Bio Final - Brain Anatomy PDF

💜 Theme 1 Brain controversies: Myth or Fact? (brain, neurons and CNS) Divisions of the Nervous System The nervous system is composed of two primary divisions: 1. Central Nervous System (CNS) Composed of the brain and sp...

💜 Theme 1 Brain controversies: Myth or Fact? (brain, neurons and CNS) Divisions of the Nervous System The nervous system is composed of two primary divisions: 1. Central Nervous System (CNS) Composed of the brain and spinal cord. 2. Peripheral Nervous System (PNS) Composed of the somatic and autonomic nervous systems. Somatic Nervous System Interacts with the external environment. Composed of: Afferent nerves: Carry sensory signals from skin, muscles, joints, eyes, and ears to the CNS. Efferent nerves: Carry motor signals from the CNS to skeletal muscles. Autonomic Nervous System Regulates the body’s internal environment. Composed of: Afferent nerves: Carry sensory signals from internal organs to the CNS. Efferent nerves: Carry motor signals from the CNS to internal organs. Theme 1 1  The autonomic nervous system has two types of efferent nerves: Sympathetic nerves: Autonomic motor nerves that project from the CNS in the lumbar and thoracic regions of the spinal cord. Parasympathetic nerves: Project from the brain and the sacral region of the spinal cord. Both sympathetic and parasympathetic nerves are two-stage neural paths: Sympathetic neurons: Synapse on second-stage neurons at a substantial distance from their target organs. Parasympathetic neurons: Synapse near their target organs. Conventional view of autonomic nerve functions includes three principles: Sympathetic nerves: Stimulate, organize, and mobilize energy resources in threatening situations. Parasympathetic nerves: Work to conserve energy. Each autonomic target organ receives opposing input from sympathetic and parasympathetic nerves, and activity is controlled by the relative levels of sympathetic and parasympathetic activity. Sympathetic activity: Associated with psychological arousal. Parasympathetic activity: Associated with psychological relaxation. Most PNS nerves project from the spinal cord, but there are 12 pairs of cranial nerves that project directly from the brain. Theme 1 2  Meninges The brain and spinal cord are protected by bone and three protective membranes called meninges: 1. Dura mater: The outermost tough membrane. 2. Arachnoid membrane: A fine, spider web-like membrane located just inside the dura mater. Beneath it lies the subarachnoid space, which contains large blood vessels and cerebrospinal fluid (CSF). 3. Pia mater: The innermost delicate membrane that adheres to the surface of the CNS. Ventricles and Cerebrospinal Fluid (CSF) The CNS is protected by cerebrospinal fluid (CSF), which fills: The subarachnoid space. The central canal of the spinal cord. The cerebral ventricles of the brain. Central canal: A small channel that runs the length of the spinal cord. Cerebral ventricles: Four large internal chambers of the brain: Theme 1 3  Two lateral ventricles Third ventricle Fourth ventricle The subarachnoid space, central canal, and cerebral ventricles are interconnected by a series of openings, forming a single reservoir. Functions of CSF: Supports and cushions the brain. Loss of CSF can cause severe headaches and stabbing pain with sudden head movements. Production and absorption of CSF: Traditionally believed to be produced by choroid plexuses. Excess CSF is absorbed from the subarachnoid space into blood-filled spaces called dural sinuses. Newer perspectives suggest that CSF production and absorption are more complex than previously understood. Theme 1 4  Hydrocephalus ("water head"): Occurs when CSF flow is blocked (often by a tumor) near a narrow channel linking the ventricles. This blockage causes a buildup of CSF, leading to the expansion of the ventricle walls and the entire brain. Anatomy of Neurons Neurons are specialized cells for the reception, conduction, and transmission of electrochemical signals. Neurons come in various shapes and sizes, but many share similar structures. Theme 1 5  Neuron Cell Membrane Composed of a lipid bilayer of fat molecules. Embedded within the bilayer are protein molecules that support key functions: Channel proteins: Allow certain molecules to pass through. Signal proteins: Transfer signals to the inside of the neuron when specific molecules bind to them on the outside of the membrane. Classes of Neurons Multipolar neurons: Have more than two processes extending from their cell body (most common type). Unipolar neurons: Have one process extending from their cell body. Bipolar neurons: Have two processes extending from their cell body. Interneurons: Have short axons or no axons at all and integrate neural activity within a single brain structure rather than transmitting signals between structures. Theme 1 6  Neurons and Neuroanatomical Structure Gross neural structures in the nervous system are composed of either cell bodies or axons. In the CNS: Clusters of cell bodies are called nuclei. Bundles of axons are called tracts. In the PNS: Clusters of cell bodies are called ganglia. Bundles of axons are called nerves. Note: "Nuclei" can refer to either the structure within a neuron's cell body or a cluster of cell bodies within the CNS. Glial Cells: The Forgotten Cells Theme 1 7  Glial cells are a key part of the nervous system, with approximately 2 glial cells for every 3 neurons in the brain. There are several different types of glial cells, each with distinct functions. Types of Glial Cells: 1. Oligodendrocytes: Located in the CNS (Central Nervous System). Have extensions that wrap around axons of neurons, forming myelin sheaths. Myelin is a fatty substance that insulates axons, increasing the speed of axonal conduction. One oligodendrocyte can provide multiple myelin segments for several neurons. 2. Schwann Cells: Found in the PNS (Peripheral Nervous System). Similar to oligodendrocytes, but each Schwann cell forms only one myelin segment. Schwann cells are unique in that they can guide axonal regeneration after nerve damage, unlike oligodendrocytes. 3. Microglia: The smallest glial cells. Play a role in responding to injury or disease by: Multiplying. Engulfing cellular debris or even entire cells. Triggering inflammatory responses. Involved in cell death regulation, synapse formation, and synapse elimination. 4. Astrocytes: The largest glial cells, shaped like stars. Theme 1 8  Their extensions cover the outer surfaces of blood vessels in the brain and make contact with neurons. Play a role in regulating the passage of chemicals between the blood and CNS neurons, allowing some to pass and blocking others. Astrocytes can contract or relax blood vessels depending on the blood flow demands of specific brain regions. Recent research shows that astrocytes: Exchange chemical signals with neurons and other astrocytes. Help in the establishment and maintenance of synapses. Modulate neural activity and form functional networks with other astrocytes. Additional Insights: The role of glial cells was once believed to be primarily supportive—providing nutrition, clearing waste, and forming a physical matrix. Research has shown that glial cells are involved in many more functions than previously understood, including modulating neural activity, regulating synapse development, and responding to injury. New types of glial cells continue to be discovered, expanding our understanding of their roles. Directions in the Vertebrate Nervous System Orientation in the Nervous System: Directions are described relative to the orientation of the spinal cord. Most vertebrates' nervous systems have three main axes: 1. Anterior-Posterior: Refers to the nose end (anterior) to the tail end (posterior). 2. Dorsal-Ventral: Refers to the top of the head (dorsal) to the bottom of the head (ventral). Theme 1 9  3. Medial-Lateral: Refers to the midline of the body (medial) to the lateral surfaces of the body (lateral). For Humans: Humans walk on hind legs, so the directional terms shift. Superior (above) and Inferior (below) are used instead of dorsal-ventral. Proximal (closer to the body) and Distal (further from the body) are commonly used to describe limb directions. Theme 1 10  Planes of the Brain: The brain can be cut into three different planes for analysis: 1. Horizontal Section: A cut that divides the brain into upper and lower parts. 2. Frontal Section: A cut that divides the brain into front and back parts. Theme 1 11  3. Sagittal Section: A cut that divides the brain into left and right halves. Midsagittal Section: A cut down the center of the brain, dividing it between the two hemispheres. Cross Section: A section cut at a right angle to any long and narrow structure. Spinal Cord The spinal cord comprises two distinct areas: Inner H-shaped core – Gray matter (cell bodies and unmyelinated interneurons). Surrounding area – White matter (myelinated axons). Theme 1 12  The spinal gray matter forms four arms: Dorsal horns – Two dorsal arms. Ventral horns – Two ventral arms. Spinal nerves (31 pairs) are attached at different levels (one left, one right). 