004 Microscopic Anatomy of the Nervous System

Question 1

• list and describe the cells of nervous tissue

Answer 1

??

Question 2

Describe a sensory neuron

Answer 2

(AKA afferent)
 Carry nerve impulses from sensory receptors to the CNS
 Two types
• Somatic afferents – associated with the somatic nervous system
• Visceral afferents – associated with the autonomic nervous system

Question 3

Describe a motor neuron

Answer 3

(AKA efferent)
 Carry nerve impulse from the CNS to effector organs
 Two types
• Somatic efferents – associated with the somatic nervous system
• Visceral efferents – associated with the autonomic nervous system

Question 4

Describe interneurons

Answer 4

 Carry nerve impulses between two other neurons
 Form communication and integration networks between sensory and motor neurons
 99.9% of neurons fall into this category, with almost all of the neurons in the CNS belonging to this type

Question 5

Anatomical types of neurons

Answer 5

multipolar, bipolar, and pseudounipolar

Question 6

Multipolar neurons

Answer 6

o	Multipolar neurons
	One axon
	Two or more dendrites
•	Usually have many that form the dendritic tree
	The main anatomical types of neurons

Question 7

Bipolar neurons

Answer 7

	One axon
	One dendrite
	Compared to multipolar neurons there are relatively few in the body
	Examples
•	Bipolar cells of the retina
•	Vestibulocochlear apparatus in the ear
•	Olfactory epithelium

Question 8

Pseudounipolar neurons

Answer 8

 Appear to only have one process attached to the soma
 During development the neuron had one axon and one dendrite that fused into a single process
• The single process has the staining qualities of an axon
 Exclusively first order sensory neurons found in spinal (dorsal root) ganglia and sensory ganglia of cranial nerves
 Peripheral (distal) process
• Part between the soma and the peripheral receptor
 Central (proximal) process
• Part between the soma and the CNS

Question 9

• describe the common types of multipolar neurons found in the CNS

Answer 9

o Pyramidal cells
o Purkinje cells
o Stellate cells
o Granule cells

Question 10

Pyramidal cells

Answer 10

(AKA Golgi type I neurons)
 MOST COMMON IN ENTIRE BODY
 Found exclusively in the gray matter of the cerebral cortex
 Cell body is triangular, with one angle always pointing towards the pia mater and the flat surface toward the white matter
 Dendrites arise from the angles of the soma
• Apical dendrite – from the pial surface
• Basal dendrite – from the other angles
 Axon always arises from the flat surface facing the white matter
• They always arise in the grey matter then enter the white

Question 11

Purkinje Cells

Answer 11

 Large neurons located exclusively in the gray matter of the cerebellar cortex
 Cell body is flask shaped with
 Large dendritic tree extending from the side of the soma facing the pial surface
 Axon always arises from the side of the cell body facing the white matter
• It exits the grey matter to enter the white
• Usually quite long and target is far away

Question 12

Satellite cells

Answer 12

(AKA Golgi type II neurons)
 Smaller, star-shaped cells in both the cerebral and cerebellar cortex – also in other gray matter
 Cell body is star shaped
 Axons are relatively short
• Stellate cells involved in more local connections

Question 13

Granule cells

Answer 13

 A specific subset of small stellate-type cells that resemble grains of sand
 Found in the cerebellar cortex and other regions of gray matter

Question 14

• describe the soma of a neuron, including the Nissl substance and axon hillock

Answer 14

o The dilated region containing the nucleus and nucleolus
o Has many mitochondria
o Has a large, well developed Golgi apparatus
o Nissl substance
 Clumps of basophilic material located within the perinuclear cytoplasm
• Polyribosomes
• Rough endoplasmic reticulum
 Gives the neuron cell body a basophilic staining quality
 NOT present in the axon hillock, which is where the axon attaches to the cell body

Question 15

Dendrites

Answer 15

	Receptive structures associated with neurons
	Carry information to the cell body
	Compared to axons 
•	shorter
•	larger diameter
•	tapered
•	unmyelinated
	Dendritic spines
•	Present on most dendrites (not all)
•	Small knob-like projections for synaptic contacts
	Structure is similar to the soma
•	Does not have nucleus nor Golgi apparatus
•	DOES have
o	Nissl substance
o	Mitochondria
o	Cytoskeletal components

Question 16

Axons

Answer 16

 Effector portion of a neuron
 Carry information away from the soma
 Only one per neuron
 Collateral branches
• Branches of the axon
 Compared to dendrites
• Longer
 Axon hillock
• Region of the cell body that attaches to the axon
• Devoid of Nissl substance
 Initial segment of the axon
• The region of the axon between the apex of the axon hillock and the beginning of the myelin sheath
• Site of the generation of action potentials
 Axon terminal (AKA axon bouton)
• Dilated region at the end of the axon that helps form the presynaptic portion of a synapse
 Structures found in axons
• Neruofilaments
o Intermediate filaments involved in axonal transport
• Vesicles
o Transported by neurofilaments
• A few mitochondria
o Usually near the termination of the axon

