Fundamentals of Neurocircuitry
Senstory Stimulus for Startle
Mauthner Cells - Response Gating
Mauthner Efferent output
Integration of C-Start Circuitry and Behavior
text:Kandel pages 180
Figures of C-Start Behavior
Escape Behavior VII. Response Gating - Mauthner Cells A. Pair of large neurons in the caudal midbrain 1. found in lamprey, sharks, teleost fishes, and amphibians a. similar to identified neurons in invertebrates i. functional analogy to indentified large axon systems that mediate escape in invertebrates ii. 'identified neuron concept' b. much larger than adjacent neurons i. 1.0 mm length (goldfish) ii. 0.1 mm diameter B. always have the same characteristic morphology 1. T shaped: 2 lateral dendrites a. coming together at an axon 2. axons extend to the spinal cord a. axons decussate i. each Mauthner neuron controls the contralateral side b. innervate motor neurons i. e.g. right Mauthner stimlates left side C. also the same neurophysiology, receptor mechanisms, and synaptic connectivity 1. 1o input VIIIth cranial nerve a. mixed Bidirectional synapse i. gap junction/connexin electrical centrally ii. chemical-Glu around edges b. complex & specialized regional input: i. Glu-AMPA everywhere ii. Gly (glycine, G) and GABA throughout (1) Gly/GABA inhibitory interneurons iii. Glu-NMDA, DA-DA1 only on distal dendrites (1) enhanced by NO synthesis (a) NOS stimulated by NMDA activity iv. colocalization of somatostatin (Glu or GABA) v. 5-HT (serotonin) inputs not on dendrites D. Part of a Reticulospinal Network E. Axon cap/hillock 1. surrounds the base of the axon: specialized morphological adaptation a. determines the character/quality of startle response i. Absent or less developed cap (1) withdrawal response, but less like a C start (eel like fish) ii. goldfish have a prominent cap b. divided into peripheral and central regions 3. Fibers that enter the Central cap excitatory a. integrates inputs & triggers the common C start response 4. Fibers that enter Peripheral cap a. reciprocal inhibitory firing: limits length of action potential b. inhibition: only one M-cell fires at a time F. Large size of neurons results in large/fast response 1. Mauthner cell Action Potential (AP) lasts 1 ms 2. 3-5 ms after acoustic/somatosensory stimuli 3. 2 ms for synaptic transmission to spinal motor neuron + motorneuron AP + neuormuscular junction 4. entire signal 6-8 ms, entire behavior < 100 ms 5. large size also facilitates speed of inhibition: more important a. mediate very fast inhibition of ipsilateral or contralateral Mauthner neuron i. reciprocal inhibitory neurons impinging on axon cap (1) produce discrete pulses of muscle stimulation iii. inhibitory signals block contralateral movement b. blocks contraditory command by non-Mauthner cell G. What is Mauthner functional output?