Structure of Nerve Tissue

What is a peripheral nerve?

An organ consisting of axons, some myelin sheaths, 3 connective tissue types, and blood vessels.

What happens a few hours after transection of a nerve?

Anterograde Wallerian Degeneration of the axon begins distal to the injury or cut.

What are the categories of synapses?

Axo-dendritic - most are of this type. axon synapses on a dendrite (b) Axosomatic: axon synapses on cell body of another neuron (a) Axoaxonic: axon synapses on another axon (c) Dendrodentric: synapse between dendrites of two neurons

Which modality's nerves are myelinated? What does the myelination cause?

Axons transmitting fine discriminatory touch and proprioception are heavily myelinated and therefore have fast nerve conduction velocities.

What are bipolar neurons? What are the most common types?

Bipolar neurons have one axon and one dendrite. They are associated with receptors for special senses.

What are the 3 connective tissue layers around a peripheral nerve?

Epineurium - surrounds the entire nerve Perineurium - surrounds a bundle of nerve fibers, creating a FASCICLE Endoneurium - surrounds individual axons

T/F: The perineurium does not need to be rejoined during microsurgery for limb attachment.

False. The perineurium must be rejoined during microsurgery for limb attachment.

What is the fuction of somatic motor neurons (GSE)?

GSE are for voluntary, skeletal muscles that moves body parts.

Where are the ganglia for the parasympathetic nervous system located?

Ganglia located in the wall or close to effector organ for parasympathetic nervous system.

Where is the parasympathetic nervous system located?

In craniosacral regions. Cell bodies of second order neurons located in autonomic ganglia

Where is the sympathetic nervous system located?

In thoracolumbar (T1-L2) region

What are the components of the nerve?

The cell body - contains nucleus and other organelles Axons + dendrites - processes Typically there is one axon per nerve that transmit to the dendrites of the next neuron or effector organ via synapse. There can be multiple dendrites

What is the axon hillcock?

The initial segment of axon which does not contain Nissl bodies or golgi, but does have other cytoplasmic structures (mitochondria, RER, free ribosomes). It is the most electrically excitable part of the axon where a nerve impulse (action potential) is generated.

What contributes to formation of a blood-nerve barrier in peripheral nerves? What is this barrier called?

The perineurium that surrounds a nerve fascile contributes to formation of a blood-nerve barrier. It is called the Permeability Barrier.

What is the rate of axonal regeneration down a peripheral nerve?

The same rate as slow axonal transport: 1-4mm/day

What is the synapse?

The synaptich cleft where the Bouton abuts the postsynaptic cell. There may be up to 80,000 synapses on a single neuron, called a convergence.

What types of channels are highly concentrated at the nodes of ranvier and what is its purpose?

There are a lot of Na+ channels. They form the basis for nerve impulse propagation in a fast "saltatory" fashion from one node to the next.

Describe neurofilaments of the neuronal cytoskeleton.

They are an intermediate filament. They are a ropelike assembly of polymers in the cytokeratin family They are a support function and play a role in neuronal development and regeneration The silver stains allow visualization as "neurofibrils" in all parts of the neuron.

Describe the microtubules of the neuronal cytoskeleton

They are cylindrical assemblies of the protein tubulin with additional assocated proteins involved in axonal transport (but they exist in both axons and dendrites) They are bidirectional: Anterograde - moves away from cell body, mediated by kinesin (atpase provides energy for motor) Retrograde - moves towards the body by dynein, also by atpase.

What are satellite cells?

They are flattened cells that surround neuronal cell bodies in peripheral ganglia (autonomic and dorsal root) and are analogous to Schwann cells. They function in maintaining a controlled microenvironment around neuron, providing electrical insulation and providing a path for metabolic exchange.

What is the role of neuroglial cells?

They are for support and myelination in PNS.

What is the area between two nodes of ranvier?

They are internodal segment where there is myelin sheath.

What are the layers of schwann cell cytoplasm wrapped around the axon?

They are lamellae.

What are autonomic ganglia?

They are multipolar neurons with eccentric nuclei that often contain lipofuscin pigment granules. Synapes do occur here There are satellite cells but less numerous.

What are schmidt-lanterman's clefts?