62 nerves in total. Each nerve divides near the spinal cord, and axons attach through either: Dorsal root (afferent – sensory). Ventral root (efferent – motor). Dorsal root axons: Sensory unipolar neurons. Cell bodies form dorsal root ganglia (outside the spinal cord). Synapse in the dorsal horns. Ventral root neurons: Motor multipolar neurons. Project to: Skeletal muscles (somatic). Ganglia → Internal organs (autonomic). Theme 1 13  Five Major Divisions of the Brain Begins in the embryo. The CNS develops from a fluid-filled tube. First sign of brain development: Three swellings appear at the anterior end of the tube. These swellings form the: Forebrain. Midbrain. Hindbrain. Theme 1 14  Before birth, the three swellings become five due to further growth: Forebrain and hindbrain swellings each develop into two distinct regions. Five final divisions: 1. Telencephalon (Forebrain). 2. Diencephalon (Forebrain). 3. Mesencephalon (Midbrain). 4. Metencephalon (Hindbrain). 5. Myelencephalon (Hindbrain, located within the head). Theme 1 15  Myelencephalon (Medulla) The myelencephalon, also known as the medulla, is the most posterior division of the brain. It primarily consists of tracts that carry signals between the brain and the body. A key structure within the myelencephalon is the reticular formation – a complex network of ~100 tiny nuclei running along the central core of the brainstem from the posterior boundary to the anterior boundary of the midbrain. The reticular formation is sometimes called the reticular activating system because certain parts play a role in arousal. Theme 1 16  However, calling it a "system" can be misleading as the nuclei perform diverse functions, including: Sleep. Attention. Movement. Cardiac reflexes. Circulatory reflexes. Respiratory reflexes. This broad involvement highlights the essential regulatory roles of the myelencephalon in maintaining vital bodily functions. Mesencephalon Has two divisions: Tectum. Tegmentum. Theme 1 17  Tectum Forms the dorsal surface of the midbrain (roof). Composed of two pairs of bumps called the colliculi: Inferior colliculi (posterior): Responsible for auditory function. Superior colliculi (anterior): Responsible for visual-motor function. Directs the body’s orientation toward or away from visual stimuli. In lower vertebrates, the tectum is entirely visual-motor and is referred to as the optic tectum. Tegmentum Contains three colorful structures of interest: Periaqueductal gray: Gray matter situated around the cerebral aqueduct (connecting the 3rd and 4th ventricles). Plays a role in mediating the analgesic effects of opioid drugs. Substantia nigra: Known as the black substance. An important component of the sensorimotor system. Red nucleus: Works alongside the substantia nigra in the sensorimotor system. Diencephalon Composed of two structures: Thalamus. Hypothalamus. Thalamus Theme 1 18  Large, two-lobed structure at the top of the brainstem. Lobes: One lobe sits on each side of the third ventricle. Lobes are joined by the massa intermedia (runs through the ventricle). Visible features: White lamina – layers composed of myelinated axons. Composition: Contains many pairs of nuclei. Most nuclei project to the cortex. Sensory Relay Nuclei (most well understood): Receive signals from sensory receptors. Process and transmit signals to appropriate areas of the sensory cortex. Hypothalamus Located below the anterior thalamus. Role: Regulates motivated behaviors (e.g., eating, sleeping, sexual behavior). Exerts influence by controlling hormone release from the pituitary gland. Pituitary Gland: Dangles from the hypothalamus on the ventral surface of the brain. Initially believed to produce nasal mucus (discovered in gelatinous form behind the nose of cadavers). Other structures on the inferior surface: Optic chiasm: Point where the optic nerves meet. Theme 1 19  Some fibers decussate (cross to the opposite side of the brain). Decussating fibers are ipsilateral (stay on the same side of the body). Mammillary bodies: Pair of spherical nuclei located just behind the pituitary gland. Telencephalon Largest division of the human brain and mediates the most complex functions. Initiates voluntary movement. Interprets sensory input. Mediates complex cognitive processes such as learning, speaking, and problem-solving. Cerebral Cortex Theme 1 20  Cerebral hemispheres are covered by a layer of tissue called the cerebral cortex. Mainly composed of small, unmyelinated neurons: Gray Matter. Beneath the cortex: Composed of large myelinated axons: White Matter. The cerebral cortex is deeply convoluted and increases the overall volume of the brain. Lissencephalic: Mammals without convoluted cortexes. Cortical Convulsions: Fissures: Large furrows. Sulci: Small furrows. Gyri: Ridges between fissures and sulci. The longitudinal fissure almost separates the two cerebral hemispheres. Cerebral Commissures: Tracts connecting the two hemispheres. Corpus callosum: The largest cerebral commissure. Lobes of the Cerebral Hemispheres The two major fissures on the lateral surface of each hemisphere: Central fissure Lateral fissure These fissures partially divide each hemisphere into four lobes: 1. Frontal lobe 2. Parietal lobe 3. Temporal lobe 4. Occipital lobe Major Gyri: Precentral gyrus (frontal lobe). Postcentral gyrus (parietal lobe). Theme 1 21  Superior temporal gyrus (temporal lobe). Functions of Each Lobe Occipital lobe: The main function is to analyze visual input to guide behavior. Large areas of the cortex are dedicated to this task. Parietal lobe: Postcentral gyrus: Analyzes sensations from the body (e.g., touch). Other areas perceive the location of objects and our bodies, as well as direct attention. Temporal lobe: Superior temporal gyrus: Involved in hearing and language. Inferior temporal cortex: Identifies complex visual patterns. Medial portion: Plays a role in memory. Frontal lobe: Precentral gyrus and adjacent cortex: Motor functions. Frontal cortex anterior to motor cortex: Performs complex cognitive functions such as planning, evaluating behavior outcomes, and assessing the significance of others' behavior. Neocortex About 90% of the cerebral cortex is neocortex (or isocortex). The neocortex is made up of six layers numbered from 1-6. Neocortical Anatomy: Pyramidal cells: Large, multipolar neurons with pyramid-shaped cell bodies, a large apical dendrite, and a long axon. Stellate cells: Small, star-shaped interneurons (with short or no axons). Theme 1 22  Layers: Differ in size, density of cell bodies, and the proportion of pyramidal and stellate cells. Vertical flow of information: Forms the columnar organization of the neocortex, where neurons in a vertical column form a mini-circuit. Layer IV: Specialized for receiving sensory signals from the thalamus and is thicker in sensory areas. Layer V: Contains pyramidal cells that conduct signals to the brainstem and spinal cord and is thicker in motor areas. Hippocampus Not neocortex. Located at the medial edge of the cerebral cortex, folding back on itself. The folding creates a shape like a sea horse. Plays a major role in certain kinds of memory. Theme 1 23  Limbic System and Basal Ganglia In the telencephalon, alongside the neocortex, several large subcortical nuclear groups exist. Theme 1 24  These nuclei are considered part of either the limbic system or the basal ganglia. The term "system" suggests more certainty than warranted but is a useful way to conceptualize subcortical structures. Limbic System Definition: Circuit of midline structures circling the thalamus. Function: Regulates motivated behaviors (fleeing, feeding, fighting, sexual behavior). Major Structures: Amygdala: Almond-shaped, in the anterior temporal lobe. Involved in emotion, especially fear. Hippocampus: Posterior to the amygdala, beneath the thalamus (medial temporal lobe). Plays a role in memory. Cingulate Cortex: Large strip in the cingulate gyrus, superior to the corpus callosum. Encircles the dorsal thalamus. Fornix: Major tract of the limbic system. Arches from the hippocampus to the septum and mammillary bodies. Septum: Midline nucleus at the anterior tip of the cingulate cortex. Mammillary Bodies: Linked by tracts to the hippocampus and amygdala. Circuit Path: 1. Amygdala → Hippocampus → Cingulate Cortex 2. Fornix loops to the septum and mammillary bodies 3. Completes the limbic ring through connections between the septum, mammillary bodies, amygdala, and hippocampus. Key Functional Insights: Hippocampus: Memory formation. Theme 1 25  Hypothalamus: Drives motivated behaviors (eating, sleeping, sexual behavior). Amygdala: Central to emotional regulation, particularly fear. Basal Ganglia Definition: Subcortical structures involved in motor control and decision- making. Major