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The muscle fibers of a motor unit are widely distributed within the skeletal muscle. The strength of a muscle contraction depends on the number of motor units involved and the frequency of stimulation by the neuron. The Cranial Nerves: There are 12 pairs of cranial nerves. Functional groups of cranial nerves: Three cranial nerves are entirely sensory: 1,11, and V111. The other cranial nerves have both sensory and motor fibers; they are mixed nerves. Olfactory-sense of smell. Optic-sense of vision. Vestibulocochlear-hearing and equilibrium. The five primary motor cranial nerves include CN111 Oculomotor for the eye, CN 1v trochlear controls the superior oblique, abducens which controls eye movement (lateral rectus), CNx1 Accessory and CNx11 Hypoglossal control muscles under the tongue. Dermatomes are segments of the skin supplied by the cutaneous branches of each of the pairs of spinal nerves. Branches of the thorocolumbar spinal nerves: each pair of spinal nerves innervates a single dermatome and consists of 4 branches: Dorsal ramus: supplies skin and muscles of the back Ventral ramus: innervates body wall and limbs White ramus: carries myelinated visceral motor axons to autonomic ganglia.