Motor Pathways CNS/PNS: Lower Motor Neurons Local Circuit Neurons Flashcards
Organizational control of movement
The neural circuits responsible for the control of movement can be divided into four distinct but highly interactive subsystems, each of which makes a unique contribution to motor control
The first of these is the local circuitry within the gray matter of the spinal cord and the analogous circuitry in the brainstem indicated by the grey and purple boxes in this figure. The relevant neurons include the local circuit neurons and the lower motor neurons which send their axons out of the brainstem and spinal cord to innervate the skeletal muscles of the head and body, respectively. The local circuit neurons are the major source of synaptic input to the lower motor neurons
All commands for movement, whether reflexive or voluntary, are ultimately conveyed to the muscles by the activity of the lower motor neurons; thus these neurons comprise the so called ‘final common pathway’ for control of movement.
The local circuit neurons receive sensory inputs from the periphery as well as from descending projections from higher centers. Thus, local circuit input to the lower motor neurons “provide much of the coordination between different muscle groups that is essential for coordinated movements.” This local circuit neuron/motor neuron interaction creates the basic anatomical machinery responsible for many patterned or rhythmic movements such as walking, chewing, etc.
One can think of these movements as ‘pre-programed’ movements that may be initiated independent of higher brain input. In fact, in some lower animals it is possible to disconnect the higher brain centers from the spinal cord and still maintain the ability to move all four legs in a walking pattern.
“The second motor subsystem consist of the upper motor neurons whose cell bodies lie in the brainstem or cerebral cortex and are shown as the tan boxes in the figure.
Upper motor neuron axons descend to synapse with the local circuit neurons or, less frequently, with the lower motor neurons directly.
The upper motor neuron pathways that arise in the cortex are essential for what?
the initiation of voluntary movements and for complex spatio-temporal sequences of skilled movements such as fine finger control.
In particular, descending projections from cortical areas in the frontal lobe, including the primary motor cortex, the premotor cortex, and the supplementary motor cortex are essential for planning, initiating, and directing sequences of voluntary movements.
Upper motor neurons originating in the brainstem are responsible for what?
regulating muscle tone and for orienting the eyes, head, and body with respect to vestibular, somatic, auditory, and visual sensory information.
The latter contributions are critical for basic navigational movement, and for the control of posture.”
The third and fourth subsystems are complex circuits with output pathways that have no direct access to either the local circuit neurons or the lower motor neurons;
instead, they control movement by regulating the activity of the upper motor neurons.
The third and larger of these subsystems is the cerebellum, shown as the green box in the figure.
How does the cerebellum work?
The cerebellum acts as a servomechanism, detecting the difference or motor error between an intended movement and the movement actually performed.
The cerebellum uses this information about discrepancies in movement to mediate both real-time and long-term adjustments in these motor errors. The long term adjustments reflect a motor learning process that improves the long term coordination of a motor task.” Think of a ballerina doing a perouette or a baseball player hitting a home run
The fourth subsystem, the basal ganglia shown as the auburn box in the figure, does what?
suppresses unwanted movements and prepares upper motor neuron circuits for the initiation of movements.
What is an UMN?
Amotor neuron contained entirely within the CNS (no part of the neuron leaves the central nervous system).
NOTE: A LOWER MOTOR NEURON is a neuron with the cell body in the CNS projecting its axon into the PNS to innervate something such as a skeletal muscle fiber
How does central motor output work (aka the corticospinal tract)? (Initial steps)
1) An UMN located in the precentral gyrus of the primary motor cortex, the so-called “motor strip”, sends its axon via the corticospinal tract (also called the pyramidal tract), forming the corona radiata and converging to pass through the posterior limb of the internal capsule
What is the 2nd step of the cortiospinal tract?
corticospinal fibers descend through the middle 3/5ths of the crus cerebri (basis pedunculi) in the anterior part of the midbrain
What are the 3rd and 4th steps of the cortiospinal tract?
3) In the pons, the fibers are broken up into many bundles
4) the corticospinal tract descends as the pyramids in the anterior part of the medulla
What is the 5th step of the corticospinal tract?
