Class 6

Question 1

Cortical areas associated with motor processes and descending tracts

Answer 1

• Frontal lobe
  
  • Precentral gyrus, primary motor cortex (M1), Area 4
  • Adjacent cortex, supplementary and premotor cortex (Area 6)
• Parietal lobe
  
  • Post-central gyrus, primary somatosensory projection area (S1), Areas 3-1-2
  • Second somatosensory, Area 5

Question 2

humunculus

Answer 2

• Map of body on lateral surface of M1 and S1 cortex
  
  • Distortion in map represents number of cortical cells projecting to muscles of a particular body part
  • Much larger areas associated with muscles of face, speech apparatus, hands (precision of motor control)
  • Electrical stimulation of a specific area within motor cortex yields muscle contraction of specific body part; damage (e.g. stroke) produces paralysis of those muscles

Question 3

What is the corticospinal tract?

Answer 3

Direct pathway to spinal cord; output from motor cortical areas (“pyramindal” tracts)

Question 4

Describe the corticospinal tract.

Answer 4

• Crossed: lateral corticospinal tract
  
  • Projects to motor neurons and interneurons supplying muscles (eg. hands)
  • Mediates fine motor control of limbs and hands (e.g. reaching and manipulation)
• Uncrossed: ventromedial corticospinal tract
  
  • Projects to spinal interneurons which, in turn, project to motor neurons supplying proximal muscles
  • Mediates axial movement and postural control

Question 5

Describe the indirect pathways to the spinal cord.

Answer 5

“Extrapyramidal”; more precisely designated as corticobulbar projections to brain stem nuclei which then give rise to several descending pathways (e.g. cortex to red nucleus: rubrospinal tract; cortext to reticular nuclei: reticularspinal tract)

Question 6

Describe the motor tracts originating in the motor cortex.

Answer 6

• Corticospinal tract
  
  • Lateral corticospinal tract: fingers, hands, arms, lower leg, and feet
  • Ventral corticospinal tract: trunk and upper legs
• Corticobulbar tract
  
  • Face and tongue

Question 7

Describe the motor tracts originating in the subcortex.

Answer 7

• Ventromedia tracts
  
  • Vestibulospinal tract: leg and lower trunk movement, control of posture
  • Tectospinal tract: head and upper trunk movement, visual tracking
  • Lateral reticulospinal tract: flexor muscles of the legs
  • Medial reticulospinal tract: extensor muscles of the legs
• Rubrospinal tract
  
  • Hands, lower arms, lower legs, and feet

Question 8

What are the inputs to the motor cortical areas?

Answer 8

1. Cortical areas associated with sensory/perceptual processes (e.g. visual, somatosensory, auditory)
2. Cortical/subcortical processes associated with motivational processes (limbic system)
3. “Loops” involving the cerebellum and basal ganglion
   
   • The basal ganglia or the cerebellum do not give rise to pathways that directly descend to the spinal cord
   • Output from these structures is to cortical motor areas or brain stem nuclei

Question 9

Describe the function, parts, and disorders associated with the basal ganglia.

Answer 9

• Integrates movement and control of posture
• Caudate nucleus, putamen, globus pallidus
• Disorders: cerebral, Parkinson’s disease, Huntington’s disease, ballismus

Question 10

Describe the inputs to and outputs from the cerebellum.

Answer 10

• Inputs: climbing fibers, mossy fibers
• Outputs: purkinje cells, basket cells (inhibitory influence)

Question 11

Cerebellar damage may cause…

Answer 11

• Intentional tremor: at onset or termination of movement
• Disruption of coordination (asynergies, dysarthrias)
• Locomotor ataxia and postural disturbances

Question 12

Describe spinal cord output motor neurons.

Answer 12

• Cell bodies located in anterior gray (ventral horn)
• Axons exit cord through ventral root (motor) and join with dorsal root (sensory) to form spinal nerve (mixed) to supply muscles and sensory receptors
• Axon endings terminate on motor end plate of muscle fiber
  
  • Release ACh, opens Na+ channels, causes AP, muscle contracts (shortening of muscle fiber brought about by interdigitation of action and myosin - sliding filament theory)

Question 13

Describe neuromuscular relationships.

