Nervous system I - Nervous Tissue

Axon of multipolar neuron

Location:

• Central nervous system

• Peripheral nervous system

Description:

• Single, cylindrical process extending from soma (cell body) of neuron

• Arises from axon hillock

• Usually without branches near cell of origin

• Terminates as axon terminal on other neurons or effectors

Function:

• Conveys efferent nerve impulses (i.e., away from soma)

• Conveys information to other neurons or effectors

Also known as:

• Nerve fiber

Comment:

• Axons with diameter greater than 2 µm usually myelinated

Dendrites of multipolar neuron

Location:

• Central nervous system

• Ganglia of autonomic nervous system

Description:

• Tapered, highly branched processes extending from soma (cell body) of neuron

• May have specialized sensory receptor (e.g., pressure receptor in hypodermis)

Function:

• Convey afferent nerve impulses (i.e., towards soma)

• Receive information from other neurons

Comment:

• Not myelinated

Multipolar neuron

Location:

• Central nervous system

• Ganglia of autonomic nervous system

Description:

• Composed of soma (cell body), multiple dendrites, and a single axon

Function:

• Soma contains nucleus and other cytoplasmic organelles that support neuron structure and function

• Dendrites convey afferent nerve impulses (i.e., towards soma)

• Axon conveys efferent nerve impulses (i.e., away from soma)

Comment:

• Neurons are the functional unit of nervous system

• Three structural categories of neurons; unipolar, bipolar, and multipolar

Soma of multipolar neuron

Location:

• Central nervous system

• Ganglia of autonomic nervous system

Description:

• Cell body of neuron

• Two types of processes: dendrites and axons

• Contains Nissl bodies

Function:

• Nucleus and other cytoplasmic organelles support neuron structure and function

Comment:

• Axons convey efferent nerve impulses (i.e., away from soma)

• Dendrites convey afferent nerve impulses (i.e., towards soma)

• Nissl bodies (dark-staining region) represent aggregates of rough endoplasmic reticulum

Cytoplasm of Schwann cell

Location:

• Schwann cell (soma and outermost layer of myelin sheath)

Description:

• Amorphous substance between plasma membrane and nuclear membrane

• Contains nucleus and other organelles

• Contains cytoskeleton, enzymes, nutrients, and other proteins

Also known as:

• Cytoplasm also known as cytosol

Comment:

• Inner layers of myelin sheath lack cytoplasm

Myelin sheath

Location:

• Surrounds myelinated axon

Description:

• Formed by oligodendrocyte in central nervous system or Schwann cell in peripheral nervous system

• Cell membrane wrapped spirally (up to 100 times) around axon

• Lacks cytoplasm between myelin layers

• Composed of a series of short segments (0.2-1.0 mm) of myelin wrappings on axon

• Myelin sheath segments are called internodes

Function:

• Maintains action potential conduction along axon

• Myelin serves as electrical insulator

• Increases speed of nerve impulse conduction

Also known as:

• Neurilemma (neurolemma)

Myelinated axon

Location:

• Central nervous system (CNS)

• Peripheral nervous system (PNS)

Description:

• Axon (usually 2 µm) wrapped by myelin sheath (layered spirals of myelin)

Function:

• Conveys efferent nerve impulses (i.e., away from soma)

• Conveys information to other neurons or effectors

• Myelin sheath increases speed of nerve impulse conduction

Comment:

• Nerve impulses travel faster (3-15 m/sec) in small myelinated axons than in unmyelinated axons of similar size (0.5-2.0 m/sec)

• Myelinating cell is oligodendrocyte (CNS) or Schwann cell (PNS)

• Aggregates of myelinated axons in CNS known as white matter

Schwann cell

Location:

• Peripheral nervous system

Description:

• Neuroglial cell

Function:

• Forms myelin sheath around a single axon

• Myelin sheath increases speed of nerve impulse conduction

• Structural and metabolic support for axons

Also known as:

• Neurolemmocyte

Comment:

• Myelin sheath is cell membrane wrapped spirally (up to 100 times) around axon

• Oligodendrocyte in CNS can form myelin sheath around more than one axon

Unmyelinated axon

Location:

• Central nervous system (CNS)

• Peripheral nervous system (PNS)

Description:

• Axons lacking a myelin sheath

• Unmyelinated axons rest in invaginations of Schwann cell or oligodendrocyte

Function:

• Conducts nerve impulses

Comment:

• Nerve impulses travel slower in unmyelinated axons (0.5-2.0 m/sec) than in small, myelinated axons of similar size (3-15 m/sec)

• Myelinating cell is oligodendrocyte (CNS) or Schwann cell (PNS)

