KAAP309 Unit 4 Notes PDF

Textbook Chapters: 12.1-12.6, 13.1, 13.2, 13.3, 14.1-14.5, 14.6, 15.1- 15.4, 16.1, 16.2, 16.3 16.5 11/7/23 and 11/9/23 Nervous Tissue Overview of the nervous system ● Utilizes neurons to send signals ○ Receptor → processor → effector ● Two major divisions ○ Central nervous system (CNS) ■ Brain and...

Textbook Chapters: 12.1-12.6, 13.1, 13.2, 13.3, 14.1-14.5, 14.6, 15.1- 15.4, 16.1, 16.2, 16.3 16.5 11/7/23 and 11/9/23 Nervous Tissue Overview of the nervous system ● Utilizes neurons to send signals ○ Receptor → processor → effector ● Two major divisions ○ Central nervous system (CNS) ■ Brain and spinal cord ○ Peripheral nervous system (PNS) ■ Nerves and ganglia Functional divisions of PNS Universal properties of neurons ● Excitability - ability to respond to stimuli ● Conductivity - produce electrical signals that are conducted to other cells ● Secretion - when signal reaches end of axon, the neuron secretes a neurotransmitter that stimulates the next cell Functional classes of neurons Structure of a neuron ● Soma ○ Cell body ○ Nucleus and organelles ● Neurites (extensions) ○ Dendrites ■ Signal reception ○ Axons ■ Signal transmission Classes of Neurons ● Mutlipolar ● Bipolar ● Unipolar ● Anaxonic Axonal transport ● Two-way movement ○ Anterograde transport - movement away from the cell body, down the axon ○ Retrograde transport - movement up the axon, toward the cell body ● Speed ○ Fast = 200-400 mm/day, anterograde and retrograde ○ Slow = 0.2-0.5 mm/day, always anterograde Supportive cells Glia in CNS ● Oligodendrocytes ● Ependymal cells ● Microglia ● Astrocytes Glia in PNS ● Schwann cells, or neurolemmocytes - envelop axons of PNS, form myelin sheath, and assist in the regeneration of damaged fibers ● Satellite cells - surround nerve cell bodies in the ganglia of PNS; provide insulation around cell body and regulate chemical environment Myelin ● Spiral layers of insulation around an axon ○ Formed by Schwann cells in PNS, oligodendrocytes in CNS ○ 20% protein and 80% lipid Myelin in CNS ● Oligodendrocytes Myelin segmentation ● Myelin sheath gap (node of ranvier) ● Internodal segments ● Initial segment ● Trigger zone Unmyelinated axons Conduction speed of axons ● The speed at which a nerve signal travels down an axon depends on 2 factors ○ Diameter: larger axons have more surface area and conduct signals more rapidly ○ Presence or absence of myelin: myelin speeds signal conduction ○ Examples: ■ Small, unmyelinated fibers: about 0.5 to 2.0 m/s ■ Small, lightly myelinated fibers: 3 to 15.0 m/s ■ Large, myelinated fibers: up to 120 m/s Electrophysiology of neurons Ionic basis of the resting membrane potential Potentials ● Sensory neurons can be stimulated by chemicals, light, heat, or mechanical forces ● Stimulation triggers local, temporary change in membrane potential ○ Temporary, short-range change in voltage is a local potential ■ Graded ■ Decremental ■ Reversible ○ Can be either excitatory or inhibitory ■ Depolarization is excitatory ■ Hyperpolarization is inhibitory Action potential Actions of the sodium and potassium channels during an action potential Characteristics of action potentials ● All or none law - if threshold reached, neuron fires up to maximum voltage; if threshold not reach, it does not fire ● Non-decremental - do not get weaker with distance ● Irreversible - once started, an action potential travels all the way down the axon cannot be stopped The refractory period Signal conduction in nerve fibers ● Continuous conduction Saltatory conduction Synapse —----------------------------------------------------------------------------> ● Point where an axon terminal meets the next cell (another neuron, gland cell, muscle cell) ○ For neuron-to-neuron synapses: ■ Action potential arrives at the end of the axon of presynaptic neuron ■ Presynaptic neuron releases neurotransmitters ■ The postsynaptic neuron responds to it Chemical synapse structure ● Synaptic cleft - gap between presynaptic neuron and postsynaptic neuron; typically only 20 um wide ○ Cell adhesion molecules (CAMs) reach into the cleft ■ Lik the two neurons together ● Axon terminal of presynaptic neuron contains synaptic vesicles containing neurotransmitter ● The postsynaptic neuron membrane contains a postsynaptic density of neurotransmitter receptors and ion channels ○ Ligand-gated ion