Structures: Caudate: Long, tail-like structure. Putamen: Adjacent to the caudate; together with the caudate, forms the striatum (striped structure). Theme 1 26  Globus Pallidus: Pale, circular structure medial to the putamen and between it and the thalamus. Functional Role: Facilitates voluntary motor responses and decision-making. Receives input from the neocortex and outputs to the globus pallidus. Clinical Relevance: Pathway from substantia nigra to the striatum deteriorates in Parkinson’s disease (leads to rigidity, tremors, reduced voluntary movement). Nucleus Accumbens: Located in the medial ventral striatum, involved in reward and reinforcement (e.g., addictive drugs). Theme 1 27  The Binding Problem The binding problem is a concept in neuroscience and psychology that refers to the question of how the brain integrates information from different sensory modalities and areas to create a unified perception of an object or experience. The Problem in Simple Terms Imagine you're holding a red apple. Your visual cortex processes the shape and color. Your somatosensory cortex feels the smooth texture. Theme 1 28  Your olfactory system detects its smell. Your gustatory system anticipates the taste. Despite each part of the brain processing different features separately, you perceive the apple as a single object, not as disconnected attributes like "red," "round," and "smooth." Why This is a Challenge Distributed Processing: Different brain areas handle different sensory inputs. Timing: These processes happen at slightly different speeds, yet your brain combines them seamlessly. Lack of Central Hub: There isn't a single area in the brain where all sensory information converges for final processing. How Does the Brain Solve It? Researchers propose several theories: 1. Neural Synchrony: When neurons in different parts of the brain fire at the same time (in sync), the brain links that activity, interpreting it as related to the same object. 2. Attention: Your brain focuses on one object at a time, selectively grouping features that belong together. 3. Spatial Location: The brain associates sensory inputs that occur in the same place as part of the same object (e.g., light, texture, and temperature from the same apple). 4. Top-Down Processing: Your expectations and prior knowledge help the brain piece together sensory inputs into a cohesive whole. Example Watching a movie: You see the actor’s lips move. You hear their voice. Theme 1 29  Even though the sound is processed in your auditory cortex and the visual in your occipital lobe, you experience them as one seamless event. Theme 1 30  💜 Theme 2 I Left brain, right brain? The Left and Right Hemispheres 1. Hemisphere Control and Body Connections: Left Hemisphere – Connects to the right side of the body. Right Hemisphere – Connects to the left side of the body. Exception – Both hemispheres control trunk muscles and facial muscles. 2. Sensory Information Processing: Visual Field – Left hemisphere sees the right half of the visual field. Right hemisphere sees the left half of the visual field. Auditory Information – Each hemisphere receives input from both ears. Slightly stronger input from the contralateral ear. Taste and Smell – Taste and smell are uncrossed. Each hemisphere receives taste from both sides of the tongue and smell from the nostril on its own side. 3. Speech and Language Lateralization: Left Hemisphere – Dominant for speech production in: 95% of right-handers. Theme 2 1  80% of left-handers. Right Hemisphere – Does not produce speech but understands it. Left-Handed Exceptions – Some strongly left-handed people have right-hemisphere speech dominance. Most left-handers have either left-hemisphere control or a mixture of left- and right-hemisphere control. 4. Lateralization and Communication Between Hemispheres: Lateralization – Division of labor between hemispheres. Corpus Callosum and Commissures – Hemispheres exchange information through: Corpus callosum. Anterior commissure. Hippocampal commissure. Other small commissures. These structures allow each hemisphere to: Process information from both sides. Coordinate movement. Lateralization Evidence – Only apparent after damage to: Corpus callosum. One hemisphere. 5. Anatomical Differences Between Hemispheres: Planum Temporale – Larger in the left hemisphere in 65% of people (Geschwind & Levitsky, 1968). Also larger in the left hemisphere of most infants. Theme 2 2  Child Development and Speech – Children activate the right hemisphere during speech more than adults. As we age, the left hemisphere becomes dominant, and the right hemisphere is gradually suppressed during speech. Corpus Callosum Development (Rare Cases) – If the corpus callosum does not develop, both hemispheres remain active during speech throughout life. Left hemisphere is predisposed to dominate for speech. Visual and Auditory Connections to the Hemispheres 1. Visual System Connections: Hemisphere and Eye Connection: Each hemisphere is connected to half of each eye. Central Vision (Vertical Strip): A 5-degree vertical strip down the center of each retina connects to both hemispheres. Optic Chiasm (Axon Crossing): About half of the axons from each eye cross to the opposite hemisphere at the optic chiasm: Right visual field → Left half of each retina → Left hemisphere. Left visual field → Right half of each retina → Right hemisphere. 2. Auditory System Connections: Information Flow: Each ear sends information to both hemispheres. Localization of Sound: Brain areas involved in sound localization compare input from both ears. Contralateral Attention: Theme 2 3  Each hemisphere pays more attention to input from the ear on the opposite side. The Corpus Callosum and Split-Brain Operation 1. Epilepsy and Treatment: Epilepsy – Characterized by repeated excessive synchronized neural activity. Treatment Options: 90% of patients respond well to anti-epileptic drugs. For severe cases, physicians may remove the focus (origin of seizures). Focus removal not an option if: Multiple foci exist. Focus is in an area essential for language. 2. Corpus Callosum Surgery (Split-Brain Operation): Procedure – Corpus callosum is cut to prevent seizures from spreading across hemispheres. Benefits: Seizures become less frequent. Seizures only affect one side of the body. Limitations: Patients struggle to use hands together for unfamiliar tasks. However, they can use hands independently in ways most people cannot. 3. Split-Brain Research (Roger Sperry’s Experiments): Experiment Setup: Patients stare straight ahead while stimuli (words/images) are flashed to one side of a screen. Information reaches only one hemisphere due to the cut corpus callosum. Theme 2 4  Findings: Left hand points to what the right hemisphere perceived. Right hand points to what the left hemisphere perceived. Patients can verbalize what the left hemisphere sees. Patients cannot verbalize what the right hemisphere perceives. They say, “I don’t know what it was.” Exceptions: Small commissures still allow partial information transfer between hemispheres (Berlucchi et al., 1997; Forster & Corballis, 2000). 4. Development of the Corpus Callosum (Children): Slow Development: Children may resemble split-brain adults. Study on Fabric Touch (Galin et al., 1979): 3-year-olds make more errors using both hands compared to one. 5-year-olds perform equally well with one or two hands. Suggests corpus callosum matures between ages 3 and 5. Split Hemispheres: Competition and Cooperation Initial Effects: Theme 2 5  Hemispheres act like separate individuals post-surgery. Adaptation: Conflicts reduce over time. Left Hemisphere Explanations (Interpreter Concept): Left hemisphere invents explanations for actions by the right hemisphere. Example: Left hemisphere sees a chicken claw; right hemisphere sees snow. Right hand points to chicken, left hand points to shovel. Patient explains that the shovel is for cleaning the chicken shed. Concept by Michael Gazzaniga (2000) – The Interpreter. Insight: Left hemisphere defends unconscious decisions, even in intact brains. The Right Hemisphere: Functions: Excels in spatial relationships. Processes emotional tone in communication (gestures, tone of voice). Damage Effects: Impairs understanding of humor and sarcasm. Depression and the Right Hemisphere: Increased right hemisphere activity is linked to depression. Long-term recovery shows persistent right hemisphere activity. Gaze Patterns: Most gaze right during verbal tasks (left hemisphere dominance). People with depression gaze left more often. Study on Lie Detection (Etcoff et al., 2000): Left hemisphere damage improved lie detection (60% accuracy). Theme 2 6  Intact left hemisphere relied on words, reducing accuracy. Wada Procedure (Ross et al., 1994): Right hemisphere inactivated by anesthesia. Patients recalled facts of emotional events but no emotions. When active, patients recalled strong emotions. Suggests right hemisphere