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Questions and Answers

What is the primary function of Schwann cells in the PNS?

To produce neurotransmitters

To form a myelin sheath around nerve fibers (correct)

To regulate the blood-brain barrier

To connect sensory and motor neurons

Which of the following correctly describes the composition of myelin?

20% protein and 80% lipid (correct)

40% protein and 60% carbohydrate

30% protein and 70% lipid

50% water and 50% lipid

What role do satellite cells play in the PNS?

They facilitate the transportation of neurotransmitters.

They generate action potentials in neurons.

They surround neurosomas and act like astrocytes. (correct)

They produce myelin for nerve fibers.

When does the process of myelination begin during development?

Week 14 of fetal development Signup and view all the answers

What is an important characteristic of oligodendrocytes and Schwann cells?

Both types are involved in myelination of nerve fibers. Signup and view all the answers

What is the primary method of communication in the endocrine system?

Chemical messengers Signup and view all the answers

Which of the following best describes the role of the central nervous system (CNS)?

Processes information and determines responses Signup and view all the answers

Which structure is NOT part of the peripheral nervous system (PNS)?

Spinal cord Signup and view all the answers

How does the nervous system relay information from sense organs to the brain?

Using coded electrical messages Signup and view all the answers

What is a ganglion in the context of the peripheral nervous system?

A concentration of neuron cell bodies Signup and view all the answers

Which of the following correctly sequences the basic steps of nervous system operation?

Receive information, process information, issue commands Signup and view all the answers

Which system works alongside the nervous system for internal coordination?

Endocrine system Signup and view all the answers

Which is a key difference between the functioning of the endocrine and nervous systems?

Nervous system has a rapid response; endocrine system has a slower response Signup and view all the answers

What occurs during the absolute refractory period?

No stimulus of any strength will trigger an action potential. Signup and view all the answers

What characterizes the relative refractory period?

Only especially strong stimuli will trigger a new action potential. Signup and view all the answers

How does continuous conduction occur in unmyelinated fibers?

Voltage-gated channels open along the entire length of the fiber. Signup and view all the answers

During what phase can a neuron still be stimulated, but only by a strong stimulus?

Relative refractory phase Signup and view all the answers

What happens to the neuron's membrane when an action potential is triggered?

Sodium ions enter the neuron, causing depolarization. Signup and view all the answers

What is the primary reason only a small patch of a neuron is refractory at any given time?

Adjacent segments can still be stimulated while one is refractory. Signup and view all the answers

During the action potential chain reaction in unmyelinated fibers, what mechanism allows the signal to continue propagating?

Chain reactions caused by adjacent voltage-gated channels opening. Signup and view all the answers

Which of the following best describes the term 'refractory period'?

The period during which a neuron resists further stimulation. Signup and view all the answers

What triggers the formation of a new action potential in the postsynaptic neuron?

The release of a neurotransmitter Signup and view all the answers

What type of synapse connects a presynaptic neuron to a postsynaptic neuron's cell body?

Axosomatic synapse Signup and view all the answers

How many axon terminals can a spinal motor neuron be covered by?

Roughly 10,000 Signup and view all the answers

What is the primary role of the presynaptic neuron in a synapse?

To release neurotransmitters Signup and view all the answers

What is the maximum number of synapses one neuron can have in the cerebellum?

100,000 synapses Signup and view all the answers

What happens to a nerve signal when it reaches the end of an axon?

It triggers the release of a neurotransmitter Signup and view all the answers

What effect does the distance have on an action potential signal?

The signal grows weaker with distance Signup and view all the answers

What type of synapses may a presynaptic neuron form with a postsynaptic neuron?

Axodendritic, axosomatic, or axoaxonic Signup and view all the answers

What is the primary function of Schwann cells in the peripheral nervous system (PNS)?

To myelinate a single nerve fiber Signup and view all the answers

Which layer contains the nucleus and most of the cytoplasm of the Schwann cell?

Neurilemma Signup and view all the answers

What characteristic distinguishes oligodendrocytes from Schwann cells in terms of myelination?

Oligodendrocytes myelinate several nerve fibers Signup and view all the answers

What happens to the myelination process conducted by oligodendrocytes?

It spirals inward toward the nerve fiber Signup and view all the answers

Which of the following structures is NOT found around the nerve fibers in the CNS?

