Retrograde Movement

7/28/2019 Retrograde Movement

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Retrograde movement Movement away from the axon terminals. Includesorganelles being returned to the cell body fordegradation or recycling.

Function of myelin sheath Protects and electrically insulates fibers and

increases the speed of transmission of nerveimpulses.

Myelinated fibers vsUnmyelinated fibers

Myelinated fibers conduct nerve impulses rapidly andunmyelinated fibers conduct nerve impulses slowly.

Axon always have myelin sheaths and dendrites areALWAYS unmyelinated.

Why are myelin sheaths goodelectrical insulators?

They lack channel and carrier proteins

Neurilemma Husk (outside) of the Schwann cell.

nodes of Ranvier Myelin sheath gaps, which occur at regular intervalsalong the myelinated axon. Associated with Schwanncells.

Gray matter Gray area of the CNS; contains neuronal cell bodiesand their dendrites and unmyellinated fibers.

White matter White substance of the CNS; myelinated nerve fibers.

Structural classification ofneurons

1. Multipolar - 3 or more processes: one axon and therest dendrites. This is the most common type of

neuron.2. Bipolar - 2 processes - an axon and a dendrite thatextend from opposite sides of the cell body. Theseare rare and found in the retina of the eye and in theolfactory mucosa.3. Unipolar (pseudounipolar) - 1 single short processthat emerges fro the cell body and divides T-like intoproximal anddistal branches. Found in the spinal cord and cranialnerves.

Describe the 3 functionalregions of the neurons 1. Receptive region: (receives stimulus). Plasmamembrane exhibits chemically gated ion channels.2. Conducting region: (generates/transmits actionpotential). Plasma membrane exhibits voltage-gatedNa (sodium) and K (potassium) channels.3. Secretory region: (axon terminals releaseneurotransmitters). Plasma membrane exhibitsvoltage-gated Ca (calcium) channels.


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Describe the 3 functionalclassifications of neurons basedo the direction in which thenerve impulse travels

1. Sensory (afferent) neurons: transmit impulses fromsensory receptors in the skin or internal organstoward or into the CNS. Almost all are unipolar andtheir cell bodies are located in sensory gangliaoutside the CNS.

2. Motor (efferent) neurons: carry impulses away fromthe CNS to the effector organs (muscles and glands)of the body periphery. Motor neurons are multipolar.Most of their cell bodies are located in the CNS.3. Interneurons (association neurons): lie betweenmotor and sensory neurons in neural pathways andshuttle signals through CNS pathways whereintegration occurs.

Current The flow of electrical charge from one point to another

Resistance Hindrance to charge flow provided by substancesthrough which the current must pass.

Insulators Substances with high electrical resistance

Conductors Substances with low electrical resistance

Ohm's law Current (I) = voltage (V)/resistance (R)

Rules of Ohm's law 1. The greater the voltage, the greater the current.2. The greater the resistance, the smaller the current.

Chemically gated (ligand-gated)

channels

open when the appropriate chemical binds

Voltage-gated channels open and close in response to changes in themembrane potential

Mechanically gated channels open in response to physical deformation of thereceptor (as in sensory receptors for touch andpressure).

Nongated channels (leakage) are always open

Concentration gradients Ions diffusing passively from an area of higher

concentration to an area of lower concentration.

Electrical gradients Ions move toward an area of opposite electricalcharge.

Electrochemical gradient Combination of electrical and concentration gradients.

Where is Na (sodium) Outside the cell


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concentration higher?

Where is K (potassium)concentration higher?

Inside the cell.

How does the sodium-potassium pump stabilize theresting membrane potential?

It maintains the concentration gradients for sodiumand potassium by FIRST, ejecting 3 Na (sodium) fromthe cell and SECONDLY, transporting 2 K(potassium)back into the cell.

Changes in membrane potentialproduce what 2 types ofsignals?

1. Graded potentials: incoming signals operating overshort distances2. Action potentials: Long-distance signals of axons.

Depolarization Reduction in membrane potential. The inside of themembrane becomes less negative (moves closer to0) than the resting potential.

Hyperpolarization Occurs when the membrane potential increases,becoming more negative (over -70 mV) than theresting potential.

Which cells can generate actionpotential (nerve impulse)?

neurons and muscle cells. usually occurs only inaxons.

Generation of an ActionPotential

1. Resting state (-70 mV) - No ions move throughvoltage-gated channels.2. Depolarization - Na+ flowing into the cell3. Repolarization - K+ flowing out of the cell

4. Hyperpolarization - K+ continuing to leave the cell5. Resting State - No ions move through voltage-gated channels.

Note: Threshold - is always the "imaginary" line thatruns through -55 mV.

Action potential (AP) Brief reversal of membrane potential with a totalamplitude of about 100mV (from -70 mV to +30 mV).Unlike graded potentials, actions potentials do notdecrease in strength with distance.

The rate of impulse propagationdepends on 2 factors

1. Axon diameter - the larger the axon's diameter, thefaster it conducts impulses.2. Degree of myelination - the presence of a myelinsheath increases the rate of AP propagation becausemyelin acts as an insulator.

Saltatory conduction Transmission of an action potential along a


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myelinated fiber in which the nerve impulse appearsto leap from node to node.

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Retrograde Movement