Nerve (Exam II) Flashcards
Cells of the nervous system include:
Neurons & Glia
- In the CNS: oligodendrocytes, astrocytes, microglia and ependymal cells.
- In the PNS: Schwann cells & satellite cells (provide nutrients to neurons).
Classify the three types of functional neurons
- Somatic motor neurons: ventral horn of spinal cord that innervate skeletal muscle. (multipolar)
- Autonomic neurons (motor): preganglions in spinal cord & brain stem while the postganglions are in autonomic ganglia. They innervate smooth muscle, glands and cardiac conduction system. (multipolar)
- Sensory neurons: in dorsal root ganglion or sensory ganglia. Receive sensory input from receptors and free nerve endings. (pseudounipolar)
Define afferent neuron vs. efferent neuron
- Afferent neurons are sensory neurons. They receive informational input from the body via free nerve endings or a variety of specialized receptors, and transmit it to the central nervous system (CNS).
- Efferent neurons are motor neurons. They carry information from the CNS to effector organs (muscles, glands) via peripheral nerves. They control muscle contraction and glandular secretion.
- “Afferent” and “efferent” are also often used as relative terms. Specifically, afferents include every neuron that sends an axon to the region in question, while efferents are all the neurons that project from the region in question.
What is the difference between a somatic motor neuron and a visceral motor neuron?
- Somatic motor neurons innervate skeletal muscle cells.
- Visceral motor neurons mainly innervate glands, smooth muscle, and the modified cardiac muscle cells that make up the conduction system of the heart.
- Visceral motor neurons make up the autonomic nervous system, and can be subdivided into sympathetic neurons, parasympathetic neurons, and the enteric neurons of the GI tract. Autonomic axons also innervate a variety of lymphoid organs and may mediate some neuro-immune interactions. There may be other examples of autonomic innervation of non-gland, non-muscle targets.
In the spinal cord, are neuronal cell bodies and glia contained in the gray matter or in the white matter?
- They are in the gray matter.
- The white matter is made up of axons, many of which are myelinated. It is the myelin that imparts the whitish color to fresh unfixed white matter.
What is meant by the terms pseudounipolar neuron, bipolar neuron and multipolar neuron? Give an example of each.
- A pseudounipolar neuron has only one process originating from its cell body. During development, this process is formed by the fusion of two previously separate processes (i.e., the neuron starts out bipolar, but becomes “pseudo”unipolar when mature). The sensory neurons that make up the dorsal root ganglion are pseudounipolar. Their single process divides into a peripheral process that collects sensory input from the body, and a central process that carries the information into the dorsal horn of the spinal cord.
- Bipolar neurons retain two cell processes when mature. The olfactory receptor cells in the olfactory epithelium and the bipolar cells of the retina are examples of bipolar neurons.
- Multipolar neurons are the most common. Each has more than two processes – usually one axon and numerous dendrites. Autonomic neurons and somatic motor neurons are multipolar.
What is the functional difference between an axon and a dendrite? What are some of their structural differences?
- Functionally, dendrites receive input from other neurons, whereas axons carry action potentials that are triggered by these inputs.
- Structurally dendrites are usually short, rapidly tapering, highly branched processes. The repeated branching increases the receptive surface area for collecting input signals, and gives multipolar neurons a tree-like appearance (dendron = Greek for tree).
- In contrast, axons tend to be long processes that taper very gradually. They can branch, but they do so much less frequently than dendrites. Branches that occur near the cell body are either collateral branches or recurrent branches. The more abundant ones that occur near the end of the axon are called preterminal branches (telodendria).
- Axons may be myelinated or unmyelinated.
- True dendrites are never myelinated. There are many other differences between axons and dendrites that are not reflected in their structure. For example, most of the microtubules in an axon have their plus end directed distally, while the microtubules in a dendrite are not polarized in this way. Axons and dendrites also differ in the types of microtubule-associated proteins that are present, and the types of ion channels found in their membranes. NOTE: The peripheral processes of pseudounipolar neurons are sometimes referred to as dendrites, but they differ in several ways from true dendrites. For example, they are usually myelinated, they rarely branch, and they don’t receive input from other neurons. They should be called peripheral processes rather than dendrites to avoid confusion.
