Jan 28: Neural Mechanisms--intro Questions Addressed Through Study of Behavioral Mechanisms (Proximate Causes) Control of behavior in response to stimuli in environment Diversity of behavior: explain the behavioral differences and similarities among species in terms of differences in underlying mechanisms? Adaptation: What properties of the nervous system account for the fit of organisms to their environment, i.e., for the behavioral solutions to the challenges of surviving and reproducing in a particular environment? Complexity: How do we account for behavioral complexity given that neural units are very simple? Answering these questions requires understanding how sensory receptors take in information about the environment and how neurons organize this information to determine the appropriate action
Jan 28: Neural Mechanisms--overview Overview of Lecture Overall organization of nervous system The basic processing elements in the nervous system Selectivity of responses Modulation of mechanisms mediating responses to stimuli relationship Some simple neural circuits underlying simple behavior Stimuli Organism Responses
Jan 28: Neural Mechanisms--Organization of Nervous System Brain Organization Function is localized Size of brain regions are correlated with behavioral importance of their functions Topographic organization midline of brain Front view of bee brain Optic lobe Star-nosed mole (see Alcock Fig. 5-26) Human somatosensory areas Antennal lobe
Jan 28: Neural Mechanisms--Properties of Neurons and Receptors The basic processing elements in the nervous system: neurons and receptors Neuron: an excitable cell that receives signal via dendrites (from other cells) and passes it along Direction of information flow Receptor: an excitable cell that receives signal from environment and passes it along photoreceptor soma nucleus terminals
Jan 28: Neural Mechanisms--Neurons and Receptors properties Excitability General properties of neurons and receptors Code information about inputs steady-state intensity (presence/absence) tonic response Change in voltage measured at certain point along axon or in cell soma changes in intensity-- phasic response Transmit information to other cells (via synapses)
Jan 28: Neural Mechanisms--Synapses Synapses Synaptic integration Synapses can be excitatory or inhibitory Inputs may sum spatially or temporally signal
Jan 28: Neural Mechanisms--Selectivity Selectivity of Responses Toad s responses to visual stimuli are highly specific Illustrates Tinbergen s concept of a sign stimulus --a simple feature that triggers behavior What determines specificity of responses? Edible Dangerous
Jan 28: Neural Mechanisms--Selectivity Selectivity -- cont d What determines specificity of responses? Thresholds: minimal level of stimulation needed Threshold E m Stimulus (Different thresholds could produce differences in selectivity) Tuning: maximal responsiveness to narrow range of input stimuli Strength of synaptic connections (variation may be innate, or due to learning)
Jan 28: Neural Mechanisms--Selectivity Selectivity -- cont d What determines specificity of responses? Back to the frog Responses of neurons in optic tectum(to which retina projects) Note the selectivity of response: Region of visual field Shape of stimulus Movement of stimulus
Jan 28: Neural Mechanisms--Modulation Modulation of input-output relationship (mechanisms causing variation in probability that given input will produce given output) Hormones (fluctuating with age, season, sex) Internal rhythms that synchronize system to external temporal patterns Learning: thought to be mediated by experienceinduced plasticity of synaptic connection
Jan 28: Neural Mechanisms--Reflexes Reflex: Direct connection from receptor to motorneuron Little integration of information Some Simple Neural Circuits
Jan 28: Neural mechanisms--escape responses Escape responses --cockroach and frog Cock roach senses frog s attack using wind currents Cercus: location of wind detectors
Jan 28: Neural mechanisms--escape responses Escape cont d --cockroach sensors Wind detectors: tuned to different wind directions Single wind detector
Jan 28: Neural mechanisms--frog escape responses Escape cont d Escape behavior may need to be more than just a simple reflex--may need to combine information from multiple inputs
Jan 28: Neural mechanisms--motor pattern generators Motor patterns Escape behavior in Tritonia Neural equivalent of a "Fixed Action Pattern triggered by simple stimulus (starfish chemicals) sequence of actions mediated by internal program all-or-none--can t be interrupted once begun sensory feedback not necessary COMPARE TO EGG ROLLING BY GOOSE
Jan 28: Neural mechanisms--learning Learning: plasticity of the synapse Aplysia learning: classical conditioning of gill withdrawal 1. Stimulate Tail E1 E2 Response Tail Siphon Withdraw gill (UR) Tail Withdraw gill (CR) 2. Just before siphon If tail is stimulated just before siphon: Facilitating interneuron, stimulates synapse from siphon to motor neuron just as signal arrives from siphon This enhances efficacy of synapse 3. Makes this synapse work when tail alone is stimulated