Review of Cellular Neuroanatomy

Question 1

What are neurons?

Answer 1

the structural and functional unit of the NS that collects sensory info and integrates it to control effector organs like muscles and glands

Question 2

What are the main functions of glia?

Answer 2

They provide/mediate:

1) physical support (protection)
2) electrical insulation for impulse conductance
3) metabolic exchange between the vascular system and the NS

Question 3

What are the main components of neurons?

Answer 3

Neurons posses a cell body or soma and two types of processes that extend from the soma: axon (output) and dendrites (input)

Question 4

T or F. Each neuron has a single axon and one to many dendrites

Answer 4

T.

Question 5

What is housed in the soma?

Answer 5

the soma houses the cell nucleus and machinery for metabolic function and the production of proteins (lots of RER and ribosomes)

Question 6

What do dendrites do? Axons?

Answer 6

Dendrites can be considered the cells “receiver” in that synaptic inputs in most cells (especially excitatory synaptic input) are preferentially on to dendrites and dendritic spines.

The electrical output for the cell is via the axon. Typically electrical signals are initiated in the axon and travel down the axon to terminal, where neurotransmitters are released for chemical neurotransmission.

Question 7

What are dendritic spines?

Answer 7

Dendritic spines are small membranous protrusions from the dendrite. Spines receive synaptic inputs, transmit electrical signals to the dendrite, and serve as an anatomical substrate for synaptic transmission, synaptic plasticity, and memory storage. Dendrites can have hundred to thousands of spines.

Question 8

What is the difference between gray and white matter?

Answer 8

Gray matter is unmyelinated tissue – primarily somas and dendrites. White matter is tissue containing myelinated axons (myelin = lipid wrapping on axons, appearance is white in unstained tissue)

Question 9

Neuronal somas can be stained for study by using a _____ stain

Answer 9

Nissl

Question 10

What are Nissl bodies?

Answer 10

“Nissl bodies” are basophilic masses – primarily rough endoplasmic reticulum (rER) and ribosomes. These masses are concerned with protein synthesis, much of which occurs in cell bodies of neurons, so Nissl stain primarily indicates cell bodies and proximal dendrites (not axons).

Nissl substance extends into proximal dendrites but not into the axon hillock, which gives rise to the axon.

Question 11

T or F. Nissl stain primarily indicates cell bodies and proximal dendrites (not axons).

Answer 11

T. Nissl substance extends into proximal dendrites but not into the axon hillock, which gives rise to the axon.

Question 12

Describe the composition of neuron soma

Answer 12

Neuronal cell bodies contain a nucleus (shown as light areas in the Nissl stained cells) with an nucleolus (dark spot in light areas: green arrows).

Other organelles in the soma include rough and smooth endoplasmic reticulum (rER), Golgi apparatus, lysosomes, and mitochondria. The soma is particularly rich in rER, as befits its role in producing proteins. The large rER stacks appear as islands in the Nissl stain.

Question 13

Three multipolar neuron cell bodies from ventral horn gray matter of the spinal cord are shown (Nissl stain).

Typical features of neuronal perikarya include a prominent nucleolus (green arrow) within a large nucleus and clumps (red arrows) of Nissl substance (rER & polyribosomes).

Answer 13

Nissl substance (chromatic substance) extends into the base of each dendrite (D) but not into the axon hillock that gives rise to the axon.

Question 14

What is the purpose of dendritic spines?

Answer 14

These are places where dendrites receive information from other neurons via synapses. This is especially true for excitatory inputs (e.g., glutamatergic).

Spines can be dynamic in their shape and number

Question 15

Typical neuron with dendrites (top) and a singular axon (bottom)

Answer 15

The neuron shown in the figure on the front is a pyramidal neuron, named for its pear-shaped soma and prominent apical dendrite (shown extending upwards from soma). Pyramidal cells typically have a group of basal dendrites as well as the apical dendrite and its branches.

Question 16

T or F. Except in experimentally induced pathology, neurons only have a single axon

Answer 16

T.

Question 17

Where do axons originate?

Answer 17

Most frequently, the axon originates at the soma and forms an axon hillock which is free of organelles.

Axons dont have to come off the soma. E.g. in many dopaminergic neurons (i.e. the ones that die in Parkinson’s), probably 50% of axons come off of primary dendrites. Axons can also branch like dendrites

Question 18

In a myelinated axon, the portion of the axon from the axon hillock to the beginning of myelination is known as the ___________.

