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
 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
Thoracic:
 Smallest diameter, more circular than oval
 Less gray matter
 Lateral gray horn –T1-L2
Lumbar:
 Round, circular shape
 Fasiculus gracilis only
 Increased proportion of gray to white matter
Sacral:
 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!!!