Neurilemma Signup and view all the answers

What is the composition within the layers of myelin formed by Schwann cells?

Membranes stacked without cytoplasm Signup and view all the answers

What is the structural role of the endoneurium?

It serves as a connective tissue layer for individual nerve fibers Signup and view all the answers

Which of the following statements is true regarding Schwann cells?

They lay down multiple layers of membrane around a single nerve fiber Signup and view all the answers

What happens to myelin in multiple sclerosis?

Myelin deteriorates and is replaced by scar tissue. Signup and view all the answers

Which age range is most commonly associated with the onset of multiple sclerosis?

20 to 40 years Signup and view all the answers

What are common symptoms of multiple sclerosis?

Double vision, tremors, and speech defects. Signup and view all the answers

What are the two primary factors affecting the speed of nerve signal conduction?

Diameter of the fiber and presence of myelin. Signup and view all the answers

How does myelin affect nerve conduction speed?

Myelin speeds up signal conduction. Signup and view all the answers

What may trigger the autoimmune response in multiple sclerosis?

Virus. Signup and view all the answers

What characterizes unmyelinated nerve fibers?

They lack additional layers of myelin around the axon. Signup and view all the answers

What is a long-term outcome of multiple sclerosis after diagnosis?

Fatality typically occurs 25 to 30 years after diagnosis. Signup and view all the answers

Study Notes

Overview of the Nervous System

Endocrine and nervous systems coordinate internal functions.
Endocrine system uses chemical messengers (hormones) released into the bloodstream.
Nervous system employs electrical and chemical signals to transmit messages between cells.

Nervous System Steps

Sense organs detect changes, sending coded messages to the central nervous system (CNS).
The CNS processes this information, considering past experiences, to determine an appropriate response.
CNS sends commands to muscles and glands to execute the response.

Nervous System Subdivisions

Central Nervous System (CNS):
- Brain and spinal cord, encased within the cranium and vertebral column.
Peripheral Nervous System (PNS):
- All nervous system components excluding the brain and spinal cord, forming nerves and ganglia.
- Nerves are bundles of nerve fibers (axons) wrapped in fibrous connective tissue.
- Ganglia are knot-like swellings in nerves, containing neuron cell bodies.

Peripheral Nervous System Divisions

Sensory (afferent) division:
- Carries signals from receptors to CNS.
- Somatic sensory division: Carries signals from skin, muscles, bones, and joints.
- Visceral sensory division: Carries signals from internal organs (heart, lungs, stomach, and bladder).
Motor (efferent) division:
- Carries signals from CNS to effectors (muscles and glands).
- Somatic motor division: Controls skeletal muscles; includes voluntary movements and reflexes (involuntary).
- Visceral motor division (autonomic nervous system): Controls glands, cardiac, and smooth muscles; responses are involuntary (e.g., reflexes).

Visceral Motor Division (Autonomic Nervous System)

Sympathetic division:
- Arouses the body for action.
- Accelerates heart rate and respiration, inhibiting digestive and urinary functions.
Parasympathetic division:
- Calming effect.
- Slows heart rate and breathing, stimulating digestive and urinary functions.
These divisions often work in opposition to each other.

Universal Properties of Neurons

Excitability (irritability): Respond to environmental changes (stimuli).
Conductivity: Respond to stimuli by producing electrical signals that are quickly transmitted.
Secretion: When an electrical signal reaches a nerve fiber's end, the cell secretes a chemical neurotransmitter.

Functional Classes of Neurons

Sensory (afferent) neurons: Detect stimuli and transmit information toward the CNS.
Interneurons: Located entirely within the CNS, connecting motor and sensory pathways. They constitute approximately 90% of all neurons. They receive signals from multiple sources and integrate information to determine a response.
Motor (efferent) neurons: Send signals out to muscles and glands (effectors).