gates open when neurotransmitters bind to them Neurotransmitters ● Neurotransmitters are released at chemical synapses ● There are also electrical synapses ○ Occur between some neurons, neuroglia, and cardiac and single-unit smooth muscle ○ Gap junctions join adjacent cells; electrical signals spread directly from cell to cell ○ Advantage- much faster; no delay for release, diffusion, and binding of neurotransmitter ○ Disadvantage - cannot integrate information Neurotransmitter classification ● Purines gases Synaptic transmission ● Synapses are variable in their modes of action ○ Some neurotransmitters are excitatory, others inhibitory, and sometimes a transmitter’s effect differs depending on the type of receptor on the postsynaptic cell ○ Some receptors are ligand-gated ion channels; others act through intracellular second messengers ● Examples of three kinds of synapses: ○ Excitatory cholinergic synapse ○ Inhibitory GABA-ergic synapse ○ Excitatory adrenergic synapse Transmission at excitatory or inhibitory synapse ● An excitatory cholinergic synapse ○ Cholinergic synapse - acetylcholine (ACh) is the neurotransmitter ○ Entry of Na+ causes depolarizing postsynaptic potential ● An inhibitory GABA-ergic synapse ○ GABA-ergic synapse - y-aminobutyric acid (GABA) is the neurotransmitter ○ Cl- entry hyperpolarizes the postsynaptic membrane Transmission at adrenergic synapse ● Excitatory ● Norepinephrine (NE) is the neurotransmitter ● Uses secondary messenger ○ G-protein coupled receptors ○ Activates cyclic AMP (cAMP) ● Slower response Cessation of the signal —-----------------------------------------------------> ● Neurotransmitter stays bound to receptor for about 1 ms ● Clearance once signal stops ○ Neurotransmitter degradation ○ Reuptake ○ Diffusion Neural integration Integration ● The ability to process, store, and recall information and use it to make decisions ● Chemical synapses allow for decision-making ● Brain cells are incredibly well connected, allowing for complex integration ○ Pyramidal cells of cerebral cortex have about 40,000 contacts with other neurons ● Trade-off: chemical transmission involves a synaptic delay that makes information travel slower than it would if there was no synapse Postsynaptic potentials ● Excitatory postsynaptic potential (EPSP) ● Inhibitory postsynaptic potential (IPSP) Summation ● The process of adding up postsynaptic potentials and responding to their net effect ● Occurs in the trigger zone ● Some incoming nerve fibers may produce EPSPs while others produce IPSPs ● ● A neuron’s response depends on whether the net input is excitatory or inhibitory The balance between EPSPs and IPSPs enables the nervous system to make decisions Facilitation ● Neurons can work in groups to modify each other’s actions ○ Presynaptic facilitation - occurs when one presynaptic neuron enhances another one ○ Presynaptic inhibition - occurs when one presynaptic neuron suppresses another one Neural processing ● Serial processing - neurons and neural pools relay information along pathways in a relatively simple linear fashion ○ Can process only one flow of information at a time ● Parallel processing - information is transmitted along diverging circuits through different pathways that act on it simultaneously, for different purposes 11/14/23 Central Nervous System The brain ● Directional terms ○ Rostral - toward the forehead ○ Caudal - toward the spinal cord ○ Major portions of the brain: forebrain, cerebellum, and brainstem ○ Cerebrum is largest part of the forebrain, 83% brain volume ○ Cerebral hemispheres - pair of half globes of cerebrum ○ Gyri - thick folds on cerebrum surface ○ Sulci - shallow grooves between gyri ○ Longitudinal cerebral fissure - deep groove that separates cerebral hemispheres ○ Corpus callosum - thick nerve bundle at bottom of longitudinal fissure that connects hemispheres ○ Cerebellum is second-largest part of brain ■ Located in posterior cranial fossa ■ Separated from cerebrum by transverse cerebral fissure ■ Also contains fissures, sulci, and gyri (folia) ○ Brainstem is the rest of the brain ■ Includes midbrain, pons, and medulla oblongata Surace anatomy of the brain Medial aspect of the brain Gray and white matter ● The brain is composed of gray and white matter ● Gray matter contains nerve cell bodies, dendrites, and synapses ○ Cortex - surface