part 2 nervous tissue

NS:

Astrocytes assist, Microglia munch debris, Ependymal cells line fluid spaces, Oligodendrocytes make CNS myelin.

PNS:

Schwann cells make PNS myelin; Satellite cells surround cell

myelin

is a fatty, insulating covering around many axons. It protects the axon and speeds nerve-impulse conduction.

Myelinated axons

conduct signals faster than unmyelinated axons. Larger axon diameter also increases conduction speed.

In the PNS,

Schwann cells wrap around one segment of an axon. The outer layer with the Schwann-cell nucleus is the neurilemma, which supports repair.

Nodes of Ranvier

are gaps between neighboring myelin segments. In myelinated axons, the impulse appears to jump from node to node; this is saltatory conduction.

In the CNS

oligodendrocyte processes make myelin; there is no outer neurilemma like in the PNS.

what does gray matter contain

mainly contains neuron cell bodies, dendrites, synapses, and unmyelinated parts.

Multipolar:

one axon and many dendrites; common for motor neurons and interneurons.

Bipolar:

one axon and one dendrite; found in certain special-sense pathways.

Unipolar (pseudounipolar):

one process that splits into branches; common for sensory neurons.

Voltage (potential difference):

Voltage (potential difference): stored energy due to a separation of electrical charges; measured in volts or millivolts (mV).

Current:

flow of electrical charge. In neurons, moving ions create electrical current.

Resistance:

how much something opposes current. Higher resistance means less current for the same voltage.

Ohm’s law:

current = voltage ÷ resistance (I = V/R).

Leak channels

are usually open and allow ions to move across the membrane.

Chemically gated (ligand-gated) channels

open when a particular chemical, such as a neurotransmitter, binds.

Voltage-gated channels

open or close when membrane voltage changes.

Mechanically gated channels

respond to physical deformation, such as pressure or stretch.

A resting neuron is usually around −70 mV:

the inside is negative compared with the outside. This separation of charge is called polarization.

The membrane is more permeable to K⁺ than Na⁺ at rest why?

because of K⁺ leak channels. K⁺ movement is a major reason the inside is negative.

The sodium-potassium pump

uses ATP to move 3 Na⁺ out and 2 K⁺ in, maintaining the ion concentration differences needed for signaling.

where are Na⁺ and K⁺ located?

There is more Na⁺ outside the cell and more K⁺ inside the cell.

Depolarization:

the inside becomes less negative (moves toward zero). This generally makes an action potential more likely.

Hyperpolarization:

the inside becomes more negative than the resting level. This generally makes an action potential less likely.

Graded potentials:

small, local changes that vary in size and fade with distance. They commonly occur in dendrites and cell bodies.

Action potentials:

large, brief electrical signals that travel along axons without fading in strength.

An action potential

is an all-or-none electrical event. If threshold is reached, a full action potential occurs; if threshold is not reached, it does not.

Range of action potential mV

Threshold is often about −55 mV (the slide gives a range near −55 to −50 mV).