At the junction of the medulla and spinal cord, most (85-90%) of fibers cross the midline in the decussation of the pyramids. These crossed fibers go on to form the lateral corticospinal tracts.
Uncrossed (10-15%) fibers descend as the anterior corticospinal tract
What is the 6th step of the corticospinal tract?
the lateral corticospinal tract terminates on the LMNs in the anterior horn of the spinal cord. The anterior corticospinal tract fibers cross the midline at the level where they terminate on the LMNS
. The lower motor neuron then sends its axon out through the spinal peripheral nerve to reach its target organ, the skeletal muscle
Not shown in this slide is the corticobulbar tract that is analogous to the corticospinal tract but synapses on lower motor neurons in brainstem motor nuclei.
Cross section of the spinal cord. The lower motor neurons are shown in red and located in the ventral or anterior horn of the gray matter.
Note the UMNs from the lateral corticospinal tract, shown in purple, synapse only ipsilaterally to innervate limb musculature while the anterior corticospinal tract (also purple) synapse bilaterally to innervate axial musculature.
Fibers from the LMNs leave the spinal cord between the juncture of the lateral and ventral columns and pass out into the peripheral (spinal) nerve via the ventral roots.
Again, UMNs from the lateral corticospinal tract synapse only ipsilaterally to innervate ______ while the anterior corticospinal tract synapse bilaterally to innervate ______
Lateral: limb musculature
Anterior: axial musculature.
This slide presents a cross section of the cervical spinal cord showing Rexed’s Laminae. What neurons are found in layers I-VI?
Intemediate sensory neurons
What neurons are found in layers VII-VIII of Rexed’s Laminae?
Local circult, autonomic, and commisural neurons
What neurons are found in layer IX of Rexed’s Laminae?
LMNs
This slide presents a cross section of the spinal cord and a color rendering of the corticospinal tracts.
The corticospinal tracts are the major motor fibers carrying motor instructions from higher brain centers to the local circuit and lower motor neurons.
Note that the lateral corticospinal tract is shown in red and the ventral corticospinal tract is shown in blue. The point of this slide is to depict the somatotopic distribution of the LMNs within the ventral horn.
Note that lower motor neurons serving the extremities (in red) are located laterally in the ventral horn while the neurons serving the more proximal trunk muscles (shown in blue) are located medially.
Note also that the lateral corticospinal tract synapses only with ipsilateral motor neurons serving the distal extremities. In contrast the ventral corticospinal tract synapses bilaterally on the medial motor neurons serving the trunk muscles.
This slide depicts a single lower motor neuron, labeled the alpha-motor neuron, innervating several muscle fibers within a single muscle. This anatomic unit is termed a “motor unit”.
T or F. One motor neuron can control several muscle fibers in a single muscle.
T.
The muscle fibers of a motor unit are distributed evenly and widely within a single muscle to assure a smooth contraction of the muscle.
How does info from a LMN get to muscle/end organ?
An action potential initiated in a lower motor neuron normally brings to threshold all the muscle fibers that it contacts thus activation of a single motor unit represents the smallest unit of force that can be generated by a single muscle.
This slide summarizes an important feature of muscle fiber types that relate to the maximum force generated by a fiber and its ability to sustain this force
The three general muscle fiber types are named SLOW FATIGUE RESISTANT, FAST FATIGUE RESISTANT, and FAST FATIGABLE. The slide shows the three muscle fiber types with their maximum sustainable force of contraction.
The figure on the left demonstrates repetitive stimulation of the three muscle types. The figure on the right demonstrates the sustainability of the three muscle types during continuous stimulation.
Muscles comprised of primarily fast fatigable fibers generate large contraction forces but fatigue quickly. Muscles with primarily slow fatigue resistant fibers generate smaller forces that are sustained for long periods without fatigue. Fast fatigue resistant muscles have properties somewhere between the two extremes.
