Motor Systems

Question 1

Somatic motor pathways

Answer 1

• control of body movement (skeletal muscles)
• direct or pyramidal pathway - primary motor cortex
  
  • initiate and control precise movements
• indirect/extrapyramidal pathways
  
  • basal nuclei help establish muscle tone and integrate semi-voluntary automatic movements
• cerebellum helps make movements smooth and helps maintain posture and balance

Question 2

Direct motor pathway

Answer 2

• originates in primary cortex (precentral gyrus)
• upper motor neurons project to spinal cord (corticospinal tract) or brain stem (corticobulbar tract)
• in the diencephalon, descending axons gather together to form internal capsule
• in the midbrain the axons form the cerebral peduncles
• in the medulla oblongata they form pyramidal tracts
• 90% cross to opposite side of medulla at the pyramidal decussation
• crossed fibres for the lateral corticospinal tract, uncrossed fibres from the anterior tract
• in the spinal cord the axons from the lateral corticospinal tract enter the ventral horn and synapse on motor neurons
• axons in the anterior corticospinal tract cross to opposite side of spinal cord and synapse on ventral horn motor neurons
• axons from the ventral horn motor neurons project in the ventral root to skeletal muscles

Question 3

Motor areas of cerebral cortex

Answer 3

• voluntary motor initiation
  
  • primary motor area: precentral gyrus
    
    • controls voluntary contractions of skeletal muscles on other side of body

Question 4

Primary motor cortex

Answer 4

• precentral gyrus initiates voluntary movement
• cells are called upper motor neurons
• muscles represented unequally (according to the number of motor units)
• more cortical area is needed if number of motor units in a muscle is high
  
  • vocal cords, tongue, lips, fingers

Question 5

Indirect pathways

Answer 5

• complex polysynaptic circuits
  
  • basal nuclei
  • cerebellum

Question 6

Basal nuclei

Answer 6

• helps to program automatic movement sequences
  
  • walking/arm swinging
  • set muscle tone by inhibiting other motor circuits
• damage is characterized by tremors or twitches
• putamen
• globus pallidus
• caudate nucleus

Question 7

Basal Nuclei pathway

Answer 7

• cerebral cortex projections to the basal nuclei
• basal Nuclei has reciprocal projections to midbrain (substantia nigra, dopamine neurons)
• basal nuclei projects to the thalamus
• thalamus projects to the cerebral cortex
• direct corticospinal pathway projects to brain stem and spinal motor neurons
• damage to basal nuclei: Parkinson’s disease

Question 8

Cerebellar pathways

Answer 8

• aspects of function
  
  • motor learning
  • coordinated and skilled movements
  • posture and equilibrium
• inferior cerebellar peduncle carries proprioceptive sensory inputs from the spinal cord and vestibular apparatus (inner ear)
• middle cerebellar peduncle carries motor commands from the cerebral cortex via the pontine nuclei
• superior cerebellar peduncle carries motor commands from cerebellum to brain stem, thalamus, basal ganglia and cerebral cortex

Question 9

Final common pathway

Answer 9

• lower motor neurons receive signals from both direct and indirect upper motor neurons
• sum total of all inhibitory and excitatory signals determines the final response of the lower motor neuron and the skeletal muscle

Question 10

Autonomic NS

Answer 10

• regulates activity of smooth muscle, cardiac muscle and glands
• involuntary control
• involves CNS motor neuron (preganglionic neuron) and PNS motor neuron (postganglionic neuron)

Question 11

Autonomic VS somatic

Answer 11

-ANS pathway is a 2 neuron pathway while somatic NS only contains one neuron

Question 12

Divisions of ANS

Answer 12

• 2 major divisions
  
  • parasympathetic
  • sympathetic
• dual innervation
  
  • one speeds up organ
  • one slows down organ

Question 13

Sources of dual innervation

Answer 13

• sympathetic (thoracolumbar) division
  
  • preganglionic cell bodies in thoracic and first 2 lumbar segments of spinal cord
• parasympathetic (craniosacral) division
  
  • preganglionic cell bodies in nuclei of 4 cranial nerves and the sacral spinal cord

Question 14

Anatomy of sympathetic NS

Answer 14

• preganglionic cell bodies found in lateral horn of thoracic and lumbar spinal cord
• postganglionic cell bodies located in the sympathetic trunk (paravertebral) or prevertebral ganglia

Question 15

Anatomy of parasympathetic NS

Answer 15

• preganglionic cell bodies found in
  
  • 4 cranial nerve nuclei in brainstem
  • S2 - S4 spinal cord
• postganglionic cell bodies very near or in the wall of target organ in terminal ganglia

Question 16

Physiological effects of ANS

Answer 16

• most body organs receive dual innervation
• hypothalamus regulated balance (tone) between sympathetic and parasympathetic activity levels
• some organs have only sympathetic behaviour
  
  • sweat glands, adrenal medulla, arrector pili muscles, many blood vessels

Question 17

Sympathetic responses

Answer 17

• dominance by sympathetic system cause by physical or emotional stress… “E stress”
  
  • emergency, embarrassment, excitement, exercise
• alarm reaction: fight or flight
  
  • dilation of pupils
  • increase heart rate and BP
  • decrease blood flow to non essential organs
  • increase blood flow to skeletal muscles and cardiac muscle
  • airways dilate and respiratory rate increases
  • blood glucose levels increase
• long lasting due to lingering of norepinephrine in synaptic gap and release of norepinephrine by adrenal gland

Question 18

Parasympathetic response

Answer 18

• enhance rest and digest activities
• mechanisms help conserve and restore body energy during times of rest
• normally dominate over sympathetic impulses
• SLUDD type of responses: salivation, lacrimation, urination, digestion and defecation
• 3 decreases: heart rate, diameter of airways, diameter of pupil