Neurons and Nervous Tissue

What is the structural unit of the nervous system?

The neuron.

What are the general characteristics of neurons?

Excitability, conductivity, secretion, extreme longevity, and amitotic nature.

What is neuronal excitability?

Responsiveness to a stimulus.

What happens to a cell's membrane potential when a stimulus occurs?

It causes a change in the cell's membrane potential.

What is neuronal conductivity?

The ability to propagate an electrical signal.

How do electrical signals propagate during conductivity?

Voltage-gated channels along the membrane open sequentially.

What is neuronal secretion?

The release of a neurotransmitter in response to conductive activity.

How is a messenger released during secretion?

It is released from a vesicle to influence a target cell.

What is meant by neurons having extreme longevity?

A cell can live throughout a person's lifetime.

What does amitotic mean for neurons?

After fetal development, mitotic activity is lost in most neurons.

What are the main parts of a neuron?

Cell body (soma), dendrites, axon, and cytoskeleton.

What does the plasma membrane of a cell body enclose?

The cytoplasm.

What does the neuron's cell body contain?

The nucleus and chromatophilic substance (Nissl bodies).

What are Nissl bodies composed of?

Free and bound ribosomes.

What is the function of the cell body regarding graded potentials?

It initiates some graded potentials, receives others from dendrites, and conducts them to the axon.

What are dendrites?

Short, unmyelinated processes branching off the cell body.

What is the function of dendrites?

To receive input and transfer it to the cell body.

What is an axon?

A long process emanating from the cell body.

What structures can an axon make contact with?

Other neurons, muscle cells, or glands.

Where does the axon attach to the cell body?

At the axon hillock, a triangular region of the soma.

What are the branches of an axon called?

Axon collaterals.

What do axon collaterals end in?

Several telodendria, or axon terminals.

What do axon terminals house?

Synaptic vesicles containing neurotransmitters.

What is the primary function of axons?

To conduct action potentials and release neurotransmitters at terminals.

What components make up the neuron's cytoskeleton?

Microfilaments, intermediate filaments, and microtubules.

What are the intermediate filaments of neurons called?

Neurofilaments.

What do neurofilaments aggregate to form?

Bundles called neurofibrils.

What is the function of neurofibrils?

To provide tensile strength.

How do axons move material in relation to the cell body?

To and from the cell body via transport mechanisms.

What is anterograde transport in axons?

Movement from the cell body toward axon terminals of newly synthesized material.

What is retrograde transport in axons?

Movement to the cell body of used materials for breakdown and recycling in the soma.

How are neurons structurally classified?

By the number of processes coming off the soma.

How are neurons functionally classified?

According to the direction they propagate action potentials.

What is the function of sensory neurons?

To conduct input from somatic and visceral receptors to the CNS.

What structural type are most sensory neurons?

Unipolar (with a few being bipolar).

What is the function of motor neurons?

To conduct output from the CNS to somatic and visceral effectors.

What is the structural classification of all motor neurons?

Multipolar.

What are the functions of interneurons?

To receive, process, and integrate information and communicate between sensory and motor neurons.

Where are interneurons located?

Within the central nervous system (CNS).

What percentage of our neurons do interneurons make up?

About 99%.

What is the structural classification of interneurons?

Generally multipolar, though they can be anaxonic.

What is a synapse?

A place where a neuron connects to another neuron or an effector.

What are the two types of synapses?

Chemical and electrical synapses.

Which type of synapse is far more common?

Chemical synapses.

How are presynaptic and postsynaptic neurons bound in an electrical synapse?

By gap junctions.

What is a key advantage of electrical synapses?

They are fast with no synaptic delay in passing electrical signals.

Where are electrical synapses located?

In limited regions of the brain and eyes.

In a chemical synapse, which neuron produces the signal?

The presynaptic neuron's axon terminal.

In a chemical synapse, which neuron receives the signal?

The postsynaptic neuron.

What part of the postsynaptic neuron most commonly receives the chemical signal?

One of its dendrites.

What is the synaptic cleft?

A small fluid-filled gap between the presynaptic and postsynaptic neurons.

What happens when neurotransmitter binds to postsynaptic receptors?

It initiates a postsynaptic potential (a graded potential).

What is synaptic delay?

The time it takes for all events of chemical synaptic communication to occur.

What are glial cells (neuroglia)?

Nonexcitable support cells found in the CNS and PNS.

How do glial cell numbers compare to neuron numbers?

There is approximately the same number of glial cells as neurons.

What proportion of nervous system volume do glial cells account for?

About half the volume.

What are the general capabilities of glial cells?

They are capable of mitosis, protect and nourish neurons, provide physical scaffolding, and guide development.

What are astrocytes?

Star-shaped glial cells with processes ending in perivascular feet.

Which glial cell is the most abundant in the CNS?

Astrocytes.

How do astrocytes help form the blood-brain barrier?

By wrapping perivascular feet around brain capillaries.

What is the purpose of the blood-brain barrier formed by astrocytes?

It controls which substances have access to the brain.

How do astrocytes regulate tissue fluid composition?

By regulating the chemical environment around neurons, such as potassium concentration.

What role do astrocytes play regarding dying neurons?

They occupy the space of dying neurons.

Where are ependymal cells located?

Lining cavities in the brain and spinal cord.

What structure are ependymal cells a part of that produces cerebrospinal fluid?

The choroid plexus.

What are microglia?

Small cells that wander the CNS and replicate during infection.

What is the immune function of microglia?

They act as phagocytic cells of the immune system to engulf infectious agents and remove debris.

What are oligodendrocytes?

Large cells with slender extensions that wrap around neuron axons.

What do oligodendrocytes form around neuron axons?

The myelin sheath.

What is the role of glial cells at neural synapses?

They are critical for normal function at neural synapses.