Lecture 2-Exam 1 Flashcards
What are the subdivisions of the nervouse system?
What are the different horns and roots of the spinal cord? What horn is special and why?
Lateral horn – serves SNS. Only in thoracolumbar spinal cord
What are descending and ascending tracts?
What are each track for:
* Lateral corticospinal
* Dorsal columns
* Lateral spinothalamic
* Ventral spinothalamic
- Lateral corticospinal: motor
- Dorsal columns: Fine touch, propioception, vibration
- Lateral spinothalamic: pain and temperture
- Ventral spinothalamic: light touch
- The motor system is comprised of what?
- Muscle contraction only occurs in response to what? What does it constitute?
- The motor system is comprised of skeletal muscles and the neurons that control them.
- Muscle contraction only occurs in response to action potentials in alpha motor neurons, which originate in the ventral gray matter of the spinal cord (and brainstem nuclei of certain cranial nerves) and constitute the final common pathway for motor control
Where do Action potentials in alpha motor neurons originate in?
in the ventral gray matter of the spinal cord (and brainstem nuclei of certain cranial nerves)
What is the hierarchy of motor control within the CNS? What is the purpose of each component?
Association cortex and the basal ganglia
* determine the goal of movements.
Primary motor cortex and cerebellum
* determine the correct sequence of commands that will allow the goal to be achieved.
Neuronal circuits in the spinal cord
* implement descending commands
There are three inputs to alpha motor neurons that determine which muscle fibers will contract, explain
- Upper motor neurons from the cortex or brainstem regulate voluntary movements
- Spinal interneurons form an extensive circuitry within the spinal cord. Basic motor programs (e.g., walking) are encoded in spinal circuits known as central pattern generators.
- Sensory neurons from muscle proprioceptors provide feedback about muscle length and tension.-> (muscle spindle and tendon golgi)
What is the alpha motor neuron?
The final common path, to release Ach onto the muscle
* Somotic motor control
What influences the motor neurons?
Peripheral sensory input and spinal cord tracts that descend from the brainstem and the cerebral cortex influence the motor neurons
What does the cerebellum and basal ganglia contribute to?
The cerebellum and basal ganglia contribute to motor control by modifying the brainstem and the cortical activity.
What is a motor unit? What is the difference of high threshold motor unit and lower threshold motor unit?
a motor neuron and all the muscle fibers it innervates
* High: More AP needed to fire
* Lower: smaller motor neurons, less APs are needed to fire
Larger vs smaller motor units?
Larger motor units are harder to activate, so a greater stimulus is needed to activate the larger motor units. Thus, smaller motor units always get activated first in any movement.
- Motor unit consist of what?
- What is determined by the motor neuron?
- A motor unit consists of an α motor neuron and a group of extrafusal muscle fibers it innervates.
- Functional characteristics, such as activation threshold, twitch speed, twitch force, and resistance to fatigue, are determined by the motor neuron
Proprioception includes what? What are they derived from?
Proprioception includes conscious sensation derived from receptors in the skin and joint capsules (e.g., Pacinian corpuscles).
Unconscious sensation about muscle length and tension is relayed from what? (2)
from muscle spindles and Golgi tendon organs (GTOs)
Muscle spindles:
* What does it provided?
* What does it consist of?
* Expanded where?
- Provide information to the CNS about muscle stretch (length) and the speed with which muscle length is changing.
- Consist of specialized muscle fibers called intrafusal fibers contained in a fibrous capsule and are connected at both ends to the force-generating extrafusal muscle fibers.
- Expanded in the middle where sensory axons are wrapped around the intrafusal fibers. The sensory fibers are large myelinated axons (type Ia) and have very fast conduction speeds.
Muscle spindles
- What are gamma motor neurons?
- Where are their cell bodies located?
- provide the motor supply to contractile filaments within muscle spindles (referred to as the fusimotor system).
- The cell bodies of gamma motor neurons are located in the ventral gray matter of the spinal cord
- Contraction of intrafusal fibers alters what?
- What is an example?
- Contraction of intrafusal fibers alters the sensitivity of muscle spindles.
- For example, intrafusal fibers and extrafusal fibers must shorten at the same time to prevent the muscle spindles from becoming slack
- How are intrafusal and extrafusal muscles fibers organized? Muscle length is monitored by what?
