EXCI_206_LECTURE_NOTES_2013_nervous_systemp. 57

Determination that a joint action will be made this

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determination that a joint action will be made, this decision is passed along to the cerebral motor cortex of the brain. The cerebral motor cortex then sends a directive down through the spinal cord within descending white matter tract pathways. Most but not all voluntary movements are directed by the brain via the spinal cord to peripheral spinal nerves. The directions for some movements leave the brain directly via peripheral cranial nerves. Initiation of Reflex Movement Reflex movement is much simpler than voluntary movement. A sensory stimulus enters the spinal cord via a sensory neuron. The sensory neuron either synapses directly with an LMN within the spinal cord, or it synapses with a short interneuron that then synapses with an LMN within the spinal cord. This LMN is the same LMN involved in voluntary movement (mentioned previously). The LMN then travels out of the spinal cord to end at the neuromuscular junctions of the fibers of a muscle, where the muscle fibers are directed to contract. Because of the arclike shape of the sensory neuron into the spinal cord and the motor neuron out of the spinal cord, the pathway of a reflex is often called a reflex arc. Reflexes are “hardwired” in the body—in other words, they are innate (inborn). These reflexes are meant to be protective in nature. Before a child can know from experience which circumstances are safe and which are not, reflexes give automatic responses that are meant to protect the child from possible danger. For example, the startle reflex causes the child to turn toward the source of any loud noise. This brings attention to a potentially dangerous stimulus. Conscious awareness of a reflex is not part of the reflex arc itself. However, we do know that we have performed a reflex after it has happened. This knowledge occurs because of connections between the reflex arc and the brain that travel upward within ascending white matter tracts of the spinal cord to the brain; connections within the brain then bring
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70 EXCI 206 2013 the information to the cerebral cortex. These connections give us conscious awareness of what reflex has just occurred. Motor reflex = preprogrammed response
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71 EXCI 206 2013 Muscle spindle reflex The muscle spindle reflex is also known as the myotatic reflex or stretch reflex When the tendon of the muscle is tapped with the reflex hammer, the tendon elongates and creates a pulling force on the muscle belly. This in turn stretches the muscle spindles located within the belly, triggering a stretch reflex. The stretch reflex occurs when a sensory neuron from the muscle spindle carries a nerve impulse into the spinal cord, where it synapses with the lower motor neuron (LMN) that returns to the muscle and causes it to contract (as well as causing synergistic muscles that perform the same action to contract as shown). Any strong or fast stretch of a muscle may result in the stretch reflex, causing the muscle to contract.
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72 EXCI 206 2013 Golgi Tendon Organs A Golgi tendon organ
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  • Winter '12
  • ChantalComeau

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