Extra! The reflex arc does just more than just pull our hands away from danger

Stepping on a sharp object! Not only pull your foot away Reflex arc allows other muscles to react Keep balance until your mind realizes what is happening Then voluntary movement takes over

What is the advantage of this reflex arc?

Saves Time Allows us to respond to a potentially harmful stimulus in less time than if a message was sent all the way to the brain and back. Brain actually interprets pain after your reaction

How can this be used clinically?

Reflex Tests Patellar Reflex!! When the tendon at the front of the knee is struck, it stretches the tendon and the large muscles at the front of the thigh Your body senses a sudden stretching these muscles and thinks you are falling over backwards. Instinctively, to correct this situation, the thigh muscle contracts to extend the knee.

Questions?

LO #3 Students will be able to describe the corticospinal tract.

The corticospinal tract allows for voluntary movement

Motor Cortex “I want to move” The motor cortex, a region of the cerebral cortex (remember me?) is divided into three parts - Primary motor cortex – controls specific movements - Premotor cortex – fine-tunes movement (including spatial awareness) - Supplementary cortex – planning of movement

A note on the primary motor cortex The primary motor cortex is somatotopically organized, meaning different areas control different muscles. Notice how the region that controls leg movements is small compared the region that controls hand movement. - But your leg is far bigger than your hand…. Why do you think that is? Homunculus

Corticospinal Tract The motor cortex sends its ‘movement signal’ to the muscles via a special type of pyramidal cells called a Betz cell to the spinal cord. - Betz cells are the largest neuron in the CNS! The cell axons travel down from the motor cortex, meeting up with axons from other parts of the cerebral cortex to form the Corticospinal Tract - 60% of the fibers are from the motor cortex - 30% are from the somatosensory cortex (touch) - 10% are from the posterior parietal cortex (movement and spatial perception)

Corticospinal Tract 2 Don’t worry about these details, this is just an overview of the structure Along the way down some axons cross over one another, splitting into the anterior and lateral corticospinal tracts DO notice that most motor nerves switch sides (left to right and vice versa) The lateral tract is significantly larger than the lateral tract, which ends much higher up in the spinal cord (see next slide)

Corticospinal Tract 3 The corticospinal tract travels down through the spinal cord, terminating at the vertebral level of their target muscle There in the spinal cord the Betz cell axon synapses (remember me?) with alpha, beta, or gamma motor nerves - Many of these interactions are through an interneuron Motor nerves translate/refine the motor cortex’s signal into a precise contractions of the target muscle

Alpha and Gamma Alpha nerves ‘innervate’ skeletal muscle, which is used for voluntary movement - They control contraction of muscles Gamma nerves innervate sensory skeletal muscle as well - They control contraction of sensory skeletal muscle This alpha-gamma co-activation allows for muscles to detect whether how much they’re contracted in the right way - Think: These are sensory muscles, so they send signals back to the brain

Clinical connect What happens if the motor cortex or the corticospinal tract is damaged?

Clinical connect What happens if the motor cortex or the corticospinal tract is damaged? Weakness/Paralysis

Questions?