Nervous System Flashcards
Broadly speaking, the nervous system consists of two major cell types: ____ and _____.
Broadly speaking, the nervous system consists of two major cell types: neurons and glia.
The cells you see in the CNS that are not neurons are mostly glia. There are two major types in the CNS: astrocytes and oligodendrocytes.
The pathway of sympathetic fibers to internal organs
The preganglionic neurons pass through the sympathetic trunk without synapsing and form splanchnic (visceral) nerves that will synapse in prevertebral (pre= anterior to, in front of) ganglia on second order neurons which innervate the target organs.
The prevertebral ganglia include the aorticorenal ganglion and the celiac, superior mesenteric, and inferior mesenteric ganglia (which innervate the digestive tract and are associated with the vessels of the same name).
Node of Ranvier Cartoon
The submucosal (Meissner’s) plexus is less well-organized and harder to find. Look for clusters of cells with large, round nuclei surrounded by basophilic cytoplasm (i.e., not fibroblasts) in the submucosal connective tissue. You may see small vessels nearby and capillaries intimately associated with the ganglia.
Dorsal Root Ganglion
The neuron cell bodies are large and round, with a round, euchromatic nucleus with prominent nucleoli. Each neuron contains a single long process that bifurcates (you can’t see it in this image). One branch carries information from the periphery, having received sensory information from a receptor; the other branch leads centrally into the spinal cord.
Note the nuclei of the satellite cells (glia), which surround the ganglion cell body – another characteristic of sensory neurons.
Spinal cord H and E
Astrocytes
The main “supporting” cells of the CNS, and proliferate in response to injury.
Which four cranial nerves carry parasympathetic nerve fibers?
III, VII, IX and X
But only X, the vagus nerve, leaves the head to innervate viscera in the thorax and abdomen.
Spinal white matter
Schwann cells
Peripheral nerves consist of axons and the glial cells, known as Schwann cells, that myelinate them.
The myenteric (Auerbach’s) plexus can be seen here sandwiched between the two layers of smooth muscle. It typically forms a well-organized layer.
Spinal gray matter regions
The spinal cord gray matter contains three regions where most of the cell bodies are located which are of major functional importance:
(1) The anterior (ventral) horn – this is where the cell bodies of motor neurons that innervate skeletal muscle are located.
(2) The posterior (dorsal) horn – this is where somatic sensory neurons are located. Note that the cell bodies of these neurons are not found here – they are in the dorsal root ganglia, as we will see shortly.
(3) The intermediate horn, also known as the lateral horn or intermediolateral cell column (ICC) – this is where the cell bodies of efferent (“visceral motor”) neurons of the sympathetic nervous system are located. Note that this region will not be present at all levels of the spinal cord.
Neurons
Neurons, the functional cells of the nervous system, contain a cell body and its processes, axons and dendrites.
Glial cells support the neuron and its processes.
Nodes of Ranvier allow for ____.
Nodes of Ranvier allow for saltatory conduction.
They allow action potentials to “jump” from one node to the next rather than traveling continuously along the axon. This significantly accelerates the velocity of nerve conduction.
The sympathetic first order neuron originates from _____.
The sympathetic first order neuron originates from the lateral gray horns (AKA intermediate horn or intermediolateral cell column) of the spinal cord in spinal levels T1 to L2.
These fibers exit the spinal cord as motor efferents in the ventral rootlets, follow the ventral root and leave the spinal nerve to synapse on second order ganglia.
To supply the body wall and limbs, the preganglionic neuron synapses on the second (postganglionic) neuron in the ganglia of the sympathetic trunks. The sympathetic trunks are a pair of nerve trunks and ganglia which run from the base of the skull to the coccyx (see schematic diagram on next page). This effectively distributes the sympathetic nerve fibers, so they join all 33 pairs of spinal nerves and follow the dorsal and ventral rami and their branches.
Microglia
Tissue-resident macrophages of the CNS
enteric nervous system
regulates contraction of smooth muscle in the gut. It consists of two networks of nerves, the myenteric (a.k.a. Auerbach’s) plexus and the submucosal (a.k.a. Meissner’s) plexus.
The myenteric plexus (myo = muscle), the larger of the two plexuses, is located between the two perpendicular layers of smooth muscle in the gut wall and regulates their contraction.
The submucosal plexus is located just below (sub-) the epithelium (mucosa) in the connective tissue, and regulates absorption and secretion.
The relationship of nerve endings to the skeletal muscle and organs they innervate can be difficult to see using standard methods. In this image, a gold staining technique has been used to show the relationship of the axons of motor neurons to striated skeletal muscle fibers.
The Adrenal Medulla
Staining myelin
Many of the axons are myelinated, and myelin (which is largely lipid) is poorly rendered on H&E stained sections, hence the amorphous appearance.
It is better visualized by a special stain for myelin (known as a Luxol fast blue or LFB-PAS stain), which highlights myelin blue
Autonomic nervous system
Nodes of Ranvier
The myelin sheath of an individual axon is provided by multiple Schwann cells, with each Schwann cell covering only a segment of the axon. Between the Schwann cells, there are short intervals where the axon is not covered by a myelin sheath. These points are known as nodes of Ranvier.
Peripheral nerve cross-section
On H&E stained sections, you can “see” the myelin sheath surrounding each axon as an unstained area surrounding each axon (the lipid-rich myelin is extracted during processing, so it appears unstained).
The nuclei belong to the Schwann cells – remember that there are no neuron cell bodies in a peripheral nerve!
Sympathetic vs Parasympathetic outflow
Spinal cord LFB-PAS
At high power, neurons in the spinal cord often have a vaguely triangular appearance (they are known as pyramidal cells), thanks to the axon which creates a tapered appearance at one end.
The nuclei are large, round, central and euchromatic, indicative of a high level of transcription and translation.
Oligodendrocytes
Oligodendrocytes synthesizes the myelin that ensheaths axons in the CNS; they can be identified by the clear “halo” around the nucleus (from the myelin, which is lost in preparation).
Nissl staining of spinal tissue.
The internal structure of the neuron is poorly visualized by H&E stain; special stains are often necessary to see it. This stain highlights the spiculated appearance of the neuron cell body; many dendrites are associated with each cell. You may also see dark clusters of basophilic ribosomes in the rough endoplasmic reticulum (Nissl bodies or Nissl substance) in the cytoplasm.
Neurons also have a clear area at the base of the axon called the axon hillock; this is where the axon emerges from the cell body
Gray matter vs White matter
Nerve cell bodies and supporting cells are located in the spinal cord gray matter;
nerve cell processes and supporting cells are located in the spinal cord white matter.
Spinal gray matter
Neuropil
The area around the cells in the CNS (which makes up the majority of the tissue) appears acellular and largely amorphous on H&E stain. This material is known as neuropil, and contains the axons and dendrites emanating from the neurons.
Location of nerve fiber bodies in spinal cord, roots, and rami
