Motor Systems

Dr. DeBello, Fall 2024, Lecture 13

Motor Neurons

• Exit the spinal cord via the ventral root ganglion and synapse onto skeletal muscles

• This is the final common pathway for voluntary and involuntary movement

Neuromuscular Junction

• Motor endplate (postsynaptic side)

• Where the neuron and muscle make contact

• Very similar to neuron-neuron synapses

What are the difference between neuron-neuron synapses and the neuromuscular junction?

1. EPSPs are typically large enough to cause spikes in the postsynaptic cell (muscle)

2. Transmission is rapidly terminated with assistance from acetylcholinesterase

Acetylcholinesterase

• An extracellular facing enzyme that breaks down the transmitter acetylcholine

Myasthenia Gravis

• Autoimmune attack on acetylcholine receptors

• Causes muscle weakness

• Symptoms can be partially alleviated by systemic administration of neostigmine, which blocks acetylcholinesterase and therefore prolongs the dwell of acetylcholine in the synaptic cleft, increasing total activation of the spared acetylcholine receptors

Withdrawal Reflex

• Afferent/ dorsal root sends a signal to the spinal cord

• 3 synapses

  • Brain for conscious perception (crosses the midline)

  • Excitatory neuron excites motor neuron that activates/ contracts the flexor muscle

  • Inhibitory neuron inhibits motor neuron that inhibits/ slacked the extensor muscle

  • Final two oppose each others action

Extensor Reflex

• Afferent/ dorsal root sends a signal to the spinal cord

• 3 synapses that cross the midline

  • Brain for conscious perception

  • Excites the extensor

  • Inhibits the flexor

• Works in tandem with the withdrawal reflex so if you pick up your leg you don’t fall over

Voluntary Movement

• Initiated by the motor cortex which contains a topographic map of body parts

• Descending pathways cross the midline exactly once before innervating muscle

Basal Ganglia

• Involved with planning and initiation of movement, and suppression of unwanted movement

• Degeneration leads to Parkinson’s disease

  • Difficulty initiating movement and resting tremors

Cerebellum

• Compares actual movement with intended movement and makes adjustments

• Degeneration leads to Multiple Sclerosis

  • Difficulty making precise movements and action tremors