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Nervous system 1) Nervous systems consist of circuits of neurons and supporting cells 2) Ion pumps and ion channels maintain the resting potential of a neuron 3) Action potentials are the signals conducted by axons 4) Neurons communicate with other cells at synapses 5) The vertebrate nervous system is regionally specialized 6) The cerebral cortex controls voluntary movement and cognitive functions Nervous systems consist of circuits of neurons and supporting cells Human brain - 100 billion neurons Complex information-processing circuits, networks Functional magnetic resonance imaging (fMRI) – view inside A functional magnetic resonance Record of increased blood flow Circuits for different tasks Cambrian explosion (500 mil.) – neurons in their present form Nervous systems consist of circuits of neurons and supporting cells All animals except the sponges No differences in units – neurons Differences – in organization of circuits/networks – nets, nerves, brains Nervous systems consist of circuits of neurons and supporting cells Information processing three stages: sensory input, integration, and motor output Sensory neurons – external and internal stimuli Interneurons – analysis (context, past), integration Motor neurons => effector organs, tissues Nervous systems consist of circuits of neurons and supporting cells Simple nerve circuit => reflex Automatic responses to stimuli Nervous systems consist of circuits of neurons and supporting cells Neuron structure Structure: cell body, dendrites, axon, axon hillock, myelin sheath, synaptic terminal, synapse, neurotransmitter Nervous systems consist of circuits of neurons and supporting cells Neuron structure Up to 100,000 synapses on one neuron Nervous systems consist of circuits of neurons and supporting cells Supporting cells Glia – 10-50 times more than neurons Types: astrocytes, radial glia, oligodendrocytes, Schwann cells Structural support, regulate extracellular concentrations of ions, transmitters Facilitating information transfer – maybe part of learning and memory Increase blood flow Astrocytes => tight junctions between cells around capillaries => blood brain barrier Nervous systems consist of circuits of neurons and supporting cells In embryo, radial glia => tracks for neurons migration Radial glia, astrocytes - stem cells Oligodendrocytes (in CNS), Schwann cells (PNS) => electrical insulation Nervous systems consist of circuits of neurons and supporting cells summary Organization of nervous systems Invertebrate nervous systems range in complexity from simple nerve nets to highly centralized nervous systems having complicated brains and ventral nerve cords. In vertebrates, the central nervous system (CNS) consists of the brain and the spinal cord, which is located dorsally. Information processing Nervous systems process information in three stages: sensory input, integration, and motor output to effector cells. The CNS integrates information, while the nerves of the peripheral nervous system (PNS) transmit sensory and motor signals between the CNS and the rest of the body. The three stages are illustrated in the knee-jerk reflex. Neuron structure Most neurons have highly branched dendrites that receive signals from other neurons. They also typically have a single axon that transmits signals to other cells at synapses. Neurons have a wide variety of shapes that reflect their input and output interactions. Supporting cells Glia perform a number of functions, including providing structural support for neurons, regulating the extracellular concentrations of certain substances, guiding the migration of developing neurons, and forming myelin, which electrically insulates axons. Ion pumps and ion channels maintain the resting potential of a neuron In all cells - across plasma membrane = membrane potential In neurons = between – 60 and – 80 mV Ion pumps and ion channels maintain the resting potential of a neuron The resting potential Na+ gradient = 150/15 = 10 K+ = 5/150 = 1/30 maintained by sodium-potassium pump Ion pumps and ion channels maintain the resting potential of a neuron The resting potential Two model membranes - ion channels selective for K+ only or for Na+ only Start = 150 : 5 mM KCl 15 : 150 mM NaCl Equilibrium potential (Eion) - given by Nernst equation For K+ = - 92 mV (at 37°C) For Na+ = + 62 mV real neuron - many channels open for K few for Na → closer to K value than to Na Ion pumps and ion channels maintain the resting potential of a neuron Gated ion channels Open channels – ungated Gated channels – open or close in response to stimuli 1) Stretch-gated – membrane mechanically deformed 2) Ligand-gated – neurotransmitter binds to channel (synapses) 3) Voltage-gated – by potential changes (axons) Ion pumps and ion channels maintain the resting potential of a neuron - summary Every cell has a voltage across its plasma membrane called a membrane potential. The inside of the cell is negative relative to the outside. The resting potential The membrane potential depends on ionic gradients across the plasma membrane: The concentration of Na+ is higher in the extracellular fluid than in the cytosol, while the reverse is true for K+. A neuron that is not transmitting signals contains many open K+ channels and fewer open Na+ channels in its plasma membrane. The diffusion of K+ and Na+ through these channels leads to the separation of charges across the membrane, producing the resting potential. Gated Ion channels Gated ion channels open or close in response to membrane stretch, the binding of a specific ligand, or a change in the membrane potential. Action potentials are the signals conducted by axons Stimuli influence the gated channels Hyperpolarization – opening of gated K+ channels → near to EK = – 92 mV Depolarization – opening of gated Na+ channels → near to ENa = 62 mV Graded potential – correlation to the stimulus Graded up to threshold → Action potential = all-or-none phenomenon (1-2 msec) Action potentials are the signals conducted by axons Action potential – voltage-gated channels opened - Na+ channels before K+ Na+ channels before K+ 1) Resting state – activation gates closed 2) Depolarization – stimulus opens Na+ channels 3) Rising phase – most Na+ channels open 4) Falling phase – Na+ channels