Answer 18

axonal initial segment (AIS)

The AIS is generally the site of action potential initiation. As the axon extends from the soma it branches and extends to contact other neurons or effector organs (e.g., muscle, glands etc.).

Question 19

_______ are major branches of an axon.

Answer 19

Collaterals

Question 20

Neurons whose axon contacts muscle or glands are known as what?

Answer 20

motor neurons (or motoneurons).

NOTE: Some long axons are covered with a lipid called myelin. This myelination serves as an electrical insulator and facilitates faster nerve conduction of action potentials along the axon.

Question 21

Neurons can be classified on the basis of dendritic structure (the number and pattern of dendrites). What is a unipolar neuron?

Answer 21

Unipolar neurons have a single neurite (process – axon or dendrite). While common in insects, true unipolar neurons do not exist in mature vertebrate nervous systems

Question 22

The sensory neurons with cell bodies in the dorsal root ganglia (DRGs) are considered to be ________ in structure

Answer 22

pseudounipolar (peripheral and central processes of a single axon mostly bypass the soma).

Question 23

Some neurons (mostly local circuit interneurons) are bipolar. What does this mean?

Answer 23

[2 primary neurites - dendrites in this case) that leave the soma at opposite ends of the cell].

Many neurons are multipolar, with several primary dendrites leaving the soma. Examples include pyramidal neurons of cortical regions, Purkinje cells of the cerebellum, or motoneurons (spinal cord and brain stem).

Question 24

Question 25

What are principle cells?

Answer 25

Principal cells (pyramidal neuron in example) are projection neurons. Projection neurons integrate information and send axons to other brain areas (a projection is the path of an axon from one brain area to another).

Question 26

Principal or projection neurons are also referred to as _______

Answer 26

Golgi type I cells (long projecting axon).

Question 27

What are interneurons?

Answer 27

Interneurons are cells that do not send their axon out of the local brain area.

Question 28

What are some exs of interneurons?

Answer 28

Chandelier cells, basket cells, and double bouquet cells are all examples of interneurons in the cortex.

Question 29

Interneurons are also known as what?

Answer 29

local circuit neurons. They are also classified as Golgi type 2 cells (either no axon or short, local axon).

Question 30

NOTE:

The brain is organized into numerous neural circuits defined by the cell types present and their connections. Throughout this course, when we study a particular brain region, we will emphasize the principal neurons and interneurons of that region as an initial step in understanding the relevant neural circuits.

Question 31

What are ‘synapses’?

Answer 31

Specialized junctions that allow neural signals to be communicated from one cell to another (or from neuron to another effector).

There are two main types of synapse: electrical or chemical.

Question 32

How do electrical synpases work?

Answer 32

Electrical synapses are formed as direct connections from one cell to another via gap junctions. Electrical signals as well as small molecules can pass directly between the pre- and postsynaptic cell at an electrical synapse.

Question 33

How do electrical synapses work?

Answer 33

At chemical synapses, the electrical signal in the presynaptic neuron is transduced into release of a chemical transmitter that traverse a synaptic cleft between cells to bind to receptors on the postsynaptic neuron (or effector).

Question 34

Synapses an be classified by their subcellular target. What is an axsomatic synapse?

Answer 34

A contact from an axon to a soma. (NOTE: Axodendritic refers to an axon contacting a dendrite (could be spine or shaft)).

Question 35

Most excitatory synapses are formed where?

Answer 35

on dendritic spines (axospinous = subset of axodendritic).

An axoaxonic synapse is axon to axon synaptic contact. Synapses can also be dendrodendritic (dendrite to dendrite). When a synapse is formed between a motoneuron and muscle, the synapse is called a neuromuscular junction or NMJ.

Question 36

Vertebrate chemical synapses are unidirectional – they only transmit information from the presynaptic neuron to the postsynaptic neuron. Thus the transmitter is found in presynaptic vesicles and the receptors are found on the postsynaptic membrane. How does neurotransmitter release occur?

Answer 36

1. The presynaptic bouton (or axonal varicosity) contains membranous synaptic vesicles called synaptic vesicles that contain neurotransmitter.
2. On an incoming nerve impulse, entry of Ca2+ causes synaptic vesicles to release their contents into the synaptic cleft.
3. Specialized receptor proteins located in the postsynaptic density bind the neurotransmitter and generate another electrical signal in the postsynaptic cell.

