Afferent Mechanisms05 II

B.Afferent neurons 1. Classification according to diameter, myelination, shape of action potential a. According to Lloyd (I-IV) GroupMyelinDiameterConduction Vel. I  m m/sec b. According to Erlanger and Gasser (A,B,C) II  m36-72 m/sec III  m18-36 m/sec IV 0 < 3  m< 10 m/sec

B.Afferent neurons 1. Classification according to diameter, myelination, shape of action potential b. According to Erlanger and Gasser (A,B,C) GroupMyelinDiameterConduction Vel. A  m   m   m   m   m B  m (No negative after potential; autonomic preganglionic) C 0< 3  m = IV

Relationship of fiber diameter to: Conduction velocity –Myelinated fibers, CV (m/sec) = 6D (  m) –Non-myelinated fibers, CV (m/sec)   D (  m) Susceptibility to pressure –Conduction of impulses in larger diameter fibers is more likely to be interrupted by pressure on fibers Susceptibility to local anesthetic, cooling –Conduction of impulses in small fibers is more likely to be interrupted by these agents

Peripheral distribution of exteroceptive afferent fibers Embryologic derivation Adult pattern –“Dermatome” a region of skin derived from a particular dermatome and still receiving the major part of its innervation from the corresponding spinal nerve (ganglion), or a cutaneous area supplied by a single dorsal root and its ganglion.

“Dermatome” – Region of skin derived from a particular dermatome and still receiving the major part of its innervation from the corresponding spinal nerve (ganglion), or a cutaneous area supplied by a single dorsal root and its ganglion.

Central distribution of afferent fibers

Dorsolateral fasciculus (of Lissauer) Contains relatively short ascending and descending branches of the smaller diameter afferent fibers Branches from the ascending and descending branches in the dorsolateral fasciculus run medioventrally to terminate in the adjacent dorsal gray column

Central distribution of afferent fibers Dorsal funiculus Fasciculus gracilis - contains ascending branches of larger diameter afferent fibers, which enter the spinal cord via the caudal, sacral, lumbar, and lower thoracic dorsal roots. The fibers continue in the fasciculus garcilis to the nucleus gracilis in the brain stem Fasciculus cuneatus - contains ascending branches of larger diameter afferent fibers, which enter the spinal cord via the upper thoracic and cervical dorsal roots. These fibers continue to the nucleus cuneatus in the brain stem Locally terminating branches of the larger diameter fibers course over the dorsal gray column and enter it from the medial side

Central distribution of afferent fibers Dorsolateral fasciculus Dorsal funiculus

Routes through the spinal cord and brain stem to “higher centers” in the CNS Propriospinal tract Multisynaptic, bilateral system Impulses eventually relayed to the reticular formation of the brain (spinoreticular fibers) Neurons in the brain stem reticular formation project axons to nuclei in the midbrain and forebrain

Spinothalamic tract Originates in the ipsilateral dorsal gray column in the segment in which the primary afferent fiber enters the spinal cord Axon crosses the midline (decusates) in the ventral white commissure and ascends in the contralateral lateral funiculus to the thalamus (diencephalon)

Medial lemniscus Receives fibers which originate from the gracile and cuneate nuclei in the medulla on the side on which the primary afferent fibers entered the spinal cord (The gracile and cuneate nuclei receive terminals of ascending branches of primary afferent fibers; these branches ascend ipsilaterally in the fasciculi of the same names The medial lemniscus fibers decussate and ascend contralaterally, ventally, near the midline

Spinocervical tract Originates in the ipsilateral dorsal gray column (horn) Axon ascends in the ipsilateral lateral funiculus to the lateral cervical nucleus Fibers originating in the lateral cervical nucleus cross the midline and ascend in the contralateral medial lemniscus

Comparison: SpThT vs Lemn Primary afferents Ascent/spinal cd Funiculus Somatotopy/SpCd Crosses midline Ascent/ brain Somatotopy/brn III, IVI, II IpsiContra LateralDorsal PelvLatPelvMed SpCd Medula Contra PlvLat Ascending Tracts: Spinothalamic vs DF-ML