Invertebrate Zoology
Week 10
Chris Foster
Control System
Sensory systems / nervous system [integrating / communicating / acting on sensory info] /
neural and endocrine signals.
1. Neurons and Neurotransmitters;
- Neurons [nerve cells] transduction; sensory cells and neurons receive diversity of
signals converted to electrical energy [potential difference changes across
membrane]
- Main mechanisms; involve receptor molecules in membrane
a. Mechanical stimuli impinge on receptor cells alter permeability of ion
channels formed = change membrane potential
b. Activation of receptors which effect membrane potential mediated by G
proteins
c. Intracellular secondary messengers.
- Transduction mediated by G proteins [homogenous in animals] activated
rhodopsin in mollusks in motion [biochemical cascade in vert vision]
- Conduction; Graded Potential
o Nervous elements ability to carry info – conduction of changes in
membrane potential
o Nerve fibers = electrical cable, local changes in potential difference spread
passive electronic spread
o Magnitude of effect gradually fades [current leaks through membrane]
o Non-spiking neurons; graded potential
- Conduction; Action Potential
o Partial depolarization of axon, triggers rapid change involve loss and
reversal of p.d “spikes”
o Threshold effect initiated by stimulus at min. level
o AP self perpetuates spread to adj membrane
o Velocity of conduction dependent on diameter of fiber [increase fibers =
fast conduction]
o Speed of conduction = increased insulating fiber with glial cells [saltatory
conduction]
- Spiking VS non-spiking;
o Graded potential [non-spiking] associated with receptors [photoreceptors
and motor neurons] provides smooth control
o Spike activity = sophisticated coding [combo of frequency, duration and
pattern]
o Intermediate cells common.
- Neurons possess elongate process, ability to conduct electrical potentials
a. Sensory neurons; convey info from CNS
b. Motor neurons; carry message from center to effector
c. Interneurons; link the 2 types of neurons.
- Neurosecretory cells which release hormones, cells which combo of
characteristics described
Basic common pattern of functional morphology;
, Invertebrate Zoology
Week 10
Chris Foster
- Input [dendritic] zones
- Graded receptor/synaptic potential arise and spread across membrane
- Spiking neurons impulse generation zone
- Axon conducts potentials to nerve terminals with synaptic transmission occurs
- Dendrites sites of transmission [reception, pre-synaptic terminals receive and
transmit info] nerve cells have axons which transmit in both directions // lack
process recognizable as an axon.
- Glial Cells;
1. Mechanical; support, separate and unsheathe nervous elements
2. Electrical; insulate fiber, enhance rate of conduction
3. Metabolic; control environment / break down neurotransmitter
o Nourish receptor cells, photoreceptors metabolizes pyruvate [produced
from alanine] to generate ammonium ions [stimulate glial cells to take up
glucose and convert to alanine]
- Chemical synapses;
o Transmit info to secrete NT [increase localized, selective manner at
synapses]
o Formed by terminal regions of nerve fibers [increase characteristics
ultrastructure]
o Synaptic contact between 2 cells [amplify signals]
- Neurotransmitters;
1. Acetylcholine [verts] and Octopamine [inverts]
o Produced in nerve fiber terminals, stored in synaptic vessels, released at
low levels, arrival of AP provokes exocytosis.
2. Neuropeptides
o Synthesized in neuronal cell bodies – cocktail of peptides
o Remarkable affinity with vert peptides.
3. Gases;
o Cannot be stored, produced by enzymatic action
o Insect retina [photoreceptor signals to monopolar cells release histamine
[transmitter] cells produce NO; diffuses into receptors [signalling back]
- Electrical synapses;
o Enable one cells to stimulate another without intervention of transmission
[discovered 1959 in Crayfish]
o Depends on presence of gap junction where protein molecules [connexons]
form hollow cylinders, current can flow and span intracellular space
o Transmission rapid, in either direction or rectifying [low resistance one
direction and shut down if reversed]
Organization of Nervous System;
a. Neuroid System;
- Sponges have no true nervous system but clear canals through exhalent oscula or
ejected larvae by coordinated contractions
- Response to stimulation due to spread of mechanical effect [electrical impulse
conducted]
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