Organization of motor system
Spinal reflexes
Squire Ch 29
Rosenbaum Ch 3 pp 46-61
Pathways for signals to muscles
Red nucleus
Hierarchical Organization of the motor system
Somatosensory ctx
Planning/ sequences
Supplementary motor ctx
Selection of trajectory
Activity prior to movement
Target selection
Posterior parietal ctx
Pre-motor cortex
Primary motor ctx
Muscle commands
Cortico-spinal tract
Initiation of movement
“Efferent copy”
of movement
Smoothness, timing
Learning new skills
Monitor feedback
80 msec
Spinal feedback: 30-50 msec
Proprioceptive signals (muscles, joints)
Somatosensory ctx
Pre-motor ctx
Primary motor ctx
Intra-parietal sulcus
A “motor unit” is defined as one motor neuron
And all the muscle fibers it innervates. In
mammals, each muscle fiber is innervated
by only one motor neuron, but one motor
neuron innervates many muscle fibers
as its axon branches in the muscle.
Hand: 100 muscle fibres per motor unit
Leg: 1000 muscle fibres per motor unit
Figure 34-1 A typical muscle consists of many thousands of muscle fibers working in parallel and organized
into a smaller number of motor units. A motor unit consists of a motor neuron and the muscle fibers that it
innervates. The motor neurons innervating one muscle are usually clustered into an elongated motor
nucleus within the ventral spinal cord that may extend over 1-4 segments. In the example shown here the
motor neuron A1 plus other motor neurons innervating muscle A form motor nucleus A. Muscle B is
innervated by motor neurons lying in a separate motor nucleus B.
Motor Unit
Intrafusal fibre connected
to a single extrafusal fibre
Contracts Intrafusal
Contracts Extrafusal
Alpha and gamma motor neurons co-activate
1a afferents respond to a difference in length between
The intra and extrafusal fibres
Golgi (Ib) thresholds regulated by central signals.
Activity of Ia afferents corresponds to awareness of stretch
Golgi tendon organ is located at the transition between muscle and tendon. It senses any
active tension produced by the muscle fibers, being in series between muscle and tendon.
The muscle spindle is located in parallel with the extrafusal muscle fibers. It signals the
muscle length and dynamic changes in muscle length. Both muscle spindle and Golgi
tendon organ have fast conducting afferent nerve fibers in the range of ∼100 m/s.
Mechanism of muscle contraction
myasthenia gravis, caused by antibodies
that bind to AChRs at the neuromuscular junction
B. The sarcomere is the functional unit of the muscle.
It contains contractile proteins, the thick and thin
filaments, bounded by thin Z disks, from which the thin
filaments arise. Thick and thin filaments overlap, creating
alternating dark bands that give skeletal muscle its
characteristic striated appearance. This banded pattern
changes as the overlap between the thin and thick filaments
changes during shortening or lengthening of the muscle fiber.
Contraction is produced by cyclical attachment and
detachment of myosin heads on adjacent thin filaments.
Figure 34-5 The amount of active contractile force developed during contraction depends on the degree
of overlap of thick and thin filaments. When the sarcomere is stretched beyond the length at which the
thick and thin filaments overlap (length a), no active force develops because the myosin heads are not
near any binding sites and thus cross bridges cannot form. As the filaments overlap (lengths a-b) the force
that can develop increases linearly as length decreases because of the progressive increase in the number
of binding sites for myosin heads. Around the muscle's optimal length (L0, between lengths b-c) the level
of force remains constant because the central portion of the thick filaments is devoid of myosin heads.
With further reductions in length (lengths c-d) the progressive overlap of thin filaments with each other
occludes potential attachment sites and the force begins to fall. Once the thick filaments abut the Z disks
(lengths d-e), they act like compression springs opposing the active force generated by the cross bridges.
Passive force exists in muscle regardless of activation, starting at about L0 and rising at first exponentially
and then linearly as progressive lengthening of the muscle stretches the connectin filaments that tether the
thick filaments between the Z disks. Total force is the sum of active and passive force.
Muscle can exert most force when joints are in the mid-range. At extreme joint angles
the muscle is less capable of exerting force.
