Motor System

Question 1

Pyramidal motor system

Answer 1

Motor function traveling through the
“pyramids” of the medulla

Newer
* Origin in cerebral cortex
* Voluntary
* Skilled movements

The pyramidal motor system controls the
contralateral side of the body

Question 2

Extrapyramidal motor system

Answer 2

• Refers to non-pyramidal motor systems
  (e.g. motor systems NOT traveling through
  pyramids of the medulla)
• Classically, used to describe aspects of
  basal ganglia motor function
• But, also applies to cerebellar motor
  function– And a few other minorish motor systems
• Older
• Origin in basal ganglia,
  cerebellum, red nucleus
• Axial and postural control,
  coordination, balance

The cerebellar motor system controls the ipsilateral side of
the body (double-crosses)]

• involuntary (non-conscious)
• modulate a-motor neuron function but do
  not directly synapse on them
• Reticulospinal
• Vestibulospinal
• Rubrospinal
• Tectospinal
• Cerebellum
  – ipsilateral control
• Basal ganglia
  – contralateral control
• Their pathways are distinct from pyramidal motor
  systems

Question 3

Upper Motor Neuron (Motor Cortex)

Answer 3

precentral gyrus
Betz cells: giant pyramidal cells
– send long axons that synapse onto alpha-motor
neurons in the spinal cord
– largest neurons of the CNS, cell bodies are in
layer V
– long axons

• Other cortical areas
  involved in motor control:
  – Posterior parietal cortex (PPC)
• production of planned movements
  – Supplementary motor area (SMA)
• projects directly to spinal cord
• can control both sides of body; many muscle groups; sequences of movements

frontal eye fields
* in prefrontal cortex
initiation of eye movements by
stimulating saccadic eye movements
* have a topographic representation of
space in retinotopic coordinates

Broca’s area
- speech production

Question 4

Corticospinal tract

Answer 4

Originates in multiple
cortical areas
– Primary motor cortex
– Pre-motor cortex
* Route via:
– Internal capsule
* Between Thalamus &
Basal Ganglia
– Cerebral peduncles
* Contralateral crossing at
the pyramids (medulla)

Question 5

internal capsule (coronal view)

Answer 5

Question 6

internal capsule (horizontal view)

Answer 6

Question 7

Where do pyramidal motor tracts travel?

Answer 7

Pyramidal motor tracts travel in
ventral brain stem/ pons

Question 8

pyramidal dessucation

Answer 8

Pyramidal
decussation
– Near complete
crossing of the
pyramidal tracts
* 90% contralateral
* 10% ipsilateral
– Ipsilateral tract
thought to be partially
responsible for
recovery after
unilateral motor
cortex damage

Question 9

Corticobulbar tract

Answer 9

• motor cortex to cranial nerve
  nuclei in the brainstem
• muscles of face, head, and
  neck
• usually innervate bilaterally
• travel in the genu of the
  internal capsule
• do NOT have major crossing
  in the pyramids

Question 10

Reticulospinal tract

Answer 10

• extrapyramidal
• arises from the
  reticular formation
• primary roles are for
  postural control and
  locomotion
  – particularly proximal
  and axial muscle
  groups

Question 11

Vestibulospinal Tract

Answer 11

• extrapyramidal
• relays information from
  the vestibular nuclei
  (VIIIth-
  vestibulocochlear nerve)
• support posture and
  balance, control head
  movements

Question 12

Rubrospinal
tract

Answer 12

• extrapyramidal
• originates in red nucleus
  – in midbrain, neighboring
  substantia nigra
• axon fibers cross/decussate
  in midbrain
• function
  – cerebellar inputs: carry out planned
  movements
  – alternative to the corticospinal system

Question 13

Tectospinal tract

Answer 13

• extrapyramidal
• originates in midbrain tectum,
  terminates in spinal cord
• mediates reflex postural
  movements of the head, based
  on visual input arriving at the
  superior colliculus

Question 14

Spinal Cord Motor Organization

Answer 14

• Gray Matter
  – Dorsal/Posterior Horn
  – Ventral/Anterior Horn
• Where the a-motor neurons live
• White Matter
  – Posterior Funiculus
  – Lateral Funiculus
• Carries the descending corticospinal
  tract (from the opposite side)
  – Anterior Funiculus
• Dorsal Root Ganglion
  – Sensory Input
• Ventral Root (aka
  Anterior root)
  – Where the motor output heads out to
  the muscles

Question 15

a-motor neuron

Answer 15

the “lower motor neuron”
* cell body is in the spinal cord, but axon extends
into the PNS
* axon can be up to 3-4 feet long
* innervates extrafusal (skeletal) motor fibers
* motor unit: a-motor neuron + its muscle fibers
* the number of a-motor neurons is proportional to
the degree of control
* #finger a-motor neurons»_space; #quadriceps a-motor neurons

• cell bodies sit in the ventral cord
  – aka ventral horn
  – aka anterior horn

Question 16

Neuromuscular
Junction

Answer 16

• synaptic connection from the
  nervous system to the muscle
• Action potential triggers calcium
  release
• Docking and then release of vesicles
  containing ACh

Question 17

The Motor Unit

Answer 17

*a-Motor neuron and
the extrafusal muscle
fibers it innervate
* The building block
of movement

Question 18

• Extrafusal (skeletal) muscle

Answer 18

– innervated by a-motor neurons
– generate tension by contracting

Question 19

• Intrafusal muscle

Answer 19

– innervated by g-motor neurons or sensory
neurons
– comprise the muscle spindle
– proprioceptor reporter of muscle contraction

Question 20

Golgi Tendon Organ

Answer 20

• Forcereceptor in tendon of muscles
• a proprioceptive sensory receptor organ
  – tree-like sensory ending enclosed in a spindle-like connective tissue capsule, that
  lies near the junction of a tendon with a muscle
• Sensory input
  reporting on muscle
  contraction
• Strongly activated
  by muscle contraction
  – Less activated by
  passive muscle
  stretch

Question 21

Muscle Spindles

Answer 21

• sensory receptor in
  extrafusal muscles
  – senses changes in
  length of muscle
• Innervated by g-motor
  neurons
  – to contract sensory
  fibers
  – Keeps gamma
  fiber/sensor at same
  proportional length as
  the rest of the muscle

Question 22

Step reflex

Answer 22

• Present at birth
• When sole of foot touches a flat surface,
  infant will “walk” by placing one foot in
  front of another
• Reflex suppressed ~ 6 weeks of age
• alternating contractions of flexor and
  extensor muscles
  does not require
  cortical input

Question 23

Walking is controlled by which muscle tracks?

Answer 23

extrapyramidal. use gait analysis to test this

Question 24

• CPG: central pattern generator

Answer 24

– located in ventral spinal cord
– functions independently:
“a neural network that produces rhythmic
patterned output without motor or sensory input”
– exist for locomotion, swallowing, respiration…

Question 25

Locomotor CPG

Answer 25

* vertebrate locomotor CPG comprises a distributed network of interneurons and motor neurons * generates an organized motor rhythm * Descending inputs from the brainstem, basal ganglia and cortex control the selection and shaping of outputs from the locomotor CPG – basal ganglia – corticospinal – rubrospinal – reticulospinal * additional modulation coming from sensory and vestibular pathways that converge on CPG neurons – tectospinal – vestibulospinal

Question 26

CPG Organization in Mammals

Answer 26

* CPGs at different levels in the spinal cord have different functions CPG interneurons have unique properties and have distinct developmental profiles

Question 27

Answer 27