Lecture Notes: Motor Neurons, Reflexes, and Damage to Motor Neurons

Introduction

• Presenter: Dr. Mike
• Topics:
  • How muscle contraction occurs (e.g., leg muscles)
  • Motor neurons and reflexes
  • Effects of damage to motor neurons

Muscle Contraction

• Starting Point: Brain, specifically the motor cortex
• Process:
  1. Brain sends a signal from the motor cortex.
  2. Signal travels through the brain stem, crosses at the medulla.
  3. Descends to spinal cord, synapses with another neuron at the level of exit.
  4. Second neuron (lower motor neuron) leaves the spinal cord to the muscle.
• Neuron System
  • Upper Motor Neuron: Brain to spinal cord level
  • Lower Motor Neuron: Spinal cord to muscle

Reflexes

• Stretch Receptors: Located in muscles and tendons to prevent overstretching.
  • Reflexively contracts muscle if overstretched.
• Example: Patellar tendon reflex
  • Hammer hits below kneecap, stretching tendon and quadriceps.
  • Receptors signal spinal cord to lower motor neuron, initiating contraction.

Role of Motor Neurons

• Lower Motor Neuron: Tendency to contract
• Upper Motor Neuron: Initiates contraction and inhibits unnecessary contraction

Damage to Motor Neurons

Upper Motor Neuron Damage

• Effects:
  • Muscle mass loss is minimal
  • Power reduction
  • Reflexes are exaggerated (hyperreflexia)
  • Muscle remains contracted (hypertonia/spasticity)

Lower Motor Neuron Damage

• Effects:
  • Significant muscle mass and power loss
  • Absence of reflexes (hyporeflexia)
  • Muscle is flaccid (hypotonia/flaccidity)

Real-Life Application: Spinal Cord Injuries

• Example: Cervical injury
  • Arm: Lower motor neuron issue (loss in mass, power, reflexes, more flaccidity)
  • Leg: Upper motor neuron issue (less pronounced mass and power loss, hyperreflexia, spasticity)

Conclusion

• Overview of upper and lower motor neurons
• Discussion on reflexes and motor neuron lesions
• Practical implications for spinal cord injuries