is essential for emotional memory. Avoiding Overstatements: Caution: Research is exciting but may lead to oversimplified or unscientific conclusions. Cerebral Lateralization of Function: Introduction 1. Early Discoveries in Cerebral Lateralization: Marc Dax (1836): Country doctor who presented a report at a medical society meeting in France. Observed 40 brain-damaged patients with speech problems; none had damage restricted to the right hemisphere. His findings implied that the left hemisphere played a crucial role in speech. The report generated little interest due to the prevailing belief that the brain acted as a whole. Dax died in 1837, unaware he had anticipated key discoveries in neuropsychology. Paul Broca (1861-1864): Reported postmortem examinations of two aphasic patients. Aphasia: A brain damage-induced deficit in language production or comprehension. Theme 2 7  Both patients had left hemisphere lesions in the frontal cortex, anterior to the primary motor cortex (later known as Broca’s area). Initially unaware of Dax’s work, Broca did not link aphasia to left hemisphere damage immediately. By 1864, after examining seven more aphasic patients, Broca confirmed all had inferior prefrontal cortex damage in the left hemisphere. Hugo-Karl Liepmann (Early 1900s): Discovered that apraxia (inability to perform movements on command despite intact motor ability) was linked to left hemisphere damage. Noted that apraxic symptoms were bilateral, affecting both sides of the body. Reinforced the concept that the left hemisphere is critical for voluntary movement. Theme 2 8  Theory of Cerebral Dominance: Evidence from Broca and Liepmann suggested the left hemisphere plays a specialized role in language and movement. This led to the notion of the left hemisphere as the "dominant" hemisphere and the right as the "minor" hemisphere. 2. Tests of Cerebral Lateralization: Early Research Approaches: Focused on comparing deficits following left vs. right hemisphere lesions. Modern Techniques: Sodium Amytal Test (Wada Procedure): Administered before neurosurgery to assess language lateralization. A small amount of sodium amytal is injected into the carotid artery on one side, anesthetizing that hemisphere for a few minutes. Patients recite series (e.g., alphabet, days of the week) and name objects. When the left hemisphere (typically language-dominant) is anesthetized, patients experience temporary mutism and speech errors. Anesthesia of the right hemisphere rarely results in mutism. Dichotic Listening Test (Kimura): Noninvasive and used on healthy individuals. Simultaneous auditory presentation of three pairs of digits to each ear via headphones. Patients report all digits heard. Better performance from the right ear (linked to left hemisphere processing) indicates left hemisphere language dominance. Kimura found that patients with right hemisphere language specialization performed better with left ear input. Theme 2 9  Functional Brain Imaging (PET/fMRI): Monitors brain activity while subjects perform language tasks. Shows greater activation in the left hemisphere during language processing. 3. Speech Laterality and Handedness: Lesion Studies (Russell & Espir, 1961; Penfield & Roberts, 1959): WWII brain injury studies and patients undergoing unilateral brain excisions for neurological disorders. Left hemisphere lesions resulted in aphasia in: 60% of right-handers (dextrals). 30% of left-handers (sinestrals). Right hemisphere lesions resulted in aphasia in: 2% of dextrals. 24% of sinestrals. Findings confirmed left hemisphere dominance for language in most dextrals and many sinestrals. Sodium Amytal Test Results (Milner, 1974): 92% of right-handers without early left hemisphere damage had left hemisphere speech specialization. 69% of left-handers and ambidextrous patients without early left hemisphere damage had left hemisphere speech specialization. Early left hemisphere damage reduced left hemisphere specialization to 30% in left-handed/ambidextrous patients. 4. Sex Differences in Brain Lateralization: Levy (1972) Hypothesis: Proposed male brains are more lateralized than female brains. McGlone (1977, 1980) found male stroke victims were three times more likely to develop aphasia than female stroke victims. Theme 2 10  Current Research: Subsequent studies failed to replicate McGlone’s findings. A meta-analysis of 17 brain imaging studies found no significant sex- based differences in lateralization. Levy’s hypothesis is no longer widely supported. The Split Brain 1. Corpus Callosum in the 1950s: Largest cerebral commissure, connecting two hemispheres with ~200 million axons. Paradox: Its size and location suggested importance, but studies from 1930- 1940 indicated no apparent function. 2. Groundbreaking Experiment of Myers and Sperry (1953): Objective: Investigate corpus callosum’s function in cats. Key Findings: 1. Information Transfer: Corpus callosum transfers learned information between hemispheres. 2. Independent Functioning: After corpus callosum is cut, hemispheres function independently—acting like two separate brains. Method: Cats trained on visual discrimination task (circle vs. square). Optic chiasm and corpus callosum cut, one eye patched—visual info restricted to one hemisphere. Theme 2 11  Results: Cats learned task normally with one eye patched. When patch switched to the other eye, performance dropped to chance (50%). No transfer of learning occurred between hemispheres, suggesting independent memory systems. Theme 2 12  3. Commissurotomy in Humans with Epilepsy: Epileptic discharges spread between hemispheres via corpus callosum. Vogel and Bogen's Rationale: Cutting corpus callosum could limit seizure spread, reducing severity. Procedure initially controversial but proved highly effective—many patients stopped experiencing major seizures. Sperry and Gazzaniga’s Tests: Visual stimuli flashed to one hemisphere (left or right) for 0.1s to prevent eye movement. Tasks performed by one hand under a ledge—ipsilateral hemisphere unable to monitor visually. Theme 2 13  Results: Each hemisphere functioned independently, displaying unique consciousness, abilities, and memories. Left hemisphere capable of speech; right hemisphere lacked verbal ability. 4. Evidence for Independent Hemispheres in Split-Brain Patients: Visual Field Tests: Objects in left visual field (processed by right hemisphere) couldn’t be named by patients, but left hand could pick the object correctly. Tactile Tasks: Objects in the left hand (right hemisphere) couldn’t be verbally identified but could be selected by touch. 5. Cross-Cuing: Definition: Indirect communication between hemispheres via external cues. Example: Patient guesses wrong color verbally (left hemisphere), but right hemisphere detects error and causes a frown or head shake—leading the left hemisphere to correct the guess. 6. Doing Two Things at Once: Split hemispheres can simultaneously learn different tasks. Example: Theme 2 14  Pencil in left visual field (right hemisphere) and orange in right visual field (left hemisphere). Each hand selects the correct object independently. Helping Hand Phenomenon: Right hemisphere detects error by left hand and corrects it by redirecting the hand to the appropriate object. Visual Completion Phenomenon: Split-brain patients demonstrate visual completion, similar to how individuals with scotomas are unaware of visual gaps. Each hemisphere functions as if it has a scotoma covering the ipsilateral visual field. Chimeric Figures Test (Levy, Trevarthen, & Sperry, 1972): Photos with fused half-faces of different people were shown to split- brain patients. Patients described seeing a bilaterally symmetrical face, typically completing the half shown to the right visual field (left hemisphere). Even when prompted, patients rarely noticed any peculiarity. Theme 2 15  7. Dual Mental Functioning and Conflict: Case Study - Peter: Post-commissurotomy, Peter’s left hemisphere experienced frustration with the autonomous actions of his left hand and leg. Left hand would act against his will (e.g., turning off TV or striking others). 