What is the correlation between degree of myelination, axon diameter and conduction speed?
The larger the diameter of the axon, the thicker the myelin layer tends to be and the faster the speed at which action potentials are conducted.
What is the name for a group of neuronal cells bodies located in the peripheral nervous system (PNS)? In the central nervous system (CNS)?
A cluster of neuronal cell bodies in the PNS is called a ganglion.
In the CNS, such a grouping of neuronal cell bodies is called a nucleus (not to be confused with the organelle contained within each individual neuron).
Are there synapses in ganglia?
- Autonomic ganglia contain synapses. These are the sites where the preganglionic neurons end on the postganglionic.
- In contrast to this, sensory ganglia (e.g., dorsal root ganglia) contain neuronal cell bodies but no synapses.
Name several morphological criteria that you could use to distinguish between a dorsal root ganglion and an autonomic ganglion by light microscopy.
- The motor neurons in autonomic ganglia are smaller on average than the sensory neurons of the dorsal root ganglia, and are more likely to have eccentrically placed nuclei.
- The cells bodies of dorsal root ganglion cells usually appear to be surrounded by a greater number of satellite cell nuclei. In actuality, satellite cells completely surround the cell bodies of neurons in both types of ganglia, but the numerous processes of the multipolar autonomic neurons interrupt this layer at many points, making it appear less complete by light microscopy.
What are the structural components of a peripheral nerve? Can a single peripheral nerve include sensory fibers as well as motor fibers? Can it include myelinated as well as unmyelinated fibers?
- A peripheral nerve is defined as a group of neuronal processes (axons and peripheral processes of sensory neurons) that are bound together and surrounded by various connective tissue layers.
- Most peripheral nerves are mixed nerves that include sensory and motor fibers, which can be either myelinated or unmyelinated.
Where else can peripheral nerves originate other than from the spinal cord?
In the simplest view of the nervous system, peripheral nerves arise either from the spinal cord (spinal nerves) or from the brainstem (cranial nerves).
Which of the connective tissue layers of a peripheral nerve (endoneurium, perineurium, epineurium) directly surrounds each individual fascicle? What specialized function does this layer carry out?
- The perineurium directly surrounds each fascicle.
- The endoneurium surrounds each nerve fiber within a fascicle, while the epineurium fills the spaces between fascicles and covers the outer surface of the entire nerve.
- The perineurium is composed of unusual squamous myoepithelial cells that are contractile and often connected by tight junctions. They contribute to the formation of the blood-nerve barrier that controls the metabolic environment within the fascicle.
Do unmyelinated axons have nodes of Ranvier? Clefts of Schmidt-Lanterman?
No, unmyelinated nerves have neither nodes of Ranvier nor clefts (incisures) of Schmidt-Lanterman.
A node of Ranvier is structurally defined as the gap between two adjacent Schwann cells along the length of a myelinated axon. Although gaps certainly exist between the Schwann cells along an unmyelinated axon, they were not visible by light microscopy where the term “node of Ranvier” was coined, nor do they play a significant role in the transmission of the action potential the way nodes of Ranvier do in myelinated axons.
Unmyelinated nerves can’t have clefts of Schmidt-Lanterman, because these clefts are located in the myelin sheath itself. If there is no sheath, there are no clefts. In a longitudinally sectioned nerve fiber, a cleft looks like a pale V-shaped structure centered on the axon. In fact, a cleft is really a long tubular passageway that contains Schwann cell cytoplasm. It forms where the cytoplasmic faces of the Schwann cell plasma membrane fail to fuse with one another during the formation of myelin. It spirals inward through each turn of the myelin sheath, connecting the two layers of Schwann cell cytoplasm – one on the outside of the myelin sheath (where the nucleus is) and the other on the inside of the myelin sheath adjacent to the axon. A cleft represents a lifeline through which material can pass to keep this inner layer of cytoplasm viable.
If you see a peripheral nerve in your section, what other two structures are likely to be running with it?
- Nerves are often accompanied by an artery and one or more veins (companion veins or venae commitantes). This grouping of structures is called a neurovascular bundle.
