Brain and Body PDF

Checklist ​Three major divisions of the brain: hindbrain, midbrain, forebrain ​The smaller subdivisions of these subdivisions ​Hindbrain : metencephalon, myelencephalon ​Midbrain: mesencephalon ​Forebrain: diencephalon, telencephalon ​Major structur...

Checklist ​Three major divisions of the brain: hindbrain, midbrain, forebrain ​The smaller subdivisions of these subdivisions ​Hindbrain : metencephalon, myelencephalon ​Midbrain: mesencephalon ​Forebrain: diencephalon, telencephalon ​Major structures of the subdivisions, location and function ​The division of the cerebral cortex into the temporal lobe, parietal lobe, frontal lobe and occipital lobe ​The binding problem ​Divisions of the nervous system ​Central Nervous System ​Brain ​Spinal cord ​Peripheral Nervous System ​Somatic nervous system ​Autonomic nervous system ​Sympathetic nervous system ​Parasympathetic nervous system ​Afferent and efferent nerves ​Cranial nerves general function and amount ​The blood-brain barrier ​Features of a neuron ​Types of neurons: interneurons, motor neurons, sensory neurons ​Neuroanatomical directions: dorsal, anterior, posterior, ventral, medial, lateral ​Planes of the brain: sagittal, horizontal, coronal Major divisions of the brain and their subdivisions ⭑ The telencephalon is on the top and the other four divisions are below it in alphabetical order. ◦ The telencephalon (cerebral hemispheres) undergoes the greatest growth during development. The other four divisions are referred to collectively as the brain stem, the stem on which the telencephalon sits. Hindbrain (rhombencephalon) ◦ The hindbrain consists of the myelencephalon/medulla and the metencephalon. ◦ The myelencephalon/medulla can be regarded as an enlarged extension of the spinal cord. ◦ The medulla controls breathing, heart rate and other vital functions through the cranial nerves. ◦ The metencephalon is divided into the pons and the cerebellum. ◦ Pons is Latin for “bridge,” reflecting the fact that in the pons, axons from each half of the brain cross to the opposite side of the spinal cord so that the left hemisphere controls the muscles of the right side of the body and the right hemisphere controls the left side. ◦ The cerebellum has long been known for its contributions to the control of movement and many describe it as important for balance and coordination. ◦ It is an important sensorimotor structure; cerebellar damage eliminates the ability to precisely control one’s movements and to adapt them to changing conditions. ◦ However, the fact that cerebellar damage also produces a variety of cognitive deficits suggests that its functions are not restricted to sensorimotor control. Midbrain (mesencephalon) ◦ The mesencephalon has two divisions: 1.​ The tectum is composed of two pairs of swelling: the inferior colliculi have an auditory function and the superior colliculi have a visual-motor function, meaning they direct the body’s orientation toward or away from particular visual stimuli. 2.​ In addition to the reticular formation, the tegmentum contains three colourful structures. The periaqueductal grey is the grey matter situated around the cerebral aqueduct. The substantia nigra gives rise to a dopamine-containing pathway that facilitates readiness for movement. Along with the red nucleus, it’s also an important part of the sensorimotor system. Forebrain (prosencephalon) ◦ The diencephalon is composed of two structures. 1.​ The thalamus is the two-lobed structure that’s part of the top of the brain stem and is the main source of input to the cerebral cortex. The two lobes are joined by the massa intermedia. ◦ Many nuclei of the thalamus receive input from and project to the cerebral cortex. ◦ Also involved in memory. 2.​ The hypothalamus plays an important role in the regulation of motivated behaviours. It does so by regulating the release of hormones from the pituitary gland, which dangles from it.​ ◦ The optic chiasm is the point at which the optic nerves from each eye come together and then decussate (cross over to the other side of the brain).​ ◦ The limbic system is a circuit of structures that circle the thalamus and is involved in the regulation of motivated behaviours (basic needs/survival). Major structures are: ◦ The basal ganglia are the caudate and putamen, known together as the striatum. ◦ Their major output is to the globus pallidus. ◦ The basal ganglia play a role in the performance of voluntary motor responses and decision making. A part of the basal ganglia is the nucleus accumbens, which is thought to play a role in the rewarding effects of addictive drugs. ◦ The telencephalon (cerebral hemispheres) is the largest division. It’s involved in the most complex functions – it initiates voluntary movement, interprets sensory input and mediates complex cognitive processes like learning and problem solving.​ ◦ The cerebral hemispheres are covered by a layer of tissue called the cerebral cortex that’s mainly composed of grey matter (unmyelinated neurons). The layer beneath the cortex is mainly composed of white matter (myelinated neurons). ◦ Each hemisphere is organised to receive sensory information, mostly from the contralateral side of the body. ◦ The cerebral hemispheres are directly connected by a few tracts spanning the longitudinal fissure (the largest fissure) called cerebral commissures, the largest of which is the corpus callosum. The division of the cerebral cortex into four types of lobes ◦ The­central fissure and the lateral fissure divide each hemisphere into four lobes: the frontal lobe, the parietal lobe, the temporal lobe and the occipital lobe. ◦ Among the largest gyri are the precentral gyri (frontal lobe), the postcentral gyri (parietal lobe) and the superior temporal gyri (temporal lobe). ◦ The occipital lobe is the main target for visual information. ◦ The parietal lobe processes sensations. Information about touch and body location is important not only for its own sake but also for interpreting visual and auditory information. The parietal lobe monitors the information about eye, head and body positions and passes it on to brain areas that control movement. ◦ The temporal lobe contributes to hearing, complex aspects of vision and processing of emotional information. Temporal lobe damage can lead to a set of behaviors known as the Klüver-Bucy syndrome, where the most common symptoms are inappropriate sexual behaviors, overeating and excessive lip-smacking or other mouth movements. ◦ The frontal lobe includes the precentral gyrus, which controls fine movements. It also includes the prefrontal cortex, which is important for planning, aspects of memory and cognition and decision making. ◦ The area where the parietal lobe and the temporal lobe meet is the temporoparietal junction. It serves multiple functions, including attention, body awareness and social cognition. ◦ Several areas in the prefrontal cortex and the temporoparietal junction have come to be known as the default network. These are the areas that dominate activity when you don’t need to concentrate on anything in particular. Binding problem The question of how various brain areas produce a perception of a single object is known as the binding problem, or large-scale integration problem. Binding requires perceiving that two types of stimuli (such as sight and sound) occurred at the same place at the same time. Divisions of the nervous system ◦ The central nervous system (CNS) is composed of the brain and the spinal cord. ​ ◦ The spinal cord comprises two different areas: an inner H-shaped core of gray matter and a surrounding area of white matter.  ◦ Gray matter is composed largely of cell bodies and unmyelinated interneurons, whereas white matter is composed largely of myelinated axons. (It is the myelin that gives the white matter its glossy white sheen.) ◦ Each segment of the spinal cord sends sensory information to the brain and receives motor commands from the brain. ◦ The peripheral nervous system (PNS) connects the brain and the spinal cord to the rest of the body and has two divisions: 1.​ The somatic nervous system (SNS) interacts with the external environment. It controls voluntary muscles and is composed of afferent nerves (signals from sensory organs → CNS) and efferent nerves (motor signals from CNS → skeletal muscles). ⭑ afferent = approaching the CNS, efferent = exiting the CNS 2.