Anatomy and Physiology / Unit 6: The Nervous System and Senses
6.5 The Spinal Cord, Nerves and Reflexes
Notes
The spinal cord extends from the brain stem to about L1-L2 and has 31 pairs of spinal nerves. It has gray matter (cell bodies, butterfly shape in center) and white matter (myelinated tracts that conduct up and down).
- The dorsal (posterior) root carries sensory input into the cord; the ventral (anterior) root carries motor output. Spinal nerves are mixed.
- Ascending tracts carry sensory information up to the brain; descending tracts carry motor commands down.
- A reflex is a rapid, automatic, involuntary response. The reflex arc: (1) receptor, (2) sensory neuron, (3) integration center (spinal cord), (4) motor neuron, (5) effector.
- Example: the knee-jerk (patellar) reflex is a stretch reflex. The withdrawal reflex pulls the hand from a hot object before the brain is aware of the pain.
- Spinal cord injury in the cervical region can cause quadriplegia; injury lower can cause paraplegia.
Practice
Which is the correct order of a reflex arc?
The signal passes from receptor to sensory neuron to the spinal cord, then out through the motor neuron to the effector.
Sensory information enters the spinal cord through which structure?
The dorsal root carries sensory fibers; the ventral root carries motor fibers.
Why does a person pull their hand from a hot stove before feeling the pain?
Reflexes are processed at the spinal level, making them very fast.
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