8. Current Perspective on Hemisphere Independence: Hemisphere independence varies by task and complexity. Simple tasks processed by one hemisphere; complex tasks require both hemispheres. Emotional responses easily transfer between hemispheres, even when stimuli are limited to one side. Theme 2 16  Differences between left and right hemispheres For many functions, there are no substantial differences between the hemispheres; and when functional differences do exist, these tend to be slight biases in favor of one hemisphere or the other—not absolute differences. Disregarding these facts, the popular media inevitably portray left–right cerebral differences as absolute. As a result, it is widely believed that various abili ties reside exclusively in one hemisphere or the other. Yet, even in this most extreme case, lateralization is far from total; there is substantial language-related activity in the right hemisphere. Examples of cerebral lateralization of function superiority of the left hemisphere in controlling ipsilateral movement When complex, cogni tively driven movements are made by one hand, most of the activation is observed in the contralateral hemisphere, as expected. However, some activation is also observed in the ipsilateral hemisphere, and these ipsilateral effects are substantially greater in the left hemisphere than in the right Consistent with this observation is the finding that left-hemisphere lesions are more likely than right-hemisphere lesions to produce ipsilateral motor problems—for example, left hemisphere lesions are more likely to reduce the accuracy of left-hand movements than right- hemisphere lesions are to reduce the accuracy of right-hand movements superiority of the right hemisphere in spatial ability Levy (1969) placed a three-dimensional block of a particular shape in either the right hand or the left hand of split-brain patients. Then, after they had thoroughly palpated (tactually investigated) it, she asked them to point to the two-dimensional test stimulus that best Theme 2 17  represented what the three-dimensional block would look like if it were made of cardboard and unfolded. She found a right-hemisphere superiority on this task, and she found that the two hemispheres seemed to go about the task in different ways. The performance of the left hand and right hemisphere was rapid and silent, whereas the performance of the right hand and left hemisphere was hesitant and often accompanied by a running verbal commentary that was difficult for the patients to inhibit. Levy concluded that the right hemisphere is superior to the left at spatial tasks. This conclusion has been frequently confirmed specialization of the right hemisphere for emotion Analysis of the effects of unilateral brain lesions indicates that the right hemisphere may be superior to the left at performing some tests of emotion for instance in identifying correctly facial expressions of emotion Although the study of unilateral brain lesions suggests a general right- hemisphere dominance for some aspects of emotional processing, functional brain-imaging studies have not provided unambiguous support for this view superior musical ability of the right hemisphere Kimura (1964) compared the performance of 20 right-handers on the standard digit version of the dichotic listening test with their performance on a version of the test involving the dichotic presentation of melodies. In the melody version of the test, Kimura simultaneously played two different melodies—one to each ear—and then asked the participants to identify the two they had just heard from four that were subsequently played to them through both ears. The right ear (i.e., the left hemisphere) was superior in the perception of digits, whereas the left ear (i.e., the right hemisphere) was superior in the perception of melodies. This is consistent with the observation that right temporal lobe lesions are more likely to disrupt music discriminations than are left temporal lobe lesions Theme 2 18  hemispheric differences in memory The two hemispheres have similar abilities that tend to be expressed in different ways. The study of the lateralization of memory was one of the first areas of research on cerebral lateralization to lead to this modification in thinking. You see, both the left and right hemispheres have the ability to perform on tests of memory, but the left hemisphere is better on some tests, whereas the right hemisphere is better on others 2 approaches to studying lateralzation of memory; one approach is to try to link particular memory processes with particular hemispheres— for example, it has been argued that the left hemisphere is specialized for encoding episodic memory. The other approach is to link the memory processes of each hemisphere to specific materials rather than to specific processes the left hemisphere has been found to play the greater role in memory for verbal material, whereas the right hemisphere has been found to play the greater role in memory for nonverbal material Theme 2 19  What is lateralized? broad clusters of abilities or individual cognitive processes? The problem is that categories such as language, emotion, musical ability, and spatial ability are each composed of dozens of different individual cognitive activities, and there is no reason to assume that all those activities associated with a general label (e.g., spatial ability) will necessarily be lateralized in the same hemisphere. Many researchers are taking a different approach to the study of cerebral lateralization. They are basing their studies on the work of cognitive psychologists, who have broken down complex cognitive tasks—such as reading, judging space, and remembering—into their constituent cognitiveprocesses. Once the laterality of the individual cognitive elements has been determined, it should be possible to predict the laterality of cognitive tasks based on the specific cognitive processes that compose them. Anatomical assymetries of the brain Most efforts to identify interhemispheric differences in brain anatomy have focused on the size of three areas of cortex that are important for language, the most lateralized of our cognitive abilities: the frontal operculum, the planum temporale, and Heschl’s gyrus. The frontal operculum is the area of frontal lobe cortex that lies just infront of the face area of the primary motor cortex; in the left hemisphere, it is the location of Broca’s area. The planum temporale and Heschl’s gyrus are areas of temporal lobe cortex. The planum temporale lies in the posterior region of the lateral fissure; it is thought to play a role in the comprehension of language and is often referred to as Wernicke’s area. Heschl’s gyrus is located in the lateral fissure just anterior to the planum temporale in the temporal lobe; it is the location of primary auditory cortex Theme 2 20  nature of assymetries is a function of sex and age 2 difficulties in studying anatomical asymmetry of the language areas; First, their boundaries are unclear, with no consensus on how best to define them. Second, there are large differences among healthy people in the structure of these cortical language areas Any report that one of the three cortical language areas tends to be larger in the left hemisphere typically leads to the suggestion that the anatomical asymmetry might have caused, or have been caused by, the lateralization of language to the left hemisphere. However, there is little support for such conjectures (see Bishop, 2013). The fact that a particular cortical area is on the average larger in the left hemisphere does not suggest that it is causally linked to language lateralization, even if the cortical area has been linked to language. At a bare minimum, it must be shown that the anatomical and functional asymmetries are correlated—that the degree of anatomical lateralization in a person reflects the degree of language lateralization in the same person. Theme 2 21  In 2018, Kong et al. conducted a large-scale analysis of the MRI data from over 17,000 healthy individuals. They reported large asymmetries in the size of the frontal operculum and Heschl’s gyrus. Heschl’s gyrus was larger on the left, as predicted. However, different parts of the frontal operculum showed different directions of asymmetry: Its anterior portion was larger on the right, and its posterior portion was larger on the left. Clearly, if there is a relationship between anatomical asymmetries and language function, it is far more complex than previously thought. In short, the search for anatomical differences between the two hemispheres has been only partially successful. Many anatomical asymmetries have been discovered, but few have been clearly related to functional asymmetries. Several researchers have suggested that studies of differences in the microstructure (e.g., differences in cell type, synapses, and neural circuitry) between the two hemispheres may prove more informative than comparisons of differences in the size of vaguely defined areas. Evolution of cerebral lateralization and language theories of the evolution of cerebral lateralization Many theories have been proposed to explain why cerebral lateralization of function evolved. Most are based on one of two general premises: (1) that it is advantageous for areas of the brain that perform similar functions to be located in the same hemisphere, and (2) that it is advantageous to place some functions on one side of the brain and others on the other side so as to minimize redundancy of function between hemispheres analytic-synthetic theory of cerebral asymmetry two basic modes of thinking—an analytic mode and a synthetic mode become segregated during the course of evolution in the left and right hemispheres, respectively according to the theory, left