- Sometimes a neurovascular bundle will also be accompanied by one or more small lymphatic vessels. Neurovascular bundles run through connective tissues, enter their target organs (skin, muscles, the gut, etc.), and often remain together as they divide into smaller and smaller branches.
With H&E it is sometimes difficult to distinguish between smooth muscle and peripheral nerves. Name one or two criteria that can help you do this.
- There is usually a more well-defined layer of connective tissue around a peripheral nerve than around smooth muscle, giving almost the appearance of a connective tissue capsule. In addition, individual nerve fibers are quite wavy, so that you may sometimes see a group of cross-sectioned nerve fibers side by side with a group of longitudinally sectioned fibers, even within a single nerve fascicle.
- This is probably due to abrupt changes in direction of the wavy fibers, and may explain why nerves can be stretched by a considerable amount without causing serious damage.
- In contrast, the cells in a smooth muscle bundle usually tend to be oriented in a single direction.
Define the term “motor unit”
A motor unit consists of one neuron and all the muscle cells that it innervates.
Name one ultrastructural criterion that can be used to distinguish a neuromuscular junction (motor end plate) on skeletal muscle from an autonomic terminal on smooth muscle.
- The most reliable criterion is the presence or absence of junctional folds on the muscle cell plasma membrane (sarcolemma) at the site of innervation.
- In skeletal muscle the part of the sarcolemma that lies immediately beneath the nerve terminal is thrown into numerous deep invaginations called junctional folds. In an autonomic ending on smooth muscle, junctional folds are absent. The width of the synaptic cleft can also be a useful secondary criterion.
- In skeletal muscle the synaptic cleft is very narrow, and the nerve terminal actually lies in a shallow depression in the sarcolemma, with junctional folds radiating out from this shallow trough. In large motor units of smooth muscle (“unitary” smooth muscle), such as in the GI tract, the nerve enlargements (boutons en passant) that contain the synaptic vesicles are located further from the muscle cells. When they release neurotransmitter it diffuses across the wide synaptic clefts to several nearby muscle cells rather than just one, and causes all of them to contract.
- The complicating factor is that there is a rarer type of smooth muscle (“multiunit” smooth muscle) where each muscle cell is individually innervated (e.g., the iris of the eye). Here the synaptic cleft is narrow as it is in skeletal muscle, but as with all smooth muscle, there are no junctional folds.
How do the synaptic vesicles in a cholinergic neuron differ morphologically from those in an adrenergic neuron?
- Cholinergic neurons use acetylcholine as their neurotransmitter. It is normally contained within small, round, empty-looking vesicles.
- Adrenergic neurons use adrenalin (epinephrine) as their neurotransmitter. It is normally contained within small, round, dark-staining (“dense-cored”) vesicles.
- The full story is a bit more complicated because other neurotransmitters do exist (e.g., serotonin, GABA), and the synaptic vesicles that contain some of them can resemble the vesicles of cholinergic or adrenergic endings.
Suppose you saw a nerve terminal that contained dense-cored synaptic vesicles. Which of the following is this neuron is likely to be: preganglionic sympathetic, preganglionic parasympathetic, postganglionic sympathetic, postganglionic parasympathetic, or a somatic motor neuron ending on skeletal muscle?
Since it contains dense-cored synaptic vesicles, this neuron is probably adrenergic. Of all the possibilities listed above, only postganglionic sympathetic neurons are normally adrenergic. All the others are cholinergic. In a very few locations (e.g., eccrine sweat glands) the postganglionic sympathetics are also cholinergic.
Name an encapsulated sensory ending that you would be most likely to find in the deep dermis or the hypodermis of the skin. Name one that you would you find in the dermal papillae.
Pacinian corpuscles are characteristically found deep in the dermis or in the hypodermis of the skin, while Meissner’s corpuscles are located in the dermal papillae just beneath the epidermis.
Name a sensory receptor other than a free nerve ending that you would find within the epidermis.
Merkel cells are sensory receptors that are derived from the epithelial stem cells of the epidermis. They are associated with sensory nerves that pierce the basement membrane of the epidermis and widen into flat plates (Merkel disks) that contact the cells. Small cytoplasmic vesicles are clustered in the Merkel cell cytoplasm near the nerve ending. Merkel cells act as mechanoreceptors.
