55-101 Study Guide - Quiz Guide: Antibody, Retina, Anti-Gravity

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Neuroscience notes for Motor Quiz
The Motor System: Lecture 1: Spinal Motor System
I. Major components of motor system
Parietal cortex: visual and proprioceptive processing gives positional info of an object
relative to body (i.e. the hand) and sends this info to the motor cortex.
Motor cortex: compute forces needed to cause desired motor action, which results in
commands that are sent to the brainstem and spinal cord.
Brainstem: tells spinal cord how to maintain balance during movement
Spinal cord: Motor neurons send the commands received form the motor cortex and the
brainstem to the muscles. During movement, sensory info from the limb is acquired and
transmitted back to the cortex. Reflex pathways unsure stability of limb. A motor unit refers to
a single motor neuron and the muscles fibers it controls. Each mature muscle fiber is innervated
by only one motor neuron, although each motor neuron innervates many muscle fibers. Motor
neurons innervating a single muscle are situated close to each other in the ventral horn, and are
called a motor pool. In the ventral horn, there are two groups of motor pools. In the medial part
of the ventral horn, the motor pools innervate axial muscles which help us stand up and maintain
posture. In the lateral part, motor pools innervate limb muscles.
Cerebellum: coordination of multi-joint movements, postural stability, and motor
learning
Basal ganglia: learning and stability of movements, initiation of movements, emotional
and motivational aspects of movement
II. Motor system diseases
A) How spinal cord injury results in paralysis
- Initial injury in small region growth of damaged area hemorrage of blood
vessels cyst, swelling, and cell death
- Injured neurons release Glu at high levels neuron excitotoxicity and glia
apoptosis
- Cyst and glu kill myelin producing cells. The death of the glial sells results in
the demylination of healthy axons, making them unable to conduct impulses in a normal way.
- After a few weeks, a wall of glial cells forms
- Paralysis can be partially overcome with direct electrical stimulation from neural
prosthetic
B) Other diseases/conditions
- Polio: virus kills motoneurons directly or allows them to survive as a giant motor unit
with 10x as many sprouts. Post-polio syndrome includes motor unit enlargement
followed by eventual muscle breakdown. Hypothesis is that eventual cell death is
due to an overuse of the few remaining motor neurons during the previous
decades (i.e. a kind of metabolic exhaustion that leads to an inability to regenerate
new axon sprouts to replace degenerated ones).
- ALS: slow degeneration of α-motorneurons in spinal cord and eventually in
motor cortex
- Motor stroke: spinal motoneurons not affected, but central control is weak
- Rigor mortis: stiff muscles after death, no ATP available for myosin detachment
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III. Properties of muscle fibers and muscle movement
- Extrafusal vs intrafusal fibers: extrafusal fibers attach to tendons, which in turn attach to
the skeleton. They produce the force that acts on bones and other structures. Intrafusal fibers
attach to the extrafusal fibers, produce a negligible force, and instead play a sensory role. They
contain muscle spindles which, innervated by muscle spindle afferents, provide the CNS with
info about muscle length.
- There are two major types of motor neurons: α motor neurons innervate large,
extrafusal muscle fibers that generate force; γ motoneurons innervate intrafusal fibers that house
the muscle’s sensory system.
- There is an optimum force of contraction with respect to muscle length (i.e. not
monotonic!)
- Rapid limb movements follows a 3-phase pattern (agonist, antagonist, agonist) directed
by descending commands
- Types of muscle fibers: The exact content of the myosin molecule determines the
functional characteristics of the muscle fibers. Type I slow (S): long contraction time and show
little or no loss of force with repeated stimulation. S type motor units are composed of muscle
fibers with small diameter, and are supplied by slow conducting, small diameter axons from
small motor neurons. Type IIa fast fatigue resistant (FR); have intermediate contraction time and
can maintain force longer than FF. Type IIx fast fatiguing (FF): have small contraction time and
produce high twitch tension, but tend to fade quickly; composed of muscle fibers with relatively
large diameter and are supplied by fast conducting, large diameter axons from large motor
neurons.