hemisphere operates in a logical, analytical, computerlike fashion, analyzing and abstracting stimulus input sequentially and attaching verbal labels; the right hemisphere is Theme 2 22  primarily a synthesizer, which organizes and processes information in terms of gestalts, or wholes. its vagueness is a problem as it cannot specify the degree to which any task requires either type of processing and has to subjectthe theory to empirical tests motor theory of cerebral asymmetry holds that the left hemisphere is specialized not for the control of speech specifically but for the control of fine movements, of which speech is only one category support for this theory comes from reports that lesions that produce aphasia often produce other motor deficits does not suggest why motor function became lateralized in the first palce linguistic theory of cerebral asymmetry posits that the primary role of the left hemisphere is language; this is in contrast to the analytic–synthetic and motor theories, which view language as a secondary specialization residing in the left hemisphere because of that hemisphere’s primary specialization for analytic thought and skilled motor activity, respectively. the linguistic theory of cerebral asymmetry is based to a large degree on the study of deaf people who use American Sign Language (a sign language with a structure similar to spoken language) and who suffer unilateral brain damage. the fact that left-hemisphere damage can disrupt the use of sign language but not pantomime gestures (gestures that express meaning), as occurred in the case of W.L., suggests that the fundamental specialization of the left hemisphere may be language case of W.L. congenitally deaf, right-handed male who grew up using American Sign Language. Seven months prior to test ing, W.L. was admitted to hospital complaining of right-side weakness and motor problems. CT Theme 2 23  scan revealed a large left frontotemporoparietal stroke. At that time, W.L.’s wife noticed he was making many uncharacteristic errors in signing and was having difficulty understanding the signs of others. W.L.’s neuropsychologists managed to obtain a 2-hour videotape of an interview with him recorded 10 months before his stroke, which served as a valuable source of prestroke performance measures. Formal poststroke neuropsychological testing confirmed that W.L. had suffered a specific loss in his ability to use and understand sign language. The fact that he could produce and understand complex pantomime gestures suggested that his sign language aphasia was specific to language (Corina et al., 1992) When did cerebral lateralization evolve? However, there is evidence of lateralization of function in many spe cies that evolved long before we humans were around There seem to be two fundamental advantages; First, in some cases, it may be more efficient for the neurons performing a particular function to be concentrated in one hemisphere. For example, in most cases, it is advantageous to have one highly skilled hand rather than having two moderately skilled hands. Second, in some cases, two different kinds of cognitive processes may be more readily performed simultaneously if they are lateralized to different hemispheres Article Briefing Document: Understanding Left-Handedness Introduction: This briefing document summarizes key findings from the review article "Understanding Left-Handedness," which explores the phenomenon of handedness, with a particular focus on left-handedness, from epidemiological, neurobiological, and medical perspectives. The article reviews existing literature on the topic, emphasizing the evolutionary origins of handedness, its genetic components, and its potential association with both advantages and disadvantages. Theme 2 24  Main Themes and Key Ideas: Cerebral Lateralization and Handedness: The human brain is asymmetrical, with each hemisphere specializing in different functions. This lateralization is a core concept for understanding handedness. Motor control is contralaterally organized, meaning the left hemisphere controls the right side of the body and vice-versa. This is exemplified by Maradona's left hand being controlled by his right motor cortex. "The contralateral cerebral control of movement is a manifestation of the lateralization of human brain function. It presumably redounds to the advantage of the organism by obviating functional redundancy, thereby making neural processes run faster and more efficiently." Handedness (left vs. right dominance) is a natural variation, and it's not entirely clear why one is more common than the other. An expected 50:50 distribution of left- and right-handedness hasn't occurred, suggesting possible advantages or disadvantages associated with each. Evolutionary Origins of Handedness: Handedness is an ancient trait, present in humans and their evolutionary predecessors for over a million years, indicated by archaeological findings like tools and cave paintings. "It seems that motor processes in man and the evolutionary predecessors of mankind have been lateralized for more than a million years, and that right-handedness was more common than left-handedness even in our remote forebears." The development of motor lateralization may have laid the foundation for the development of language, with hand gestures preceding vocal speech. Prevalence and Cultural Views on Left-Handedness: Left-handedness prevalence ranges from 5% to 25.9% across cultures, and is more common in men than in women, with reasons for variation remaining unclear. Theme 2 25  "Across all human cultures, left-handedness is found in 5% to 25.9% of individuals and is more common in men than in women" Historically, left-handedness has been stigmatized in some cultures, even considered a sign of inferiority, leading to attempts to convert left-handers to right-handers, particularly in children. "As with other natural variants, such as sexual orientation or skin color, handedness has also been an object of stigmatization. In Europe, left- handedness was considered undesirable, and even a sign of inferiority, until well into the 20th century." Even today, in some cultures the left hand is considered "impure" as it is used for hygiene purposes. Genetic and Developmental Aspects: Handedness is partly hereditary, with a higher chance of left-handed offspring if one or both parents are left-handed, suggesting maternal transmission. Twin studies show a higher concordance rate for handedness in identical twins than fraternal twins, indicating a genetic component. "Left-handers are more likely to have left-handed parents, particularly left- handed mothers; this indicates possible maternal transmission." Handedness likely begins to develop in the uterus with fetal preferences like thumb-sucking and head position Some forms of handedness may arise from developmental disturbances or genetic defects, particularly in the case of extreme right-handedness or left-handedness. The article suggests that these "disturbances" can be related to intrauterine or perinatal issues like infections or hypoxia, which may alter cerebral development. Elevated testosterone concentrations during intrauterine life are also implicated in potentially influencing handedness. "In most cases, left-handedness appears to be the result of natural variation, but some types of handedness may be the expression of an early developmental disturbance or genetic defect." Theme 2 26  Associations with Disease and Developmental Abnormalities: Left-handedness is found to be more common among people with epilepsy, schizophrenia and autism, and also associated with neural tube defects. "Left-handedness is reportedly 1.2 to 2 times more common among schizophrenics than in the normal population." The article notes that perinatal stress (premature birth, lower Apgar scores) and birth season (more common with spring births) can be linked with left-handedness. "Left-handedness is significantly more common in persons who were born in the spring or early summer (March to July)." Left-handers, on average, tend to have a delayed onset of sexual maturity, later development of secondary sexual characteristics and are generally shorter. Cognitive and Behavioral Implications of Left-Handedness: Left-handedness is associated with more bilateral cognitive processing. "There are multiple indications that left-handedness is associated with more pronounced bilaterality of cognitive processing." Compared to right-handers, left-handers have a more variable pattern in the location of their speech center, with a greater proportion having language processing in both hemispheres or the right hemisphere. "97% of right-handers have their motor speech area located exclusively in the left hemisphere but only 60% of left-handers have it exclusively in the left hemisphere." Left-handers tend to have a larger corpus callosum, implying increased interhemispheric connectivity which may be related to enhanced