MCB 163: Mammalian Neuroanatomy
Name __Key ______________
Fall 2005 Seat_______
CLASS EXAMINATION #1
Short essay questions (4 points each; ink only; please write only on this side of the page)
1. How does the spatial location of synapses on a neuron affect the excitability of the cell?
The spatial distribution of synapses selectively influences the excitability of the postsynaptic
cell. Thus, synapses near the axon hillock are maximally effective in controlling discharge, and
those on the most distal dendritic spines are weakest since dendrites are unmyelinated and
because of the high impedance of the spine head to the flow of current. Neurons thus use the
spatial locus of synapses as a filter for a synaptic hierarchy: inhibitory axosomatic synapses
follow those on the axon hillock, and those on the proximal dendrites are next most powerful,
though significantly weaker. As in real estate, location is everything.
2. What features distinguish axons from dendrites?
Axons are singular (one/cell), dendrites multiple and diverse in shape. Axons are smooth, thin,
and sinuous and may project locally; dendrites are thicker, rougher in texture, and divide only a
few times, while axons may branch profusely. Axons are commonly presynaptic, dendrites rarely
so. Axons can propagate all-or-none impulses over vast distances rapidly, dendrites convey
decremental and graded signals over comparatively shorter distances. Axons are capable of
rapid transport, while transport in axons is far slower, Axons have high levels of putative
neurotransmitters at the terminals, while dendrites have few such molecules. Axons have a
pathfinding role in choosing which targets to innervate.
3. Describe two circuits in the spinal cord in which inhibitory neurons play a role.
Renshaw cells are Golgi type II local circuit neurons that receive input from  motoneurons and
which then project recurrently onto them to regulate their global excitability with synapses on the
soma near the axon initial segment. Since the  motoneuron thus creates its own inhibition
autogenetically, reducing the likelihood of prolonged discharge which might injure the cell
through excitotoxicity, and maintaining the motoneuron in a ready-to-fire state. In contrast,
reciprocal inhibition between matched sets of flexor and extensor motoneurons ensures that a
high discharge rate in one elicits a concomitant reduction in the  motoneurons innervating
muscles that are antagonistic.
4. Draw a picture of the spinal cord and include and define the following structures: ventral
horn, dorsal horn, laminae I-VII, flexors and extensors, dorsal columns.
The dorsal horn is the route by which all ganglion cells enter the spinal cord, including those
from receptors destined for the dorsal column, spinothalamic, and spinothalamic systems.
The ventral horn contains the  and  motoneurons, Renshaw cells, interneurons for feedforward
and feedback operations, and it is the target of the direct corticospinal system. Damage to it
renders muscle flaccidly. Laminae I-VII are sensory (laminae I-V are devoted to the
spinothalamic system, lamina VI contains parts of the spinocerebellar pathway), lamina VII is
part of the interneuron-rich spinal intermediate zone. Flexors and extensors correspond to the
motoneuron pools in laminae VIII and IX, and the dorsal columns are the first order axons from
ganglion cells innervating skin, joint and muscle receptors (conscious) and for spinocerebellar
(subconscious) information.
1
5. What are the three parallel pathways in the pain and temperature system?
The spinothalamic system consists of two subdivisions: neospinothalamic, which conveys
information from rapidly adapting nociceptors along A along thick axons and which targets the
posterior thalamus, from which it is distributed widely to the cerebral cortex. The
paleospinothalamic system contains two parallel streams, each of which arise from free nerve
endings and travel along slow-conducting C fibers to convey burning, grinding, non-adapting
nociceptive information to the reticular formation) where it exerts effects on attention) or to the
central gray, whose neurons either project to the hypothalamus for visceral regulation or to the
raphe for raphespinal descending control of nociception. These three pathways are related but
independent.
Matching (1 point each)
b
h
c
a
g
e
d
f
6. non-adapting
7. syphilis
8. raphespinal
9. SII
10. Golgi, basket, stellate
11. VII
12. pinprick
13. climbing fiber
a.
b.
c.
d.
e.
f.
g.
h.
texture
burning, grinding pain
serotonin
rapidly adapting
spinocerebellar
complex spike
GABA
dorsal columns
Multiple choice (2 points each)
B
14. Ganglion cells
(A) usually have long dendrites with a few short spines, and are myelinated lightly.
(B) convert graded receptor potentials into synaptic potentials.
(C) usually have cell bodies of a uniform size in a dorsal root usually
(D) use GABA as a transmitter and a peptide as a co-transmitter.
(E) are myelinated peripherally by oligodendroglia cells and form part of the plexus.
(F) none of the above.
B
15. Transecting a single dorsal root
(A) has no effect on sensation in the corresponding dermatome.
(B) weakly affects sensation, altering cutaneous sensation more than pain and
temperature.
(C) causes loss of power and precision in movement and weakness and wasting in
muscle.
(D) has no effect on sensation and a profound effect on motor behavior.
(E) none of the above.
E 16. Group A epsilon fibers are critical for
(A) the perception of burning, grinding pain.
(B) conduction velocities of 150 meters/sec.
(C) inducing the decussation of descending medial lemniscal fibers.
(D) the expression of pathology in shiverer mutant mice.
2
(E) none of the above.
A 17. In SI, information from slowly- and rapidly adapting mechanoreceptors
(A) is segregated in alternating columns for parallel processing.
(B) is combined through convergence of Renshaw cell input.
(C) synapses in Lissauer’s tract create monosynaptic receptive fields.
(D) represents mainly the ipsilateral body surface.
(E) produces postsynaptic cells with a wide dynamic range through convergence.
D 18. The cerebellum
(A) has powerful and monosynaptic projections directly to  motoneurons.
(B) is responsible for the strength of muscle contractions in smooth and skeletal muscle.
(C) contains Renshaw cells that provide bradykinin for feedforward inhibition.
(D) influences spinal motoneurons through the reticulo- and vestibulospinal tracts.
(E) none of the above.
E
19. The conduction velocity of the thinnest unmyelinated autonomic efferent axons is
(A) just a little slower than that in the dorsal columns, ~ 20 meters/sec.
(B) is much faster than that in the dorsal columns.
(C) affected by corticofugal influences that reach the spinal cord and target Renfrew
cells.
(D) governed by the rule of 4, which states that conduction velocity is inversely
proportional to the square root of the fourth power of the number of nodes of Ranvier.
(E) none of the above.
3