Question 17

• describe the three (3) general types of axons, based upon what they connect: projection, association, and commissural

Answer 17

o Projection
 Connects cerebral cortex with subcortical structures
o Association
 Connect structures within the cerebral cortex (usually on the same side)
o Commisural
 Connects like structures on opposite sides of the CNS

Question 18

• list the morphological and physiologic types of synapses

Answer 18

o	Axodendritic
	Between axon and dendrite
	Vast majority of synapses are this type
	Axospinous 
•	Axon synapses with dendritic spines
o	Axosomatic
	Between axon and soma
o	Axoaxonic
	Between axon and axon
o	Dendrodendritic
	Between dendrite and dendrite

Question 19

• compare and contrast styles of synaptic contacts: single bouton, en passant, and en grappe

Answer 19

o Single bouton
 Stand alone synapse
o En passant
 Multiple string of synapses from a single axon
o En grappe
 Special type ONLY found between motor axons and large muscle fibers of EOMs

Question 20

• describe the structure and function of a chemical synapse

Answer 20

o the conduction of information across the synapse is achieved by the release of chemical substances (neurotransmitters)
o neurotransmitters diffuse across the synaptic cleft to stimulate the next neuron
o Presynaptic component
 Contains synaptic vesicles containing neurotransmitters
 Usually has a thin presynaptic density along the plasma membrane
 May also be a few mitochondria
o Synaptic cleft
 Physical gap between cells
o Postsynaptic component
 Cytoplasmic side of the plasma membrane
 Two varieties
• Asymmetric synapse
o Postsynaptic density noticeably thicker than presynaptic density
o Thought to be excitatory synapses
• Symmetric synapse
o Pre and postsynaptic densities are about the same size
o Thought to be inhibitory synapses
o Astrocytic processes
 In the CNS
 Surround the synapse
 Appear to have some support function

Question 21

• describe axonal transport

Answer 21

o	carries components between the soma and the axon
o	Two types
	Slow
•	Carries components up to 5 mm/day
•	Appears to use microtubules and neurofilaments
•	Cytoskeletal proteins and enzymes are carried by this system
	Fast
•	Carries components up to 400 mm/day
•	Appears to use only microtubules (no neurofilaments)
•	Transports
o	Mitochondria
o	Lysosomes
o	Vesicles
o	Two directions
	Anterograde
•	From soma to periphery
•	Fast and slow use this direction
	Retrograde
•	From periphery to soma
•	Only fast mechanisms move in this direction

Question 22

PNS supporting cells

Answer 22

 Schwann Cells
• In PNS
• Create the necessary microenvironment necessary for axons to function properly
• Axon + Schwann cell = nerve fiber
• Functions
o Offer structural support to the axon
o Help mediate nutrition of axon
o Can form myelin (electrochemical insulation)
 Satellite Cells
• In PNS
• Found exclusively surrounding the cell bodies of neurons in the ganglia (sensory and autonomic ganglia)
o Ganglia=collection of nerve cell bodies in the PNS
• Create a microenvironment for the cell bodies in a way similar to Schwann cells
• Appears as a single layer of cuboidal or squamous cells surrounding the somata in ganglia

Question 23

Schwann cells

Answer 23

• In PNS
• Create the necessary microenvironment necessary for axons to function properly
• Axon + Schwann cell = nerve fiber
• Functions
o Offer structural support to the axon
o Help mediate nutrition of axon
o Can form myelin (electrochemical insulation)

Question 24

Satellite Cells

Answer 24

• In PNS
• Found exclusively surrounding the cell bodies of neurons in the ganglia (sensory and autonomic ganglia)
o Ganglia=collection of nerve cell bodies in the PNS
• Create a microenvironment for the cell bodies in a way similar to Schwann cells
• Appears as a single layer of cuboidal or squamous cells surrounding the somata in ganglia

Question 25

Neuroglia

Answer 25

 Supporting cells of the CNS  In standard preparations, only the nuclei of glial cells can be seen, but the structures in the rest of the cells can be seen with special stains

Question 26

oligodendrocytes

Answer 26

o AKA oligodendroglia, oligodendrogliocytes o Produce and maintain myelin sheaths in the CNS o Small cells with few processes o Usually, each cell will myelinate more than one axon (with one process per axon) o Each process forms an internode region o There are nodes of Ranvier between each oligodendrocyte process o Unmyelinated axons in the CNS are NOT surrounded by any processes and are truly “bare”

