1 Bi/CNS 150 Lecture 15 Monday November 3, 2014 Motor Systems Kandel, Chap. 14:p , 37 Bruce Cohen

2 Overview Motor neurons can be divided into two classes, “lower” and “upper” “Lower” motor neurons are located in the spinal cord and brainstem They initiate skeletal muscle contraction and are cholinergic (release acetylcholine) Lower motor neurons send axons directly to muscle fibers Local circuits in the brainstem and spinal cord primarily determine the spatial and temporal activation of lower motor neurons

3 Upper motor neurons “Upper” motor neurons are located in the brainstem and cerebral cortex, and are glutamatergic (release glutamate) Their axons form descending pathways such as the corticospinal tract that modulate the activity of the lower motor neurons Upper motor neurons govern voluntary motor movements such as locomotion Damage to the descending pathways of the upper motor neurons can cause weakness, spasticity (increased muscle tone), and the loss of the ability to perform fine motor movements

4 Primary Motor Cortex BA 4 Premotor/supplementary Motor cortex BA 6 Frontal Eye Fields BA 8 Broca’s Area (left side) BA 44, 45 Prefrontal Cortex (Frontal Association Areas) Motor Areas of Cortex

5 Layer 5 is particularly prominent in primary motor cortex 5

Flow chart of motor system hierarchy 6

Corticospinal tract: A key motor tract 7 Decussation in hindbrain

Motor unit consists of a single  motor neuron and all the muscle fibers that it innervates Most muscle fibers in mature mammals are innervated by a single  motor neuron When the motor neurons fires, all innervated muscle fibers contract because the endplate potential typically exceeds threshold voltage Motor unit is the smallest unit of force that can be activated to produce movement To increase the force of a muscle contraction additional motor units are recruited Motor Unit 8

Fewer Myelinated Fibers in Lower Spinal Cord 9

Myelin Dorsal Horn Sensory Ventral Horn Motor Ventral Root Motor Motor neuron in spinal cord cross section Motoneuron 10

Force exerted by muscle contraction is increased by recruiting more motor units The size of motor units varies from small to large Small  motor neurons innervate relatively few muscle fibers, large  motor neurons innervate many muscle fibers Small motor units generate fine motor movements, large motor units generate gross motor movement Synaptic input to the pool of motor neurons excites small ones first because they have the greatest input resistance The size principle states that, during voluntary and reflexive movements, the smallest motor units are recruited first and larger motor units are recruited later Size principle 11

12 Orderly recruitment of motor units

Spinal Reflexes Stretch reflex is a monosynaptic spinal reflex triggered by stretching a muscle Muscle spindles sense, and signal, changes in muscle length Axons from spindle contact motor neurons that drive the muscle they are in (homonymous), synergist muscles, and inhibitory interneurons Inhibitory interneurons inhibit motor neurons driving the antagonist muscles They inhibit extensors when flexors are commanded, and vice- versa 13 Figure 35-2B

The stretch reflex acts as a negative feedback loop to maintain muscle length 14

Intrafusal fibers in parallel with extrafusal muscle fibers Two types of sensory fibers – primary (Group Ia fibers) and secondary (Group II fibers) spindle afferents Group Ia – change in length (dynamic) Group II – length (static) Golgi tendon organ measures tension of muscle contraction (not shown) Sensory information goes to spinal cord segment, dorsal column nuclei (proprioception), and cerebellum Fine structure of muscle spindle Extrafusal fibers 15

Gamma motor neurons are small MNs that project from ventral roots to intrafusal fibers Activity in gamma-MNs contracts the intrafusal muscles and makes the spindle apparatus more sensitive In turn, the group Ia and II fibers become more active Gamma-bias impacts muscle tone Gamma motor neurons regulate muscle tone Extrafusal fibers 16

Amyotrophic lateral sclerosis (ALS) “Lou Gehrig’s Disease” 17 “Upper” motor neurons also degenerate ALS symptoms Loss of motor unit innervation leads to weakness or paralysis of muscle Fasciculations (spontaneous contractions of muscle fibers); detected with electromyography (EMG) Atrophy of muscles, due to loss of trophic factors from motoneuron Hyporeflexia or areflexia Average time from diagnosis to death ~ 3 yr

Effects of damage to the upper and lower motor neurons Lower Motor NeuronUpper Motor Neuron ParalysisParesis (weakness) Muscle atrophyNo atrophy Areflexia & atoniaHyperreflexia, hypertonia, spasticity Ipsi-lateral deficit in spinal cord Contra-lateral deficit above decussation; Ipsilateral deficit below decussation 18

19 Stimulation in human motor cortex. An array is implanted... to localize an epileptic focus

20 Anterior Cingulate Cortex Lesions in ACC cause impair one of the hierarchically highest levels of the motor system: the will to act. Patients with ACC lesions can exhibit "akinetic mutism": they are not paralyzed and are conscious but respond poorly to their surroundings. They sometimes exhibit conditioned responses, like picking up a phone that rings next to their bedside (but then say nothing). They often recover, and then explain that while in this state, they were fully conscious but just lacked motivation to do anything and so did not respond or act on their surroundings.

End of Lecture 15