layer of gray matter in cerebrum, cerebellum ○ Nuclei - deeper masses of gray matter, surrounded by white matter ● White matter composed of tracts - bundles of nerve fibers (axons) ○ Deep to cortical gray matter in brain ○ Connect one part of the brain to another and to the spinal cord Mininges ● Three membranes surrounding brain and spinal cord ○ Lie between the nervous tissue and bone ○ Protect the brain and provide structural framework for its arteries and veins ○ From outermore to innermost: ■ Dura mater ■ Arachnoid mater ■ Pia mater ○ The cranial dura mater is comprised of 2 layers ■ Outer periosteal layer - equivalent to periosteum of cranial bones ■ Inner meningeal layer - continues into vertebral canal and forms dural sheath around spinal cord ○ Dural sinuses - spaces located where periosteal and meningeal layers separate ■ Collect blood circulating through the brain ■ Superior sagittal sinus - just under calvaria along median line ■ Transverse sinus - runs horizontally from rear of head toward each ear ○ Dura mater presses closely against cranial bones ■ No epidural space (unlike spinal cord) ■ Not directly attached to bone except around foramen magnum, sella turcica, crista galli, and sutures of the skull ○ Folds of dura mater extend inward, separate some brain regions ■ Falx cerebri separates the two central hemispheres ■ Tentorium cerebelli separates cerecum from cerebellum ■ Falx cerebelli separates the right and left halves of the cerebellum ○ Arachnoid mater ■ Transparent membrane over the brain surface subarachnoid space separates it from the pia mater below: filled with cerebrospinal fluid and blood vessels ○ Pia mater ■ Very thin membrane, not usually visible without a microscope ■ Follows all contours of the brain ■ Follows arteries as they penetrate into cerebrum The meninges of the brain Ventricles and cerebrospinal fluid ● Ventricles - four internal, fluid-filled chambers of the brain ○ ○ ○ ○ ○ ○ ○ ● Two lateral ventricles (one in each cerebral hemisphere) Third ventricle - narrow medial space beneath corpus callosum Fourth ventricle - small triangular chamber between pons and cerebellum The ventricles are connected Interventricular forament - pore that connects lateral ventricles to third ventricle Cerebral aqueduct - tube running through the midbrain that connect the third ventricle to the fourth ventricle Central canal - tube that connect to fourth ventricle and runs through the center of the spinal cord Cerebrospinal fluid (CSF) - clear, colorless liquid that filled the ventricles, canals of CNS and bathes its external surface ○ Production of CSF begins with filtration of blood plasma through capillaries of the brain ○ Choroid plexus - spongy mass of blood capillaries on the floor of each ventricle ○ Ependymal cells - neuroglia that lines ventricles and covers choroid plexus ■ Ependymal cells mody the filtrate ■ Compared to plasma, CSF has more sodium and chloride, less potassium, calcium, glucose, and very little protein ○ CSF is continuously flowing through the CNS ■ Driven by its own pressure, beating of ependymal cilia, and pulsations of the brain produced by each heartbeat ■ Path through ventricles: ● Secreted in lateral ventricles ● Through intervertebral foramina into third ventricle ● Down the cerebral aqueduct into the fourth ventricle ● Third and fourth ventricles add more CSF along the way ■ All CSF ultimately escapes through three pores that lead into subarachnoid space of brain and spinal cord surface ● Median aperture ■ ○ ● Two lateral apertures CSF is reabsorbed by arachnoid granulations ● Cauliflower­shaped extensions of the arachnoid meninx ● Protrude through dura mater into superior sagittal sinus ● CSF penetrates the walls of the villi and mixes with the blood in the sinus Functions of CSF: ■ Buoyancy ● Allows brain to attain considerable size without being impaired by its own weight ● If brain rested heavily on floor of cranium, pressure would kill the nervous tissue ■ Protection ● Protects brain from striking cranium when head is jolted ● Shaken child syndrome and concussions still occur from severe jolting ■ Chemical stability ● Flow of CSF rinses away metabolic wastes from nervous tissue and homeostatically regulates chemical environment The hindbrain and midbrain The medulla oblongata ● Adult brain region that develops from embryonic