Slow fatigue resistant muscle is also called ______ muscle and fast fatigable muscle is known as Type II muscle. (This Type I and II muscle terminology arises from the histological appearance of muscle with certain stains)
Type I
Muscle stretch reflex involved in maintaining muscle tone. Describe the initial steps of this process.
1) As the reflex hammer strikes the patellar tendon, the muscle is stretched.
2) Within the quadriceps muscle, and all other muscles, lie sensory organs called muscle spindles that sense stretch of the muscle and send signals via sensory nerves called 1a sensory nerves, shown in blue, to synapse directly on alpha lower motor neurons.
Describe the later steps of a muscle reflex
3) The alpha motor neuron (shown in red) is activated by this synaptic volley and fires off an action potential to its quadriceps muscle fibers causing contraction of the muscle.
3b) Simultaneously, the 1a sensory fibers activate inhibitory interneurons (shown in green) that inhibit the lower motor neurons of the antagonistic muscle, in this case the hamstring muscle.
This monosynaptic reflex, i.e. the one linking the muscle spindle to the lower motor neuron and back to the muscle, is known as the “muscle stretch” reflex. This reflex is sometimes inappropriately referred to as the deep tendon reflex when in fact is should be called the ‘muscle stretch reflex’.
Let’s consider the types of connections made on alpha-motor neurons. Alpha-motor neurons are the neurons that innervate skeletal muscle fibers to form motor units. Alpha-motor neurons in the ventral horn of the spinal cord can receive feedback from what?
Sensory organs within the bed of the muscle. The afferent neurons from these sensory organs have cell bodies in the dorsal root ganglion. If these afferent nerves provide excitatory information to the motor neuron, the nerve has an excitatory monosynaptic connection with the motor neuron. Common excitatory neurotransmitters are acidic amino acids.
In the case where the afferent nerve arising from a sensory organ within the muscle bed provides inhibitory feedback to the motor neuron, an inhibitory interneuron is necessary to form an inhibitory synapse on the motor neuron. Note that this arrangement allows an afferent nerve to provide excitatory information to some motor neurons, and inhibitory information to others. Common inhibitory neurotransmitters are _______ and _______.
GABA and glycine.
In addition, motor neurons receive feedback from neurons in the DRG within the same spinal cord segment in which they reside AND from dorsal root ganglion neurons projecting from spinal cord segments above and below.
In addition, motor neurons also receive information from higher motor centers.
When excitatory connections are made to motor neurons from afferents projecting from above or below that particular spinal cord segment in which the motor neuron resides, the connection is made how?
Through an excitatory interneuron. All excitatory connections from higher motor centers are through excitatory interneurons.
Inhibitory connections on motor neurons, no matter what spinal level they originate from, all occur through inhibitory interneurons.
Consider the sensory organs within the muscle bed that give rise to afferent nerves. What are they?
One sensory organ, the muscle spindle, may be associated with a particular motor unit or several motor units, depending upon the muscle.
Another set of sensory organs are located at the semitendonous junction where muscle fibers terminate to form a tendon. These sensors, the Golgi tendon organs, give rise to the type Ib afferent nerves
This slide presents the anatomical structure of the muscle spindle found amongst a group of muscle fibers. The primary function of the MS is to ______.
The muscle spindle generates the sensory information that triggers the muscle reflex arc
What are muscle spindles comprised of?
]It is comprised of small, intrafusal muscle fibers, contained in a connective tissue sheath shown here as a gray covering.
These intrafusal spindle fibers lie in parallel with the extrafusal muscle fibers that make up most of the muscle.
How does the muscle spindle work?
When a muscle is stretched, for example the lengthening of your biceps as you extend your arm at the elbow, the intrafusal muscle fibers of the spindle are also stretched since they are connected in parallel with the extrafusal muscle.
Specialized sensory nerve endings, called annulospiral endings (shown in blue) encircle the intrafusal fibers and these sensory endings possess membrane channels that are sensitive to mechanical stretch, thus firing off an action potential.