Question 27

Astrocytes

Answer 27

o Most numerous type of cell in the nervous system o Largest type of glial cell o Star shaped o Body has several radiating processes o Have glial fibrillar acidic protein  Makes up a special type of intermediate filament unique to astrocytes  Used as a basis for staining astrocytes o Only the nuclei can be seen with H&E stains o Perivascular feet (AKA end feet)  On the ends of some astrocyte processes  Cover large areas of blood vessels in the CNS to help establish a microenvironment for the neurons  Help form the blood brain barrier  Regulate movements of metabolites and ionic concentrations  The processes also surround the bare areas of axons (i.e. nodes of Ranvier, synapses and the entire length of unmyelinated axons)  Play a role in regulation of ions and neurotransmitter metabolism in these structures o Help form glial limitans  Consists of expanded ends of astrocyte processes, called subpial feet that are associated with the basal lamina of the pia mater  A relatively impermeable layer surrounding the PNS o Two types  Protoplasmic  Fibrous

Question 28

Microglia

Answer 28

``` o Relatively small cells o Usually have an elongated nucleus o Have few, short, twisted processes  Need a special stain to be seen o Macrophages  Phagocytic cells that originate in bone marrow  Part of monocyte lineage o Play a role in defense o Help remove debris after injury ```

Question 29

Ependymal Cells

Answer 29

o Simple cuboidal epithelium o lines the fluid filled cavities of the CNS (i.e. brain ventricles and central canal of the spinal cord) o NO basal lamina associated with this epithelium  Basal domain is associated with astrocytic processes o These cells can be modified to produce CSF  They form, along with their capillaries, the choroid plexus

Question 30

Myelinated axons

Answer 30

o Myelinated  Schwann cell wraps around the axon multiple times (like a cinnamon roll)  Myelin sheath = multiple fused layers of Schwann cell plasma membrane that directly surround the axon • Lipid rich layer • Hydrophobic • Acts as an insulator • Cytoplasm gets squeezed out o Ends up peripheral to myelin sheath, forming the neurilemma (AKA Sheath of Schwann) o Contains the organelles of the Schwann cells • Clefts of Schmidt-Lanterman o Small islands of cytoplasm that remain between the layers of the myelin o Important for metabolic maintenance of the myelin • Segmented along the axon o Node of Ranvier  Junction between two Schwann cells  Has no myelin to allow for saltatory conduction o Internodes of Ranvier  Areas between the nodes of Ranvier  Corresponds to one Schwann cell

Question 31

Unmyelinated axons

Answer 31

 Axons that do not have multiple layers of Schwann cell membranes to enclose them  Still are associated with Schwann cells but do not have enough membrane to serve as an insulator or cause salutatory conduction (it is surrounded by only one layer) • The axon runs in the groove of the Schwann cell • Each Schwann cell usually has multiple grooves • Each groove can contain one or more axons o Particularly in autonomic neurons

Question 32

• describe the functional importance of myelin in the nervous system

Answer 32

???

Question 33

o Protoplasmic Astrocytes

Answer 33

 Pretty exclusively found only in gray matter  More numerous, shorter and more branching processes than fibrous type  Usually have fewer intermediate filaments than fibrous types

Question 34

Fibrous Astrocytes

Answer 34

 Pretty exclusively only found in white matter  Have fewer, longer and straighter processes than protoplasmic types  Usually have more intermediate filaments than protoplasmic types

Question 35

Peripheral nerves

Answer 35

``` o Nerve  A bundle of nerve fibers (i.e. axons and supporting cells) in the PNS  THERE ARE NO NERVES IN THE CNS o Connective tissue layers are formed by fibroblasts -endoneurium -perineurium -epineurium ***MAJOR EXCEPTION IS THE OPTIC NERVE ```

Question 36

endoneurium

Answer 36

 Loose CT that directly surrounds each nerve fiber

Question 37

Perineurium

Answer 37

 Dense Irregular CT that directly surrounds a fascicle of nerve fibers  Metabolically active barrier  Helps form the blood-nerve barrier

Question 38

Epineurium

Answer 38

 Dense Irregular CT that surrounds the entire nerve and fills the space between nerve fascicles  May contain a variable amount of adipose tissue

Question 39

The major exception to the nerve rule

Answer 39

 Optic Nerve • Has a different structure than the other cranial nerves • Has oligodendrocytes (instead of Schwann cells) • Does NOT have endo-, peri-, or epineurium o Has pia, arachnoid, and dura mater • It is a direct extension of the CNS into the periphery • The optic nerve is more accurately a tract rather than a cranial nerve