*
- Intrafusal muscle fibers are arranged in parallel with the extrafusal muscle fibers.
- Muscle length is monitored by myelinated Ia afferent neurons coiled around the midsection of intrafusal fibers.
Gamma motor neurons contract what? What does this maintain?
Gamma motor neurons contract intrafusal fibers to maintain spindle tension when surrounding extrafusal fibers contract
* Allos the intrafusal muscle fibers to reset
- Golgi tendon organ arranged how?
- Sensory endings of Ib afferent neurons are arranged how?
- A Golgi tendon organ arranged in series with muscle fibers at the junction between the muscle and tendon.
- Sensory endings of Ib afferent neurons are intertwined with collagen filaments and detect the force of muscle contraction.
What is the myotatic reflex?
- A lesion in any part of the myotatic reflex circuit will result what?
- The following deep tendon reflexes are important to know when performing a neurologic examination: (5)
- A lesion in any part of the myotatic reflex circuit will result in areflexia.
- The following deep tendon reflexes are important to know when performing a neurologic examination: brachioradialis (C5–C6), biceps (C5–C6), triceps (C6–C7), knee (L2–L4), ankle (S1)
Myotatic reflex cicuitry
- Muscle contraction is stimulated by what?
- What occurs simultaneously and how?
- Muscle contraction is stimulated by a monosynaptic pathway following activation of muscle spindle afferents
- Reciprocal inhibition of antagonist muscles occurs simultaneously, via inhibitory spinal interneurons
Myotatic reflex circuitry
Ia afferent axons from the muscle spindle does what?
- Ia afferent axons from the muscle spindle make excitatory monosynaptic contact with homonymous motor neurons and with inhibitory interneurons that synapse on motor neurons of antagonist muscles
- What are golgi tendon organs?
- How are they arranged?
- GTOs are sensory nerve terminals that are encapsulated within tendons.
- GTOs are arranged in series with extrafusal muscle fibers, with one end attached to the extrafusal muscle fibers and the other end attached to the collagen fibers of the tendon
Sensory axons from GTOs are smaller than what? What are they called?
Sensory axons from GTOs are smaller than type Ia fibers and are designated type Ib
- What is The Golgi tendon reflex?
- What is the purpose of this reflex?
- The Golgi tendon reflex is an inhibitory effect on the muscle resulting from the muscle tension stimulating Golgi tendon organs of the muscle, and hence it is self-induced.
- The reflex arc is a negative feedback mechanism preventing too much tension on the muscle and tendon.
Antagonist muscle: activated
Intrafusal muscle fibers are arranged in parallel with what?
Intrafusal muscle fibers are arranged in parallel with the extrafusal muscle fibers
Inverse myptatic reflex circuitry:
* How does this work
Contraction of the agonist muscle activates the Golgi tendon organ and Ib afferents that synapse on interneurons that inhibit agonist motor neurons and excite the motor neurons of the antagonist muscle.
What is the flexor and crossed externsion reflexes?
- If a painful stimulus is applied to one side of the body, flexors contract (quad) and extensors (hamstring) relax on that side to rapidly remove the body part from the stimulus.
- The opposite response occurs on the contralateral side of the body to maintain posture; this crossed-extensor reflex is mediated by spinal interneurons.
The brain communicates with the spinal motor circuitry through two major groups of descending pathways, named according to their location in the spinal white matter:
Lateral pathways and ventromedial pathways
Lateral pathways is concerned with what?
concerned with voluntary movement of the distal muscles (e.g., muscles of the arm and hand).
What are the two major lateral pathways? Where do they originate?
Corticospinal (pyramidal) tract
* originates in the primary motor cortex
Rubrospinal tract.
* originates in the brainstem (midbrain region)
What does the ventromedial pathways originate where? What does it innervate?
- originate in the brainstem
- innervate the proximal and axial muscles to help maintain head position and posture.
What are the major ventromedial pathways?
- Tectospinal tract
- Reticulospinal tract
- Vestibulospinal tract
decerebrate rigidity:
* Associated with what?
* What is the result?
- Its occurrence is associated with lesions of the mesencephalon that eliminate the influence of higher brainstem and cortical centers.
- The abnormal posture is a result of extreme antigravity extensor muscle activation by the unopposed action of the reticulospinal and vestibulospinal tracts.
