MA4 - PNS and Muscle Flashcards
What are the two components of the PNS?
somatic nervous system; autonomic nervous system
What are the divisions of the somatic nervous system?
afferent (sensation); efferent (locomotion)
What is the function of the autonomic nervous system? (3)
[broadly speaking, homeostatic control of internal tissues] control of heart rate; control of peristalsis/blood pressure; control of glandular secretions
What are the divisions of the autonomic nervous system?
sympathetic; parasympathetic; enteric
What is the function of the sympathetic nervous system? (2)
fight or flight; efferent functions
What is the function of the parasympathetic nervous system? (2)
rest and digest; efferent functions
What is the function of the enteric nervous system? (2)
digestion; mostly efferent but some afferent functions
What are the three components of neurons?
soma; dendrites; axons
Define soma.
neuronal cell body
Describe the activity of the soma’s nucleus.
highly euchromatic nucleus with a large nucleolus
What is the Nissl substance?
granules of ribosomes and rough ER
What is the function of dendrites?
receive synaptic inputs from other neurons on small projections called dendritic spines
Neurofilaments are a type of
intermediate filament
What is the composition of the axon membrane skeleton?
actin-spectrin network
What are the functions of the axon membrane skeleton? (2)
supports plasma membrane; anchors ion conductance proteins necessary for saltatory conduction
What is the function of microtubules in axons?
provide a highway for vesicular transport within axons
What are the two forms of microtubule-dependent transport?
anterograde transport; retrograde transport
Define anterograde transport.
movement of vesicles + cargo from soma to axonal periphery via kinesin
Define retrograde transport.
movement of vesicles + cargo from axonal periphery to soma via dynein
Where does biosynthesis occur in neurons?
synthesized in soma and then transported
Define synapse.
cell-to-cell junction that mediates intercellular signaling from neuron to target cell
List and describe the sequence of events in synaptic communication.
axonal action potential reaches terminal; voltage gating of Ca2+ channels triggers fusion of synaptic vesicles w/ presynaptic membrane; diffusion of neurotransmitters across synaptic cleft; binding of neurotransmitters to receptors on postsynaptic membrane; activation of response in target cell
What are the three types of neurons?
afferent; efferent; interneurons
In what direction do afferent neurons conduct impulses?
periphery to CNS
What are the possible myelination states of afferent neurons?
can be either myelinated or unmyelinated
What is the morphology of afferent neurons?
pseudounipolar morphology
In what direction do efferent neurons conduct impulses?
CNS to periphery
What are the possible myelination states of efferent neurons?
can be either myelinated or unmyelinated
What is the morphology of efferent neurons?
multipolar morphology
In what direction do interneurons conduct impulses?
between neurons
What is the distribution of interneurons?
abundant in CNS, but restricted to enteric ganglia in PNS
What is the function of glial cells?
serve as support cells for neurons
What are the two types of Schwann cells?
myelinating Schwann cells; non-myelinating Schwann cells
Myelinating Schwann cells are recruited by
axons that are >1 micron in diameter
Non-myelinating Schwann cells are recruited by
axons that are <1 micron in diameter
What are satellite cells?
cells that associate with soma and proximal dendrites of neurons in ganglia of somatosensory, sympathetic, and parasympathetic divisions
What is the function of satellite cells? (2)
provide physical and trophic support to associated neurons; act as barrier that controls access of substances to neuron
What is the distribution of enteric glial cells?
only present in ganglia of enteric division
Describe the morphology of enteric glial cells. (2)
resemble astrocytes of CNS; bigger than satellite cells
What is the function of enteric glial cells? (2)
provide physical and trophic support to associated enteric neurons; act as barrier that controls access of substances to enteric neuron
What are ganglia?
clusters of neuronal cell bodies and supporting glial cells in PNS
What makes up a nerve fiber?
nerve fiber = axon + myelin sheath
Nerve fibers are bundled into
fascicles
Organize the hierarchy of nerves, nerve fibers, and fascicles.
nerve fibers → fascicles → nerves
What are the two broad types of nerves?
