Unit 4 Part 1.pdf

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 Univ...

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