Fill in the desending (motor) tracts
Lateral pathways
The corticospinal tract (pyramidal tract)
* Contains what?
* Controls what?
* Fibers descend through what and where does it cross?
- contains fibers that mostly originate from the primary motor cortex
- motor cortex in each hemisphere controls the muscles on the contralateral side of the body
- the fibers descend through the internal capsule and upper brainstem to the medullary pyramids, where the tract crosses the midline (decussate).
In the corticospinal tract (pyramidal tract): what happens for a lesions below and above the meduallary pyramids?
- lesions above the medullary pyramids will result in contralateral muscle weakness;
- lesions below the pyramidal decussation will produce an ipsilateral muscle weakness.
The corticospinal tract:
* Axons arise from where? Where do they descend through?
Axons arising from cortical motor areas descend through the internal capsule, decussate in the medulla, descend in the lateral funiculus of the spinal cord as the lateral corticospinal tract, and terminate on motor neurons and interneurons in the ventral horn areas of the spinal cord.
Rubrospinal tract:
* Originates where? What does it receives?
originates in the red nucleus of the midbrain, which in turn receives input from the motor cortical areas.
Rubrospinal tract:
* Many rubrospinal tract neurons are redundant in humans but they are active in what? What can this help in?
Many rubrospinal tract neurons are redundant in humans but they are active in mediating flexion, which can help to diagnose the site of neurologic injury in comatose patients where “posturing” can clue the physician to the site of neurologic damage
What is decorticate and decerebrate posturing?
- Decorticate posturing, characterized by flexion of the elbows and wrists and supination of the arms, indicates damage immediately rostral (anterior) to the red nucleus of the midbrain. An intact red nucleus allows the upper extremities to undergo flexion via the rubrospinal neurons.
- In contrast, decerebrate posturing, characterized by extension of the elbows and wrists with pronation, indicates a midbrain lesion that involves the level of the red nucleus.
- The red nucleus is where the rubrospinal tract originates from, which runs how?
- What is the tract thought to modulate?
- The red nucleus is where the rubrospinal tract originates from, which runs in the lateral motor neurone pool alongside the lateral corticospinal tract.
- The rubrospinal tract itself is thought to modulate flexion of upper limbs given that it terminates in the cervical spinal cord.
What happens when the lesion is above the level of the rubrospinal tract?
- A lesion above the level of the rubrospinal tract I.e. above the red nucleus would inhibit the corticospinal tract’s ability to modulate the function of the rubrospinal, meaning that the rubrospinal tract would be uninhibited to engage flexion of upper limbs.
- This manifests in decorticate rigidity whereby a patient displays flexed upper limbs and extended lower limbs due to the rubrospinal tract acting unchecked.
What happens if there was a lesion below the red nucleus?
A lesion below the the red nucleus would then damage not only the corticospinal but also the rubrospinal tract, meaning that the rubrospinal tract’s ability to control upper limb flexion is now gone. This tends to manifest in decerebrate rigidity whereby patients display extension of upper and lower limbs
Ventromedial pathways provide what?
Provide sensory information from the visual and vestibular systems about the body position and balance
Vestibulospinal tract
* Originates in what?
* Provides what?
- originates in the vestibular nuclei
- provides one of the links between the sensors for balance and the extensor muscles, which are important for maintaining posture
Tectospinal tract:
* Originates in what?
* Main function is what?
- originates in the superior colliculus of the midbrain, which receives input from the retina and visual cortex and has reciprocal connections with the vestibular nuclei
- main function to direct the head and eyes to move toward a selected object in the visual field (gaze)
head and eyes move together
Reticulospinal tract
* Originates in what?
* What is the purpose?
- originates in the reticular formation and consists of two antagonistic pathways: one from the pontine reticular area and the other from the medullary reticular area
- a balance between the activities of these pathways facilitates fine control of posture through actions on the extensor muscles of the lower limb
Spinal motor neurons
Organization of spinal motor neurons is consistent with the function and location of the descending motor tracts, with a medial to lateral organization:
- Distal muscles of the limbs are represented most laterally.
- Axial muscles of the trunk are represented medially.
- Motor neurons for the proximal limb (girdle) muscles are located in an intermediate position.
What are UMN lesion and LMN lesion Characteritics?