cognitive abilities like language fluency and retentiveness. Left-handers are reported to have higher IQ scores, mathematical ability, and more often are proficient musicians. Theme 2 27  Left-handedness is unusually common among successful athletes, particularly those in one-on-one sports, perhaps due to the element of surprise that can occur when using the left side. "Furthermore, left-handedness is unusually common among successful high-performance athletes, particularly in one-on-one sports such as tennis, boxing, and judo." Left-handers may face disadvantages in the world designed for right- handed people, like a higher risk of accidents using machinery. Left-Handedness as a Polymorphism: The persistence of left-handedness is an example of continuous polymorphism, where multiple types persist in a population. Game theory suggests that the rarity of left-handedness might confer an evolutionary advantage, such as a "surprise effect" in confrontations. "This might be called “survival of the unexpected,” rather than “survival of the fittest”: Left-handedness is advantageous in such situations only because it is rare." Left-handers may benefit at a societal level due to their increased interhemispheric connectivity. Lateralization Beyond Handedness: Lateralization isn't exclusive to handedness; it's found in other functions including footedness, eyedness, and sensory processing (hearing, taste, smell). Emotional processing also exhibits lateralization, with positive emotions associated with the left hemisphere and negative emotions with the right. Whether this applies to left-handers in a similar manner is being debated. Lateralization Across the Animal Kingdom: Cerebral lateralization is not exclusive to humans and is found in many animals, including birds, dogs, and chimpanzees. Conclusion: The review article emphasizes that handedness, particularly left- handedness, is a complex trait influenced by both genetic and environmental Theme 2 28  factors, and while it's usually a normal variant, it can be indicative of a developmental disturbance in rare cases. The study of handedness is beneficial to our understanding of cerebral lateralization, a key component in brain evolution. This phenomenon impacts not just motor function but potentially also cognitive abilities, language processing, and emotional regulation. Furthermore, the existence of a continuous polymorphism highlights the advantages of having diverse traits in the overall population. Key Messages (from the original document): "Handedness has probably existed for more than a million years and was a precondition for the evolutionary development of language and fine motor function." "Left-handedness and extreme right-handedness usually occur as normal variants but, in rare cases, can be an expression of disturbances of cerebral development." "Lateralizing tendencies are seen for other cerebral functions as well, including language and emotion." "Understanding the origin of handedness helps us understand the evolutionary development of the brain." Theme 2 29  💜 Theme 3 Inside the neuron The action potential the resting potential of the neuron resting potential- difference in electrical charge between the inside and outside of a neuon all parts of a neuron are covered by a membrane about 8 nanometers thick membrane is composed of 2 layers of phospholipid molecules and embedded among the phospholipids are cylindrical protiens that let certain chemicals pass Theme 3 1  when at rest, membrane maintains an electrical gradient aka polarization- a difference in electric charge in and out of the cell inside of membrane has a slightly negative charge compared to outside because of negatively charged protiens on the inside and this difference in voltage is the resting potential researchers measure resting potential by inserting a very thin microelectrode into cell body- most common electrode is a fine glass tube filled with a salt solution. a reference electrode outside the cell completes the circuit. connecting electrodes to a voltmeter we find neurons interior has a negative potential relative to exterior (typical level is 70 millivolts) forces acting on sodium and potassium ions if charged ions could flow freely in membrane, positive ions would enter to depolarize the membrane but the membrane is selectively permeable letting some chemicals pass more freely than rest oxygen, carbon dioxidem urea and water are unchagred and flow freey through channels that are always open sodium, potassium, calcium and chloride are charged ions that cross through channels and gates that are sometimes open and sometimes closed- when membrane is at rest, channels are closed hence sodium cannot flow and very little potassium can flow and only stimulation can open the channels to permit flow sodium-potassium pump is a protien complex that repeatedly transports three sodium ions out of the cell while drawing two potassium ions in becuase of this pump, sodium ions are 10 times more concentrated outside than inside the membrane and vice versa for potassium the pump is effective only because the selective permeability of membrane prevents sodium ions that were pumped out from leaking in hence when sodium is pumped out it stays out however the same is not for potassium as it slowly leaks out carrying a positive charge and this increases the electrical gradient across membrane Theme 3 2  when neuron is at rest two forces push sodium into the cell; one is the electrical gradient because sodium is positively charged and inside of cell is negatively charged and becuase opposite electric charges attract the gradient attracts sodium into the cell. the other force is concentration gradient, the difference in distribution of ions across membrane- sodium is more concentrated outside than inside so it is more likely to enter cell than leave why a resting potential? the resting potential prepares the neuron to respond rapidly excitation of the neuron opens sodium channels letting sodium enter cell rapidly because membrane did its work by keeping so much sodium outside, the cell is prepared to respond vigourosly to a stimulus the action potential axons send messages called action potentials when axon membrane is at rest, a microelectrode shows negative potential inside the axon and if we use a different electrode to increase the negative charge we can produce hyperpolarization which means increased polarization when stimulation ends, charge returs to original resting level and it looks like: Theme 3 3  applying brief current to depolarize neuron and reduce polarization toward zero, we get: slightly stronger depolarizing current and potential rises slightly higher and again returns to resting level: a stronger current applied: and this time when potential reaches threshold the membrane opens its sodium channels and permits sodium ions to flow into cell driving membrane potential upward Theme 3 4  any subthreshold stimulation produces a small response that quickly decays and any stimulation beyond threshold, regardless of how far beyond, produces a big response like the one above, known as the action potential- the peak of the action potential, varies from one axon to another, but it is consistent for a given axon. the all-or-none law any depolarization that reaches or passes the threshold produces an action potential for a given neuron, all action potentials are approximately equal in amplitude and velocity- variations occur spontaneously the all-or-none law is that the amplitude and velocity of an action potential are independent of the intensity initiated by stimulus provided the stimulus reaches the threshold the law constrains how an axon can send a message all an axon can change is frequency of its action potentials or their timing to signal a strong or weak stimulus and the nervous system uses both types of coding the molecular basis of the action potential Theme 3 5  chemical events behind action potential can be remembered with 3 principles: At the start, sodium ions are mostly outside the neuron, and potassium ions are mostly inside. Depolarizing the membrane opens the sodium and potassium channels. At the peak of the action potential, the sodium channels close. a neurons membrane contain cylindrical protiens in different shapes and sizes of opening a protien that allows sodium to cross is a sodium channel and one that lets potassium pass is a potassium channel these are called voltage-gated channels because they open and close based on voltage across membrane at resting potential, sodium channels are fully closed and potassium is almost fully closed as membrane becomes depolarized, both channels begin to open allowing freer flow at first potassium opens making a little difference but its much bigger when the sodium channel opens because both electric and concentration gradient drive sodium into the neuron sodium ions enter rapidly and electrical potential across membrane quickly passes beyond 0 to