How does the AP generated by annulospiral endings propagate?
the AP travels via the 1a sensory nerve (shown in blue) into the dorsal horn of the spinal cord and on to the ventral horn, where it reaches an alpha motor neuron which in turn fires an action potential to contract the extrafusal muscle (shown in red).
Since the muscle spindle is connected in parallel with the extrafusal muscle, as the latter contracts and shortens, so does the muscle spindle.
Without an additional mechanism, the muscle spindle would only function under conditions when the extrafusal muscle fibers were fully extended (that is stretched). Since muscles must maintain graded tone and strength throughout their full range of motion, the muscle spindle is also served by what?
a motor neuron called the gamma motor neuron (also shown in red).
This neuron fires in concert with the alpha motor neurons and thus causes simultaneous contraction of the spindle’s intrafusal muscle fibers, keeping the mechanical stretch sensors from becoming relaxed and nonfunctional. Accordingly, the muscle spindle monitors and regulates muscle tone throughout the full extension and flexion of a muscle such that this becomes a smooth process with relatively constant muscle tone throughout the movement.
The muscle spindles, sometimes called intrafusal muscle fibers, are sensory organs that have cross striations similar to those observed in skeletal muscle fibers.
Anatomically, these fibers are much smaller than skeletal muscle fibers in both length and diameter, and are attached to one or more muscle fibers by their extracellular matrix.
Microscopically, two general forms of muscle spindles are observed based upon the location of their nuclei, namely:
nuclear chain and nuclear bag. (For our purposes, we will treat them identically)
Type II afferent from the muscle spindle. This afferent nerve encodes muscle fiber length information in the frequency of its action potentials. Increasing length increases the frequency of action potentials, decreasing length decreases the frequency of action potentials.
Type Ia afferent arising from the muscle spindle. This nerve encodes both length and velocity information in the frequency of its action potentials. Considering first a rapid change in muscle fiber length, the frequency of action potentials increases dramatically during a rapid increase in muscle fiber length, and decreases dramatically during a rapid decrease in muscle fiber length. While length is static, the frequency of action potential in the nerve reflects the length of the muscle fiber.
A less rapid change in muscle fiber length produces less dramatic changes in type Ia afferent nerve activity. An increase in the frequency of action potentials is still apparent during lengthening, and a decrease in action potential frequency occurs during shortening, both indicating velocity. Again, when muscle fiber length is static, the action potential frequency reflects the fiber length.
How do the type Ia and II afferent nerves arising in the muscle spindles feedback on their associated motor neurons?
through a monosynaptic, excitatory connection. Thus, the motor unit is monitoring its length. This allows controlled movement, or lack of movement. For example, you are able to hold forceps in a static position without observing your hands, or you are able to close your eyes and touch your nose.
What is the Golgi tendon organ composed of?
The organ is comprised of encapsulated afferent nerve endings (shown in blue) located at the junction of the muscle and tendon and connected in series with the muscle/tendon.
This slide demonstrates the Golgi tendon organ mediating negative feedback to the contracting muscle causing it to relax. How does this work?
Stretch of the Golgi tendon organ, shown in blue, stimulates an inhibitory circuit neuron (shown in black) to inhibit the lower motor neuron (shown in brown) to cause the biceps to relax.
Simultaneously, the 1b afferent stimulates an excitatory interneuron (shown in purple) to activate the lower motor neuron (shown in red) of the antagonist muscle, in this case the triceps, to contract.
In addition to intrinsic muscle reflex arcs, there are other muscle reflex connections with varying degrees of complexity. One simple example is the flexor reflex. What is this?
An uncomfortable stimulus on the skin will produce a reflex withdrawal. The flexor reflex has both the components of agonist muscle activation and antagonist muscle inhibition. Note that the agonist muscle excitatory stimulus occurs through an excitatory interneuron.
This is a summary slide that I will not expand on other than to emphasize the higher level of control from cortex and brainstem that is maintained over the muscle spindle and Golgi tendon organ reflexes. These descending pathways terminate almost entirely on local circuit neurons that in turn regulate the lower motor neuron.
Clinical signs differentiating UMN vs LMN lesion