myelencephalon ● Begins at foramen magnum of skull ● ● ● Extends about 3 cm rostrally and ends at a groove just below pons Slightly wider than spinal cord Anatomical features: ○ Pyramids - ridges on anterior surface, resemble side-by-sude baseball bats ■ Separated by anterior median fissure ○ Four parts of cranial nerves begin or end in medulla VIII (in part), IX, X, XII ○ Olives - prominent bulges lateral to each pyramid ○ Gracile and cuneate fasciculi of spinal cord continue as two pairs of ridges on posterior medulla ■ Contains sensory fibers; synapse in gracile and cuneate nuclei The brainstem ○ All ascending and descending fibers connecting the brain and spinal cord pass through the medulla ■ Medial lemniscus: axons of gracile and cuneate nuclei descussate and form ascending (sensory) tract to thalamus ■ Corticospinal tracts - descending motor tracts in pyramids; carry signals down to skeletal muscles ○ Medulla contains numerous nuclei ■ Inferior olivary nucleus - relay center for signals to cerebellum ■ Reticular formation - loose network of nuclei extending through the entire brainstem; contains cardiac center, vasomotor center, and respiratory centers Cross sections of the brainstem (medulla oblongata) The pons ● Adult brain region that develops from embryonic metencephalon ● Measures 2.5 cm ● Broad anterior bulge rostral to medulla ● Posteriorly, consists of thick stalks (cerebellar peduncles) ● Communication center to the cerebellum ● Anatomical features: ○ Cerebellar peduncles—thick stalks on posterior pons that connect it (and the midbrain) to the cerebellum ● Cranial nerves 5, 6, 7, and 8 ○ Sensory roles: hearing, equilibrium, taste, facial sensations ○ Motor roles: eye movement, facial expressions, chewing, swallowing, urination, and secretion of saliva and tears ● Reticular formation in pons contains additional nuclei concerned with sleep, respiration, posture Medial aspect of the brain (pons) Cross sections of the brainstem (pons) The midbrain ● Brain region that develops from embryonic mesencephalon ○ Short segments of the brainstem that connects the hindbrain to the forebrain ● Anatomical features: ○ Cerebral aqueduct ○ Surrounded by central (periaqueductal) gray substance involved in pain awareness ○ Continuations of medial lemniscus and reticular formations ○ Motor nuclei of two cranial nerves that control eye movements: CN III (oculomotor) and CN IV (trochlear) ○ Tectum—roof­like part of the midbrain posterior to cerebral aqueduct ■ Tectum has four bulges: ● Two superior colliculi—visual attention, tracking moving objects, and some reflexes ● Two inferior colliculi—relays signals from inner ear to thalamus and other parts of the brain ○ Cerebral peduncles—two anterior midbrain stalks that anchor the cerebrum to the brainstem ■ Each peduncle has three parts: tegmentum, substantia nigra, and cerebral crus ○ Tegmentum ■ Within cerebral peduncle; dominated by red nucleus ■ Pink color due to high density of blood vessels ■ Connections go to and from cerebellum for motor control ○ Substantia nigra ■ Nucleus within peduncle; dark nucleus pigmented with melanin ■ Motor center that relays inhibitory signals to thalamus and basal nuclei suppressing unwanted body movement ■ Degeneration of neurons leads to tremors of Parkinson’s disease ○ Cerebral crus ■ Bundle of nerve fibers that connect cerebrum to pons ■ Carries corticospinal tracts ○ Medial aspect of the brain (midbrain) Cross section of the brainstem (midbrain) The reticular formation ● Loose web of gray matter that runs vertically through all levels of the brainstem and into the upper spinal cord ● Occupies space between white fiber tract and brainstem nuclei ● Has connections with many areas of the cerebrum ● Consists of more than 100 small nuclear without distinct boundaries ● Functions of reticular formation nuclei ○ Somatic motor control ■ Adjust muscle tension to maintain tone, balance, and posture, especially during body movements ■ ■ ○ ○ ○ ○ Relay signals from eyes to ears to cerebellum Integrate visual, auditory, balance, and motion stimuli into motor coordination ■ Gaze centers - allows eyes to track and fixate on objects ■ Central pattern generators - neural pools that produce rhythmic signals to the muscles of breathing and swallowing Cardiovascular control ■ Cardiac and vasomotor centers of medulla oblongata