Question 37

This an electron-microscopic view of a synapse between an axon (and terminal) to a dendritic spine (axodendritic: specifically an axospinous synapse). Note the synaptic vesicles (small round objects) in the presynaptic neuron. Note also the postsynaptic density (PSD), an electron-dense part of the postsynaptic membrane. Hundreds of proteins have been identified in the postsynaptic density including transmitter receptors, scaffold proteins, and numerous signaling molecules.

Question 38

Synapses can also be classified by function – that is, what happens when the transmitter binds to its receptors. On the basis of their actions, neurotransmitters can be divided into three classes:

Answer 38

• Excitatory
• Inhibitory
• Modulatory

Question 39

Excitatory synapses increase the probability that the postsynaptic neuron will fire an action potential. What are some ex?

Answer 39

synapses containing acetylcholine (at nicotinic receptors) or glutamate (AMPA, NMDA receptors)

Question 40

Inhibitory synapses reduce the probability that the postsynaptic neuron will fire an action potential. What are some exs?

Answer 40

Examples are synapses with glycine or gamma-aminobutyric acid (GABA – at GABAA receptors).

Question 41

Modulatory: influences how excitatory and inhibitory signals are integrated. Examples?

Answer 41

Synapses using dopamine or norepinephrine (Also acetylcholine at muscarinic receptors and GABA at GABAa receptors).

Question 42

Excitatory synapses are referred to as _______

Answer 42

Gray’s type 1 synapse. They are asymetric, with a pronounced postsynaptic density.

Inhibitory synapses are Gray’s type 2. They are more symmetrical in density between pre- and postsynaptic membranes

Question 43

Question 44

Electrical synapses, generally composed of gap junctions, are close appositions of the pre- and postsynaptic membranes. A direct, passive flow of electrical current from one cell to the next is achieved via gap junctions. Wgat are gap junctions composed of?

Answer 44

Gap junctions are made up of a connexon from each cell.

Question 45

What are connexons made of?

Answer 45

Connexons are made up of molecules called connexins. In each cell, 6 connexins form a connexon. When the connexon from each cell become opposed, a low resistance pathway (pore) called a gap junction is formed between the two cells.

Question 46

T or F. Electrical synapses can be bi-directional (so either cell can be both pre- and postsynaptic).

Answer 46

T.

Question 47

The soma is where most protein synthesis takes place. Proteins synthesized in the soma can be transported to sites in axons or dendrites. Messages (e.g. protein, virus, etc.) received in dendrites or axons can also be transported to the soma. Cells therefore have systems in place for protein transport in processes (axons, dendrites).

Answer 47

In general, proteins synthesized in the soma are packed into cargo vesicles and transported along microtubules toward their destination via ratchet-like action.

Question 48

There are separate engines for movement toward or away from the soma. What are they?

Answer 48

Kinesin moves the cargo vesicles anterograde – away from the soma. Dynein moves the cargo vesicles retrograde – towards the soma.

Question 49

There are several types of non-neuronal cells known as supporting cells. What are some examples?

Answer 49

neuroglia (glia: Schwann cells, Oligodendrocytes, Astrocytes) as well as other cell types (microglia, satellite cells, ependymal cells).

Question 50

In addition to neurons, the nervous system contains several types of supporting cells. These are not “excitable cells” – they do not generate action potentials. They perform important roles in structural support, facilitating electrical signaling, isolating cells electrically and biochemically, assisting in repair in response to injury (glial scar, tubes for regeneration, phagocytosis), and producing cerebrospinal fluid.

One important group of supporting cells are neuroglia. Glia (from Latin word for “glue”) outnumber neurons ~ 10 to 1 and make up ~ 50% of brain volume. What are some exs?

Answer 50

• Schwann Cells; that provide myelination in the peripheral NS,
• Oligodendrocytes; that provide myelination in the CNS, and
• Astrocytes.
• Microglia are another type of glia cell that have important functions mediating phagocytosis and response to inflammation in the CNS. They are sometimes considered to be equivalent to immune cells in the brain.

Question 51

What are the types of astrocytes?

Answer 51

There are two major types of astrocytes: fibrous and protoplasmic astrocytes.

Astrocytes provide general support and serve as part of the blood brain barrier in the CNS.

Question 52

What do satellite cells do?