Answer 13

• A given muscle fiber typically supplied only by one motorneuron
• Motor neuron pool: all motor neurons innvervating one muscle
• Motor unit: motor neurons terminate on many muscle fibers (endings diverge); all muscle fibers supplied by one motor neuron are activated as a unit when impulse travels down axon
• Innervation ratio: number of muscle fibers supplied by one axon (10:1 yields less precise force gradation than 3:1)

Question 14

What are the two types of motor neurons?

Answer 14

1. alpha motor neurons: terminate on extrafusal fibers resulting in the generation of force and motion at joints
2. gamma motor neurons: terminate on intrafusal fibers attached to the muscle spindle (a sensory receptor) resulting in regulating the sensitivity of this receptor (efferent control of afferent input)

Question 15

Describe the gamma motor system.

Answer 15

• Small motor neurons originating in the spinal cord
• Innervate the distal ends of intrafusal fibers
• This contraction actiates the stretch reflex
• The result is increased muscle tone

Question 16

Describe the inputs to the spinal cord.

Answer 16

• Tracts from brain stem nuclei and motor cortext descend in white matter and terminate on interneurons and motor neurons in gray matter
• Afferents from receptors located in the skin, joints, and muscles (spindle, tendon Golgi body) enter cord through dorsal root
  
  • Axons ascend to terminate in brain
  • Axon collaterals terminate on interneurons and motor neurons contributing to local reflexes

Question 17

Describe the monosynaptic reflex (myotactic stretch reflex).

Answer 17

• Muscle stretch is transduced by muscle spindle (a sensory receptor) and leads to action potentials in Group IA afferents
• Group IA afferents enter cord through dorsal roots and synapse with MNs that innervate that same muscle (as was stretched) causing contraction
• Monosynaptic stretch reflex: maintains position; intrafusal fibers serve as detectors
  
  • When stretched, receptor activated; sends signals to motor neuron activating extrafusal fibers; their increased contraction reduces stretch; works because connected in series with extrafual fibers
• Reciprocal innervation: muscles arranged in pairs that act in opposite directions at a joint (agonists/antagonists); for joint displacement, agonist/antagonist MN pools activated in opposite direction (excitation of agonist MN, inhibition of antagonist MN)

Question 18

Describe the multisynaptic reflex.

Answer 18

• Flexor withdrawal reflex
• Sensory inputs from pain receptors (or pressure and thermal receptors at high-intensity stimulation) terminate on interneurons
• After interneuronal synapses, input to MN that broadly activate flexor muscles and inhibit extensor (on same side as input; reverse pattern on opposite side of body)
• Involves 3 or more neurons; receptor sends signal to spinal cord and synapses on interneuron; interneurons may excite and/or inhibit to accomplish a withdrawal reflex

Question 19

Describe cerebral palsy.

Answer 19

• A disability caused by brain damage before or during birth or in the first years, resulting in a loss of voluntary muscular control and coordination. It is non-progressive and results in activity limitation.
• Statistics:
  
  • ~2 in 1000 births
  • 10% postnatal
  • ~500,000 individuals in the US

Question 20

Describe the causes of cerebral palsy.

Answer 20

• Usually caused by damage to one or more specific areas of the brain, usually during fetal development; before, during, shortly following birth, or during infancy
• CP in term infants most often reflets phenomena preceding the onset of labor
• CP in preterm infants more often include prenatal and perinatal factors
• Major risks are low birth weight and birth asphyxia (often involving CNS maldevelopment); the latter is usually associated with quadriplegia
• Other factors include physical trauma, abnormalities of neural migration, viral agents (rubella, cytomegalovirus) and toxoplasmosis
• Multiple births constitute 10% of CP

Question 21

What are the three main types of cerebral palsy?

Answer 21

1. spastic (pyramidal): still and difficult movement (hemiplegia, diplegia, quadraplegia)
2. athetoid (extrapyramidal): involuntary and uncontrolled movement
3. ataxic: disturbed sense of balance and depth perception

* There may be a mixture of these types for any individual.

Question 22

What are the treatment options for cerebral palsy?

Answer 22

• pharmacological: reduce muscle tone, seizures
• surgical: tendon release, transfer
• physical/occupational/speech therapy
• neuroplasticity (practice-induced brain changes arising from repetition, increasing movement complexity, motivation, and reward) plays a large role
• Early and intensive treatments