Somatic nervous system | EPFL Graph Search

The somatic nervous system (SNS), or voluntary nervous system is the part of the peripheral nervous system associated with the voluntary control of body movements via skeletal muscles. The somatic nervous system consists of nerves carrying afferent nerve fibers, which relay sensation from the body to the central nervous system (CNS), and nerves carrying efferent nerve fibers, which relay motor commands from the CNS to stimulate muscle contraction. The a- of afferent and the e- of efferent correspond to the prefixes ad- (to, toward) and ex- (out of). There are 43 segments of nerves in the human body. With each segment, there is a pair of sensory and motor nerves. In the body, 31 segments of nerves are in the spinal cord and 12 are in the brain stem. Besides these, thousands of association nerves are also present in the body. Thus the somatic nervous system consists of two parts: Spinal nerves: They are mixed nerves that carry sensory information into and motor commands out of the spinal cord. Cranial nerves: They are the nerve fibers that carry information into and out of the brain stem. They include smell, eye muscles, mouth, taste, ear, neck, shoulders, and tongue. The somatic nervous system controls all voluntary muscular systems within the body, and the process of voluntary reflex arcs. The basic route of nerve signals within the efferent somatic nervous system involves a sequence that begins in the upper cell bodies of motor neurons (upper motor neurons) within the precentral gyrus (which approximates the primary motor cortex). Stimuli from the precentral gyrus are transmitted from upper motor neurons, down the corticospinal tract, to lower motor neurons (alpha motor neurons) in the brainstem and ventral horn of the spinal cord: upper motor neurons release a neurotransmitter called glutamate from their axon terminal knobs, which is received by glutamate receptors on the lower motor neurons: from there, acetylcholine is released from the axon terminal knobs of alpha motor neurons and received by postsynaptic receptors (nicotinic acetylcholine receptors) of muscles, thereby relaying the stimulus to contract muscle fibers.

Epithelial stem cells and mechanical signal transduction

Vincent Bernard Cattin

Hair follicles are cutaneous structures characteristic of mammals. These sensitive organs cycle and contain multipotent epithelial stem cells which are implicated in hair growth and hair cycle. A single whisker follicle of the rat hosts up to 1500 stem cells, many more than necessary to regenerate the entire follicle throughout the animal's life. These cells are preferentially located in the most innervated region of the hair follicle. We hypothesised that stem cells might have an additional function aside from tissue renewal: stem cells may be involved in the mechanical signal transduction between hair follicle and afferent nervous system. This supplemental role may either take place through the interaction of the multipotent epithelial stem cells with nerve endings, or through neuroepithelial Merkel cells acting as intermediates. Merkel cells are located in the basal layer of the whisker follicle outer root sheath (ORS) and share a close relationship with their neighbouring nerve endings. These cells specifically express the transcription factor Math1 and their function is poorly understood. We demonstrate here by clonal analysis and transplantation assays that Merkel cells are not derived from epithelial stem cells. In addition, Math1-expressing cells isolated from the epidermis are unable to form colonies and to participate in the generation epidermal lineages when transplanted, failing to show stemness properties. Furthermore, when cultured in conditions that favour the growth of neurospheres, they are unable to proliferate, revealing a different behaviour than that of skin-derived neural crest cells. We also demonstrate that multipotent epithelial stem cells express several proteins implicated in glutamate neurotransmission, as well as other molecules usually found in the nervous system. The expression of NMDAR and AMPAR subunits, as well as their scaffolding proteins and neurotransmitter vesicular transport-implicated proteins are uncovered for the first time in the multipotent epithelial stem cells from the bulge. Our results support the mechanical signal transduction hypothesis and confirm the increasingly important role of the recently described neural-like structures of the skin. This work paves the way for a better comprehension of multipotent epithelial stem cell behaviour and stem cell-related pathologies.

EPFL2008

Epithelial stem cells and mechanical signal transduction

Vincent Bernard Cattin

EPFL2008