Different kinds of muscle fiber
Red muscles - type I fibers
type IIA fibers
type IIB fibers
Slow rise
fatigue resistant
Smaller, exert less force
Exert more force- use different form of myosin
When a motor nucleus begins to be activated by peripheral or descending inputs, individual motor
neurons begin firing at a slow regular rate (5-10 impulses per second in humans). This results in a
partially fused train of contractions in the target muscle fibers. As the net excitatory synaptic input in
the nucleus increases, the firing rate of the cells increases and other, slightly larger motor neurons reach
their threshold for firing, adding their force as well. In this way the mean level of force produced in the
muscle gradually increases (Figure 34-12). The overall force of a contraction depends on both the
number and size of active motor units and their individual firing rates.
Spinal Cord
Feedback from muscle receptors to motoneurons: the stretch reflex.
The muscle spindle activates a-motoneurons directly, which causes the muscle fibers to contract.
The sensitivity of the muscle spindle can be actively regulated by g-motoneurons, which are more
slowly conducting. The muscle spindle provides negative feedback. If the muscle with its muscle
spindle is lengthened, the afferent activity from the muscle spindle increases, exciting the a-motoneurons
and leading to an increased muscle contraction, which in turn counteracts the lengthening. This
is called the stretch reflex. The Golgi tendon organ provides force feedback. The more the muscle
contracts, the more the Golgi tendon organ and its afferent are activated. In the diagram the interneuron
between afferent nerve fiber and motoneuron is inhibitory. Thus increased muscle force leads to
inhibition of the a-motoneuron, which results in a decrease of muscle force. The efficacy of
length and force feedback can be regulated independently in the spinal cord and via g-motoneurons.
Thus their respective contributions can vary considerably between different patterns of motor
Role of gamma motor neurons is to
prevent the spindle sensory fiber from
falling silent when the muscle shortens
as a result of active contraction,
enabling it to signal length changes
over the full range of muscle lengths.
Reciprocal inhibition in the spinal cord
Modulation of spinal reflexes by
descending innervation from M1 and
A. The Ia inhibitory interneuron allows higher centers to coordinate opposing muscles at a joint through a single command.
This inhibitory interneuron mediates reciprocal innervation in stretch reflex circuits. In addition, it receives inputs from
corticospinal descending axons, so that a descending signal that activates one set of muscles automatically leads to
relaxation of the antagonists. Other descending pathways make excitatory and inhibitory connections to this interneuron.
When the balance of input is shifted to greater inhibition of the Ia inhibitory interneuron, reciprocal inhibition will be
decreased and co-contraction of opposing muscles will occur.
B. Renshaw cells produce recurrent inhibition of motor neurons. These spinal interneurons are excited by collaterals from
motor neurons and then inhibit those same motor neurons. This negative feedback system regulates motor neuron
excitability and stabilizes firing rates. Renshaw cells also send collaterals to synergist motor neurons (not shown) and Ia
inhibitory interneurons. Thus, descending inputs that modulate the excitability of the Renshaw cell adjust the excitability of
all the motor neurons around a joint.
Renshaw cells modulate alpha
motor neurons
Short-latency and Long-latency responses: M1 = short, M2 = long
Central Pattern Generations: neural networks for control of rhythmic movements
Central Pattern Generators
A. Transection of the spinal cord of a cat at the level of b-b¢ isolates the hind limb segments of the cord, but
but the hind limbs are still able to step on a treadmill
B. Depending on the exact level of the transection of the brain stem, locomotion either occurs spontaneously
(cut 1) or can be initiated by electrical stimulation of the mesencephalic locomotor region (MLR) after a more
caudal transection (cut 2). Thal = thalamus; SC = superior colliculus; MB = mammillary body.
C. The spinal cord is removed from a neonatal animal (0-5 days of age) and placed in a saline bath.
Addition of N-methyl- D-aspartate (NMDA) and serotonin (5-hydroxytryptamine, or 5-HT) to the bath elicits
rhythmic bursting in the motor neurons supplying leg muscles.
Complex behavior in the spinal frog. Spinal cord must be able to relate the
sensory and motor maps.

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