​ The autonomic nervous system (ANS) regulates the internal environment. It controls involuntary muscles and is composed of afferent nerves (sensory signals from internal organs → CNS) and efferent nerves (motor signals from CNS → internal organs). ◦ The ANS has two kinds of efferent nerves: sympathetic nerves and parasympathetic nerves. ◦ All sympathetic and parasympathetic nerves are two-stage neural paths – they project from the CNS and go only part of the way to the target organs before they synapse on second-stage neurons that carry the signals the rest of the way. ◦ There are three important principles of the functions of the sympathetic and parasympathetic nervous systems: 1.​ Sympathetic nerves expend energy in threatening situations, whereas parasympathetic nerves act to conserve energy. 2.​ Each autonomic target organ receives opposing sympathetic and parasympathetic input, so its activity is controlled by relative levels of sympathetic and parasympathetic activity. 3.​ Sympathetic changes are indicative of arousal, whereas parasympathetic changes are indicative of relaxation.  ◦ Sympathetic and parasympathetic activities are usually the opposite of each other. ◦ A special part of the ANS is the enteric nervous system, which controls the digestive system. Meninges ◦ The CNS is protected by three membranes. In order of innermost to outermost: 1.​ Pia mater 2.​ Arachnoid membrane 3.​ Dura mater Cerebrospinal fluid ◦ The cerebrospinal fluid supports and cushions the brain. ◦ Occasionally, the flow of cerebrospinal fluid is blocked by a tumor. The resulting buildup in the ventricles causes the walls of the ventricles and thus the entire brain to expand, producing a condition called hydrocephalus. Cranial nerves general function and amount ◦ While most of the nerves of the PNS project from the spinal cord, the 12 pairs of cranial nerves project from the brain. The head and the organs connect to the medulla and adjacent areas by 12 pairs of cranial nerves. ◦ They’re involved in a variety of sensory and motor functions. ◦ The functions of the various cranial nerves are commonly assessed by neurologists as a basis for diagnosis. Because the functions and locations of the cranial nerves are specific, disruptions of particular cranial nerve functions provide excellent clues about the location and extent of tumors and other kinds of brain pathology. Blood-brain barrier ◦ The blood-brain barrier prevents the passage of many toxic substances from the blood into the brain. Unlike the loosely packed cells that compose the walls of blood vessels in the rest of the body, those in the brain are tightly packed and form a barrier to large molecules, except those that are critical for brain function. ◦ Many CNS disorders are associated with impairment of the blood–brain barrier. Features of a neuron ◦ Neurons are cells in the nervous system that receive information and transmit it to other cells. A.​ Dendrite – Collection of information from other neurons. B.​ Cell nucleus – Location of DNA (the genetic information). C.​ Cell body (soma) – Integration of incoming information and generation of outgoing signal to the axon. D.​ Axon – Passing the signal over long distances. E.​ Axon branches – Passing the signal in different directions. F.​ Presynaptic terminal – Here the signal is passed to the dendrites of other neurons or other cells. Types of neurons ◦ One way of classifying neurons is based on the number of processes (projections) emanating from their cell bodies. ◦ Most neurons are multipolar (unipolar, bipolar), which means more than two ◦ processes extend from their cell bodies. ◦ Interneurons have a short axon or no axon at all. Their function is to integrate neural activity within a single brain structure, not to conduct signals from one structure to another. ◦ A motor neuron receives excitation through its dendrites and conducts impulses along its axon to a muscle. ◦ A sensory neuron is specialized at one end to be highly sensitive to a particular type of stimulation, such as touch. Neuroanatomical directions and planes of the brain Rostral, caudal and ipsilateral are not in the checklist Summary of major brain structures Checklist ​Corpus callosum and commissures – central, anterior, posterior, hippocampal ​Contralateral organization of the cerebral cortex ​Right hemisphere vs left hemisphere processes ​Split-brain experiments and phenomena ​Cross-cueing ​Helping-hand ​Interpreter ​Visual half-field paradigm ​Optic chiasm ​Planum temporale and Heschl’s gyrus asymmetry ​Theories about cerebral lateralization and criticism ​Analytic-synthetic theory ​Motor theory ​Linguistic theory ​Differences between right-handers and left-handers ​Evolutionary perspective ​Advantages and disadvantages ​Causes of left-handedness ​Genetic influences ​Pathological view ​Prenatal influences Corpus callosum and commissures ◦ The hemispheres are connected by and exchange information through sets of axons called the corpus callosum //caLOWsum, the anterior commissure, the posterior commissure, the hippocampal commissure and other small commissures. They enable each hemisphere to process information from and coordinate movement on both sides. ◦ Only after damage to the corpus callosum or to one hemisphere do we see clear evidence of lateralization. Contralateral organization of the cerebral cortex ◦ Lateralisation (of function) – The division of labour between the two cerebral hemispheres. ◦ The left hemisphere of the cerebral cortex connects to skin receptors and muscles on the right side of the body, the right hemisphere connects to those on the left side. ◦ The left hemisphere sees the right half and the right hemisphere sees the left half of the visual field (what one sees). ◦ At the optic chiasm, axons from the right half of the retina cross to the right hemisphere, and axons from the left half of the retina cross to the left hemisphere. ​ ⭑ Right visual field ––> left half of each retina ––> left hemisphere ⭑ Left visual field ––> right half of each retina ––> right hemisphere ◦ Most efforts to identify anatomical differences between the hemispheres have focused on the size of three areas of the cerebral cortex that are important for language: the frontal operculum, the planum temporale and Heschl’s gyrus. The planum temporale is thought to play a role in the comprehension of language and is often referred to as Wernicke’s area. It’s larger in the left hemisphere for 65% of people. 🔍 In 2018, Kong et al. conducted an analysis of MRI data from over 17,000 healthy individuals. Heschl’s gyrus was larger on the left, as predicted. For the frontal operculum, its anterior portion was larger on the right and its posterior portion was larger on the left. Right hemisphere vs left hemisphere processes ◦ Also left – local processing (details); right – global processing (big picture) ◦ Both get auditory information from both ears but slightly stronger information from the contralateral ear; both get taste information from both sides of the tongue but smell information from the nostril on its own side. ◦ The right hemisphere is better at comprehending spatial relationships and is more responsive to the emotional tone of communication than the left. ◦ A happier mood tends to correlate with left-hemisphere activity, while depression is associated with increased activity in the right prefrontal cortex. Increased right-hemisphere activity is also common in people who have recovered from depression, implying that it’s a long-term predisposing factor rather than a reaction to depression. ◦ When the right hemisphere is inactive, people don’t experience or remember feeling strong emotions. ◦ The theory of cerebral dominance says that the left is the dominant hemisphere in the control of all complex behavioural and cognitive processes and the right is the minor hemisphere as it only plays a minor role in these processes. ◦ It appears that simple tasks are best processed in the hemisphere specialised for the specific activity, but complex tasks require the cognitive power of both hemispheres. Split-brain experiments and phenomena ◦ People with have undergone surgery to the corpus callosum have split-brain syndrome. They maintain their intellect and their motivation but struggle to use both hands when doing unfamiliar tasks. ◦ Information going to one hemisphere couldn’t cross to the other because of the damage to the corpus callosum. ◦ Because a small amount of information travels between the hemispheres through several smaller commissures, some people with split-brain get enough information to give a partial description of what the right hemisphere saw. 