Neural Networks Notes PDF

NS1. Neural networks Physiology IIB Andrea Yool andrea.yool @ adelaide.edu.au Objectives: 1. Outline the principle divisions of the nervous system. 2. Network organisation uses convergent and divergent inputs, with fast and slow mechanisms of synaptic...

NS1. Neural networks Physiology IIB Andrea Yool andrea.yool @ adelaide.edu.au Objectives: 1. Outline the principle divisions of the nervous system. 2. Network organisation uses convergent and divergent inputs, with fast and slow mechanisms of synaptic signaling. 3. Roles of inhibitory interneurons in networks.  Principle divisions of the nervous system An integrated system subdivided for convenience into a brain hierarchical series of CNS classifications. spinal cord The broadest categories: nerves PNS Central (CNS): brain and spinal cord. Peripheral (PNS): nerves umc.rochester.edu leading to and from the central nervous system. Brain size and evolution Through phylogeny, the brain area that shows the largest increase is the cerebral cortex. en.citizendium.org In humans and other higher mammals, the cerebral cortex surface is deeply folded to increase area. High energy and good temperature control are needed.  Central nervous system: CNS The brain-- center for integrative, cognitive, and higher- order information processing. The spinal cord-- a "superhighway" that connects the brain and periphery (body); and processes information at the involuntary and reflex levels. Peripheral nervous system: PNS Afferent (ascending) is "flowing to" the brain and carries sensory information Efferent (descending) is "flowing from" the brain: 1. somatic (mainly voluntary) 2. autonomic (mainly involuntary). important Central (CNS) to know Peripheral (PNS) Ascending Descending afferent efferent sensory visceral somatic autonomic RECEPTORS sympathetic parasympathetic MUSCLE enteric Names of CNS structures Many of the "place names" in the nervous system reflect the shape or position Paul Broca or are named after a person who described it. These structurally distinct regions correlate with functional specialisations. Carl Wernicke Names describe a location, appearance, person Hippocampus “sea horse” Cerebellum “little brain” Hypothalamus “under the thalamus” Corpus callosum “tough body” Thalamus “inner chamber” Substantia nigra “dark substance” Limbic system “edge” Broca's and Wernicke's areas are named for scientists Carlsbad Cavern, New Mexico Patterns of cellular connections in networks of neurons CONVERGENT DIVERGENT One neuron One neuron is regulates multiple regulated by targets multiple inputs Synaptic terminal: Signaling from the presynaptic neuron to the postsynaptic cell (neuron, muscle, secretory gland) www.cise.ufl.edu/ ~psze/glias.html Synaptic terminal Large numbers of synapses, both excitatory and inhibitory, provide input to the dendrites. Responses are excitatory (EPSP) or inhibitory (IPSP) depending on the neurotransmitter released by the Corry Shores, 2009 presynaptic neuron.  Responses are defined by the ion channels that are activated (or inhibited) Ligand gated channels produce responses that are fast and localized. G protein coupled receptors produce responses that are relatively slower, smaller, longer in duration, and widespread within the cell. Excitatory and inhibitory responses are defined by the ion flowing through the open channel ACh GABA Cl- Na+ transmitters ionic flux effect glutamate, cation excitatory (depolarisation; acetylcholine VM +) GABA, anion inhibitory glycine (hyperpolarisation; VM -) Synaptic transmission Excitatory glutamate and acetylcholine (ACh) are used for both fast and slow EPSPs. Inhibitory GABA is used for fast and slow IPSPs. Transmitter evoked VM (mV) responses are: fast IPSPs (i) fast (ligand gated) slow IPSP (ii) slow (GPCR) time (ms) dendrites Signal generation EPSPs and IPSPs are integrated in the soma by summation. graded integration AP conduction response Thompson, 1967 The level of contribution (strength) of synaptic inputs is set in part by position (distance from the soma) distal inhibitory hippocampal (CA1 pyramidal) interneuron neuron proximal N Spruston. 2008. Nature Rev Neurosci 9: 206-221 Spatial control of synaptic inputs distal EPSP (distant) small proximal effect IPSP (close) large effect N Spruston. 2008. Nature Rev Neurosci 9: 206-221  – + + Inhibition by interneurons + Inhibitory interneurons provide local negative control of activity, important for network functions such as enhancing signals, and switching between output pathways. Example 1. Education versus experience? The “luminance” (physical parameter) of the two gray disks stays constant at all times. Their seeming change in “brightness” (subjective perception) is due to an effect described as simultaneous contrast. http://www.michaelbach.de/ot/lum-inducedContrastAsym/index.html MICHAEL BACH Universitätsklinikum Freiberg, Germany “Visual Phenomena & Optical Illusions”  Sensory processing enhances the signal by suppressing background. Pinterest: “Picasso” light drawing contest The neural network pattern that achieves this is called an “inhibitory surround” STIMULUS excitatory inhibitory neuron inhibitory interneuron interneuron IPSPs IPSPs on to the CNS  Information flow is regulated by increasing and decreasing in the ongoing (basal) firing rate STIMULUS EXCITATORY CENTER FIRING RATE inhibitory surround (basal rate) POSITION OF NEURON IN SENSORY TISSUE Example 1. The circle that is surrounded by light areas appears darkest. Where fields of inhibitory surround overlap, their inhibitory effect is increased. Note the central intersection is very dark.  The circle with a dark surround is brighter. Surrounding zones are Activated surrounding fields all off, leaving the overlap and inhibit the center central circle bright. (darker grey circle) + + + + + + + + + + + + + + + + + + + + + + + + dark surround light surround INHIBITION BY INTERNEURONS Example 2: Creating a decision point in a network Neurons that use an excitatory transmitter pacemaker cell Neuron that uses an inhibitory transmitter  INHIBITION BY INTERNEURONS Example 2: Creating a decision point in a network motor neuron 1 pacemaker cell BEHAVIOURAL PATTERN 1 INHIBITION BY INTERNEURONS Example 2: Creating a decision point in a network motor neuron 2 “disinhibition” BEHAVIOURAL PATTERN 2  Motor neuron 1 Motor neuron 2 inhibited disinhibited inhibited Reciprocal patterns of activity Inhibition by interneurons Inhibitory interneurons nested in a neural network provide a mechanism for switching between alternate pathways. Examples include rhythmic alternating behaviors such as: – respiration, – locomotion, – feeding, – sleep and wakefulness cycles, etc.  Example: Control of sleep and wake cycles in the brain Activity of “thalamocortical” neurons in the thalamus correlates with sleep -vs- wake cycles. GABA signaling from the cortex enables thalamic pacemaker neurons to shift to a slow waveform pattern of activity, associated with sleep. thalamocortical rhythmic firing in sleep/wake control membrane potential (mV) time (hours) Two stable firing states: slow oscillations during sleep, and rapid firing during wakefulness. cortex --> increase activity of GABAergic neurons --> disinhibition of slow waveform pacemakers --> sleep Neuroscience, 2nd ed. Purves et al.  Revision Are classes of interneurons identified as “inhibitory” because their firing activity is reduced by all synaptic input? True or false? The CNS consists of the brain and spinal cord; whereas the PNS consists of the afferent (sensory and visceral), and efferent (somatic and autonomic) subdivisions. How are fast and slow responses created at neuronal synapses? Revision Which synaptic events have a greater effect on summation at the soma (cell body)? Inputs at the distal (distant) or the proximal dendrites (close to the cell body)? Give an example of the role of inhibitory interneurons in a network for: (i) focusing the source of a stimulus; (ii) alternating between two behavioural patterns.