Pain modulation ■ Some pain signals ascend through the reticular formation ■ Some descending analgesic pathways begin in the reticular formation, end in the spinal cord where they block transmission of pain signals Sleep and consciousness ■ Reticular formation plays a central role in consciousness, alertness, and sleep ■ Injury here can result in irreversible coma Habituation ■ Reticular activating system modulates activity in cerebral cortex so that it ignores repetitive, inconsequential stimuli 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 The Spinal Cord and Peripheral Nervous System Functions of the spinal cord ● Conduction - nerve fibers conduct sensory and motor information up and down the spinal cord ● Neural integration - spinal neurons receive input from multiple sources, integrate it, and execute appropriate output (for example, bladder control) ● Locomotion - spinal cord contains central pattern generators: groups of neurons that coordinate repetitive sequences of contractions for walking ● Reflexes - involuntary responses to stimuli that are vital to posture, coordination, and protection Spinal cord anatomy ● Divided into the cervical, thoracic, lumbar, and sacral regions ● Two areas of the cord are thicker than elsewhere: ○ Cervical enlargement ○ Lumbosacral enlargement ● Medullary cone (conus medullaris) ● Cauda equina Cross-section of the spinal cord ● Gray matter ○ Two posterior horns - sensory ○ Two anterior horns - motor ○ Lateral horns T2-L1 - ANS sympathetic ○ Commissural fibers connect two sides ● White matter ○ Bundles of axons - posterior, lateral, and anterior funiculus on each side ■ Subdivided into tracts Spinal tracts Ganglion ● Cluster of nerve cell bodies outside the CNS Spinal Nerves ● 31 pairs ○ 8 cervical (C1 to C8) ○ 12 thoracic (T1 to T12) ○ 5 lumbar (L1 to L5) ○ 5 sacral (S1 to S5) ○ 1 coccygeal (Co1) ● 5 plexuses ○ Cervical (C1 to C5) ○ Brachial (C5 to T1) ○ Lumbar (L1 to L4) ○ Sacral (L4 to S4) ○ Dermatome Coccygeal (S4 to Co1) Autonomic Nervous System (ANS) General properties of ANS ● A motor nervous system that controls glands, cardiac muscles, and smooth muscle ○ The primary target organs of the ANS ■ Viscera of thoracic and abdominal cavities ■ Some structures of the body wall - cutaneous blood vessels, sweat glands, and arrector muscles ○ Carries out actions involuntary Divisions of ANS ● Sympathetic division ○ Prepares the body for physical activity: exercise, trauma, arousal, competition, anger, or fear; fight or flight reaction ○ Increases heart rate, BP, airflow, blood glucose levels, etc; reduces blood flow to the skin and digestive tract ● Parasympathetic division ○ Calms many body functions reducing energy expenditure and assists in bodily maintenance; resting and digesting state ○ Digestion and waste elimination ● Autonomic tone ○ Normal background rate of activity that represents the balance of the two systems according to the body's needs Efferent pathways Sympathetic nervous system ● Thoracolumbar division -------------------------------------------------> ● Short preganglionic/long postganglionic fibers ○ Preganglionic fibers leave T1 - L2 ○ Lead to chain ganglia parallel to spinal cord ■ 3 cervical ■ 11 thoracic ■ 4 lumbar ■ 4 sacral ■ 1 coccygeal ● Neuronal divergence ● Craniosacral division --------------------------------------------------> ● Long preganglionic/short postganglionic fibers ○ Preganglionic fibers leave ■ CN3 ■ CN7 ■ CN9 ■ CN10 ■ S2-S4 ○ End in terminal ganglia near target organ ● Very little neural divergence Enteric plexus ● The nervous network of the digestive tract ○ Composed of 500 million networks neurons found in the walls of the digestive tract (5x more neurons than spinal cord) ○ Unlike the rest of the ANS, it does not arise from the brainstem or spinal cord ○ Like the rest of ANS, it innervates smooth muscle, glands ● Has its own ganglia and reflex arcs ● Regulates motility of the esophagus, stomach, and intestines and secretion of digestive enzymes and acid ● Normal digestive function also requires regulation by sympathetic and parasympathetic systems Neurotransmitter and their receptors ● Opposing effects in ANS ○ Sympathetic and parasympathetic fibers secrete different neurotransmitters (norepinephrine and acetylcholine) ○ The receptors on target cells vary ● Neurotransmitters ○ ACh - all