🔍Gazzaniga (2000) proposed the concept of the interpreter, the tendency of the left hemisphere to invent and defend explanations for actions, even when the true causes are unconscious. This external route of communication between the hemispheres when split is called cross-cuing. ◦ Suppose an experimenter flashes a picture in the left visual field and asks, “Was it green?” The left hemisphere takes a guess: “Yes.” If that’s wrong, the  right hemisphere, knowing the correct answer, makes the face frown. The left hemisphere, feeling the frown, says, “Oh, I’m sorry, I meant ‘no.’ 🔍Similarly, there’s the helping-hand phenomenon: ◦ A pencil was presented to the left visual field and an orange to the right at the same time and the patient was asked to pick up the presented object from a table. As the right hand reached out to pick up the orange under the direction of the left hemisphere, the right hemisphere saw what was happening and thought an error was being made. The left hand shot out, grabbed the right hand away from the orange and redirected it to the pencil. 🔍Visual completion / visual half field paradigm – When a split-brain patient sees an image of two faces merged together, the left hemisphere sees a single normal face that is a completed version of the half face on the right. At the same time, the right hemisphere sees a single normal face that is a completed version of the half face on the left. However, the patient will say they saw one completed face because of the left hemisphere’s dominance over language/speech. Theories about cerebral lateralization and criticism ◦ Most theories are based on one of two ideas: (1) that it’s advantageous for areas of that perform similar functions to be located in the same hemisphere and (2) that it’s advantageous to place some functions on one side and others on the other side to minimise redundancy of function between hemispheres. Analytic-synthetic theory ◦ There are two basic modes of thinking, analytic and synthetic, which became segregated as the hemispheres evolved. ◦ The left hemisphere operates logically and analytically; the right hemisphere is primarily a synthesiser, which organises and processes information wholly. -​ Because it’s not possible to specify the degree to which any task requires analytic or synthetic processing, it’s difficult to empirically test. Motor theory ◦ The left hemisphere is specialised not for the control of speech specifically but for the control of fine movements, of which speech is only one category. +​ Lesions that produce aphasia often produce other motor deficits. -​ Doesn’t suggest why motor function became lateralized. Linguistic theory ◦ The primary role of the left hemisphere is language (instead of analytical thought and movement) ◦ The fact that left-hemisphere damage can disrupt the use of sign language but not pantomime gestures (gestures that express meaning) suggests that the fundamental specialisation of the left hemisphere may be language. ◦ There are two main survival advantages of lateralisation: 1.​ It may be more efficient for the neurons performing a particular function to be concentrated in one hemisphere. 2.​ Different kinds of cognitive processes may be more readily performed simultaneously if lateralised to different hemispheres. Differences between right-handers and left-handers ◦ The left hemisphere is dominant for language-related abilities in both sinistrals (left-handed people) and dextrals (right-handed people), while the right hemisphere doesn’t produce speech but understands it. ◦ The left hemisphere is predisposed to dominate for speech and gradually suppresses the speech capacity of the right hemisphere. ◦ Sinistrals are less predictable than dextrals when it comes to lateralisation. This increased variability also applies to other aspects of brain function; for example, sinistrals show greater variability in their lateralisation of attention and face recognition. ◦ The corpus callosum of sinistrals tends to be larger. This may be a sign of greater interhemispheric connectivity and may be associated with certain cognitive skills, such as language fluency and retentiveness. This greater interhemispheric connectivity might explain the findings that sinistrals are more likely to have a high IQ and exceptional mathematical ability. Causes of left-handedness ◦ It remains unclear why left-handedness is less common than right-handedness. If neither was more advantageous than the other, a 50:50 distribution in the population would be expected. If one was more advantageous, then the other would be expected to decline in prevalence and then eventually die out. ◦ The tendency toward ambidexterity is more pronounced among left-handed people. Genetic influences ◦ Handedness is thought to be partly hereditary. Left-handers are more likely to have left-handed parents. The handedness concordance rate of identical twins is higher than that of fraternal twins (81.2% vs. 73.3%). The handedness of adopted children corresponds more closely to their biological parents than their adoptive parents. ◦ Annett’s right-shift hypothesis postulates the existence of a gene encoding the necessary information for a shift of functions such as language and manual skills into the right hemisphere. Pathological view ◦ Handedness may be the expression of an early developmental disturbance or genetic defect – left-handedness and extreme right-handedness are more common among people with certain diseases and developmental abnormalities. ◦ It has been found that the differentiation of interhemispheric connections like the corpus callosum can be affected in certain developmental phases, leading to extreme right-handedness. ◦ The development of the left cerebral hemisphere, which probably takes longer to mature than the right hemisphere, might be affected in other phases of brain development. This would result in a transfer of motor functions from the otherwise dominant left hemisphere to the right hemisphere, leading to left-handedness. Prenatal influences ◦ The handedness that arises as a fetus is thought to persist throughout one’s life. ◦ Neural tube defects and some types of cleft lip and palate that are thought to be due to intrauterine disturbances are also associated with left-handedness. ◦ Evidence suggests that elevated testosterone concentrations during intrauterine life can affect handedness by promoting the loss of callosal axons or by inhibiting the development of the left hemisphere during certain phases of cerebral maturation. However, the testosterone hypothesis is controversial as it’s mainly based on observations from animal experiments. ◦ Left-handedness is significantly more common in people who were born in the spring or early summer. Perhaps intrauterine cerebral development during the winter months has been affected by altered vitamin D metabolism owing to the lack of sunlight. Immune mechanisms related to the higher incidence of viral infections in winter might be another cause. ◦ Left-handedness is associated with perinatal (when you become pregnant or up to a year after giving birth) stress. Checklist ​Nerve impulse ​Resting potential and action potential ​All-or-none law ​Propagation of the action potential ​Different phases of the action potential ​Polarization ​Depolarization ​Hyperpolarization ​Absolute vs relative refractory period ​Membrane and neuron cell characteristics ​Ion channels ​Sodium-potassium pump ​Concentration gradient ​Myelin sheaths and nodes of ranvier ​Transmission at synapses ​Presynaptic neurons vs postsynaptic neurons ​Temporal vs spatial summation ​EPSPs and IPSPs ​Chemical transmission steps ​Ionotropic vs metabotropic receptors ​Neurotransmitters ​Synthesis ​Functions ​Types ​Storage ​Release - exocytosis Nerve impulse ◦ The axon periodically regenerates an impulse. ◦ All parts of a neuron are covered by a membrane that lets certain chemicals pass. ◦ When at rest, the membrane maintains an electrical gradient, also known as polarization – a difference in electrical charge between the inside and outside of the cell. ◦ The inside of the membrane has a slight negative charge compared to the outside. The resting potential is the numerical difference in electrical charge, about -70 mV. ◦ The resting potential prepares the neuron to respond rapidly. Excitation of the neuron opens sodium channels, letting sodium enter the cell rapidly. Because the membrane did its work in advance by keeping so much sodium outside, the cell is prepared to respond vigorously to a stimulus. ◦ If we increase the negative charge inside a neuron, we can produce hyperpolarization (decreasing towards zero). ◦ Depolarization is reducing a neuron’s polarization (increasing from zero). ◦ When the membrane is depolarized enough to reach the cell’s threshold, sodium and potassium channels open. Sodium ions enter rapidly, reducing and reversing the charge across the membrane. This event is known as the action potential. ◦ After the peak of the action potential, the membrane returns toward its original level of polarization because of the outflow of potassium ions. ◦ The action potential is regenerated at successive points along the axon as sodium ions flow through the core of the axon and stimulate the next point along the axon to its threshold. The action potential maintains a constant magnitude as it passes along the axon. Summary of the action potential ◦ When an area of the axon membrane reaches its thresh- old, sodium channels and potassium channels open. ◦ Opening sodium channels lets sodium ions rush into the axon. At first, opening the potassium channels produces little effect. ◦ Positive charge flows down the axon and opens voltage-gated sodium channels at the next point. ◦ At the peak of the action potential, the sodium gates snap shut. ◦ Because voltage-gated potassium channels remain open, potassium ions flow out of the axon, returning the membrane toward its original depolarization. ◦ A few milliseconds later, the voltage-dependent potassium channels close. ◦ The all-or-none law is that the amplitude and velocity of an action potential are independent of the intensity of the stimulus that initiated it, provided that the stimulus reaches the threshold. ◦ Therefore, to signal the difference between a weak stimulus and a strong stimulus, all that an axon can change is the frequency or timing of its action potentials. ◦ The propagation of the action potential describes the transmission of an action potential down an axon. ◦ During an action potential, sodium ions enter the axon. Temporarily, the spot where they enter is positively charged in comparison with neighboring areas along the axon. The positive charge flows to neighboring regions of the axon, where they slightly depolarize the next area of the membrane, causing it to reach its threshold and open its sodium channels. The membrane regenerates the action potential at that point. In this manner, the action potential travels along the axon. ◦ An action potential starts in an axon and propagates without loss from start to finish. However, at its start, it “back-propagates” into the cell body and dendrites. The cell body and dendrites do not conduct action potentials, but they passively register the electrical event that started in the nearby axon. This back-propagation is important: When an action potential back-propagates into a dendrite, the dendrite becomes more susceptible to the structural changes responsible for learning. Different phases of the action potential ◦ Polarization, depolarization, hyperpolarization. ◦ At the peak of the action potential, the sodium channels shut tightly and remain tightly shut for approximately the next millisecond. This is the absolute refractory period, the time when the membrane can’t produce an action potential regardless of the stimulation. ◦ After that millisecond, the sodium channels relax a bit, but the rapid departure of potassium ions has driven the membrane potential farther into negative territory than usual. This is the relative refractory period, when a stronger-than-usual stimulus is necessary to initiate an action potential. ◦ The refractory period depends on two facts: The sodium channels are closed and potassium is flowing out of the cell. Membrane and neuron cell characteristics ◦ If charged ions could flow freely across the membrane, enough positive ions would enter to depolarize the membrane. However, the membrane is selectively permeable. ◦ When the membrane is at rest, the sodium and potassium channels are closed, permitting almost no flow of sodium and only a small flow of potassium. Stimulation can open these channels, permitting freer flow. ◦ The sodium–potassium pump repeatedly transports three sodium ions out of the cell while drawing two potassium ions in. Because of this pump, sodium ions are more concentrated outside and potassium ions are more concentrated inside. ◦ The sodium–potassium pump is effective only because the selective permeability of the membrane prevents the sodium ions that were pumped out from leaking back in. ◦ However, some of the potassium in the neuron slowly leaks out, carrying a positive charge. This leakage increases the electrical gradient across the membrane. ◦ When the neuron is at rest, two forces tend to push sodium into the cell: 1.​ The electrical gradient – Sodium is positively charged and the inside of the cell is negatively charged. Opposite electrical charges attract, so the electrical gradient attracts sodium into the cell. 2.​ The concentration gradient is the difference in distribution of ions across the membrane. Sodium is more concentrated outside than inside, so it’s more likely to enter the cell than to leave. Sodium would enter rapidly if it could, but the sodium channels are closed. ◦ Potassium is positively charged and the inside of the cell is negatively charged, so the electrical gradient tends to attract potassium into the cell. However, potassium is more concentrated inside the cell than outside, so the concentration gradient tends to drive it out. ◦ The potassium channels, almost completely closed, permit a small amount of potassium flow (more outward than inward) but the sodium–potassium pump continues pulling potassium back into the cell. ◦ To increase the speed of the action potential, axons evolved a special mechanism: sheaths of myelin, an insulating material composed of fats and proteins. ◦ The myelin sheath is interrupted periodically by short sections called nodes of Ranvier. In myelinated axons, the action potential starts at the first node of Ranvier. ◦ The action potential can’t regenerate along the membrane between nodes because the axon has few if any sodium channels between nodes. ◦ After an action potential occurs at a node, sodium ions enter the axon and diffuse, pushing a chain of positive charge along the axon to the next node, where they quickly regenerate the action potential. ◦ The jumping of action potentials from node to node is referred to as saltatory conduction. ◦ In addition to providing rapid conduction of impulses, saltatory conduction conserves energy – instead of admitting sodium ions at every point along the axon and pumping them out, a myelinated axon admits sodium only at its nodes. Transmission at synapses Summation ◦ Temporal summation is the phenomenon that repeated stimuli within a brief time combine their effects. A light pinch of the dog’s foot didn’t evoke a reflex, but a few rapidly repeated pinches did. ◦ The neuron delivering transmission is the presynaptic neuron and the one receiving it is the postsynaptic neuron. ◦ Although a single subthreshold excitation in the postsynaptic neuron decays over time, it combines with a second excitation that follows quickly. With a rapid succession of excitations, each adds its effect to what remained from the previous ones, until the combination exceeds the threshold of the postsynaptic neuron, producing an action potential. ◦ Spatial summation is the summation of