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NEUROSCIENCE MSP
Problem Set #1
Question 1: Spinal Cord Organization:
Draw a basic cross section of the spinal cord depicting the regions of white and gray matter (e.g., top
diagram on page 3 of Spinal Cord I handout). Then label the following regions. Also, in a few
sentences describe what you would find in the given region.
a. Ventral Horn
The ventral horn contains the bodies of motor neurons that control the skeletal muscles of the
body. The larger neurons are known as alpha motor neurons and they innervate striated
muscles. The other, smaller neurons are known as gamma motor neurons and they innervate the
muscle spindle (See Muscle Spindle Question). Cells in this area are organized into motor pools
with somatotopic organization in regard to the muscles they innervate (See Question 2).
b. Dorsal Horn
The dorsal horn has a somatosensory function. It receives input from the dorsal root ganglia
and other neurons involved in processing sensory information. The dorsal horn does have a
laminar organization that may be discussed in regard to sensory pathways.
c. Ventral & Lateral Columns
The ventral & lateral columns contain several sensory and motor tracts. See the figure on page
4 of Spinal Cord I handout for more details. These tracts will be covered later in the class.
d. Dorsal Column
The dorsal column contains the major sensory pathway. Depending on the level of the section,
there may be a distinct fasciculus gracilis or fasciculus cuneatus present. The fasciculus gracilis
contains afferents from the lower limbs, while the fasciculus cuneatus contains afferents from
the upper limbs.
Why do you think the shape or the gray matter changes throughout the spinal cord? How does the
proportion of white to gray matter change and why?
The proportion of white matter increases as you go higher because there are more tracts present.
Question 2:
Describe in detail the morphologic differences between Cervical, Thoracic, Lumbar and Sacral
regions of the spinal cord. (i.e. think size, shape, differences of gray matter as seen in your slides)
Cervical:
 Oval Shape
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Thoracic:
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Lumbar:
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
Sacral:
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Posterior intermediate septum (division into cuneate and gracilis fasiculi) –
Gracilis is medial and cuneatus is lateral
Cervical enlargement of gray matter ie C4 – T1
Increased proportion of white to gray matter
Smallest diameter, more circular than oval
Less gray matter
Lateral gray horn –T1-L2
Round, circular shape
Fasiculus gracilis only
Increased proportion of gray to white matter
Almost rectangular in shape
Cauda equina present
Increased proportion of gray to white matter
Question 3: The Muscle Spindle
a. The muscle spindle is arranged in parallel with muscle fibers. It is a sensor of muscle stretch and
length. Describe how gamma motor neurons innervate the muscle spindle. Explain why gamma
motor neuron enervation is important in maintaining muscle spindle function, especially during
contraction.
Gamma motor neurons innervate the polar ends of the intrafusal muscle fibers. They do not
innervate extrafusal fibers, nor are they themselves contacted by primary sensor endings (Ia
fibers). Activation of the gamma motor neurons during movement leads to contraction of the
distal ends of the intrafusal fibers. As a result, the intrafusal fibers and the spindle remain taut
even as the muscle contracts. Thus the muscle spindle remains responsive during contraction.
b. Muscle movements can be controlled through the gamma loop. Please draw out and describe the
mechanism of this loop.
First the gamma motor neurons are activated. This leads to stimulation of the muscle spindle
sensory endings. The muscle spindle sensory nerves then stimulate the muscle’s alpha neurons,
which leads to a contraction of the muscle fibers.
Question 4: The Golgi Tendon Reflex:
a. Contrast the golgi tendon organ to the muscle spindle. Describe the differences.
The golgi tendon organ is located in series. There is a disynaptic pathway to the motor neurons.
Homonymous and synergistic muscles are inhibited, while antagonistic muscles are excited. The
organ responds primarily to contraction though there is some response to extreme stretch.
b. What are some of the functions of the golgi tendon organ?
The golgi tendon organ may serve to (1) modulate muscle tension (2), contribute to reciprocal
movements (3), protect muscles at the limits of their range of motion, and (4) form the substrate
of the clasp-knife reaction seen in spasticity.
Question 5: Motor Syndromes:
a. Define what a lower motor neuron syndrome means.
A lower motor neuron syndrome is the result of direct damage to the motor neurons that
innervate the skeletal muscles. Such damage can affect the motor neuron cell body as in the case
of poliomyelitis, or the damage can affect the axons of the motor neuron as in the case of
peripheral neuropathies. A lower motor neuron syndrome can cause hypotonia and
hyporeflexia.
Question 6: Clinical Scenario
Feeling very bold after returning from vacation, a young rider decides to jump a horse he has never
ridden. Unexpectedly, the horse refuses the jump and the rider is thrown into the jump. Because
the rider’s hands are caught in the reins, he smashes directly into the jump with nothing but his head
and neck to break the fall. Unable to move, he is rushed to the ER. After receiving and reviewing
the MRI, you see that his spinal cord has a left hemisection at the level of C4. What neurologic
symptoms will this young man be suffering from?? (Be Specific!!) Understand the pathways
involved with his symptoms.
Answer:
Levels C4 and below are affect:
 Loss of fine discriminative touch, 2 point discrimination, pressure, vibration
sense on his left side (C4 dermatome and below)
 Loss of pain and temperature sense, crude touch on his right side
 Loss of motor control on his left side
 Impaired respiration (C3-C5 = phrenic nerve)
**As time permits – please make sure students know the sensory and motor pathways and their
locations throughout the spinal cord, brainstem, thalamus and cortex!!!