Central Nervous System (cont.) PDF

11/16/23 Central Nervous System (cont.) The Cerebellum ● Cerebellum - the largest part of the hindbrain, the second-largest part of the brain as a whole, and contains more than half of all brain neurons ● Granule cells - found in the cerebellum; the most abundant type of neuron in the entire brain ●...

11/16/23 Central Nervous System (cont.) The Cerebellum ● Cerebellum - the largest part of the hindbrain, the second-largest part of the brain as a whole, and contains more than half of all brain neurons ● Granule cells - found in the cerebellum; the most abundant type of neuron in the entire brain ● Purkinje cell - large cerebellar neurons; axons project to deep nuclei to synapse with neurons that lead to brainstem ● Anatomical features: ○ The right and left cerebellar hemispheres are connected by a bridge (vermis) ○ Superficial cortex of gray matter with folds (folia), branching white matter (arbor vitae), and deep nuclei ■ Input to the cerebellum goes to the cortex, output comes from deep nuclei ○ Cerebellar peduncles - three pairs of stalks that connect the brainstem and cerebellum; their fibers carry signals to and from the cerebellum ■ Inferior peduncles connected to medulla oblongata; most spinal input enters the cerebellum through the inferior peduncle ■ Middle peduncles connected to pons; most input from the rest of the brain enters through muddle peduncle ■ Superior peduncles connected to the midbrain; carry cerebellar output ● Functions of the cerebellum ○ Motor coordination and locomotor ability ● Sensory, linguistic, emotional, and other nonmotor functions including ○ Comparing textures of objects ○ Perceiving space (as tested by pegboard puzzles) ○ Recognizing objects from different views ○ Keeping judge of elapsed time and maintaining tapping rhythm ○ Directing eye movements to compensate for head movement ○ Judging pitch of tones, distinguishing between spoken words ○ He;ping in verbal association tasks ○ Planning, scheduling, and emotional control The forebrain ● Forebrain consists of two parts ○ Diencephalon ■ Encloses third ventricle ■ Most rostral part of the brainstem ○ ● ● Telencephalon ■ Develops chiefly into the cerebrum Components of the forebrain The diencephalon ○ The diencephalon has three major components: thalamus, hypothalamus, and epithalamus ○ The thalamus: ■ Ovoid mass on each side of the brain ● Interthalamic adhesion joins right and left thalami medially ■ Perched at the superior end of the brainstem beneath the cerebral hemispheres ■ Constitutes about four-fifths of the diencephalon ■ Composed of at least 23 nuclei within five major groups ● Anterior, posterior, medial, lateral, and ventral ■ Gateway to the cerebral cortex ● Nearly all input to cerebrum synapses in thalamic nuclei ● Processes information on its way to the cerebral cortex ● Not all information is passed along; thalamus screens out most of the information it receives ■ Plays a key role in motor control ● Relays signals from cerebellum to cerebrum ● Provides feedback loops between cerebral cortex and basal nuclei ■ Involved in memory and emotion ● Limbic system includes some of the anterior thalamic nuclei ○ ● The hypothalamus ■ Forms part of walls and floor of the third ventricle ■ Entends anteriorly to optic chiasm ■ Extends posteriorly to mammillary bodies ● Each mammillary body contains three or four mammillary nuclei that relay signals from the limbic system to the thalamus ■ Attaches to the pituitary through a stalk-like structure called the infundibulum ■ Contains many nuclei with a wide variety of visceral, emotional, and behavioral functions ● The major control center of the autonomic nervous system, the endocrine system ● Homeostatic regulation of all body systems Functions of the hypothalamic nuclei ○ Hormone secretion ■ Controls anterior pituitary, thereby regulating growth, metabolism, reproduction, and stress responses ■ Produces posterior pituitary hormones for labor contractions, lactation, and water conservation ○ Autonomic effects ■ Major integrating center for the autonomic nervous system ■ Influences heart rate, blood pressure, gastrointestinal secretions, motility, etc. ○ Thermoregulation ■ ● ● The hypothalamic thermostat monitors body temperature and activates mechanisms to adjust the temperature if necessary ○ Food and water intake ■ Regulates hunger and satiety, responds to hormones influencing hunger, energy expenditure, and long-term control of body mass ■ Osmoreceptors monitor the osmolarity of blood and can stimulate the production of antidiuretic hormones to help conserve water ○ Sleep and circadian rhythms ■ The suprachiasmatic nucleus controls a 24-hour (circadian) rhythm ○ Memory ■ Mammillary nuclei relay signals from the hippocampus to the thalamus ○ Emotional behavior and sexual response ■ Anger, aggression, fear, pleasure, contentment, and sexual drive The epithalamus is a small mass composed of: ○ The pineal gland, an endocrine gland ○ Habenula - relay from the limbic system to the midbrain ○ A thin roof over the third ventricle The cerebrum ○ Cerebrum - develops from the telencephalon and is the largest, most conspicuous part of the human brain ■ The seat of sensory perception, memory, thought, judgment, and voluntary motor actions ○ Gross anatomy ■ Two cerebral hemispheres divided by longitudinal cerebral fissure ● Connected by white fibrous tract, the corpus callosum ● Gyri and sulci increase the amount of cortex in the cranial cavity, allowing for more information processing capability ● Each hemisphere has give distinct lobes; four of them are named for the cranial bones overlying them ■ ■ ■ ■ ■ Frontal lobe ● Rostral to central sulcus ● Voluntary motor functions, motivation, foresight, planning, memory, mood, emotion, social judgement, and aggression Parietal lobe ● Between central sulcus and parieto-occipital sulcus ● Integrates general senses, taste, and some visual information Occipital lobe ● Caudal to parieto-occipital sulcus ● Primary visual center of the brain Temporal lobe ● Lateral and horizontal; below lateral sulcus ● Function in hearing, smell, learning, memory, and some aspects of vision and emotion Insula ● Deep to lateral sulcus ● Helps in understanding spoken language, taste, and integrating information from visceral receptors The five lobes of the cerebrum and some of their key functions ● The cerebral white matter: ○ Most of the volume of the cerebrum ■ Glia and myelinated nerve fibers that transmit signals ○ Tracts: bundles of nerve fibers in the CNS ○ Three types of tracts: ■ Projection tracts ■ Commissural tracts ■ Association tracts ○ Projection tracts ■ Extend vertically between higher and lower brain and spinal cord centers ○ Commissural tracts ■ Cross from one cerebral hemisphere to the other allowing communication between two sides of cerebrum ■ Most pass through corpus callosum ■ Others: anterior and posterior commissures ○ Association tracts ■ Connect different regions within the same cerebral hemisphere ■ Long association fibers connect different lobes; short association fibers connect gyri within a lobe ● ● The cerebral cortex ○ Layer of gray matter (2-3 mm thick) covering the surface of hemispheres ■ Constitutes about 40% of brain mass ■ Contains 14 to 16 billion neurons ○ Stellate cells - spheroidal cell bodies with short axons and dendrites; receive sensory input and process information on a local level ○ Pyramidal cells - tall and conical with an axon that projects into white matter below; output neurons that connect the cortex to other parts of the CNS ○ 90% of the human cerebral cortex is neocortex - six-layered tissue that has a relatively recent evolutionary origin Histology of the neocortex ● The limbic system ○ Important center of emotion and learning ○ Prominent components, present in each hemisphere ■ Cingulate gyrus - arches over corpus callosum in frontal and parietal lobes ■ Hippocampus - in the medial temporal lobe ■ Amygdala - immediately rostral to the hippocampus ○ Components are connected through a loop of fiber tracts allowing for somewhat circular patterns of feedback ○ Limbic system structures have centers for reward and aversion ● The basal nuclei ○ Masses of cerebral gray matter buried deep in the white matter, lateral to the thalamus ○ Involved in motor control ○ Basal nuclei include: ■ Caudate nucleus ■ Putamen ■ Globus pallidus ○ All three are collectively called corpus striatum due to their striped appearance ○ Putmen and globus pallidus together are called the lentiform nucleus (lens-shaped) Processing ● Cognition - the range of mental processes by which we acquire and use knowledge ○ Sensory perception, thought, reasoning, judgment, memory, imagination, and intuition ● Accomplished by widely distributed association areas of the cerebral cortex ○ Association areas constitute about 75% of all brain tissue ● Learn about cognition from studies of patients with brain lesions and from imaging studies using PET and fMRI ● Example cognitive functions in association areas of the cortex: ○ The parietal lobe helps perceive and attend to stimuli ■ Lesions can cause contralateral neglect syndrome - unaware of objects on opposite sides of the body ○ The temporal lobe helps identify stimuli ■ Lesions can cause agnosia - inability to recognize, identify familiar objects; example: prosopagnosia - cannot recognize faces ○ The frontal lobe helps us think about the world and plan and execute appropriate behaviors ● Memory ○ Information management by the brain involves: ■ Learning: acquiring new information ■ Memory: information storage and retrieval ■ Forgetting: eliminating trivial information; as important as remembering ○ Amnesia - defects in explicit memory: inability to describe past events ■ Anterograde amnesia - unable to store new information ■ Retrograde amnesia - a person cannot recall things known before the injury ○ Brain structures involved in memory: ■ Hippocampus - important limbic system area for memory ● Functions in memory consolidation - the process of “teaching the cerebral cortex” until a long-term memory is established in the cortex ● Organizes cognitive information into a unified long-term memory but does not hold the memory itself ■ The cerebellum is involved in learning motor skills ■ The amygdala plays a role in emotional memory ● Emotion ○ Emotional feelings and memories involve interactions between the prefrontal cortex and the diencephalon ○ The prefrontal cortex (frontal association area) - the seat of judgment, intent, and control over expression of emotions ○ Feelings arise from deeper brain regions: hypothalamus and amygdala ○ Amygdala roles in emotion: ■ Especially important in fear, but also affects food intake, sexual behavior, and drawing attention to novel stimuli ■ ■ ■ ● Receives input from sensory systems Outputs: ● To the hypothalamus, influencing somatic and visceral motor systems- heart races, blood pressure rises, hair stands on end, or vomiting ensues ● To prefrontal cortex, is important in controlling the expression of emotions - the ability to express love, control anger, or overcome fear Behavior is shaped by learning associations between stimuli, our responses to them, and the reward or punishment that results Sensation ○ Primary sensory cortex - sites where sensory input is first received and one becomes conscious of the stimulus ○ Association areas near primary sensory areas process and interpret that sensory information ■ Example: The primary visual cortex is bordered by visual association areas, making cognitive sense of visual stimuli ○ Multimodal association areas receive input from multiple senses and integrate this into an overall perception of our surroundings ■ Example: The orbitofrontal cortex receives taste, smell, and visual input to provide an overall impression of the desirability of a food ○ Special senses - vision, hearing, equilibrium, taste, and smell ■ Vision ● Primary visual cortex - a far posterior region of the occipital lobe; receives visual signals ● Visual association area - borders primary cortex anteriorly and occupies all the remaining occipital lobe ● ○ Much of inferior temporal lobe deals with recognizing faces and familiar objects ■ Hearing ● Primary auditory cortex - superior region of temporal lobe; receives auditory signals ● Auditory association area - temporal lobe, inferior to primary cortex; recognize spoken words, music, voices ■ Equilibrium ● Signals from inner ear project to cerebellum and brainstem nuclei ● Some fibers project to association cortex in roof of lateral sulcus area concerned with consciousness of body movements and orientation in space ■ Taste and smell ● Primary gustatory cortex - inferior end of postcentral gyrus; receives taste signals ● Primary olfactory cortex - medial cortex of temporal lobe; receives smell signals The general (somatosensory, somesthetic, somatic) senses ■ Distributes over the entire body ■ Involve relatively simple receptors include senses of touch, pressure, stretch, movement, heat, cold, and pin ■ Signals of head pass through cranial nerves to brain ■ Signals from the rest of the body ascend sensory tracts of spinal cord ■ Thalamus processes the input from contralateral side ■ Selectively relays signals to postcentral gurus of parietal lobe ● Cerebral fold that is immediately causal to the central sulcus ● Functionally known as the primary somatosensory cortex ● Provides awareness of stimulus ■ Somatosensory association area - caudal to the postcentral gyrus and in the roof of the lateral sulcus ● Mkes cognitive sense of stimulus ■ Sensory homunculus - diagram of sensory inputs to the primary somesthetic cortex in parietal lobe ● Resembles upside-down sensory map of contralateral side of the body ■ Somatotopy - point - to - point correspondence between an area of the body and an area of the CNS ● Receptors in lower limbs projecting to superior and medial parts of the gyrus ● Receptors from face projecting to the inferior and lateral parts of the gyrus ○ ● The primary motor cortex (precentral gyrus) Motor control ○ Two principle brain regions involved in voluntary motor control: ■ Motor association (premotor) area - location where the intention to contract a muscle begins; found in frontal lobes ● Where we plan our behavior ● Where neurons compile a program for degree and sequence of muscle contraction