They are small islands of schwan cell cytoplasm between successive lamellae of the myelin. This creates characteristic fishbone appearance in LM. They allow for intracellular communication.

What are dorsal root ganglion cells? (DRG)

They are the cell bodies of pseudounipolar sensory neurons. They have centrally located nuclei, with cell bodies surrounded by satellite cells which are analogous to schwann cells and are continuous with schwann cells investing axons. There are no synapses in DRG.

What is the fuction of GVE, autonomic/general visceral efferent?

They carry involuntary impulses to smooth and cardiac muscles and glands (sweat glands) and smooth muscles located in wall of blood vessels and organs.

What is the structure of GVE?

They come in a two neuron chain. The cell body of the first neuron is located in the brain stem or spinal cord. The second is outside the CNS, in the autonomic ganglia (synapse occurs here). These are typically the sympathetic chain (paravertebral)

What is the node of ranvier?

They exist between two adjacent schwanna cells where there is no myelin sheath.

What are unipolar/pseudo-unipolar neurons? What are the most common types?

They have one long process (axon) that divides close to the cell body into 2 long axonal branches. True unipolar are only seen in invertebrates. Vertebrate unipolars begin as bipolar and during development the cell body expands asymmetrically to leave a stalk from which both processes emerge. Example is the dorsal root ganglion cells containing cell bodies of peripheral sensory neurons projecting into the spinal cord.

What organelles do dendritic trees contain?

They have the same organelles as axons (RER, free ribosomes, mitochondria). Dendritic spines

Describe microfilaments of the neuronal cytoskeleton.

Thinnest Twisted pairs of actin filament Anchor molecules to membrane (such as receptors) Transfer molecules to and from cell membrane Cause movement of growing axonal tips during development and regeneration.

What are dendritic spines?

Tiny protuberances that are sites of synaptic contacts. Numbers may change reflecting synaptic plasticity (modifiability of synapses).

T/F: Typically there are no lymphocytes or plasma cells found within the perinerium or endoneurium compartments

True

What are the stages of anterograde Wallerian Degeneration?

1. Axon swells and disintegrates and is removed by macrophages 2. Schwann cells dedifferentiate and the myelin sheath and blood/nerve barrier breaks down along its entire length (debris removed by invading migratory macrophages) 3. Schwann cells secrete growth factors, divide and line up as guidance tubes for new axon sprouts (inner lining has neurotrophic substance laminin).

What is involved in retrograde chromatolysis?

1. The cell body swells 2. The nucleus shifts to the periphery (eccentric) 3. The Nissl substance breaks down. If the loss of axoplasm is extensive (more proximal cut or more severe injury), the neuron is more likely to die and not be replaced.

What is a neuromuscular junction? What is another name for this type of junction?

A specialized synapse on skeletal muscle with acetylcholene as the neurotransmitter and junctional folds on muscle fiber membrane. This is also called a motor end plate.

What neurotransmitter is used in neuromuscular junctions?

Acetylcholine

What route does information flow to GSA cell bodies?

Cell bodies are located in the dorsal root ganglion and information flows into the dorsal horn of the spinal cord via the dorsal root.

What are the second order neurons?

Cell bodies of the second order neuron are located in the autonomic ganglia, typically located in the sympathetic chain (or paravertebral).

What is a chemical synapse?

Contains specific neurotransmitters. They come in Excitatory vs inhibitory depending on the neurotransmitter type.

What are interneurons?

Interneurons interconnect other neurons, sometimes they connect sensory and motor neurons. They serve an integrative function. 99.9% of all neurons are of this type.

What is the purpose of the myelin?

It is a lipid rich structure that insulates the axon, increasing the speed of nerve conduction. The thicker the myelin sheath, the faster the rate of nerve conduction. Larger diameter axons have thicker myelin sheaths.

Where is the postsynaptic membrane thickened? What is the term for this thickening?

It is called the postsynaptic density. It indicates the location of receptor and other molecules important in synaptic transmission.

What is the neuronal cytoskeleton composed of?

It is composed of 3 types of long filamentous protein polymers located throughout neurons and processes, all playing a role in support. 1. Microtubules 2. Neurofilaments (an intermediate filament) 3. Microfilaments

What is the dendritic tree?

It is the total array of dendrites on a particular neuron and specific morphology depends on cell type and location.