- Upper motor neuron lesions are characterized by spastic paresis (an incomplete paralysis), hyperreflexia, hypertonia
- Lower motor neuron lesions are characterized by flaccid paralysis, areflexia, hypotonia, muscle twitches, and atrophy.
- What is poliomyelitis?
- What is amyotrophic lateral sclerosis?
- Poliomyelitis is an example of a pure lower motor neuron lesion. The poliovirus infects and selectively destroys the motor neuron cell bodies located in the ventral horn of the spinal cord, typically in the lumbar region.
- Amyotrophic lateral sclerosis (also known as ALS or Lou Gehrig’s disease) is a progressive motor neuron disease that affects both upper and lower motor neurons
Patients with both an upper and a lower motor neuron lesion at the level of the cervical spine can present with what?
present with flaccid paresis of the upper extremities and spastic paresis of the lower extremities
CEREBRAL CORTEX ROLE IN MOTOR CONTROL
What are the serveral distinct cortical areas participate in voluntary movement?
- Area 4 primary motor cortex (aka M1)
- Area 6 premotor cortex
- Area 8 influences eye movements.
- Areas 1, 2, 3, 5, 7 have sensory functions but also contribute to corticospinal tract
- Central sulcus divides areas 4 and 3
- What is area 6?
- What area 8 influence?
- The central sulcus divides what?
- area 6 is the premotor cortex on the lateral and supplementary motor area on the medial aspect of the hemisphere.
- Area 8 influences voluntary conjugate eye movements.
- The central sulcus divides areas 4 and 3.
T/f: Cortical gray mater is not evenly distributed among the different body parts
TRUE
Cortical gray mater:
* Produce what?
* Destruction of of any part of the primary motor cortex leads to what?
- Produces movements in the contralateral side of the body
- Destruction of any part of the primary motor cortex leads to immediate paralysis of the muscles controlled by that area.
- What is somatosensory association area?
- What is the premotor cortex?
- Somatosensory association area - movements we decide to make volitionally
- Premotor cortex - encoding movement intention and the selection of movements based on environmental sensory inputs
The middle cerebral artery supplies what?
the majority of the lateral surface of the cortex, including the section of primary motor cortex (and primary sensory cortex) responsible for movement (and sensation) of the face and upper extremities
An occlusion of the middle cerebral artery will cause what?
cause contralateral spastic paresis (and impaired sensation) of the face and upper extremities
The area responsible for the lower extremities is supplied by what?
the anterior cerebral artery
* Occlusion of the anterior cerebral artery will have similar effects on the lower extremities.
Where is the supplementary motor cortex located?
SMA is on the medial aspect of the hemisphere
The basal ganglia:
* What is it?
* What is it broken up into?
- a group of subcortical nuclei located primarily in the base of the forebrain
- the striatum (divided into the caudate nucleus and putamen), the globus pallidus, and subthalamic nucleus
The function of the basal ganglia in normal individuals appears to be what?
modulation of cortical output
What happens in parkinson and huntingtons disease?
- In Parkinson’s disease, there is degeneration of the dopaminergic neurons that project from the substantia nigra to the striatum.
- Huntington’s disease is a fatal autosomal dominant disorder affecting the huntingtin gene. Severe idiopathic degeneration of GABAergic neurons (decrease inhibition), particularly affecting the indirect pathway of the basal ganglia.
What is the main pathway for basal ganglia influence on motor control?
The neuronal circuit of cerebral cortex to striatum to globus pallidus interna (GPi) to thalamus and back to the cortex
- globus pallidus interna flows how?
- What is important in eye movements?
- GPi output also flows to the midbrain extrapyramidal area (MBEA)
- The substantia nigra pars reticulata (SNr) to superior colliculus (SC) pathway is important in eye movements
Cerebellar lesions produce a characteristic movement disorder called what?
ataxia, in which movements become inaccurate and are poorly coordinated; there is also decreased muscle tone (hypotonia)
An alternative description of the cerebellum is based on the evolution of cerebellar functions:
- How is the cerebellum divided?
- What are the dunctional regions?
- he output pathways of the functional cerebellar areas via what?
- The cerebellum is divided into anterior, posterior, and flocculonodular lobes by surface fissures (shown on the left).