required for an action ■ Program transmitted to neurons of the precentral gyrus (primary motor area) ● Most posterior gyrus of the frontal lobe ● These neurons send signals to the brainstem and spinal cord leading ultimately to muscle contraction ○ The precentral gyrus also exhibits somatotopy; diagrammed as a motor homunculus ○ Neurons for toe movement are deep in the longitudinal fissure of the medial side of the gyrus ○ The summit of the gyrus control the trunk, shoulder, and arm ○ The inferolateral region controls the facial muscles ○ Homunculus looks distorted because the amount of cortex devoted to a given body region is proportional to the number of muscles and motor units in that body region (not body region size) ○ Boundaries of cortical areas controlling body regions overlap, not sharply defined- a muscle is controlled by neurons at several points within a general area of the gyrus ○ The primary somatosensory cortex (postcentral gyrus) ○ ○ ○ Neurons involved in motor control: ■ Upper motor neurons - pyramidal cells of the precentral gyrus ● Their fibers project caudally ● About 19 million fibers ending in nuclei of the brainstem ● About 1 million forming the corticospinal tracts ● Most fibers decussate in lower medulla oblongata ● Form lateral corticospinal tracts on each side of the spinal cord ■ In the brainstem or spinal cord, the diners from upper motor neurons synapse with lower motor neurons whose axons innervate skeletal muscles Basal nuclei and cerebellum are also important in muscle control ■ Motor functions of basal nuclei: ● Onset and cessation of intentional movements ● Repetitive hip and shoulder movements in walking ● Highly practiced, learned behaviors such as writing, typing, or driving a car ● Lie in a feedback circuit from the cerebrum and substantia nigra of midbrain, to the basal nuclei, to the thalamus, and back to the cerebrum and midbrain ● Dyskinesias - movement disorders caused by lesions in the basal nuclei involving abnormal movement initiation (Parkinson's) Other brain regions involved in motor control ■ The cerebellum is highly important in motor coordination ● Aids in learning new motor skills ● Maintains muscle tone and posture ● Smooths muscle contraction ● Coordinates eye and body movements ● Coordinates motions of different joints with each other ● Lesions can cause ataxia - clumsy, awkward gait ■ Motor pathways involving the cerebellum ● Language ○ Different brain regions are responsible for components of language: reading, writing, speaking, and understanding words ○ Posterior speed area (Wernicke area) - posterior to the lateral sulcus, usually in the left hemisphere; recognition of spoken and written language ○ Motor language area (Broca area) - the inferior prefrontal cortex, usually in the left hemisphere; generates a motor program for the muscles of the larynx, tongue, cheeks, and lips for speaking and for hands when signing ■ Transmits program to primary motor cortex for commands to the lower motor neurons that supply relevant muscles ○ When we intend to speak, Wernicke area formulates phrases and transmits plan of speech to Broca area ○ Broca area transmits program to primary motor cortex for commands to the lower motor neurons that supply relevant muscles ○ Emotional aspect of language controlled by regions in opposite hemispheres ■ Affective language area usually in right hemisphere ■ Lesions produce prosody - flat, emotionless speech ○ Aphasia - any language deficit from lesions in Wernicke, Broca areas ● Cerebral lateralization ○ Cerebral lateralization - difference in structure, function between the two cerebral hemispheres ○ Neither hemisphere is “dominant” but each is specialized for certain tasks ○ Everyone uses the two hemispheres about equally ■ The idea that some people are “left­­brained” (such as a mathematician or scientist) and others “right­­brained” (such as a musician or artist) is only a discredited popular myth ○ ○ ○ Left hemisphere - usually the categorical hemisphere ■ Specialized for spoken and written language ■ Sequential and analytical reasoning (math and science) ■ Breaks information into fragments and analyzes it Right hemisphere - usually the representational hemisphere ■ Perceives information in a more integrated way ■ Seat of imagination and insight ■ Musical and artistic skill ■ Perception of patterns and spatial relationships ■ Comparison of sights, sounds, smells, and taste Lateralization is correlated with handedness, age, and sex ■ Left hemisphere is the categorical hemisphere in 96% of right-handed people and 70% of left-handed people ■ Lateralization develops with age ● Children are more resilient to lesions on one side ■ Males exhibit more lateralization than females and suffer more functional loss when one hemisphere is damaged