Structure of a Neuron

Neurosoma (cell body): Control center of the neuron, containing a single nucleus and nucleolus. Cytoplasm includes mitochondria, lysosomes, extensive rough ER (creating chromatophilic substance), Golgi complex and inclusions (lipid droplets, melanin).
Dendrites: Branching extensions off the neurosoma, primary sites for receiving signals from other neurons.
Axon (nerve fiber): A single, typically long projection originating from the axon hillock, transmitting signals to distant points. Has collaterals (branches) that extend to target tissues.
- Axoplasm: Cytoplasm of the axon.
- Axolemma: Plasma membrane of the axon.
Distal End of Axon: Has extensive terminal arborization (complex branches). These end in axon terminals, which form synapses with the next cell (postsynaptic).
- Axon terminal: Contain synaptic vesicles filled with neurotransmitters to transmit signals to the next cell.

Axonal Transport

Axonal transport: Two-way passage of organelles, proteins and materials along the axon.
Anterograde transport: Movement down the axon away from the neurosoma. Utilizes the motor protein kinesin.
Retrograde transport: Movement up the axon toward the neurosoma. Utilizes the motor protein dynein.

Axonal Transport (continued)

Fast axonal transport: Rate of 20-400 mm/day. Used to transport organelles, enzymes, synaptic vesicles, and small molecules.
Retrograde fast transport: Carries recycled materials and pathogens.
Slow axonal transport: Rate of 0.5-10 mm/day. Used for moving enzymes, cytoskeletal components, and new axoplasm along the axon.

Supportive Cells (Neuroglia)

Neuroglia outnumber neurons 10 to 1.
They protect and support neurons; including binding neurons together to form a framework.
In the fetal stage, neuroglia guide migrating neurons.
Mature neurons not in synaptic contact are usually covered with glial cells.

Types of Neuroglia (CNS)

Oligodendrocytes: Form myelin sheaths within the CNS, speeding signal conduction along axons. Arm-like processes wrap around nerve fibers.
Ependymal cells: Line internal cavities of the brain; secrete and circulate cerebrospinal fluid (CSF). Are cuboidal epithelium with cilia on apical surface.
Microglia: Wander through CNS looking for debris or damage. Develop from white blood cells (monocytes) and are concentrated in damaged areas.
Astrocytes: Most abundant glial cell in CNS, covering brain surface and nonsynaptic regions of neurons in gray matter. Support framework, form blood-brain barrier, monitor neuron activity, supply lactate to neurons.

Types of Neuroglia (PNS)

Schwann cells: Envelope nerve fibers in peripheral nervous system (PNS), producing a myelin sheath around nerve fibers. Wind repeatedly around nerve fibers. Assist in regeneration of damaged fibers.
Satellite cells: Surround the neurosomas in ganglia of the PNS; act like astrocytes, providing electrical insulation and regulating the chemical environment around the neurons.

Myelin

Myelin sheath: Formed from oligodendrocytes in CNS and Schwann cells in PNS; a lipid-rich insulating layer around an axon. 20% protein, 80% lipid.
Myelination: Process of myelin sheath formation. Begins in fetal development and proceeds rapidly during infancy, completing in late adolescence
PNS Myelin: Schwann cells wrap repeatedly around a single nerve fiber, creating several myelin sheath layers with neurilemma (thick outermost coil), containing the nucleus and cytoplasm. Has external basal lamina and endoneurium (thin fibrous connective layer).
CNS Myelin: Oligodendrocytes form myelin sheaths around several nerve fibers in their vicinity; cannot migrate.

Myelin (continued)

Nodes of Ranvier: Gaps between myelin segments along the axon.
Internodes: Myelin-covered sections of axon between Nodes of Ranvier.
Initial segment: Short section of the nerve fiber between the axon hillock and the first glial cell.

Myelin and Diseases

Multiple Sclerosis (MS): A degenerative disorder of the myelin sheath of CNS, with oligodendrocytes and myelin sheaths deteriorating, leading to hardened scar tissue. This interrupts nerve conduction, leading to symptoms like double vision, tremors, numbness, and speech defects. Onset typically between ages 20-40, progressing to death, usually 25-30 years after diagnosis.

Unmyelinated Nerve Fibers

Many CNS and PNS nerve fibers are unmyelinated.

Conduction Speed of Nerve Fibers

Nerve signal speed depends on fiber diameter and myelin presence.
- Larger diameter fibers conduct faster; more surface area.
- Myelin speeds signal conduction.
Unmyelinated fibers have slower conduction speeds (0.5-2.0 m/s), less than myelinated ones (3-15.0 m/s). Large myelinated fibers can reach up to 120 m/s.

