Lecture 2a_ Neuroscience.pdf

what role does neuroscience play in understanding cognition or mind? brains are a physical structure supporting the control of behavior in biological systems brains vary immensely in size, complexity and structure 104 neurons 106 neurons 108 neurons 1010 neuro...

what role does neuroscience play in understanding cognition or mind? brains are a physical structure supporting the control of behavior in biological systems brains vary immensely in size, complexity and structure 104 neurons 106 neurons 108 neurons 1010 neurons 109 connections 1014-15 connections how we approach and study brain depends on what behavior we’re trying to explain different levels of analysis (abstraction) explain different types of brain function different levels of analysis can also be more or less difficult to connect to behavior what role does neuroscience play in understanding cognition or mind? Phineas Gage is one of the earliest (and most cited) pieces of evidence for this link railroad construction foreman who, in 1848, had an accident involving a tamping rod an unplanned explosion caused a tamping rod to be launched through his face and out of the top of his head Gage not only survived, but recovered normal motor function in a few months popular accounts suggest that Gage’s personality completely changed, becoming less inhibited, more aggressive, closer to his basic “animal” instincts (significant variation in the accounts of his behavior) modern brain imaging techniques are still conflicted about the extent of the damage what role does neuroscience play in understanding cognition or mind? in 1953, HM underwent surgery to relieve epileptic seizures by removing large parts of his brain: hippocampus, amygdala and nearby areas surgery stopped the seizures, but resulted in severe anterograde amnesia (unable to create new memories) undamaged anterograde intact memory present damaging previous event 30 sec didn’t recognize himself in the mirror, would fail to remember people who left the room and returned a few minutes later, some motor/spatial impairments however, on certain tasks, HM was able to demonstrate learning implicitly (more on this later) what do cognitive psychologists need to know about neuroscience? broadly, we’ll need some anatomy and an understanding of how neurons work the cortex can be divided into several large-scale regions based on their functionality frontal lobe parietal lobe working memory somatic sensation planning future actions body image fine motor control spatial relationships occipital lobe temporal lobe cerebellum visual processing auditory processing spinal cord learning/memory emotion these divisions occur in both hemispheres of the cortex (with some small variation) what do cognitive psychologists need to know about neuroscience? we can observe through various techniques that the cortex in most species consists of layers of neurons specifically, six layers of neurons same number of layers and basic structure over the entire cortex, despite different areas having different functions these layers are not stacked like a cake, there are connections between different layers and types of neurons in most areas there is also a columnar structure as well what do cognitive psychologists need to know about neuroscience? brains consist of many distinct cells of two types: neurons (receive, process, and transmit information) like any cell, have a cell body, as well as two types of branching extensions, dendrites and axons dendrites are relatively short and thick, with multiple branches, thinning as they split and get further from the cell body (input) axons generally consist of one long thin extension away from the cell body that terminates in a complex and highly branched structure (output) glia (support, maintain, and facilitate neural function) come in several varieties with many different functions, such as: holding neurons in place, connecting neurons to their blood supply, forming fatty myelin sheaths that increase the efficiency of axons, and regulating blood flow what do cognitive psychologists need to know about neuroscience? how do we classify different types of neurons? one possibility is their shape within a given brain area there are often several different shapes Purjinke stellate pyramidal rosehip what do cognitive psychologists need to know about neuroscience? while different types and subtypes of cells play different functional roles, we can also classify them based on their axon type, or more specifically, axon length long axons connect different nuclei (functional sub areas) while short axons have connections within a given nuclei more highly developed organisms have more short axon neurons relative to long axon neurons what does this suggest about the role they might play in neural function? this structure repeats at different scales over different areas of the brain what do cognitive psychologists need to know about neuroscience? what distinguishes neurons from other cells? structurally, aside from axons and dendrites, not much neurons are hungry: despite being ~2% of the body by weight, it consumes ~20% of total oxygen and ~25% of total glucose as a result, neurons require oxygen in a way that other parts of the body do not, and will die off relatively quickly (~5 min) if deprived of oxygen the other key difference is that neurons do not regenerate like other cells (with a few exceptions) at 1 year old, you effectively have all of your neurons loss occurs due to damage, but also through normal aging (loss of branching, neural size and volume) what do cognitive psychologists need to know about neuroscience? the branches of neurons consist of connections between neurons called synapses in rare cases they may be in physical contact, but functionally the communication is chemical axon terminals release specific chemicals that receptor sites on the dendrites are sensitive to current neuroscience explains most of the function of brains in terms of synaptic function most drugs affect synaptic function in some way current theories of most mental disorders are similarly explained in terms of synaptic function (or disfunction) because of its importance, we’ll be discussing synaptic function in much more detail