- The functional regions of the cerebellum are the vestibulocerebellum, spinocerebellum, and cerebrocerebellum (shown on the right)
- The output pathways of the functional cerebellar areas via the cerebellar nuclei are shown (note: the nuclei are bilateral but are only shown on the right).
Circadian rhythms
* What does the hypothalamus generate?
* Circadian rhythms generated by what?
* Body clock created by neurons of what?
- Hypothalamus generates cyclical variation seen in most variables throughout a 24-hour period
- Circadian rhythms generated by endogenous body clock (even persist in people who are isolated from all time and light and dark cues)
- Body clock created by neurons of the suprachiasmatic nucleus (SCN), the activity of which oscillates spontaneously in a daily cycle
Circadian rhythms
* The SCN receives what?
* The pineal glands does what?
- The SCN receives afferent fibers directly from the retina via the retinohypothalamic tract, which provides information about daylight to entrain the circadian rhythm.
- The pineal gland secretes the hormone melatonin in the hours of darkness, which helps to synchronize the day/night and sleep cycles.
Body temp and alertness are what?
parallel to each other
electroencephalogram
* Monitors what?
* useful for what?
* What is it used to diagnosis?
- EEG monitors surface electrical activity of the brain waves
- Useful for studying normal brain functions as sleep and consciousness
- In diagnosis of degenerative brain diseases, metabolic abnormalities, brain tumors, etc
Lack of brain waves is a common criterion of what?
brain death
What are the four different brain waves?
The Reticular Formation and Diffuse Modulatory Systems: What is this?
An anatomic concept that describes columns of neurons that extend throughout the core of the brainstem. Neurons in the reticular formation have very widespread connectivity throughout the CNS.
The reticular formation has a strong influence on what?
on wakefulness, referred to as the concept of a reticular activating system
- Consciousness requires what?
- The principal causes of unconsciousness or coma are what?
- Consciousness requires proper functioning of the reticular activating system and both cerebral hemispheres.
- Therefore, the principal causes of unconsciousness or coma are lesions that damage the reticular activating system or when there is diffuse damage to both cerebral hemispheres
The reticular formation utilizes what?
The reticular formation utilizes diffuse modulatory systems, which are categorized according to the principal neurotransmitter that is used.
What are the four major diffuse modulatory system?
Noradrenergic system
Serotonergic system
Dopaminergic neurons
Cholinergic neurons
Noradrenergic system:
* Origianates where?
* Functions as what?
* Neurons are what?
- originates in the locus coeruleus of the pons
- functions as an “alarm center” that becomes most active when new environmental stimuli appear.
- neurons widely distributed throughout the CNS and increase the general state of arousal.
Serotonergic system:
* Originates where?
* Releases what?
* Neurons control what?
- originates in the raphe nuclei
- release the neurotransmitter serotonin
- neurons central to control of wakefulness/sleep-wake cycles, and control of mood & emotions.
What is the reticular activating system?
Noradrenergic system + serotonergic system
Cholinergic neurons
* Where?
* Ability to do what?
- widespread in the CNS. Include diffuse modulatory systems originating in the basal forebrain and brainstem.
- ability to direct selective attention to a particular task and are important for learning and memory
Dopaminergic neurons:
* Where are they?
* What are different systems and their functions?
widespread in the CNS; several dopaminergic systems identified including:
* Nigrostriatal system - functional part of the basal ganglia. Connects the substantia nigra of the midbrain to the striatum. Important in initiation of voluntary movement.
* Mesocorticolimbic system - originates in the ventral tegmental area (VTa) of midbrain and spreads throughout the limbic system. Important role in the brain’s reward system.
The Limbic System
* What is it?
* What is the cingulate cortex?
* The hippcampus is important in what?
- A functional concept - series of structures that are intimately associated with the experience and control of emotions and with learning and memory.
- The cingulate cortex is connected to the highest centers of cognition in the prefrontal and association areas of cortex, and is a site where the “sensations” of emotions are perceived.
- The hippocampus is important in the conversion of short-term memory to long-term memory.
- if issue here: memory lost
Limbic system
- The amygdaloid nucleus involved in what?
- Hypothalamus is what?
- What happens with the activity of the hypothalamus?
