Helena Parkington; Week 11 MED1011; Physiology
Dorsal root ganglion for sensory fibres is in dorsal root, sympathetic ganglion is in chain outside the roots. Groups are numbered, I to IX from dorsal to ventral generally, smaller fibres are more dorsal (sensory). Ascending sensory pathways are the dorsal column and spinothalamic tract, descending motor pathways are corticospinal tract and rubrospinal tract (lateral pathways); medullary reticulospinal tract, tectospinal tract, pontine reticulospinal tract, vestibulospinal tract (ventromedial pathways). Axons from skin are classed A and C, from muscle are classed I, II, III and IV (largest to smallest). A alpha is 13-20um in diameter, conduction speed is 80-120, are proprioceptors of skeletal muscle. A beta are 6-12 um in diameter, conduction of 35-75, and are mechanoreceptors of skin. A delta are 1-5um, 5-30m/s and are for pain or temperature. C are 0.2-1.5um, 0.2-2m/s and are used for temperature, pain or itch. Smaller axons synapse right away, larger more deeply.
Large axons travel from primary somatosensory cortex to the thalamus, decussate at the medial lemniscus in the medulla and synapse at dorsal column nuclei, travel through dorsal column to large dorsal root axons. Small axons go from primary somatosensory cortex, synapse at thalamus, travel down to medulla through spinothalamic tract to the spinal cord where they decussate to dorsal column and to large dorsal root axons.
Nerves to skeletal muscles have both motor and sensory merging at dorsal root, viscera go through their respective roots with gray ramus communicans connecting the two. Skeletal muscle reflexes have one synapse in the ventral root from Ia spindle afferent to motor neuron. There can be inhibitory interneurons between reflexes (particularly between flexors and extensor muscles).
Renshaw cells are inhibtors of muscle APs, may feed back onto many neurons.
Spontaneous oscillations are from glutamate depolarising + AP allowing entry of Ca via NMDA, increase in cytoplasmic Ca activates Ca sensitive K channels causing hyperpolarisation, decrease in cytoplasmic Ca causes closing of K channels and so on.