later what do cognitive psychologists need to know about neuroscience? atoms are composed of smaller particles: neutrons ( ), protons ( ), & electrons ( ) protons have a positive electrical charge, electrons have a negative electrical charge, and most atoms have an equal number so that the charge balances a hydrogen atom a helium atom consists of a single is slightly more proton and electron complex a potassium atom (K) has 19 electrons, arranged such that one electron is in an orbit by itself this makes it possible to lose that electron, and results in a relatively stable atomic configuration that has a net positive charge, called an ion, and designated K+ the other important ions in the brain are sodium (Na+), chloride (Cl-), and calcium (Ca++) when ions are moved around, they generate electrical current, which is the primary mechanism for signaling and communication in the brain what do cognitive psychologists need to know about neuroscience? ions cannot freely move back and forth across the cell membrane but embedded in the membrane are protein channels some of these channels function as pumps, using energy to move ions of a particular charge into or out of the cell in order to change the concentration within the cell (outside, positively charged) higher concentrations of sodium (Na+) and chloride (Cl-) ions the membrane potential is a difference in electric cell membrane charge across the cell higher concentration of membrane potassium (K+) ions (inside, negatively charged) normally in neurons, the inside of the cell is more negatively charged, resulting in a resting potential of approximately -70 millivolts results from passive ion movement, primarily driven by K+ levels what do cognitive psychologists need to know about neuroscience? receptor potentials occur in receptor cells as the result of interaction with a stimulus synaptic potentials occur in the dendrites or cell body and result from activity in other neurons by means of synaptic connections synaptic potentials can be either excitatory (increasing membrane potential) or inhibitory (decreasing membrane potential) in the excitatory case, neurotransmitters cause protein channels to open and allowing Na+ to move across the membrane both of the above are relatively small and directly proportional to either stimulus size or synaptic connection strength what do cognitive psychologists need to know about neuroscience? action potentials typically result from many synaptic potentials, starting in the cell body and traveling down the axon to its terminals as synaptic potentials increase the membrane potential, a positive feedback loop of Na+ occurs membrane depolarization more channels Na+ in open this results in a rapid and large increase in membrane potential, followed by a rapid decrease (a spike) action potentials are all or nothing, and are all roughly the same size and duration what makes action potentials meaningful is their frequency, not their amplitude: weak moderate strong what do cognitive psychologists need to know about neuroscience? the frequency of action potentials depends on the relative rate of synaptic potentials more excitatory synaptic potentials (ESP) result in a higher rate of action potentials more inhibitory synaptic potentials (ISP) result in a reduced rate of action potentials what do cognitive psychologists need to know about neuroscience? after an action potential occurs, it propagates down the axon, remaining the same size even for neurons that are several feet long action potentials travel at ~100-200mph in vertebrates and ~30-40mph in invertebrates, facilitated by myelin sheaths provided by glial cells myelin sheath allows electrical signals to jump between gaps, speeding up transmission what do cognitive psychologists need to know about neuroscience? action potential (~40mV) after an action potential, membrane (“spike”) potential falls below the resting potential membrane as charge returns to equilibrium potential threshold resting this refractory period limits the activity potential rate of neurons to ~1000/second, typical time (-70mV) activity is usually ~10-100/second this is the primary mechanism of neural activity, and integral to how neurons interact the variation in membrane potential that leads to an action potential is caused by the activity of other, upstream neurons that are “connected” to the firing neuron each connection (synapse) generates slight changes (positive or negative) in the connected neuron’s state these changes vary the membrane potential between the resting potential and threshold if these changes (excitatory postsynaptic potentials) cause the membrane potential to go above the threshold, the neuron fires, then the process repeats what do cognitive psychologists need to know about neuroscience? each action potential travels down the axon and begins a set of processes that cause the neuron to produce a signal of its own, a process called synaptic transmission action potential travels down the presynaptic axon terminal axon which has small bubbles (vesicles) containing specialized chemicals (neurotransmitter) that are released presynaptic postsynaptic into the synaptic cleft neuron neuron synaptic cleft neurotransmitter activates receptors in the postsynaptic neuron, which open ion channels, allowing charged particles to flow in and out this causes an increase (excitatory) or decrease (inhibitory) in the membrane potential of the postsynaptic neuron if enough excitatory events occur (typically across many synapses), the membrane potential will increase above threshold, it triggering an action potential in the postsynaptic neuron what do cognitive psychologists need to know about neuroscience? equally important to synaptic function is how neurotransmitter is recovered by the presynaptic neuron (why does this matter to firing rate?) vesicles become part of the cell membrane, and new ones are formed from parts of it some neurotransmitters are broken down in the synaptic cleft by enzymes others get transported back into the presynaptic neuron by specialized protein channels for reuse (reuptake) once back in the presynaptic neuron, neurotransmitter gets placed into the new vesicles what do cognitive psychologists need to know about neuroscience?