelerates the propagation of nerve impulses. - **Unmyelinated Nerve Fibers:** Nerve fibers without myelin sheaths conduct action potentials more slowly than myelinated fibers. In unmyelinated fibers, the action potential travels continuously along the entire length of the axon membrane, which requires more time and energy for ion flow. In summary, the conduction speed of a nerve fiber is determined by its diameter and the presence or absence of myelin: - Larger-diameter fibers conduct action potentials more rapidly. - Myelinated fibers conduct action potentials faster, primarily due to saltatory conduction. - Smaller-diameter fibers and unmyelinated fibers conduct action potentials more slowly. The combination of fiber diameter and myelination is critical for the efficient transmission of information in the nervous system. Motor neurons, for example, often have larger diameters and myelin sheaths to enable rapid muscle contractions, while sensory neurons vary in diameter and myelination depending on the specific sensory modality and function they serve.