potentials from different locations. A combination of excitations exceeds the threshold to produce an action potential. ◦ Temporal summation and spatial summation occur together when a neuron receives input from several axons in succession. EPSPs and IPSPs ◦ Unlike action potentials, which are always depolarizations, graded potentials may be either depolarizations (excitatory) or hyperpolarizations (inhibitory), and they decay over both time and distance. ◦ A graded depolarization is an excitatory postsynaptic potential (EPSP), which results from sodium ions entering the neuron. A graded hyperpolarization is an inhibitory postsynaptic potential (IPSP), produced by the flow of negatively charged chloride ions into the cell. ◦ Temporal and spatial summation also occurs with EPSPs. ◦ A dog raising one leg needs to extend the other legs to maintain balance. A pinch on the foot sends a message along a sensory neuron to an interneuron in the spinal cord that excites the motor neurons connected to the flexor muscles of that leg and the extensor muscles of the other legs. The interneuron also sends messages to inhibit the extensor muscles in that leg and the flexor muscles of the three other legs. ◦ At inhibitory synapses, input from an axon hyperpolarizes the postsynaptic cell, moving the cell’s charge farther from the threshold and decreasing the probability of an action potential. ◦ Most neurons have a spontaneous firing rate, a periodic production of action potentials even without synaptic input. ◦ In such cases, the EPSPs increase the frequency of action potentials above the spontaneous rate, whereas IPSPs decrease it. Chemical transmission steps 1.​ The neuron synthesizes chemicals that serve as neurotransmitters, either in the cell body or at the end of the axon. 2.​ Action potentials travel down the axon. At the presynaptic terminal, the depolarization enables calcium to enter the cell. Calcium releases neurotransmitters from the terminals and into the synaptic cleft, the space between the presynaptic and postsynaptic neurons. 3.​ The released molecules diffuse across the narrow cleft, attach to receptors and alter the activity of the postsynaptic neuron.  4.​ The neurotransmitter molecules separate from their receptors. 5.​ The neurotransmitter molecules may be taken back into the presynaptic neuron for recycling, or they may diffuse away. 6.​ Some postsynaptic cells send reverse messages to control the further release of neurotransmitters by presynaptic cells. Neurotransmitters ◦ The chemicals that a neuron releases at a synapse are neurotransmitters, or neuromodulators. Types Functions ◦ Many neurons release nitric oxide when they are stimulated. In addition to influencing other neurons, nitric oxide dilates the nearby blood vessels, thereby increasing blood flow to that brain area. ◦ Your serotonin levels rise after you eat foods rich in the amino acid tryptophan. Synthesis ◦ Neurons synthesize nearly all neurotransmitters from amino acids, which the body obtains from proteins in the diet. ◦ Note the relationship among epinephrine, norepinephrine and dopamine – compounds known as catecholamines. Storage ◦ Most neurotransmitters are synthesized in the presynaptic terminal, near the point of release. The presynaptic terminal stores high concentrations of neurotransmitter molecules in vesicles, tiny nearly spherical packets. ◦ However, neurons release nitric oxide as soon as they form it instead of storing it. Release ◦ An action potential by itself does not release the neurotransmitter at the end of the axon. Rather, depolarization opens voltage-dependent calcium channels in the presynaptic terminal. The calcium entry causes exocytosis, a release of neurotransmitters from the presynaptic neuron. ◦ The released neurotransmitter diffuses across the synaptic cleft to the postsynaptic membrane, where it attaches to a receptor. ◦ Most neurons release a combination of two or more transmitters. The result could be immediate brief excitation followed by slower inhibition, or other complex messages. Some neurons simultaneously release an excitatory transmitter and an inhibitory transmitter, the equivalent of saying “yes” and “no” at the same time. Ionotropic vs metabotropic receptors ◦ The effect of a neurotransmitter depends on how it affects its receptor. When the neurotransmitter attaches to its receptor, the receptor may open a channel, exerting an ionotropic effect, or it may produce a slower but longer metabotropic effect. ◦ When the neurotransmitter binds to an ionotropic receptor, it twists the receptor just enough to open its central channel to let one type of ion pass through. ◦ The channels controlled by a neurotransmitter are transmitter-gated or ligand-gated. A ligand is a chemical that binds to something. ◦ Ionotropic effects begin and decay quickly. ◦ Glutamate is usually the neurotransmitter used to elicit an excitatory ionotropic effect, which opens sodium channels (positively charged) and facilitates the depolarization of the neuron cell. On the other hand, the neurotransmitter GABA is usually used to elicit an inhibitory ionotropic effect, which opens chloride ions channels (negatively charged), provoking an further polarization of the neuron cell. ◦ Other receptors control metabotropic effects by initiating a sequence of metabolic reactions that start more slowly but last longer than ionotropic effects. ◦ Metabotropic synapses use many chemicals. ◦ When a neurotransmitter attaches to a metabotropic receptor, it bends the receptor protein that goes through the membrane of the cell. The other side of that receptor is attached to a G protein, a protein coupled to GTP, an energy-storing molecule. Bending the receptor protein detaches that G protein, which is then free to take its energy elsewhere in the cell. ◦ An ionotropic synapse has effects localized to one point on the membrane, whereas a metabotropic synapse influences activity in much or all of the cell and over a longer time. ◦ For vision and hearing, the brain needs immediate, brief information, the kind that ionotropic synapses bring. Metabotropic synapses are suited for more enduring effects such as taste and pain. Metabotropic effects are also important for arousal, attention, hunger, thirst, and emotion – functions that arise slowly and last longer than a sensation. Checklist ​Definition of drug addiction ​Stages of addiction  ​Initial drug taking ​Habitual drug taking ​Drug craving and relapse ​Theories about addiction: physical dependence theory, positive incentive theory, incentive-sensitization theory ​Drug Tolerance ​Cross-tolerance ​Withdrawal ​Drug sensitization ​Metabolic – functional tolerance ​Contingent drug tolerance ​Conditioned drug tolerance ​Exteroceptive and interoceptive stimuli ​Conditioned withdrawal effects ​Effects of drugs on the CNS: alcohol, cocaine, marijuana, nicotine, opioids ​Agonist versus antagonists ​Affinity and efficacy ​Genetic and environmental influences in addiction ​Mesotelencephalic dopamine system in reward and addiction ​Comparing the health hazards of commonly used drugs ​Treatment options Definition of drug addiction ◦ Drug addicts are habitual drug users, but not all habitual drug users are drug addicts. Drug-addicted individuals are habitual drug users who continue to use a drug despite its adverse effects on their health and social life. Stages of addiction Initial drug taking ◦ A defining feature of addiction is craving, an insistent search for something. Habitual drug taking ◦ Most people who become addicted to a substance don’t enjoy the experience as much as before but continue to want it. After people become addicted, the drug releases less dopamine but cues associated with the drug release more dopamine. ◦ Habitual drug use makes the nucleus accumbens less responsive to other types of motivation, making cues for the drug attract even more attention because of the decreased competition. This inability to experience pleasure in response to natural reinforcers is called anhedonia. ◦ Habitual alcohol or cocaine use decreases blood flow and metabolism in the prefrontal cortex, resulting in decreased ability to restrain impulses. Drug craving and relapse ◦ During a period of abstinence, the NAcc synapses responding to drug cues become even more sensitive before declining. These results match indications that craving increases during the early stage of abstinence and slightly declines later. ◦ Three different causes of relapse have been identified: 1.