Electrical Potentials and Currents

Electrophysiology: Study of cellular mechanisms producing electrical potentials and currents in the nervous system.
- Electrical potential = Voltage difference in charge between two points.
- Electrical current = Charges flow along a pathway to equalize potentials.
Living cells are polarized, with a resting membrane potential (usually about -70 mV).
Ions move through gated channels in the membrane.

The Resting Membrane Potential

Unequal distribution of ions across membrane creates RMP.
- Ions diffuse down their concentration gradients.
- The membrane is selectively permeable, allowing some ions to pass easier than others.
- Electrical attractions between ions influence RMP.
Potassium (K+) has the greatest influence on RMP due to high permeability. The inside of the cell is more negative than the outside due to their concentration gradient.
Sodium (Na+) leaks into the cell, but in a smaller amount.

The Resting Membrane Potential (Continued)

Sodium/Potassium pump: maintains ion concentration gradients across the membrane by moving 3 Na+ out of the cell for every 2 K+ moved in.

Local Potentials

Local potentials: Changes in membrane potential at or nearby a stimulated cell segment.
Graded, meaning their magnitude changes with stimulus strength.
Decremental, meaning their strength diminishes with distance from stimulus site.
Reversible changes in membrane potential.
Can be excitatory (positive shift) or inhibitory (negative shift).

Action Potentials

Action Potential: Large, rapid, and transient changes from resting potential to a maximum peak and then back to resting potential.
Only occurs at locations with high density of voltage-regulated ion channels.
Neurosoma has low density of channels and cannot trigger action potential.
Voltage-gated ion channels open and close in sequence when membrane polarization reaches a threshold voltage.
Action potentials follow an "all-or-none” law and are nondecremental and irreversible.
After action potentials, a refractory period creates a time where cell cannot fire again, until the hyperpolarization is over.

Signal Conduction

In unmyelinated fibers, signal conduction is continuous (a wave of depolarization moving along the membrane).
In myelinated fibers, signal conduction is saltatory (jumping from node to node). This is much faster.

Synapses

Synapses: Junctions between neurons or between a neuron and an effector (e.g., muscle or gland).
Presynaptic neurons release neurotransmitters into the synaptic cleft.
Postsynaptic neurons contain receptors which binds to the neurotransmitter stimulating responses.

Synapses (continued)

Synapse types include axodendritic, axosomatic, and axoaxonic. Number of synapses can be high, thousands or hundred thousands in some parts of the brain.
Neurotransmitters can modify a neuron signaling process.

Structure of a Chemical Synapse

Presynaptic axon terminal contains synaptic vesicles with neurotransmitter.
Postsynaptic neuron’s membrane has receptors for neurotransmitters.

Neurotransmitters

Neurotransmitters: Chemical messengers that transmit signals across chemical synapses to signal postsynaptic cells.
Classified by category (e.g., acetylcholine, amino acids, monoamines, neuropeptides, and gases).

Neurotransmitters (continued)

Acetylcholine (ACh): Key neurotransmitter that's unique. Important for nervous systems. Derived from acetic acid and choline.
Amino acids: Include variations like glycine, glutamate, aspartate, and GABA (gamma-aminobutyric acid).
Monoamines: These are derived from amino acids; examples are epinephrine, Norepinephrine, dopamine, histamine and serotonin.
Purines: Includes adenosine and ATP.
Gases: Such as nitric oxide and carbon monoxide that function as neurotransmitters.
Neuropeptides: Chains of 2 to 40 amino acids. Include substance P, endorphins, and gut brain peptides. These have a wide variety of effects.

Synaptic Transmission

Neurotransmitters can be excitatory or inhibitory. The effect depends on specific postsynaptic receptors.
Ligand-gated channels or second-messenger systems are involved.

Cessation of the Signal

Synaptic transmission ends when neurotransmitters are removed.
Methods include:
- Degradation by enzymes (e.g., acetylcholinesterase for ACh).
- Reuptake into the presynaptic terminal.
- Diffussion into surrounding tissue

Neural Integration

Neural integration: CNS’s ability to process, store and recall information; to make decisions.
- Chemical synapses and extensive neuronal connections allow for integration and decision-making in the CNS.
- In the brain, pyramidal cells alone have about 40,000 contacts with other neurons.

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