Answer 52

function as astrocytes in the ANS

Question 53

What are polydendrocytes?

Answer 53

stem cells within the brain

Question 54

What are ependymal cells?

Answer 54

these line the central canal and form a cuboidl to columnar epithelium

Question 55

_______ provide myelination to axons in the PNS. This improves electrical insulation, which increases conduction velocity for action potentials. Individual Schwann cells wrap around an axon.

Answer 55

Schwann cells

Question 56

Where two Schwann cells touch they form a ‘_________

Answer 56

‘Node of Ranvier’ (an unmyelinated area). This is the site where action potentials regenerate. Synapses are not myelinated.

Question 57

Schwann cells form the internodes (between Nodes of Ranvier). The Schwann cell membrane wraps tightly around an axon, similar to insulating tape around a wire. During this process, what happens to the cytoplasm?

Answer 57

the cytoplasm is mostly squeezed out of the Schwann cell, forming many layers (lamellae) of lipid, however small folds of cytosol remain to support the myelin. These folds are called Schmidt-Lanterman clefts (SL) (also called Schmidt-Lanterman incisures).

The major part of the cytoplasm sits on top of the internode with the Schwann cell nucleus. The proteins of the myelin are visible as neurocreatin after dehydration. Not all axons in the PNS are myelinated (but all elaborated by Schwann cells).

Question 58

T or F. Each Schwann cell forms only 1 internode.

Answer 58

T. Schwann cell myelin is in contact with the Schwann cell soma (vs. long processes in oligodendrocytes)

Question 59

Whereas Schwann cells myelinate axons in the peripheral nervous system (PNS), in the CNS this function is carried out by ______.

Answer 59

oligodendrocytes. Each oligodendrocyte has several (oligo) processes (‘dendrites’). Each process provides myelination for one axon, therefore one oligodendrocyte myelinates several axons.

Neighboring internodes originate from different oligodendrocytes. Many axons in the CNS are not myelinated.

Question 60

Unmyelinated axons are unprotected in the CNS but groups of several unmyelinated axons are enveloped by Schwann cells in the PNS. In the CNS, one oligodendrocyte forms 30-50 internodes (many different axons, 1 internode per axon).

Answer 60

In the PNS, each Schwann cell forms only 1 internode. Schwann cell myelin is in contact with the Schwann cell soma (vs. long processes in oligodendrocytes)

Question 61

Astrocytes are sometimes referred to as “servants of the CNS”. They have several important functions, including what?

Answer 61

• moving metabolites to and from neurons, i.e., metabolic exchange.
• maintaining constant ionic concentrations for optimal neuronal function by taking up ions (e.g., K+, Ca2+) and transmitters.
• Astrocyte radiating processes contact neurons (perineural feet), endothelia cells of blood vessels (perivascular feet), and myelin.
• Astrocytes facilitate angiogenesis, synaptogenesis and maintenance of the blood brain barrier.

Question 62

There is considerable morphological heterogeneity within astrocytes. Explain.

Answer 62

Several forms of astrocytes exist in the CNS including fibrous (found in white matter), protoplasmic (in grey matter: these are the most common), and radial.

Question 63

Fibrous astrocytes have “vascular feet” that physically connect them to the outside of capillary walls.

Answer 63

Radial glia (radial astrocytes) are oriented in a plane perpendicular to the axis of the ventricles, with one axis towards the pia and the other near the ventricle. They are mostly present during development and play a role in neuron migration (see figure).

Question 64

What are some ex of radial astrocytes that persist into adulthood?

Answer 64

Mueller cells (retina) and Bergmann glia (cerebellum)

Question 65

What are Satellite cells?

Answer 65

Small cuboidal cells of neural crest origin. They are modified Schwann cells (PNS) or oligodendrocytes (CNS) and function as astrocytes in peripheral ganglia.

•They surround the entire soma of ganglion cells, but only their nucleus is visible in H&E stains

Question 66

The ventricles of the brain and the central canal of the spinal cord are lined with what?

Answer 66

ependymal cells

Question 67

Describe ependymal cells

Answer 67

The cells are often ciliated and form a simple cuboidal or low columnar epithelium. The lack of tight junctions between ependymal cells allows a free exchange between cerebrospinal fluid and nervous tissue.

The apical surface is covered with cilia (C) and microvilli (M).

The basal surface is in close contact with astrocytes.

Question 68

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Question 71

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Question 72

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Question 73

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