​ Many therapists and patients point to stress as a major factor in relapse. 2.​ Drug priming (a single exposure to the formerly misused drug) is a cause of relapse. Many addicts who have managed to abstain feel they have their addiction under control. Reassured by this feeling, they sample their addictive drug just once and are immediately plunged back into full-blown addiction. 3.​ Exposure to cues (e.g., people, times, places, or objects) that have previously been associated with drug taking has been shown to precipitate relapse. Theories about addiction Physical dependence theory ◦ Physical dependence traps addicted individuals in a vicious cycle of drug taking and withdrawal symptoms. The idea was that drug users whose intake has reached a level sufficient to induce physical dependence are driven by their withdrawal symptoms to self-administer the drug each time they try to abstain. ◦ Early drug addiction treatment programs attempted to break this cycle by gradually withdrawing drugs from addicted individuals in a hospital environment. But once discharged, almost all detoxified habitual drug users relapsed. ◦ This is because some highly addictive drugs, such as cocaine and amphetamines, don’t produce severe withdrawal symptoms. Furthermore, the pattern of drug taking routinely displayed by many habitual drug users involves an alternating cycle of binges and detoxification. ◦ For example, some addicts only take drugs on the weekends to not interfere with their work, others don’t have enough money to use drugs continuously and others have it forced on them by their repeated unsuccessful efforts to shake their habit. Positive incentive theory ◦ This theory is based on the assumption that most addicts take drugs not to escape or to avoid the unpleasant consequences of withdrawal but rather to obtain the drugs’ positive effects. ◦ The primary factor in most cases of addiction is the craving for the drug’s positive-incentive properties. -​ Doesn't explain why some users become addicts and others don’t. -​ Unable to explain why addicts often experience a big discrepancy between the positive-incentive value (the anticipated pleasure associated with an action) and the hedonic value (the amount of pleasure that is actually experienced). Incentive-sensitization theory ◦ The positive-incentive value of addictive drugs increases (becomes sensitized) with repeated drug use in addiction-prone individuals. ◦ It isn’t the hedonice value that is the basis of addiction but the positive-incentive value. ◦ Initially, a drug’s positive-incentive value is closely tied to its pleasurable effects, but tolerance often develops to the drug’s effects whereas craving is sensitized. Drug tolerance ◦ Tolerance is the decreased sensitivity to a drug that develops as a result of exposure to it. ◦ It can be demonstrated by showing that a given dose of the drug has less effect than it had before exposure or by showing that it takes more of the drug to produce the same effect. ◦ Because tolerance is learned, it can be weakened through extinction. ◦ Cross tolerance – one drug can produce tolerance to other drugs that act by the same mechanism. Tolerance may develop to some effects of a drug while sensitivity to other effects of the same drug increases. ◦ Metabolic tolerance results from changes that reduce the amount of the drug getting to its sites of action. ◦ Functional tolerance results from changes that reduce the reactivity of the sites of action to the drug. Tolerance to psychoactive drugs are mostly functional. ◦ Contingent drug tolerance develops only to drug effects that are actually experienced. ◦ Conditioned drug tolerance effects are expressed only when a drug is administered in the same situation in which it has previously been administered. ◦ Most demonstrations of conditioned drug tolerance have used exteroceptive stimuli (external and public stimuli, such as the drug-administration environment) as the conditional stimuli. However, interoceptive stimuli (internal, private stimuli) are just as effective. The thoughts and feelings produced by the drug-taking ritual and the drug effects experienced soon after administration can, through conditioning, come to reduce the full impact of a drug. ◦ Conditioned withdrawal effects are also called conditioned compensatory responses. Withdrawal ◦ Withdrawal syndrome is the strong adverse reaction when a drug is absent, as the body comes to expect it. ◦ Those who suffer from withdrawal symptoms are said to be physically dependent on that drug. ◦ The strength of withdrawal ≠ the strength of addiction – cocaine is addictive even though withdrawal symptoms are mild. ◦ Receiving an addictive drug during withdrawal increases addiction, as one learns that the drug relieves the distress caused by withdrawal. One then craves the drug during other kinds of distress. ◦ Cues presented soon after drug withdrawal are less likely to elicit craving and relapse than cues presented later. This time-dependent increase in cue-induced drug craving and relapse is known as the incubation of drug craving. Effects of drugs on the central nervous system ◦ Psychoactive drugs influence subjective experience and behavior by acting on the nervous system. ◦ A drug that blocks a neurotransmitter is an antagonist. A drug that mimics or increases the effects of a neurotransmitter is an agonist. ◦ A drug has an affinity for a receptor if it binds to it ◦ A drug’s efficacy is its tendency to activate the receptor. ◦ The effectiveness and side effects of any drug vary from one person to another, because drugs affect several kinds of receptors. One person might have more dopamine type D4 receptors and fewer D1 or D2 receptors. Alcohol ◦ Alcohol is classified as a depressant because at moderate-to-high doses it depresses neural firing. However, at low doses, it can stimulate neural firing and facilitate social interaction. ◦ At moderate doses, alcohol causes various degrees of cognitive, perceptual, verbal and motor impairment as well as a loss of control. ◦ High doses result in unconsciousness and if blood levels reach 0.5 percent, there is a risk of death from respiratory depression. ◦ The red facial flush of alcohol intoxication is produced by the dilation of blood vessels in the skin; this dilation increases the amount of heat lost from the blood to the air and leads to a decrease in body temperature (hypothermia). ◦ Alcohol attacks almost every tissue in the body and chronic alcohol consumption produces extensive brain damage like Korsakoff’s syndrome (a neuropsychological disorder characterized by memory loss, sensory and motor dysfunction) or a general loss of cortical white and gray matter. ◦ Chronic alcohol consumption also causes extensive scarring, or cirrhosis, of the liver, which is the major cause of death among heavy alcohol users. ◦ The children of mothers who consume substantial quantities of alcohol during pregnancy can develop fetal alcohol syndrome (FAS). A child with FAS suffers from some or all of the following symptoms: brain damage, intellectual disability, poor coordination, poor muscle tone, low weight, delayed growth and physical deformity. Cocaine ◦ Stimulants are drugs whose primary effect is to produce general increases in neural and behavioral activity. ◦ Empathogens (ecstasy) are psychoactive drugs that produce feelings of empathy. ◦ The primary mechanism by which cocaine and its derivatives exert their effects is by altering the activity of dopamine transporters, molecules in the presynaptic membrane that normally remove dopamine from synapses and transfer it back into presynaptic neurons. ◦ Cocaine increases dopamine levels by blocking dopamine reuptake by binding to and inhibiting the dopamine transporter (DAT), which is a protein responsible for pumping dopamine back into the presynaptic neuron after it has been released into the synapse. Normally, after dopamine is released, the transporter clears it from the synaptic gap to regulate its levels and terminate