Nervous tissue is comprised of ______ and their support cells called ______.
neurons and neuroglia
The ____ is the functional and structural unit of the nervous system.
Discuss the general functions and/or contents of the cell body, axons, dendrites, oligodendryocytes, and synapse.
cell body: contains nucleus and other cellular organelles. proceses info from incoming neurons
axons: send action potential and synapse on another neuron or effector organ (muscle, gland)
dendrites: processes coming off of the cell body where neurons synapse
oligodendrocytes: type of glial cell that myelinates axons in the CNS
How are neurons classified?
1. Based on connections and types of information they carry
2. Based on number of processes extending from the cell body (dendrites)
The cell bodies of sensory neurons (GSA, GVA) are located in the ___ _____ _____. Information flows from the ____ ___ of the psinal cord via the dorsal roots.
1. dorsal root ganglion
2. dorsal horn (DH)
Motor neurons carry information from the CNS or _______ ganglia to effector organs such as muscle and glands.
What are the names of cell bodies of efferent neurons? Where are they located? Where do their axons exit the spinal cord?
1. alpha motor neurons
2. located in ventral (anterior) horn (VH) of spinal cord where their axons exit via the ventral root
Describe the 2 neuron chain system in of general visceral efferent (GVE) axons.
The cell bodies of the first order neuron are located in the brain stem or spinal cord. The second order neuron is located in autonomic ganglia. These are typically the sympathetic chain
(paravertebral) ganglia for the sympathetic NS. Ganglia of the parasympathetic nervous system are typically located in the wall or close to the effector organ. See pg 31 of course notes for visual of parasympathetic plexuses
What are interneurons?
neurons that interconnect other neurons. sometimes connect sensory and motor neurons (such as in knee jerk reflex in spinal cord). have an integrative fxn. 99.9% of all neurons are interneurons.
What are multipolar, bipolar, and psuedounipolar neurons? Where in the nervous system are they found?
multipolar: have one axon and multiple dendrites. motor neurons, interneurons, and most CNS neurons are multipolar
biploar: have 2 cytoplasmic extensions where one serves as the axon and the other a dendrit. both processes (axon and dendrite) can have many smaller branches. are associated with receptors for special senses such as in the retina
pseuodunipolar: have one long axon that divides close to the cell body into 2 long axonal branches. begin as bipolar neurons during development. Example is in dorsal root ganglion containg cell bodies of peripheral sensory neruons projecting into spinal cord. true unipolar neurons are only seen in invertebrates
Discuss the cellular contents of the cell body (soma) or nuerons.
contains a large pale-staining nucleus (euchromatin: active gene transription). has 1-2 prominent nuceloli for rRNA transcription. cytplasm has abundant RER and free ribosomes called Nissl bodies/Nissl substance reflecting the large amount of protein synthesis. Goli apparatus further processes and packages proteins. Have numerous mitochondria due to high energy needs. Please see pg 33 of course notes for an EM of the soma.
What 2 cellular components are not in the axon hillock? What does the axon hillock do?
There are no Nissl bodies or Golgi apparatuses in the axon hillock (no protein processing) but does contain all other cytoplasmic structures. The axon hillock is where an action potential is generated
What are the cellular contents of dendrites? What is the array and morphology of dendrites dependent on? What is contained on dendrites and what is their function? Discuss the plasticity of dendrites?
Dendrites contain the same organelles as axons. The total array and specific morphology depend on neuronal type and location. Dendrites may contain dendritic spines: tiny proteuberances that are sites of synaptic contacts. The number of dendritic spines may change, reflecting synaptic plasticity
What are the 3 types of cytoskeletal proteins contained in neurons? Where in the neurons are they contained?
microtubules, neurofilaments, microfilaments (actin)
are located in all portions of the neuron (cell body and processes). have support functions as well as other specialized functions.
What is the location and function of microtubules? What proteins are associated with them and what are their functions?
microtubules are cylindrical assemblies of tubulin with additional associated proteins involved in axonal transport but also exist in axons and dendrites. Transport on microtubules is bidirectional: anterograde transport (away from the cell body) is performed by the ATPase kinesin and retrograde transport (toward the cell body) is mediated by the ATPase dynein.
clinical note: some viruses (e.g. herpes) are taken up by nerve cell terminals and are retrogradely transported to cell bodies thereby gaining entrance into CNS
What are the two types of axonal transport? In what directions (anterograde/retrograde) do they occur and what types of things do they transport? Which is more important for neuronal activity?