What does the Permeability Barrier do?

It maintains the ionic environment of the nerve fibers. Tight junctions occur between perineurial cells which form basis for blood-nerve barrier. The perineurial fibroblasts have receptors/transporters regulating active transport across them.

What is contained within the cell body of a neuron (the soma)?

Large pale staining nucleus Dispersed chromatin for active gene transcription 1-2 prominent nucleoli (transcribes rRNA) Nissl bodies/substance - Cytoplasm with abundant RER and free ribosomes Golgi apparatus - further processes and packages proteins Mitochondria - energy

What happens if there is fiber mismatch during healing?

Loss of specificity and less functional recovery.

What are the degrees of injury to peripheral nerves?

Mild compression (demyelination) Nerve crush (axons disconnected) Complete transection (whole nerve cut)

What are motor neurons?

Motor neurons carry information from the CNS or autonomic ganglia to effector organs such as muscle and glands. They are also known as efferent neurons.

What are the classifications of neurons, based on the number of processes extending from the cell body?

Multipolar Bipolar Unipolar or Pseudo-unipolar

What are the support cells of neurons?

Neuroglia

Which modality's nerves are unmyelinated?

Pain Temperature

What happens to the cells proximal to the injury?

Retrograde Chromatolysis (in the DRG for sensory neurons and spinal cord for motor neurons)

What types of cells are part of the peripherial neuroglia category?

Schwann and satellite cells.

How do unmyelinated axons get nutrients?

Schwann cell cytoplasm also surround and nurtures unmyelinated axons, altho there are no concentric rings or nodes of ranvier and nerve impulse propagation is therefore much slower.

What do schwann cells do?

Schwann cells myelinate peripheral axons.

What are the types of neurons, based on connection type?

Sensory Neurons Motor Neurons

What are sensory neurons?

Sensory neurons are also known as afferent or GSA neurons. They bring in information such as somatic sensation (touch, temp, pain), proprioception, and kinesthesia (perception of movement in joint).

What are sensory receptors and what do they do?

Sensory receptors are transducers of physical stimuli into an electrical signal. They encode the nature, location, intensity, and duration of sensory stimuli.

What is the modality of sensory receptors?

Sensory receptors detect pain, temp, touch, deep pressure, proprioception, and kinesthesia.

What are the rate of motion of microtubules?

Slow (1-4mm/day) only in antegrade direction, transporting soluble proteins. It is the same rate as peripheral nerve regeneration. Fast (20-400mm/day) in both anterograde and retrograde. It transports neurotransmitter precursor vesicles, some neurotransmitters, mitochondria, lysosomes, membrane components.

What types of motor neurons are there?

Somatic motor neuron (GSE) Autonomic/General Visceral Efferent (GVE)

How does nerve regeneration happen, if at all?

Some sensory and motor axons grow into the neduneural guidance tubes formed by Schwann cells. Neurite "sprouts" containing growth cones grow from the distal stump of the axon and find the distal targets. If a motor neuron re-establishes contact with its muscle, recovery of function may occur.

What happens if there aren't proper guidance tubes?

Sprouts may grow in a disorganized fashion, resulting in painful traumatic neuroma (tumor) at the site of injury, with no reinnervation.

What is another name for the presynaptic terminal?

The Bouton

What does the axon terminal contain?

The axon terminal contains synaptic vesicles which are membrane bound packages of neurotransmitters.

What are the names of the GSE cell bodies and where are they located?

The cell bodies are called alpha motor neurons (the arrows). They are located in the ventral (anterior) horn of the spinal cord and their axons exit via the ventral root.

Where are sensory receptors located?

The cell bodies are located in the DRG and information enters the dorsal horn of the spinal cord via the dorsal root.

What is Guillain-Barre syndrome?

an acute inflammatory demyelination (autoimmune) that destroys Schwann cells. A large T cell mediated immune response directed against peripheral myelin results in a large accumulation of lymphocytes, macrophages, and plasma cells around nerve fibers (within endoneurium and perineurium). This causes loss of cutaneous sensation and symmetrical, ascending muscle weakness begining in lower extremities. Most patients survive and usually have complete recovery.

What are multipolar neurons? What are the most common types?

multipolar neurons have one axon and 2 or more dendrites. The most common types are motor neurons and interneurons