the signal. By inhibiting the reuptake process, dopamine accumulates in the synapse where it can further stimulate dopamine receptors. It inhibits dopamine reuptake in the pre-synaptic neuron. ◦ Addicts tend to go on cocaine sprees, binges in which extremely high levels of intake are maintained for periods of a day or two. During a cocaine spree, users become increasingly tolerant to the euphoria-producing effects of cocaine. Accordingly, larger and larger doses are often administered. The spree usually ends when the cocaine is gone or when it begins to have serious toxic effects. ◦ The effects of cocaine sprees include sleeplessness, tremors, nausea, hyperthermia, and in rare cases psychotic symptoms. During cocaine sprees, there is a risk of loss of consciousness, seizures, respiratory arrest, heart attack or stroke. Marijuana ◦ The psychoactive effects of marijuana are largely attributable to a constituent called THC. ◦ At low, usual “social” doses, the intoxicated individual may experience an increased sense of well-being: initial restlessness and hilarity followed by a dreamy, carefree state of relaxation; alteration of sensory perceptions including expansion of space and time; and a more vivid sense of touch, sight, smell, taste, and sound; a feeling of hunger, especially a craving for sweets; and subtle changes in thought formation and expression. ◦ At high doses, short-term memory is impaired and the ability to carry out tasks involving multiple steps to reach a specific goal declines. Speech becomes slurred, and meaningful conversation becomes difficult. A sense of unreality, emotional intensification, sensory distortion, feelings of paranoia and motor impairment are also common. ◦ There are respiratory symptoms associated with heavy marijuana smoking: bronchitis and coughing being the most common. There is evidence of an increase in the likelihood of a heart attack in individuals of all ages. ◦ There is no convincing evidence that marijuana causes brain damage and it may actually have neuroprotective effects. Nicotine ◦ It’s a teratogen, an agent that can disturb the normal development of the fetus. ◦ Nicotine exerts its effects in the brain by acting on a specific type of receptor for the neurotransmitter acetylcholine, known as the nicotinic cholinergic receptor. ◦ Some nicotinic receptors are located on the cell bodies of dopamine neurons within the VTA. Activation of these receptors increases the activity of these dopamine neurons, leading to an increase in dopamine release in the NAcc, which is thought to mediate reward. ◦ Nicotinic receptors are also located on other neurotransmitter inputs to the VTA and further increase dopamine release by removing the inhibitory influence that these other neurotransmitter inputs exert over the dopamine neurons. Opioids ◦ Opioids (morphine, codeine, heroin) exert their effects by binding to receptors whose normal function is to bind to endogenous opioids. The endogenous opioid neurotransmitters that bind to such receptors are of two classes: endorphins and enkephalins. ◦ They’re effective painkillers but are very addictive. ◦ Although many opioids are highly addictive, the direct health hazards of chronic exposure are surprisingly minor. The main direct risks are constipation, pupil constriction, menstrual irregularity and reduced sex drive. Many opioid users have taken pure heroin or morphine for years with no serious ill-effects. Genetic and environmental influences in addiction ◦ A great deal of the predisposition depends on experience but is also related to genetic influences. ◦ Several genes affect the probability of substance use, most of which are non-specific, increasing the risk of several mental disorders and risk-taking behaviors in general. ◦ Predispositions to alcohol or drug abuse arise from both environment and genetics. Early-onset alcoholism reflects a stronger genetic predisposition than later-onset alcoholism. ◦ One gene with a well-confirmed influence on alcohol abuse controls the metabolism of alcohol. After anyone drinks ethyl alcohol, enzymes in the liver metabolize it to acetaldehyde, a toxic substance. The enzyme acetaldehyde dehydrogenase then converts acetaldehyde to acetic acid, a chemical that the body uses for energy. People with a gene that produces less acetaldehyde dehydrogenase metabolize acetaldehyde more slowly. If they drink a lot of alcohol, they accumulate acetaldehyde, which produces flushing of the face, increased heart rate, nausea, headache, abdominal pain, impaired breathing and tissue damage. Mesotelencephalic dopamine system in reward and addiction ◦ The mesotelencephalic dopamine system is a system of dopaminergic neurons that projects from the mesencephalon (the midbrain) into various regions of the telencephalon. ◦ The neurons that compose the mesotelencephalic dopamine system have their cell bodies in two midbrain nuclei—the substantia nigra and the ventral tegmental area. Their axons project to a variety of telencephalic sites, in particular the nucleus accumbens. ◦ Natural and artificial rewards (food, sex, drugs) have been shown to activate the dopaminergic pathway or the mesolimbic dopamine pathway – ventral tegmental area (VTA) to the nucleus accumbens (NAcc) – increasing dopamine levels within the NAcc. ◦ Drugs modify this brain reward system either by directly influencing the action of dopamine within the system or by altering the activity of other neurotransmitters that influence it. ◦ Although it makes sense to assume that dopamine mediates pleasure, it’s important for the motivation to get something but not necessarily for enjoying it. Comparing the health hazards of commonly used drugs ◦ One way of comparing the adverse effects of commonly used drugs is to compare the prevalence of their abuse in society as a whole. In terms of this criterion, it is clear that tobacco and alcohol have a greater negative impact than marijuana, cocaine and heroin. ◦ Another method is based on global death rates: Tobacco has been implicated in approximately 6.5 million deaths per year, alcohol in approximately 3 million deaths per year and all other drugs combined in about 450,000 deaths per year. Treatments ◦ Research has revealed much about the mechanisms of addiction. Unfortunately, our increased understanding has not yet translated into better forms of treatment. ◦ The most common treatment for substance abuse, especially in the United States, is participation in Alcoholics Anonymous, Narcotics Anonymous or similar organizations. A smaller number of people consult a psychotherapist. Not many people turn to medications, but a few options are available. ◦ The liver metabolizes alcohol into acetaldehyde (toxic) and then into acetic acid (harmless). The drug disulfiram, which goes by the trade name Antabuse, antagonizes the enzyme that metabolizes acetaldehyde. Consequently, anyone who takes Antabuse becomes ill after drinking alcohol. ◦ Most studies find that Antabuse is about equal to a placebo. Ordinarily, that result would indicate that a drug is ineffective, but Antabuse is a special case because of why it is similar to the placebo. When people take Antabuse, or a placebo that they think might be Antabuse, the threat of becoming ill strongly discourages any attempt to drink alcohol. ◦ Other medications are naloxone (trade name Revia) and naltrexone, which block opiate receptors and thereby decrease the pleasure from alcohol. ◦ The idea has persisted that people who cannot quit opiates might switch to a less harmful drug. Methadone, similar to heroin and morphine, activates the same brain receptors and produces the same effects. Methadone taken orally slowly enters the blood and then the brain, avoiding a “rush” that disrupts behavior. Because methadone leaves the brain slowly, the withdrawal symptoms are also gradual. However, methadone does not eliminate the addiction. If someone quits using methadone, the cravings return. ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Checklist ​Value-Based Decisions ​Basic definition and characteristics ​Role of the basal ganglia in slow reward learning ​Functions of different prefrontal cortex regions: ​Ventromedial (fast learning) ​Orbitofrontal (rule learning, reward comparison) ​Dorsomedial (social behavior processing) ​Factual Decisions ​Basic definition and characteristics ​Speed-accuracy tradeoff principle ​Role of posterior parietal cortex in evidence accumulation ​Role of prefrontal cortex in final binary choices ​Brain Regions & Damage Effects ​Impact of prefrontal cortex damage on decision updating ​Effects of orbitofrontal cortex damage on risk assessment ​Consequences of frontotemporal dementia on social decision-making ​Social Decision-Making ​Role of oxytocin in social behavior ​Key brain regions: amygdala, medial prefrontal cortex, temporoparietal junction ​Conservation of oxytocin receptors across species ​Neural Mechanisms for factual decision-making ​How LIP neurons reflect decision-making process ​Understanding of the diffusion-to-barrier framework ​How motion coherence affects decision speed and accuracy ​Integration of sensory information over time ​Body-Brain Connection ​Relationship between heart rate variability and decision quality ​Impact of gut bacteria on cognition and decision-making Value-based decisions ◦ When you make a decision based on preferences, you estimate the probable outcome of each choice and weigh which outcome seems better. ◦ One has to learn probabilistically over many trials, relying mainly on cells in the basal ganglia. ◦ Cells in the ventromedial prefrontal cortex also participate by rapidly learning new information. If the basal ganglia learns that choice A is usually better than B, but something now favors choice B, the rapid-learning prefrontal cortex can overrule the slower-learning basal ganglia. ◦ The orbitofrontal cortex learns rules about the environment, such as if one fruit on a tree is ripe, the others probably are too. Another function is that compares an expected reward to other possible choices. Impairment or relative inactivity in the orbitofrontal cortex is often associated with poor or impulsive decisions. ◦ Frontotemporal dementia is when parts of the frontal and temporal lobes gradually degenerate. The damage usually includes the ventromedial prefrontal cortex and orbitofrontal cortex, which are important for evaluating possible rewards and also for interpreting other people’s emotional expressions. People with frontotemporal dementia also exhibit little interest in how others perceive them. Factual decisions ◦ When you make a decision about factual matters, one set of cells accumulates evidence in favor of one choice, another set accumulates evidence for another choice and a third set compares the two. Accumulating evidence = parietal cortex; comparing = prefrontal cortex. ◦ Whereas cells in the posterior parietal cortex responded in proportion to the number of clicks, responses in the frontal cortex produced an all-or-none outcome, like a scorekeeper who announces which team won the game, regardless of whether the score was close or not. ◦ Decisions about noisy sensory signals involve an inherent tradeoff between speed and accuracy. Deciding quickly can mean missing important signals. Taking more time can provide more or better signals, but that time might be wasted. Neural Mechanisms for factual decision-making How LIP neurons reflect the decision-making process ◦ Lateral Intraparietal (LIP) neurons show activity that persists during the decision-making process. Their activity ramps up or down depending on the "weight of evidence" favoring one decision over another. ◦ In tasks like motion discrimination, LIP neurons' firing rates correlate with the accumulation of sensory evidence. Their activity reflects not only the outcome of the decision but also the intermediate stages of the decision process. ◦ The ramping activity of these neurons mirrors the diffusion-to-barrier model: evidence accumulates toward a threshold, at which point a decision is triggered. For example: positive ramps correspond to evidence favoring a choice aligned with the neuron's response field, while negative ramps occur for the alternative choice. ◦ LIP activity appears to encode the decision variable over time and the firing rate reaches a consistent threshold before a motor response, indicating the completion of the decision process. Understanding of the diffusion-to-barrier framework ◦ This model likens the decision process to Brownian motion: noisy evidence accumulates toward one of two boundaries (positive or negative), representing alternative decisions. ◦ Tradeoff between speed and accuracy: Low thresholds lead to faster but less accurate decisions, while higher thresholds improve accuracy at the cost of speed. ◦ The stochastic nature of sensory signals is where momentary evidence fluctuates but drifts on average toward the correct choice with coherent signals. ◦ The barrier crossing represents the point where the decision is finalized, transitioning from sensory integration to motor execution. ◦ This framework accurately predicts psychometric and chronometric data, such as the probability of correct choices and reaction times, by modeling evidence accumulation dynamics. How motion coherence affects decision speed and accuracy ◦ Motion coherence, or the percentage of dots moving in the same direction, directly influences both the speed and accuracy of decisions: ◦ High coherence results in faster and more accurate decisions because the sensory evidence is stronger and less ambiguous. ◦ Low coherence slows the decision process and increases errors due to weaker and noisier evidence. ◦ Psychophysical experiments demonstrated: ◦ At high coherence levels, accuracy improves rapidly with viewing time but plateaus after enough evidence is accumulated. ◦ At low coherence, decisions take longer and are less accurate because evidence accumulates more slowly and variably. ◦ In response-time tasks, the stronger the motion coherence, the shorter the reaction time, as the accumulation process reaches the threshold more quickly. Integration of sensory information over time ◦ Sensory evidence from the motion stimulus is accumulated over time, which allows the brain to build a more reliable "weight of evidence" for making decisions. ◦ This temporal integration: 1.​ Improves performance by combining information across multiple time points. 2.​ Operates on a time scale much longer than the neural computations in the sensory cortex, suggesting higher-order processing. ◦ The ramp-like neural activity in LIP reflects this integration, where stronger motion coherence results in steeper accumulation slopes. ◦ The integration process enables the brain to interpret noisy or ambiguous stimuli by continuously adding new evidence to existing information until a decision threshold is reached. Social decision making 🔍In one study, rhesus monkeys could offer food. They reciprocated with monkeys who had offered them food in the past, but not those that had previously failed to offer. Neurons in the dorsomedial prefrontal cortex responded specifically to different monkeys, depending on how they had acted in the past. Disrupting those neurons left the monkeys responding unselectively, treating equally those that had or had not been helpful in the past. ◦ Neurons in the amygdala are also necessary for recognizing other individuals and establishing relationships with them. ◦ The process of inferring the intentions of the other person based on the perception of their current state (known as theory-of-mind or mentalizing) is known to engage the temporoparietal junction and the medial prefrontal cortex. ◦ Oxytocin stimulates contractions of the uterus during childbirth, stimulates breasts to produce milk and promotes maternal behavior, social approach, and pair bonding. ◦ Analysis examining the changes in gene expression patterns identified the oxytocin receptor distributions to be the most conserved through evolution. Body brain connection ◦ The vagus nerve, part of the parasympathetic nervous system, controls heart rate. People with greater heart rate variability from moment to moment tend to make better decisions, especially in situations of risk and uncertainty. A possible explanation is that changes in vagus activity indicate increased mental effort. ◦ Some types of intestinal bacteria produce inflammation, impair cognition and increase the risk of psychological and neurological disorders.