What type of sensory receptor is responsible for the knee-jerk reflex (patellar tendon reflex)? How does tapping the tendon of the muscle cause this reflex?
Muscle spindles are the sensory receptors that initiate the knee-jerk reflex. The physician elicits this reflex by tapping the patellar tendon with a reflex hammer. The patellar tendon is the continuation of the tendon of the quadriceps femoris muscle of the thigh. Tapping it stretches the quadriceps muscle, including the intrafusal fibers within the muscle spindle.
As a result of this stretch, the afferent (sensory) nerve fibers that innervate the spindle are activated. Their cell bodies are located in the dorsal root ganglia. Their central processes synapse in the spinal cord on the somatic motor neurons that innervate the extrafusal fibers of the quadriceps muscle, causing it to contract and extend the leg at the knee. This is an example of a monosynaptic reflex since only one synapse is involved in the entire circuit from muscle spindle to spinal cord and back to muscle.
Clinically the knee-jerk reflex tests the integrity of the spinal cord at the L2-L4 (second to fourth lumbar) spinal segments, since it is from these segments that the motor neurons that innervate the quadriceps arise.
What type of ganglion are these?
Doral Root Ganglion: huge, nucleus not really visible.
Identify these types of neurons
Autonomic Multipolar Neurons (Postganglionic)
Identify the layers as indicated by the arrows.
Top arrows: perineurium
Bottom arrows: epineurium
Identify the intrafusal & extrafusal fibers
Top: intrafusal fibers
Bottom: extrafusal fibers
What type of receptor is this?
Meissner Corpuscle which is an encapsulated rapidly adapting mechanoreceptor for light touch (near skin).
What two components make up this sensory receptor?
Merkel Corpuscle = Merkel cell + Merkel disk.
Slowly adapting mechanoreceptor (fires throughout duration of stimulus)
What is this a cross-section of? Where are the arrows pointing?
Top arrows: extrafusal fibers
Botom arrows: intrafusal fibers
Picture: cross-section of encapsulated muscle spindle
What do the structures in this section respond to?
This cross section of a muscle spindle responds to passive stretch of the muscle and cause extrafsual fibers to contract.
Identify:
- Internal mesaxon
- Outer mesaxon
- Schwann cell nucleus
- Inner collar
- Outer collar
- Major dense line
- Intraperiod Line.
Internal mesaxon – 5
Outer mesaxon – 11
Schwann cell nucleus = 8
Inner collar = 9
Outer collar = 10
Major dense line = 7
Intraperiod Line = 6
Which are the myelinated axons? Which are the unmyelinated axons?
4 are myelinated axons while #2 are unmyelinated axons.
Identify the function of the structures marked as “NR” and “SL”
- Clefts of Schmidt-Lanterman: small pockets of cytoplasm left behind during the Schwann cell mylenation process. They are histological evidence of the small amount of cytoplasm that remains in the inner layer of the myelin sheath created by Schwann cellswrapping tightly around a nerve.
- Nodes of Ranvier: At nodes of Ranvier, the axonal membrane is uninsulated and, therefore, capable of generating electrical activity.
How do you tell what type of incisure this is?
This is a “Node of Raniver” because of the gap in myelin where cytoplasm exists.
What type of sensory receptor is this?
Encapsulated Pacinian Corpuscle that is a rapidly adapting mechanoreceptor.
Why is this incisure important?
The clefts of SL are important because they ensure that nutrients reach the different layers of myelin.
How can you tell the function of this ganglion?
It is large and has an euchromatic nucleus so it may be a DRG so it is sensory.
What distinguishes these ganglion from others?
They are larger than typical autonomic ganglion and their nuclei are in the center so they should be sensory ganglion.
Which is the pre-synaptic neuron and which is the post-synaptic neuron?
Top arrow: pre-synaptic neuron
Bottom arrow: post-synaptic neuron
What type of stain is this? What is it staining for?
Trichrome stain that stains for collagen.
What is this structure?
Unmyelinated PNS axons. Lighter-staining structures—each one is an axon. Many unmyelinated axons associated with an individual Schwann Cell.