preganglionic fibers and parasympathetic postganglionic ○ Norepinephrine - sympathetic postganglionic ● Receptors ○ Cholinergic ■ Muscarinic and nicotinic ○ Adrenergic ■ Alpha and beta Dual innervation Cortical Pathways Communication tracts Somatic sensory pathways ● Somatotopy = functional map of primary somatosensory cortex ○ Sensory homunculus (“little human”) = somatotope showing the relative size of cortex devoted to any specific body area ○ ● ● Motor homunculus = functional map of primary motor cortex ■ Proportions reflect the number of motor units innervated and degree of fine motor control in corresponding body region Three major somatic sensory pathways ○ Spinothalamic pathway ○ Posterior column pathway ○ Spinocerebellar pathway Spinothalamic pathway ○ Anterior spinothalamic tracts ■ Crude touch and pressure sensations from body ○ Lateral spinothalamic tracts ■ Pain and temperature sensations from body ○ First-order neuron - from receptor to spinal cord; synapses with second-order neuron in posterior gray horns ○ Second-order neuron - from posterior gray horn; crosses spinal cord; ascends to thalamus ○ Third-order neuron - from thalamus to primary sensory cortex ● Posterior column pathway ○ Highly localized (“fine”) touch, pressure, vibration, proprioception ○ From peripheral receptor to primary somatosensory cortex ○ Sensory axons ascend in medial gracile fasciculus and lateral cuneate fasciculus ○ Medial lemniscus - tract leading from the gracile nucleus and cuneate nucleus to the thalamus ● Spinocerebellar pathway ○ Proprioceptive information about position of skeletal muscles, joints, and tendons; goes to the cerebellum ○ Axons of posterior spinocerebellar tracts do not cross to the other side - pass through the inferior cerebellar peduncle on the same side ○ Anterior spinocerebellar tract axons do cross to opposite side of the spinal cord Somatic motor pathways ● Always involve at least two motor neurons ○ Upper motor neuron ■ Cell body in a CNS processing center ○ Lower motor neuron ■ Cell body in a nucleus of brainstem or spinal cord ■ Upper motor neuron synpases on lower motor neuron which then innervates a single motor unit of a skeletal muscle Coricospinal pathway ● Voluntary control of skeletal muscles ● Sometimes called the pyramidal system - upper motor neurons start at pyramidal cells in primary motor cortex ● Upper motor neuron axons descend into brainstem and spinal cord ● Synapse with lower motor neurons that control skeletal muscles Tracts of the corticospinal pathway ● Corticobulbar (bulbar, brainstem) tracts - conscious control of skeletal muscles for eye, jaw, face, some muscles of neck/pharynx ○ Synapse on lower motor neurons in cranial nerve motor nuclei ● Corticospinal tracts - conscious control of skeletal muscles ○ Visible along ventral surface of medulla oblongata as pair of thick bands (pyramids) ■ About 15 percent descend uncrossed as anterior corticospinal tracts; cross over in anterior white commissure before synapsing on lower motor neurons in anterior gray horns Two main pathways for subconscious motor commands ● Commands issues from cerebrum, diencephalon, brainstem ○ Lateral pathway - muscle tone/precise movements of distal limb muscles ■ Upper motor neurons start in red nucleus ■ Axons cross to opposite side of the brain; descend through rubrospinal (ruber, red) tracts ○ Medial pathway - gross movements of trunk, proximal limb ■ Upper motor neurons located in vestibular nuclei, superior and inferior colliculi, and reticular formation ■ Three major sets of tracts in medial pathway ● Reticulospinal tracts ● Vestibulospinal tracts ● Tectospinal tracts ■ Reticulospinal tracts ● Axons of upper motor neurons in reticular formation ● Reticular formation receives input from almost every ascending/descending pathway and has extensive connections with cerebrum, cerebellum, and brainstem ■ Vestibulospinal tracts ● Start at vestibular nuclei of CN VIII (vestibulocochlear nerve) ● Get sensory information from internal ear about head position/movement; send motor commands to alter muscle tone and position of neck, head, eyes, and limbs ■ Tectospinal tracts ● Upper motor neurons in superior/inferior colliculi (midbrain) ● Reflex changed in position of head, neck, upper limb in response to bright light, sudden movement, and loud noises Multiple levels of