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Lecture 5

HMB200H1 Lecture Notes - Lecture 5: Nervi, V Speeds, Aplysia

Human Biology
Course Code
John Yeomans

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HMB200H1S L5; Jan. 24, 2012
Fast Reflexes
Speed and Timing in CNS
Nervous systems (100m/s) evolved because they are
faster than hormonal systems (3m/s; chemical
Voltage-gated Na+ channelsàAction potentials.
Axons: fast (myelinated A – 130m/s) & slow (C).
Receptors: fast ionotropic (open in ms), G-coupled &
hormonal/steroid-mediated (mins-hrs, flow thru
membrane à slow growth effects).
Circuits: # of synapses & processing at each synapse.
Simplest = monosynaptic stretch reflex w 2
Can slow down easily
Slows info flow if more synapses – but get more
info to analyze, getr more info out per unit time
Fast systems, w large neurons (easiest to record
from), are the easiest to study: Reflexes w giant
Simple f’ns – escape reflexes (1st to evolve)
Direct feedback reflexes
Neurons & Bhvr
Largest neurons (Giant neurons)
Single synapses (Squid or muscles)
Simple reflexes (Spinal Cord or Aplysia)
Starfish, anemone –fast response when touch arm à
withdraw; tentacles bring food in (interactions btwn
central viscera & response tentacles) – not always
avoidance, sometimes approach & food-seeking
Segmented animals w nerves – elongated NS,
ganglia, axons
Worm – tiny head ganglion~brain – responds to
stimuli at front of animal, have 3 giant axons –
activate fast escape response (shrinking, into
hole) by longitudinal axon from head & tail
Axon – chain of cells linked, work like linked act
pot, linked electrically
Squid – compound cell that forms giant axon
from assembled cells, divided differently in squid
than worm
Aplysia – a few ganglia in anterior end – head ganglia
chain; abdominal ganglion in centre w biggest cells
(can be identified easily, poked, coloured); 1000
micron diameter neurons (10 times bigger than
biggest human/mammalian neurons [100microns] ),
easy to see
Few synapses
Compound Action Potential
Fast & Slow Receptors
Fastest = ligand gated
Nerve growth factor – enzyme activates chemical
reaction à growth, multiple effect; mediated by
kinase not g-prot
Steroid recs intracellular, so steroid must get thru
bilipid membrane
Can regulate gene transcription, ex. primary &
secondary sex characteristics
Neurons & Bhvr
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