Slow (1-4 mm/day): only occurs in anterograde direction. transports soluble proteins such as cytoskeletal proteins and enzymes. note that this is the same rate as peripheral nerve regneration!
Fast (20-400 mm/day): occurs in anterograde and retrograde direction. transports neurotransmitter precursor vesicles, some neurotransmitters, mitochondria, lysosomes, membrane components
Fast axonal transport is more important for neuronal activity
What type of cytoskeletal protein are neurofilaments? What are their functions? How are they visualized histologically?
Neurofilaments are a type of intermediate filament in the cytokeratin family. Is a ropelike assembly of polymers. Have support functions and play role in neuronal development and regeneration. Are visualized with silver staining as "neurofibrils" in all parts of the neuron.
What are microfilaments? What are their functions in neurons?
Thin twisted pairs of actin fillaments (G-actin). Anchor molecules to the membrane (such as receptors), transfer molecules to and from the cell membrane. and cause movement of growing axonal tips during development and regeneration
Discuss what occurs at a synapse and be sure to include structural components.
The presynaptic terminal ("bouton") of an axon abuts the postsynaptic cell and is separtated by a synaptic cleft. Axon terminals of the presynatic neuron contains synaptic vesicles, membrane bound packages of neurotransmitter. The postsynaptic membrane is thickened (postsynaptic density) indicating location of receptors and other molecules important in synaptic transmission. When nerve impulses arrive at a presynaptic terminal, neurotransmitter is released, diffuses across the cleft, and binds to specific receptors on the postsynaptic membrane. There may be up to 80k synapses on a single neuron (convergence)
What are the 4 categories of synapses?
1. axo-dendritic: axon synapsing on a dendrite. majority of synapses are of this type (b in picture)
2. axosomatic: axon synpases on cell body of another neuron (a in picture)
3. axoaxonic: axon synapses on another axon (c in picture)
4. dendrodendritic: dendrite synpases on another dendrite
What are chemical synapses?
contain specific neurotransmitters that can be excitatory or inhibitory (depending on NT and receptor type)
What are the 2 neuroglial cells in the PNS and what are their general functions?
1. Schwann cells and satellite cells
2. myelination and support
What cells myelinate axons in the PNS? What is myelin rich in? What are the segments btwn myelin sheaths called? What types of receptors lie btwn myelin sheaths and what does this result in as it pertains to APs?
Schwann cells myelinate peripheral axons. Myelin sheaths consist of concentric rings of Schwann cell cytoplasm wrapped around the axons, creating lamellae (layers). Myelin sheaths are lipid rich and insulate axons, increasing the speed of AP conduction. A node of Ranvier exists btwn two adjacent Schwann cells where there is no myelin sheath. The area btwn 2 nodes of Ranvier where there is a myelin sheath is called an internodal segment. Na+ channels are highly concentrated at nodes of Ranvier to allow for nerve propagation in a fast saltatory fashion from one node ot the next. The thicker the myelin sheath, the faster rate of nerve conduction. Axons larger in diameter have thicker myelin sheaths.
How does nerve conduction velocity (NCV) relate to myelination?
Axons with thicker myelin sheaths have faster NCV while unmeylinated axons have the slowest rates of NCV. Larger diameter axons have thicker myelin sheaths and faster NCV
What are Schmidt-Lanterman's clefts? What is their function? How are they visualized in light microscopy?
Schmidt-Lantermann's clefts are small islands of Schwann cell cytoplasm btwn successive lamellae of myelin. Allows for intracellular communcation. Have characteristic fishbone appearance on light microscopy. see pg 42 and 43 of course notes
Schwann cell cytoplasm also surrounds and nurtures unmeylinated axons. Discuss the structural differences of Schwann cells in unmyelinated axons.
