lect 12 pt 5

What do Schwann cells form in the PNS?

Schwann cells form myelin sheaths around axons of PNS neurons.

How many Schwann cells can contribute to one neuron's axon?

Many Schwann cells contribute to one neuron, with potentially thousands along a single axon.

How does the number of neurons myelinated by oligodendrocytes compare with Schwann cells?

One oligodendrocyte can myelinate portions of multiple neurons, while multiple Schwann cells contribute to one neuron's axon.

How is a myelin sheath formed by a Schwann cell?

The Schwann cell wraps its cell membrane around the axon repeatedly, creating many layers of membrane.

Approximately how many times might a Schwann cell wrap around an axon?

The instructor estimates around 20 wraps, producing approximately 20 layers of phospholipid bilayer.

What is a myelin sheath made of?

A myelin sheath consists of many layers of cell membrane wrapped around an axon.

Which cells produce myelin in the CNS and PNS?

Oligodendrocytes produce myelin in the CNS, while Schwann cells produce myelin in the PNS.

What are the 3 main functions of myelination?

1. Myelin insulates axons to prevent signal loss,

2. increases conduction speed through saltatory conduction,

3. and protects axons.

How does myelin help prevent signal loss?

Its insulating layers help prevent electrical signals from dissipating along the axon.

How does myelin increase nerve impulse conduction speed?

Myelin allows impulses to travel by saltatory conduction, jumping between exposed gaps along the axon.

How does myelin protect an axon physically?

It cushions the axon and helps it withstand external forces and pressure that could otherwise damage the nerve.

How does myelin stabilize the axon's environment?

Its layers insulate and cushion the axon, helping protect and stabilize the environment around it.

What is a node of Ranvier?

A node of Ranvier is an exposed gap between adjacent myelin-forming cells along a myelinated axon.

What is another name for a node of Ranvier mentioned in the lecture?

A neurofiber node.

Why are nodes of Ranvier important for nerve impulse conduction?

They are exposed regions where conduction can occur because the myelin sheath does not cover the axon at those points.

Why doesn't the nerve impulse travel continuously beneath the myelin sheath in the same way?

The myelin sheath insulates the covered regions, so the impulse is regenerated at exposed nodes rather than along every part of the axon.

What is saltatory conduction?

Saltatory conduction is the rapid propagation of a nerve impulse from one node of Ranvier to the next along a myelinated axon.

Why is saltatory conduction faster than conduction along an unmyelinated axon?

The signal travels between nodes instead of being regenerated along the entire length of the axon.

What controls the electrical changes involved in nerve impulse conduction?

Voltage-gated ion channels control these changes,

Are all neurons myelinated?

No, some axons are myelinated and others are unmyelinated.

What does an unmyelinated axon look like when associated with a Schwann cell?

A Schwann cell may surround or bundle axons without repeatedly wrapping its membrane around them to form a multilayered myelin sheath.

Why is an axon not necessarily myelinated just because a Schwann cell surrounds it?

Myelination requires repeated membrane wrapping that creates multiple layers; simply surrounding an axon does not create a myelin sheath.

What three terms can all refer to nervous tissue support cells?

Support cells, glial cells, and neuroglial cells

What does the term fiber mean when studying nervous tissue?

A fiber is a long cellular process that may be an axon or a dendrite when the specific structure cannot be identified confidently.

Why might a nervous tissue process be called a fiber instead of an axon or dendrite?

Under the microscope, it may be impossible to distinguish an axon from a dendrite, so the broader term fiber avoids guessing.

What is a bundle of fibers called in the PNS versus the CNS?

In the PNS, a bundle of fibers is a nerve; in the CNS, it is called a tract.

What is a cluster of neuronal cell bodies called in the PNS versus the CNS?

In the PNS, it is a ganglion; in the CNS, it is called a nucleus.

What connective tissue layer surrounds an entire nerve?

The epineurium surrounds the entire nerve.

What connective tissue layer surrounds a nerve fascicle?

The perineurium surrounds each fascicle, which is a bundle of nerve fibers.

What connective tissue layer surrounds an individual nerve fiber?

The endoneurium surrounds an individual nerve fiber, including its axon and associated Schwann cell.