somatic motor control ● Many nuclei in spinal cord and brain are involved in controlling skeletal muscle contractions ● Generally, the closer the motor center is to the cerebral cortex, the more complex the motor activity ● Cerebellum coordinates motor activities at multiple levels ● ● Preparing for movement ○ Decision to move begins relay of information from: ■ Frontal lobes → premotor cortex → basal nuclei/cerebellum As movement begins: ○ Information is sent from premotor cortex to primary motor cortex ○ Commands are modified by feedback from cerebellum/ basal nuclei ● Effects of primary motor cortex damage ○ Person loses fine motor control over skeletal muscles ○ Some voluntary movements will still be controlled by basal nuclei with input from prefrontal cortex ○ Cerebellum cannot fine-tune movements because corticospinal pathway is inoperative ■ An individual is able to stand, balance, and walk, but movements are hesitant, awkward, and poorly controlled Reflexes Reflexes ● Quick, involuntary, stereotyped reactions of glands or muscle to stimulation ○ Reflexes requiem stimulation ■ Not spontaneous actions, but responses to sensory input ○ Reflexes are quick ■ Involve few, if any, interneurons and minimum synaptic delay ○ Reflexes are involuntary ■ Occur without intent and are difficult to suppress ○ Reflexes are stereotyped ■ Occur essentially the same way every time ○ Reflexes include glandular secretion and contraction of all three types of muscles ■ Somatic reflexes: reflexes involving the somatic nervous system, innervating skeletal muscles ■ Visceral reflexes: reflexes involving organs such as heart and intestines A somatic reflex involves a reflex arc with the following pathway 1. Somatic receptors in skin, muscles, or tendons 2. Afferent nerve fibers carry information from receptors to posterior horn of spinal cord or to the brainstem 3. Integrating center - a point of synaptic contact between neurons in gray matter of cord or brainstem a. Determines whether efferent neurons issue signal to muscles 4. Efferent nerve fibers carry motor impulses to muscles 5. Effectors - the muscles that carry out the response A muscle spindle and its innervation ● Many somatic reflected involve muscle spindles - stretch receptors embedded in skeletal muscles ● Serve as proprioceptors - specialized sense organs to monitor position and movement of body parts ● Inform the brain of muscle length and body movement ● Enable the brain to send motor commands back to the muscles that control coordinated movement, corrective reflexes, muscle tone, and posture Stretch reflex and reciprocal inhibition of the antagonistic muscle ● Stretch (myotactic) reflex - when a muscle is stretched, it “fights back” and contracts ○ Helps maintain equilibrium and posture ○ Head starts to tip forward as you fall asleep, muscles contract to raise the head ○ Stabilize joints by balancing tension in extensors and flexors, smoothing muscle actions ○ Stretch reflex is mediated primarily by the brain, but its spinal component can be more pronounced if muscle is suddenly stretched by a tendon tap (knee jerk) The flexor and crossed extension reflexes ● Flexor reflex - the quick contraction of flexor muscles resulting in the withdrawal of a lumb from an injurious stimulus ○ Triggers contraction of the flexors and relaxation of the extensors in that limb ○ Polysynaptic reflex arc - pathway in which signals travel over many synapses on their way to the muscle ● Crosses extension reflex - contraction of extensor muscles in limb opposite of the one that is withdrawn ○ Maintains balance by extending other leg ○ Flexor reflex uses an ipsilateral reflex arc (stimulus and response on same side) whereas crossed extension reflex uses a contralateral reflex arc (input and output are on opposite sides) ○ Intersegmental reflex - one in which the input and output occur at different levels (segments) of the spinal cord The tendon reflex ● Tendon organs - proprioceptors in a tendon near its junction with a muscle; involved in the tendon reflex ○ Is 0.5mm long, consists of encapsulated bundle of collagen fibers and one or more nerve fibers ● Tendon reflex - response to excessive tension on the tendon ○ Inhibits the muscle from contracting strongly ○ Moderates muscle contraction before it tears the tendon or pulls it loose from the muscle or bone