There are no concentric rings or nodes of Ranvier and nerve impulse propagation is therefore much slower
Peripheral ganglia (DRG, autonomic) arise from what type of embyrological cell?
neural crest cells
What are satellite cells? What is their function? Where are they found?
satellite cells are flattened cells that surround neuronal cells bodies in peripheral ganglia (DRG, autonomic) and are analogous to Schwann cells. Funciton in maintaining a controlled microenvironment around the neuron, provide electrical insulation, and proivde a path for metabolic exchange
What type of neurons are located in DRG? What are histological features of these cells? What support cells are they asscoiated with? Do synapses occur in the DRG?
DRG contains cell bodies of pseudounipolar neurons. Their nuclei are centrally located and are completely surrounded by satellite cells. There are no synapses in the DRG. See pg 44 of course notes
What type of neurons are located in autonomic ganglia What are histological features of these cells? What support cells are they asscoiated with? Do synapses occur in autonomic ganglia?
Autonomic ganglia contain multipolar neurons with eccentric nulcei that often contain lipfuscin pigment granules. Satellite cells are present but are less numerous than in DRG. Nerve fibers tend to be unmyelinated. Synapses do occur in autonomic ganglia.
What structures are contained within a peripheral nerve?
axons (some with myelin sheaths, some unmyelinated), 3 connective tissue layers, and blood vessels
What are the 3 connective tissue layers within a peripheral nerve and what cells produce them? What layers is responsible for the blood-nerve/permeability barrier? What is the permeability barriers function?
fibroblasts produce collagen and ECM for the 3 connective tissue coverings
epineurium: surrounds entire peripheral nerve
perineurium: surrounds a bundle of nerve fibers (fasicle)
endoneurium: surrounds individual axons
perineurium is responsible for forming the permeability barrier which functions to maintain the ionic environment of nerve fibers. tight junctions occur btwn perineurial fibroblasts which also have receptors/transporters that regulate transport across them.
True or false: Typically, no lymphocytes or plasma cells can be found within the perineurium or endoneurium compartments of a peripheral nerve.
What types of information do sensory nerves convey?
pain, temperature, touch, deep pressure, proprioception, and kinesthesia (detection of movement through space)
Discuss the myelination of axons that transmit fine discriminatory touch and proprioception vs those that transmit pain and temperature.
axons that transmit fine discriminatory touch and proprioception are heavily myelinated and therefore have faster NCV. axons that transmite pain and temperature tend to be unmyelinated and therefore have slower NCV.
True or false: PNS nerve injury is classified according to the degree of trauma.
Discuss anterograde Wallerian degeneration in peripheral nerve injury.
Within a few hours of injury this process begins in the axon distal to the injury
1. axons swells and disintegrates
2.. debris is removed by invading migratory macrophages. do so by breaking down the tight junctions in the perineurium (blood-nerve barrier) along the length of the entire axon
3. Schwann cell dedifferentiates once axon degenerates and myelin sheath is broken down
Discuss retrograde chromatolysis in peripheral nerve degeneration.
occurs proximally in the cell bodies of origin (DRG for sensory neurons and spinal cord for motor neurons)
1. cell body swells
2. nucleus shifts to periphery (eccentric)
3. Nissl substance breaks down.
Discuss axonal regeneration in the CNS and PNS.
Regeneration in CNS does not occur.
In the PNS, Schwann cells secrete growth factors, divide, and line up as guidance tubes for new axon sprouts (inner lining has the neurotrophic substance laminin). If loss of axoplasm is extensive (injury closer to cell body), the neuron is more likely to die and not regenerate. If a motor neuron reestablishes contact with its muscle, recovery of function may occur. Peripheral nerve regeneration occurs at rate of slow axonal transport (1-4 mm/day)
during nerve regeneration, fiber mismatch of proximal with distal axons can occur leading to loss of specificity and less functional recovery. sprouts may also grow in a disorganized fashion without proper guidance tubes (created by Schwann cells) creating a painful traumatic neuroma at the site of injury in this instance, no reinnervation occurs.
What is Guillan-Barre syndrome?
autoimmune condition that destroys Schwann cells resulting in demyelination of peripheral nerves. get large accumulation of lymphocytes, macrophages, and plasma cells around nerve fibers (within perineurium and endoneurium). results in loss of cutaneous sensation and symmetrical, ascending muscle weakness beginning in lower ext