cranial nerves (originate in brainstem); spinal nerve (originate in spinal cord)
What are the three types of connective tissue of nerves?
epineurium; perineurium; endoneurium
What is the epineurium?
outermost connective tissue layer of nerve
What is the composition of the epineurium?
type 1 collagen fibrils parallel to the long axis of nerve
The epineurium contains what kind of cells?
connective tissue cells such as fibroblasts and adipocytes
Describe the relationship between the epineurium and vasculature.
epineurium contains larger blood vessels that supply the nerve
What is the perineurium?
layer of flattened cells that surround each fascicle of nerve
What is the function of the perineurium?
contributes to blood-nerve barrier through tight junctions between perineurial cells
Describe the relationship between the perineurium and vasculature.
perineurium surrounds blood vessels (arterioles and venules) that enter endoneurium
What is the endoneurium?
connective tissue that packs between nerve fibers in nerve fascicle
Describe the composition of the matrix component of the endoneurium.
rich in collagen parallel to nerve fibers, together with ground substance
Describe the composition of the cellular component of the endoneurium.
Schwann cells with a few fibroblasts, macrophages, and mast cells
What are the two components of the endoneurium?
matrix and cellular components
What are Schmidt-Lanterman clefts?
periodically-spaced cytosolic inclusions within myelin sheath that provide metabolic support for sheath
How do Schmidt-Lanterman clefts appear in section?
appear as V-shaped discontinuities in nerve fibers
How do nodes of Ranvier appear in section?
appear in longitudinal section as butterfly-like discontinuities in nerve fibers
What are nodes of Ranvier?
specialized region of axonal membrane that are free of myelin (i.e. gap between adjoining Schwann cells)
Nodes of Ranvier have a high density of
voltage-gated sodium channels
(T/F) All sensory neurons are unmyelinated.
False. They can be either unmyelinated or myelinated.
Where are somata (cell bodies) of the somatic nervous system located?
in dorsal root ganglia
What does it mean for sensory neurons to be pseudounipolar?
lack dendrites; have a bifurcated axon instead
Describe the features of the bifurcated axon of pseudounipolar nerves.
one branch of axon projects into periphery; other branch projects through dorsal roots to form synapses in dorsal horn of spinal cord
Describe the role of axon thickness in signal transmission. Give two examples.
thick axons = faster transmission; touch-sensitive neurons are thick and myelinated (fast transmission); pain-sensitive neurons are thin and unmyelinated (slow transmission)
What are the two types of sensory neuron terminals?
non-encapsulated endings; encapsulated endings
Describe the defining feature of non-encapsulated nerve endings.
nerve ending is exposed in tissue
Non-encapsulated nerve endings are used by what kinds of nerves?
neurons that sense pain, temperature, and light touch
Give an example of a non-encapsulated ending.
Merkel ending (senses light touch)
Describe the defining feature of encapsulated nerve endings.
nerve endings are encapsulated in structure that facilitates sensation
Encapsulated nerve endings are used by what kinds of nerves?
neurons that sense pressure, texture, and muscle elongation
What are the three main types of encapsulated endings?
Pacinian corpuscle; Meissner’s corpuscle; muscle spindle
Pacinian corpuscles detect
pressure + rate of tissue displacement
Meissner’s corpuscles detect
texture
Muscle spindles detect
rate/extent of muscle elongation
Where are the somata of autonomic nerves located?
in the ventral horn of the spinal cord
Describe the myelination state of autonomic nerves.
ALL myelinated
What neurotransmitter is used by autonomic nerves?
acetylcholine
What are the three types of motor neurons?
alpha, beta, gamma
Describe the features of alpha-motor neurons.
have very large axons
What is the function of alpha-motor neurons?
stimulate contractions of skeletal muscle fibers
Describe the features of beta- and gamma-motor neurons.
have smaller axons than alpha-motor neurons
What is the function of beta- and gamma-motor neurons?
stimulate contractions of specialized myofibers of muscle spindles
(T/F) Muscle cells cannot undergo cell division.