- The amygdaloid nucleus involved in strong emotions, including fear and aggression, and linking emotions with memories
- The hypothalamus is an important output route of the limbic system for the expression of emotions because its efferent connections **coordinate autonomic and visceral functions. **
- The activity of the hypothalamus is relayed back to the cortex, via the mammillothalamic tract, from the mammillary bodies to the anterior thalamic nuclei, thereby providing a pathway linking the expression with the sensation of emotions.
Higher cognitive skills
- Learning and memory is associated with what?
- What is long term potentation?
- What is LTP associated with?
- Learning and memory is associated with changes in the structure and function of synapses and is known as synaptic plasticity.
- Repeated synaptic stimulation can result in a persistent increase in the sensitivity of a neuron, called long-term potentiation (LTP).
- LTP is associated with remodeling of synaptic contacts as well as functional changes including ion channel phosphorylation, altered intracellular second-messenger activity, and gene expression.
What are the different areas involved in memory?
What is the wenicke area, arcuate fasciculus and broca’s area?
- Wernicke’s area - Understanding the meaning of words
- Output from Wernicke’s area to Broca’s area occurs via a tract called the arcuate fasciculus.
- Broca’s area - generates the commands needed to instruct the neighboring motor cortex to produce the movements of the mouth and tongue needed to speak.
What is broca’s aphasia and wenicke’s aphasia?
- Broca’s aphasia is characterized by an inability to produce speech or “find a word,” whereas comprehension is generally preserved.
- Wernicke’s aphasia is characterized by fluent production of speech, but the sentences lack meaning and comprehension of language is poor.
Notes for houston
- What are prominent landmarks for definging the cortical lobes?
- What is the primary auditory cortex for?
- What is the limic association cortex for?
- prefrontal association cortex is responsible for?
- The premotor cortex is for?
- The primary motor cortex is responsible for what?
- The somatosensory cortex is responsible for what?
- The parietal–temporal–occipital association cortex does what?
- The central sulcus and lateral sulcus (sylvian fissure) are prominent landmarks used in defining the cortical lobes.
- The primary auditory cortex is responsible for hearing.
- The limbic association cortex is the region for motivation, emotions, and memory.
- The prefrontal association cortex is responsible for voluntary activity, decision making, and personality traits.
- The premotor cortex is responsible for coordinating complex motor movements.
- The primary motor cortex is responsible for voluntary movements.
- The somatosensory cortex is responsible for sensation and proprioception.
- The parietal–temporal–occipital association cortex integrates all sensory input and is important in language
What are the different structures of skeletal muscle?
How is the muscle broken down and covered?
What is a motor unit?
Motor unit = a motor neuron and all the muscle fibers it innervates
How many actin encircle each myosin thick filament
6 actin encircle each myosin thick filament
Where does thick and thin filaments overlap?
A band
_ = thin contractile myofilament
_ = thick contractile myofilament
Actin = thin contractile myofilament
Myosin = thick contractile myofilament
How is actin and myosin organized?
What is a crossbridge?
Crossbridge = “head” portion of the myosin filament that “bridges” the gap between myosin and actin during contraction
More crossbridge=more force
What is dystrophin?
Dystrophin is an important scaffolding protein located between the sarcolemma and myofilaments that is mutated in Duchenne muscular dystrophy (DMD).
Tropinin is what? What are each parts?
Troponin is a heterotrimer consisting of troponins T, C, and I:
- Troponin T anchors the trimer to tropomyosin.
- Troponin C binds Ca2+, which allows muscle contraction to occur.
- Troponin I inhibits interaction between actin and myosin if the intracellular Ca2+ concentration is low.
What is part of the regulatory protein complex?
- What is flexible and stiff in a myosin molecule?
- Where are actin binding sites located?
- Thin filaments of muscle are form how?
- The S2 region of the molecule is flexible, whereas the tail region is stiff.
- Actin-binding sites are located on the globular myosin heads, which also contain light chains with ATPase activity.
- Thin (actin) filaments of skeletal muscle are formed from actin monomers (G-actin) into helical F-actin strands, which associate with a troponin–tropomyosin complex to form the functional actin filament.
What are the two NMJ diseases we need to know about?
- Myasthenia Gravis: autoantibodies inhibit ach to bind to receptors in MEP
- Botulism
What are the structural and function of NMJ?
- Acetylcholine released from membrane vesicles is released from the axon terminal and diffuses to the underlying membrane where it binds with nicotinic receptors concentrated in invaginations of the sarcolemma.