a reversed polarity like: Theme 3 6  of all sodium ions near axon less than 1 percent cross membrane during an action potential and remain far more concentrated outside than inside because of concentration gradient, sodium ions continue to enter the cell but get shut at peak of action potential opening those channels at first made little difference but after sodium ions have crossed membrane the inside of cell has a slight positive charge instead of usual negative and at that point both electric and concentration gradient drive potassium ions out of cell because the potassium channels remain open after the sodium channels close, enough potassium leaves to drive the membrane back to its original level and then slightly beyond it Theme 3 7 Theme 3 8  at the end of this process the membrane has returned to resting potential but inside of neuron has slightly more sodium ions and slightly fewer potassium ions than before hence eventually the sodium-potassium pump restores to original distribution but takes time action potentials require flow of sodium and potassium, local anesthetic drugs like novocain and xylocaine attach to sodium channels of membrane preventing sodium ions from entering Propagation of the action potential during an action potential, sodium ions enter the axon for a temporary time the spot they enter is positively charged in comparison to other areas the positive charge flows to neighboring regions of the axons where they slightly depolarize next area of membrane causing it to reach threshold and open sodium channels membrane regenerates action potential at that point and hence travels along the axon the refractory period when action potential is at peak, electrical potential across membrane is above threshold at peak of action potential, sodium channels shut tightly and remain shut for approx a millisecond this period is absolute refractory period- the time when the membrane cannot produce an action potential regardless of the stimulation after the millisecond, sodium channels relax a bit but the rapid depart of potassium ions has driven membrane potential farther into negative territory than usual at that time lasts another 2-4 ms the membrane is in a relative refractory period when a stronger than usual stimulus is necessary to initiate an action potential Theme 3 9  thus the refractory period depends on 2 things; the sodium channels are closed and potassium is flowing out of the cell The myelin sheath and saltatory conduction in thinnest of axons, action potentials travel at velocity less than 1 meter/second. increasing diameter brings conduction velocity up to 10 m/s and to increase the speed even more, vertebrate axons evolved a special mechanism: sheaths of myelin, an insulating material composed of fats and protiens myelinated axons are covered with myelin sheath and are present in only vertebrates myelin axons are covered with layers of fats and protiens myelin sheath is interrupted periodically by short sections called nodes of ranvier each only 1 micrometer wide Theme 3 10  in myelinated axons, action potential stats at first node of ranvier action potential at first myelin segment cannot regenerate along the membrane between nodes because axon has few if any sodium channels between nodes after action potential occurs at a node, sodium ions enter the axon and diffuse, pushing a chain of positive charge alon the axon to the next node where they regenerate the action potential jumping of action potentials from one node to another is called saltatory conduction and in addition to providing rapid conduction of impulses this conduction conserves energy because instead of admitting sodium ions at every point along the axon and pumping it out, a myelinated axon admits sodium only at nodes Theme 3 11  local neurons many tiny neurons called local neurons have no axon and communicate only with immediate neighbors because they have no axon, they do not produce action potentials and do not follow all-or-none law in such cells, greater stimulation produces greater depolarization called graded potential that spreads over surface of tiny neuron declining in strength over distance most local neurons have inhibitory effects on neighboring cells Theme 3 12  local neurons are hard to study because it is also impossible to isert an electrode in a tiny cell without damages most knowledge comes from larger neurons and that leads to misconceptions The concept of they synapse Charles Scott Sherrington (1906) physiologically demonstrated that neurons communicate at a junction between them, the synapse propeties of synapses Sherrington based his conclusions on studies of reflexes, automatic muscular responses to a stimuli circuit from a sensory neuron to a muscle response is a reflex arc if neurons are separate, a reflex must require communication between neurons and measurements of reflexes might reveal the propeties of their communication Sherrington observed several properties of reflexes that suggest special processes at the junctions between neurons: Reflexes are slower than conduction along an axon. Several weak stimuli presented at nearby places or times combine their effects. When one set of muscles becomes excited, a different set becomes relaxed. Speed of a reflex and delayed transmission at the synapse speed of conduction through the reflex arc varied but was never more than about 15 m/s in contrast, previous research measured action potential velocities along sensory or motor nerves at about 40 m/s Sherrington concluded that some process must be slowing conduction through the reflec and he inferred that delay occurs when one neuron Theme 3 13  communicates with another this idea estabised existence of synapses temporal summation repeated stimuli within a brief time combine their effects aka temporal summation neuron delivering transmission is the presynaptic neuron and the one recieving it is the postsynaptic neuron although a single subthreshold excitation in postsynaptic neuron decays, it combines with a second excitation that follows quickly and each following adds to the effect hence producing an action potential graded potentials are either depolarizations (excitatory) or hyperpolarizations (inhibitory) and they decay over both time and distance graded depolarization is an excitatory postsynaptic potential (EPSP) results from a sodium ions entering neuron Theme 3 14  graded hyperpolarization is an inhibitory postsynaptic potential (IPSP) produced by flow of negatively charged chloride into the cell spatial summation synapses also have the property of spatial summation combinations of excitation exceeded the threshold to produce an action potential EPSPs from several axons summate their effects on a postsynaptic cell temporal and spatial summation occur together when a neuron recieves input from several axons in succession inhibitory synapses Flexor muscles are those that decrease the angle between two bones Extensor muscles are those that increase the angle between two bones. Flexor muscles work to decrease the angles between two body parts. Extensor muscles work to increase the angle between two body parts Theme 3 15  researchers physiologically demonstrated those inhibitory synapses. At these 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 and in such cases the EPSP’s increase the frequency of action potentials above spontaneous rate whereas IPSP’s decrease it spontaneous firing rate varies among neurons for a given neuron it stays within a narrow range Relationship among EPSP, IPSP and action potentials When neuron 1 excites neuron 3, it also excites neuron 2, which inhibits neuron 3. The excitatory message reaches neuron 3 faster because it goes through just one synapse instead of two. The result is a burst of excitation (EPSP) in neuron 3, which quickly slows or stops. You can now understand how inhibitory messages can regulate the timing of activity Theme 3 16  some synapses are larger than others and produce bigger effects the effect of 2 synapses at the same time can be more than double the effect of either one or less than double Chemical events at the synapse discovery of chemical transmission at synapses set of nerves called sympathetic nervous system accelerates heartbeat, dialates pupils and regulates other organs T.R.Elliot reported that applying adrenaline hormonse directly to surface of heart, stomach or pupils produces same effect as sympathetic nervous system- hence he suggested that sympathetic nerves stimulate muscles by releasing adrenaline or similar chemical nerves send messages by releasing chemicals the chemical events at a synapse Theme 3 17  chemical events at a synapse: The neuron synthesizes chemicals that serve as neurotransmitters, either in the cell body or at the end of the axon. 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. The released molecules diffuse across the narrow cleft, attach to receptors, and alter the activity of the postsynaptic neuron in any