TRUE
How are muscle fibers/myofibers formed?
formed by fusion of precursor myocytes
What are myofibrils?
specialized organelles that mediate contraction
What is the composition of myofibrils?
composed of myofilaments (thin filaments + thick filaments)
Most skeletal muscle fibers are activated by
alpha-motor neurons
Can muscle fibers be innervated by more than one neuron?
No. Each muscle fiber is innervated by only one motor neuron.
Activation by a motor neuron causes
its innervated myofiber to depolarize.
How is depolarization of a motor neuron conveyed?
depolarization is conveyed to myofibrils via T-tubules, which penetrate from sarcolemma down to myofibrils
What are T-tubules?
invaginations of plasma membrane that wrap around myofibrils to elicit excitation-contraction coupling
What are terminal cisternae?
specializations of sarcoplasmic reticulum that sit in close apposition to T-tubules
What is the function of terminal cisternae?
excitation in T-tubules triggers release of Ca2+ from terminal cisternae, which initiates muscle contraction
What is a triad?
combination of 1 T-tubule + 2 terminal cisternae
What is the function of a triad?
site where excitation-contraction coupling occurs
Where are triads located?
positioned between border of A-band and I-band
How does calcium release from the terminal cisternae activate myofilament contraction?
through the troponin/tropomyosin mechanism
What is the composition of sarcomeres?
central thick filaments flanked by thin filaments on each side
Thick filaments are polymers of
type II myosin
What is the function of thick filaments?
provides the molecular motor that drives muscle contraction
How are thin filaments oriented in sarcomeres?
minus/pointed end points toward center of sarcomere
Describe how the length of a sarcomere changes during contraction.
myosins move from (-) to (+), so when thick filaments move on thin filaments, the sarcomere shortens
Describe the activity of the thin filament at the plus/barbed end.
plus/barbed ends of thin filaments are cross-linked via alpha-actinin to plus/barbed ends of thin filament of neighboring sarcomere
What are the five elements of a sarcomere?
Z-line; I-band; A-band; H-band; M-line
Describe the Z-line.
where thin filaments of neighboring sarcomeres are cross-linked
Describe the I-band.
area with thin filaments but not thick filaments
Describe the A-band.
area with thick filaments
Describe the H-band.
area of the A-band with thick filaments but not thin filaments
Describe the M-line.
area at center of sarcomere where thick filaments are cross-linked
**Label the following diagram of a sarcomere.
**image
Contraction of a sarcomere results in shortening of which elements?
causes shortening in I- and H-bands, but NOT in A-bands
What is internal force coupling?
intermediate filaments (mostly desmin) transmit force of contraction b/w myofibrils and b/w myofibrils + rest of cell
What is lateral force coupling? (3)
force is transmitted b/w myofibers through endomysium; force transmitted through perimysium which connects adjacent fascicles; contraction force is then transmitted laterally to adjacent muscle through epimysium
What is the endomysium?
connective tissue that separates muscle fibers in skeletal muscle
What is the perimysium?
connective tissue that encapsulates fascicles of skeletal muscle
Describe the vasculature and innervation associated with the perimysium.
perimysium has arterioles, venules, and small nerves that branch of larger vessels/nerves of the epimysium
What is the epimysium?
outermost connective tissue of skeletal muscle
What are the three types of tissue associated with skeletal muscle?
endomysium; perimysium; epimysium
What is longitudinal force coupling?
force transmitted longitudinally to tendons through junctions called myotendinous junctions
Describe the sequence of events associated with the reflex circuit. (3)
sensory neuron stimulates motor neuron and interneurons in spinal cord; motor neurons signal to target muscle; interneurons can inhibit opposed motor neurons
What are varicosities?
bulges within an axon that generate synapses outside of nerve endings
Varicosities are primarily used by what kind of nerve?
efferent postganglionic nerves
(T/F) The sympathetic nervous system mediates both efferent and afferent functions.
False. It has only efferent neurons.