- Different motor units are composed of different ratios of fiber types
- Muscle fiber type is a result of myosin isotype expr; the type can change; fast type is
default
- A muscle fiber cannot split to form a new fiber. So a muscle can become bulkier only if
the individual fibers become thicker. This happens by addition of new myofibrils. The process
starts when there is additional stress at the tendons. This triggers signaling proteins to activate
genes that cause muscle fibers to make more contractile proteins (chiefly myosin).
- Grading of muscle force is accomplished in two ways: 1) in voluntary contraction, as
force requirements increase, new motor neurons are recruited. 2) as force requirements
increase, the freq of activation of already recruited motor neurons increases. Motor units
that are recruited later then to have faster contraction time and larger peak force.
IV. Role of afferent neurons in motor control
- Afferent neuron fiber types
- Ia and II: wrap themselves around intrafusal fibers and are sensitive to muscle length
changes; used for proprioception; Ia spindle afferent excites MTNs of agonist
mm. and inhibits MTNs of antagonist mm. which provides the stretch reflex. Ia
fibers, coming from the spindles, terminate in the part of the spinal cord wehre the
motor neurons are. They have monosynaptic excitatory connections on α-motor
neurons of the same muscle. A single Ia afferent sends afferents to nearly all of
the MNs in the same muscle. They make excitatory synaptic input to inhibitory
interneurons acting on alpha motor neurons of antagonistic muscles.
- Ib: afferents innervate the junction between extrafusal muscle fibers and the
tendon.; sensitive to force changes in the muscle; Golgi tendon organs (from which Ib fibers
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arise) measures force change. Group Ib fibers terminate in the regions of the spinal cord where
interneurons are located. These interneurons inhibit alpha motor neurons that go to the same
muclse.
- γ motor neuron system controls spindle sensitivity. When the extrafusal muscle fibers
shorten due to activation from a motor neuron, the intrafusal fibers can go slack. This would
lead to a sudden loss of firing in the spindle afferents, resulting in a loss of information regarding
muscle length. To prevent this, the CNS activates the γ-motor neurons during contraction to
maintain tension in the intrafusal fibers.
- Stretch reflex: short-loop response caused by Ia afferents responding to lengthening of
muscle spindles. This produces AP in the afferents, which then synapses on the motor
neurons of the stretched muscle, exciting these motor neurons and contracting the muscle.
Long-loop response is a second pathways thru which a stretch can result in a
compensating activity in motoneurons. This pathway begins with the Ia afferents, goes
up the spinal cord thru the dorsal column-medial lemniscal pathway (DC-ML), reaches
the thalamus, then the somatosensory and motor cortex and then comes back down to the
spinal cord thru the cortico-spinal tract. Unlike the short loop, its activity is
programmable by the brain: voluntarily we can change the response to a stretch (but loss
of this control in Parkinson’s disease or ipsolateral stroke)
- In absence of afferent input, accurate motor control depends strictly on visual input
The Motor System: Lecture 2: Descending Tracts
0. Motor control is a high level action, e.g. it may involve complex joint motions but smooth
trajectories.
I. CORTICOSPINAL TRACT (CST)
- Origin at premotor cortex, motor cortex (M1), somatosensory cortex (S1)
- Axons run through pyramids (medulla) and 90% cross in lower medulla
to form lateral corticospinal tract which projects to sensory neurons in the dorsal
horn, interneurons in the intermediate zone and motor neurons innervating distal
limb muscles. Therefore, the cortical motor areas have dominant control over the
contralateral limbs. 10% remain ipsolateral mostly going to ventral corticospinal
tract, which projects onto the ventromedial motor pools innervating axial muscles.
This tract originates from the neck and trunk regions of the premotor and motor
cortex. Unlike the lateral corticospinal tract, this tract has bilateral projections in
the cord.
A tiny minority of fibers do not cross and travel laterally in the spinal cord, and
allows the motor cortex to have a small amount of control over the ipsilateral
limb.
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