Mitochondrion in presynaptic terminal

Location:

• Presynaptic terminal

Description:

• Membrane-bound organelle

Function:

• Synthesize adenosine triphosphate (ATP)

Comment:

• Origin of mitochondria as an organelle: thought to be prokaryotic organisms (like bacteria) that formed symbiotic relationship with anaerobic eukaryotic cells: "mitochondria" received protection and nutrients, the cell received a chemical energy source

Presynaptic terminal

Location:

• Distal end of axon

Description:

• Bulbous swelling containing synaptic vesicles

• Contains presynaptic membrane

• Contains mitochondria and synatic vesicles

Function:

• Delivers neuronal impulses to (chemical) synapse

• Accumulates and recycles synaptic vesicles (containing neurotransmitter)

Comment:

• Presynaptic terminal is unmyelinated

Synapse

Location:

• Junction between axon or dendrite and an effector

Description:

• Contact between two neurons or a neuron and an effector

• Includes presynaptic membrane, synaptic cleft, and postsynaptic membrane

Function:

• Transmit neuronal impulse by use of neurotransmitter (chemical synapse)

Comment:

• Synaptic effectors include neuronal dendrites, axons, or somata, and non-neuronal cell types (e.g., muscle)

• Neurotransmitters are chemical agents (e.g., acetylcholine, norepinephrine) released by presynaptic cell at chemical synapse

• Electrical synapse involves current (ion) flow through gap junctions between adjacent cells

Synaptic cleft

Location:

• Chemical synapse

• Between neurons, or between neuron and effector

Description:

• Narrow intercellular gap

• Defined by presynaptic and postsynaptic membranes

Function:

• Neurotransmitter released from presynaptic membrane diffuses across cleft to postsynaptic membrane

Comment:

• Neurotransmitters are chemical agents (e.g., acetylcholine, norepinephrine) released by presynaptic cell at chemical synapse

Synaptic vehicles in presynaptic terminal

Location:

• Presynaptic terminal

Description:

• Membrane-bound organelle

• Small sac containing neurotransmitters

Function:

• Storage and release of neurotransmitters

Comment:

• Neurotransmitters are chemical agents (e.g., acetylcholine, norepinephrine) released by presynaptic cell at chemical synapse

Myelin sheath

Location:

• Surrounds myelinated axon

Description:

• Formed by oligodendrocyte in central nervous system or Schwann cell in peripheral nervous system

• Cell membrane wrapped spirally (up to 100 times) around axon

• Lacks cytoplasm between myelin layers

• Composed of a series of short segments (0.2-1.0 mm) of myelin wrappings on axon

• Myelin sheath segments are called internodes

Function:

• Maintains action potential conduction along axon

• Myelin serves as electrical insulator

• Increases speed of nerve impulse conduction

Myelinated axon

Location:

• Central nervous system (CNS)

• Peripheral nervous system (PNS)

Description:

• Axon (usually 2 µm) wrapped by myelin sheath (layered spirals of myelin)

Function:

• Conveys efferent nerve impulses (i.e., away from soma)

• Conveys information to other neurons or effectors

• Myelin sheath increases speed of nerve impulse conduction

Comment:

• Nerve impulses travel faster (3-15 m/sec) in small myelinated axons than in unmyelinated axons of similar size (0.5-2.0 m/sec)

• Myelinating cell is oligodendrocyte (CNS) or Schwann cell (PNS)

• Aggregates of myelinated axons in CNS known as white matter

Node of fancier

Location:

• Myelinated axon

Description:

• Cleft between internodes of myelin sheath

Function:

• Rapid nerve impulse conduction

• Impulses "jump" from node to node (a process called saltatory conduction)

Comment:

• Myelin sheath segmented (segments called internodes)

• Latin: saltare = to jump

Nucleus of Schwann cell

Location:

• Soma (cell body) of Schwann cell

Description:

• Largest organelle (5-10 µm in diameter)

• Spherical or ovoid structure

• Nuclear envelope comprised of double membrane (i.e., two lipid bilayers)

• Contains cellular DNA and nucleolus

Function:

• DNA replication

• DNA transcription into mRNA

• Ribosomal RNA synthesis and ribosome subunit assembly (in nucleolus)

Comment:

• Staining of DNA with basic histological dyes like hematoxylin depends on transcriptional activity: DNA undergoing active transcription, called euchromatin, stains lightly because it is less folded and, therefore, less dense; transcriptionally inactive DNA, or heterochromatin, is tightly packed and condensed and, therefore, stains more darkly

• Micrometer (µm), also known as a micron, is 1/1,000 of a millimeter