Describe the sequence of efferent signals in the sympathetic system and the structures they pass through.
preganglionic cell body → paravertebral ganglion → postganglionic cell body → innervation of target tissue
Preganglionic sympathetic neurons receive input from
the CNS
Preganglionic sympathetic neurons’ somata are located in
the spinal cord
Describe the axons of preganglionic sympathetic neurons.
short and myelinated
Postganglionic sympathetic neurons receive input from
receive input from preganglionic neurons
Postganglionic sympathetic neurons’ somata are located in
the sympathetic ganglia
Describe the axons of postganglionic sympathetic neurons.
long and unmyelinated
What neurotransmitter do postganglionic sympathetic neurons use?
norepinephrine
(T/F) The parasympathetic nervous system mediates both efferent and afferent signaling.
FALSE. It, like the sympathetic system, also only contains efferent neurons and does NOT contain afferent neurons or interneurons.
Describe the sequence of efferent signals in the parasympathetic system and the structures they pass through.
preganglionic cell body → paravertebral ganglion → visceral ganglion → postganglionic cell body → innervation of target tissue
Preganglionic parasympathetic neurons receive input from
the CNS
Preganglionic parasympathetic neurons’ somata are located in
spinal cord
Describe the axons of preganglionic parasympathetic neurons.
long, myelinated axons that contact postganglionic neurons in parasympathetic ganglia or near target tissue
Postganglionic parasympathetic neurons receive input from
receive input from preganglionic neurons
Postganglionic parasympathetic neurons’ somata are located in
the parasympathetic (visceral) ganglia, which are located near the target tissue
Describe the axons of postganglionic parasympathetic neurons.
short, unmyelinated axons
What neurotransmitter do postganglionic parasympathetic neurons use?
acetylcholine
What are myoepithelial cells?
stellate epithelial cells that express smooth muscle actin
How are myoepithelial cells induced to contract?
induced to contract by autonomic efferent neurons
How do myoepithelial cells drive luminal contents into ducts?
squeezing by myoepithelial cells in glands helps to drive luminal contents into ducts.
Sympathetic linkage to the heart involves what nerves?
sympathetic cardiac nerves
Parasympathetic linkage to the heart involves what nerve?
vagus nerve
What are the three layers of the heart?
epicardium; myocardium; endocardium
What is the function of the epicardium?
provides protection/support
What is the function of the myocardium?
mediates contraction through cardiomyocytes, which are contractile cardiac muscle cells
What is the function of the endocardium?
provides internal support
The endocardium of the heart is contiguous with
the endothelium
(T/F) Cardiac muscle cells can have more than one nucleus.
True. They can have single or double nuclei.
Describe the appearance of cardiomyocytes.
striated
Are cardiomyocytes capable of expansion or division?
cardiomyocytes are capable of hypertrophy (cell expansion), but CANNOT undergo hyperplasia (cell division)
How are cardiomyocytes linked?
linked end-to-end via intercalated discs, which forms cardiac muscle fibers
(T/F) Cardiomyocytes are arranged in fascicles.
False. Instead, groupings of fibers fuse and split, which contributes to the squeezing action of the heart.
How are myofilaments organized in cardiomyocytes?
organized into sarcomeres, like in skeletal muscle, but important to remember that cardiomyocyte myofilaments use different genes than those of skeletal muscle
Describe excitation-contraction coupling in cardiomyocytes.
use diads (1 terminal cisterna + 1 T-tubule) instead of triads
The epicardium is what layer of the heart muscle?
outermost layer
The epicardium is rich in
adipocytes
Compare the T-tubules found in cardiomyocytes to those found in skeletal muscle.
T-tubules in cardiomyocytes are bigger and are positioned near Z bands (not b/w A and I bands in skeletal muscle)
What is the function of intercalated disks?
transmit force of contraction from one cardiomyocyte to the next
What is the function of the fascia adherens in intercalated disks?
fascia adherens is the patch in which thin filaments from sarcomeres insert, allowing force of sarcomere contractions to be communicated from one cell to the next
Fascia adherens are found on what aspect of intercalated disks?
transverse aspect
What is the function of desmosomes in intercalated disks?
link intermediate filaments (rich in desmin) of adjoining cardiomyocytes together
Desmosomes are found on what aspect of intercalated disks?
both transverse + longitudinal aspects
What are the three components to intercalated disks?
fascia adherens; desmosomes; gap junctions
What is the function of gap junctions in intercalated disks?
allow excitation to pass between cells
Gap junctions are found on what aspect of intercalated disks?
only longitudinal aspect
Intercalated disks stain with
eosin (they are highly eosinophilic)
Which cells serve as the pacemakers of the heart?
specialized cardiomyocytes of the sinoatrial node (intrinsic frequency of ~100 cycles per minute)
How is the excitation generated by the sinoatrial node transferred?