- (A) An action potential from a motor neuron releases a small amount of acetylcholine into the myoneural junction. This causes a small depolarization in the motor endplate that is transmitted electrotonically with decrement through the myoplasm.
- In this instance, the amount of depolarization reaching the first available voltage-gated sodium channel in the cell membrane is below threshold for opening the channel, and no muscle action potential is formed.
- (B) With greater frequency of action potentials reaching the motor axon terminal, acetylcholine reaches a higher concentration in the myoneural junction. This creates a larger endplate potential than that seen with lesser motor neuron activation.
- Although this potential proceeds down the muscle fiber with decrement, the depolarization reaching the first sodium channels is greater than its threshold, and the muscle fiber is able to fire an action potential.
- Action potentials are self-reinforcing and travel down the muscle fiber without decrement in a manner similar to that seen in unmyelinated nerve fibers.
What is a triad?
T-tubule and the terminal sarcoplasmic reticulum.
How is calcium released from the SR?
- Excitation–contraction coupling involves opening of ryanodine receptors by action potentials in the T-tube membrane with subsequence release of calcium stored in the SR into the sarcoplasm.
- This activates actin–myosin crossbridge cycling. The SR is replenished with calcium from the sarcoplasm by Ca2+ ATPase pumps in the SR membrane.
What contols the interaction between myosin heads and active sits on thin filaments?
Calcium ions, via the troponin–tropomyosin complex, control the blocking of the interaction between the myosin heads (the crossbridges) and the active site on the thin filaments. The thin filaments are seen in cross section.
What is the threshold for muslce contraction?
What is the cross bridge cycling?
- What happens when ca is present in muscle? Why is ATP needed?
What is the length tension relationship? And the force velocity relationship?
Smooth muscle:
* Where is it?
* Important why?
- lines the walls of most hollow organs- vascular, GI, respiratory, urinary, and reproductive systems.
- important therapeutic target because it regulates variables such as blood flow, ventilation of the lungs, and gastrointestinal motility.
Ultrastructure of smooth muscle:
* What do they not have?
* How is it networked?
* What does the irreg arrangement of actin and myosin allow?
* There is no what?
- Smooth muscle cells are not striated in appearance (as are skeletal and cardiac muscle) because thin and thick filaments are not organized as sarcomeres.
- A network of dense bodies in the cytoplasm of smooth muscle cells serves as attachment points for actin filaments; thick filaments overlap thin filaments in an irregular array
- The irregular arrangement of actin and myosin allows smooth muscle cells to generate force over a larger range of preloads than is possible in striated muscle.
- There is no T tubule or muscle triad structure in smooth muscle; SR is present but it has an irregular arrangement.
What are the two general types of smooth muscle?
What is the excitation-contraction coupling in smooth muscles?
What are the major routes of calcium entry and exit from the cytoplasm of smooth muscle.
- Calcium enters the smooth muscle cell through voltage-gated, ligand-gated, stretch-activated, and “leak” calcium channels.
- Calcium can also be released into the cell from the sarcoplasmic reticulum (SR) by inositol 1,4,5-trisphosphate (IP3) and Ca2+-stimulated calcium release.
- The processes on the left side increase cytoplasmic calcium and promote contraction; those on the right side decrease internal calcium and promote relaxation.
What can some smooth muscle do? Others?
- Some smooth muscle makes rapid phasic contractions, followed by complete relaxation.
- Other smooth muscle can maintain a low level of active tension for long periods without cyclic contraction and relaxation - tonic contraction
What is tonic v. phasic?
- Tonic: ligand - gated calcium channels (hormonal, pharmacologic, or metabolic factors)
- Phasic: activation of voltage-gated calcium channels
What is the latch state?
- reduced rate of crossbridge cycling so that actin and myosin remain attached for a longer portion of its total cycle.
- maintain tonic contractions for long periods of time with a reduced energy requirement for adenosine triphosphate (ATP) and at a reduced intracellular calcium concentration
EXTRA: When myosin is dephosphorylated while attached to actin, the crossbridge cycling slows because actin and myosin remain attached for longer durations than normal. This allows the muscle to maintain tonic contractions for long periods of time with a reduced energy requirement for adenosine triphosphate (ATP) and at a reduced intracellular calcium concentration
What are two ways the smooth muscle can relax?