SA node [→ gap junctions →] atrial cardiomyocytes [→ gap junctions →] AV node [→ bundle of His →] Purkinje fibers [→ gap junctions →] excitation of ventricular cardiomyocytes
(T/F) Sympathetic and parasympathetic innervation is critical for the intrinsic depolarization/repolarization cycles of cardiomyocytes.
False. Sympathetic and parasympathetic innervation can help regulate the frequency of these cycles, but is not required for these cycles to take place.
Where can Purkinje cardiomyocytes be found in section?
located at the interface b/w myocardium and endocardium
The ganglia of the enteric system are organized into what two plexi?
submucosal Meissner’s plexus; myenteric Auerbach’s plexus
The enteric plexi are controlled by which nervous system?
autonomic nervous system (specifically, postganglionic sympathetic and preganglionic parasympathetic)
Describe the common morphological features of the enteric ganglia.
smaller than literally all the other ganglia; closely packed somata surrounded by enteric glial cells; little EC matrix
Where is the myenteric/Auerbach’s plexus located?
enteric ganglia in muscularis propria b/w outer longitudinal and inner circular muscle layers
What is the function of the myenteric/Auerbach’s plexus?
controls peristaltic contractions of muscularis propria
Where is the submucosal/Meissner’s plexus located?
enteric ganglia in submucosa
Compare the size of the submucosal/Meissner and myenteric/Auerbach plexi.
ganglia of myenteric/Auerbach are more numerous but smaller than those of submucosal/Meissner
What is the function of the submucosal/Meissner’s plexus?
controls actions of mucosa (i.e. glands, contractions of smooth muscle of muscularis mucosae, etc.)
What is the major target of efferent enteric neurons?
smooth muscle
Describe the shape of smooth muscle cells (SMCs).
fusiform (long, thin with pointy end) shape w/ central nucleus
Are smooth muscle cells capable of expansion or cell division?
capable of both cell expansion and cell division
How do smooth muscle cells appear in longitudinal section?
appear long and thin with highly eosinophilic cytosol
How do smooth muscle cells appear in transverse section?
appear small, w/ nucleus frequently out of plane
Smooth muscle cells are commonly organized into
fascicles (but they do NOT form fibers like skeletal or cardiac muscle)
What structure within smooth muscle cells mediates contractions?
myofilaments
What are dense bodies? (2)
structures that anchor myofilaments within cytosol and at sarcolemma; myofilaments radiate out of the dense bodies
(T/F) Dense bodies are striated.
False, they are NOT striated
Smooth muscle cells are surrounded by
an external lamina
What is the function of the external lamina that surrounds smooth muscle cells?
fused onto external laminae of neighboring SMCs, which pulls on adjoining SMCs during contraction
How are smooth muscle cells usually innervated/excited?
can be excited by nerves (synapses), hormones (endocrine receptors), or external force (mechanoreceptors)
Give an example of how smooth muscles use mechanoreceptors for contraction.
full bladder has relaxed smooth muscle, but as the bladder empties, mechanoreceptors on SMC activate, initiating contraction that drive remaining fluid from bladder
Describe excitation-contraction coupling in smooth muscle.
mediated through caveolae of sarcolemma (no diads or triads); if activation reaches threshold, Ca2+ released from sarcoplasmic reticulum which drives contraction by activating MLCK/MLCP system and promotes myosin activity
What is the function of caveloae in smooth muscle?
sites of calcium influx (since SMCs lack a T-tubule system)