closed, K+ opened 5) Undershoot – K+ channel opened, then closed => resting state 4,5 phase – second depolarization unable – refractory period Action potentials are the signals conducted by axons Conduction of action potentials Regenerating itself along axon Depolarization => repolarization => next segment depolarization => one direction movement Action potentials are the signals conducted by axons Several factors affect speed of AP conduction The larger the axon’s diameter, the faster the conduction – solution in invertebrates (squids, some arthropods) - from several cm to 100 m/sec (giant axons) In vertebrates – insulation by myelin sheath => AP jumps from node to node = saltatory conduction (up to 120 m/sec) Conduction – myelinated, 20 µm in diameter = 1 mm giant axon Action potentials are the signals conducted by axons - summary An increase in the magnitude of the membrane potential is called a hyperpolarization; a decrease in magnitude is called a depolarization. Changes in membrane potential that vary with the strength of a stimulus are known as graded potentials Production of action potentials An action potential is a brief, all-or-none depolarization of a neuron’s plasma membrane. When a graded depolarization brings the membrane potential to the threshold, many voltage-gated Na+ channels open, triggering an influx of Na+ that rapidly brings the membrane potential to a positive value. The membrane potential is restored to its normal resting value by the inactivation of Na+ channels and by the opening of many voltage-gated K+ channels, which increases K+ efflux. A refractory period follows the action potential, corresponding to the interval when the Na+ channels are inactivated. Conduction of action potentials An action potential travels from the axon hillock to the synaptic terminals by regenerating itself along the axon. The speed of conduction of an action potential increases with the diameter of the axon and, in many vertebrate axons, with myelination. Action potentials in myelinated axons jump between the nodes of Ranvier, a process called saltatory conduction. Neurons communicate with other cells at synapses In most cases – AP is not transmitted, only in electrical synapses - gap junctions In rapid stereotypical behaviors (in lobsters escape reaction) Vast majority - chemical synapses – neurotransmitter released Synaptic vesicles in synaptic terminals Neurons communicate with other cells at synapses AP at synaptic terminal depolarizes membrane => opens voltage-gated calcium channels => Ca2+ inside causes fuse of vesicles and membrane => exocytosis into synaptic cleft - adaptable connection Direct synaptic transmission (indirect - over second messenger – slower, long-lasting) Neurotransmitter to ligand-gated ion channels => postsynaptic potential (PSP) Na+ and K+ channels => depol. Excitatory PSP (EPSP) K+ channels => hyperp. Inhibitory PSP (IPSP) Acetylcholine degraded by cholinesterase Neurons communicate with other cells at synapses Summation of postsynaptic potentials PSP - graded Single EPSP usually too small => temporal and spatial summation => AP at hillock Neurons communicate with other cells at synapses One substance - more receptors => different effects Neurons communicate with other cells at synapses – summary In an electrical synapse, electrical current flows directly from one cell to another via a gap junction. In a chemical synapse, depolarization of the synaptic terminal causes synaptic vesicles to fuse with the terminal membrane and to release neurotransmitter into the synaptic cleft. Direct synaptic transmission The neurotransmitter binds to ligand-gated ion channels in the postsynaptic membrane, producing an excitatory or inhibitory postsynaptic potential (EPSP or IPSP). After release, the neurotransmitter diffuses out of the synaptic cleft, is taken up by surrounding cells, or is degraded by enzymes. A single neuron has many synapses on its dendrites and cell body. Whether it generates an action potential depends on the temporal and spatial summation of EPSPs and IPSPs at the axon hillock. Indirect synaptic transmission The binding of neurotransmitter to some receptors activates signal transduction pathways, which produce slowly developing but long-lasting effects in the postsynaptic cell. Neurotransmitters The same neurotransmitter can produce different effects on different types of cells. Major known neurotransmitters include acetylcholine, biogenic amines (epinephrine, norepinephrine, dopamine, and serotonin), various amino acids and peptides, and the gases nitric oxide and carbon monoxide. The vertebrate nervous system is regionally specialized Vertebrates – cephalization Distinct CNS a PNS components Spinal cord Simple responses – reflexes Conveys information from and to brain Segmental ganglia outside The vertebrate nervous system is regionally specialized CNS – derived from dorsal embryonic nerve cord – hollow => in adult – Central canal, four ventricles of brain, between two meninges – cerebrospinal fluid Supply of nutrients and hormones, removal of wastes White matter – myelinated axons Gray matter – dendrites, unmyelinated axons, cell bodies The vertebrate nervous system is regionally specialized Peripheral nervous system Mammals – 12 pairs of cranial nerves, 31 pairs of spinal nerves PNS – two functional components 1) Somatic nervous system – signals to and from skeletal muscles, external stimuli 2) Autonomic nervous system – regulates internal environment – controlling smooth and cardiac muscles, internal organs, tissues The vertebrate nervous system is regionally specialized Autonomic nervous system: Sympathetic, parasympathetic, enteric division 1) Sympathetic Arousal + energy generation “fight-or-flight” response 2) Parasympathetic Self-maintenance function “rest and digest” 3) Enteric – semiindepend. to 1 and 2 network of neurons secretion, peristalsis The vertebrate nervous system is regionally specialized Embryonic development of the brain In all vertebrates - three anterior bulges of the neural tube become evident The vertebrate nervous system is regionally specialized The brainstem Evolutionarily – old part of the brain Medulla oblongata – centers control visceral (automatic, homeostatic) functions – breathing, heart and blood vessel activity, swallowing, vomiting, digestion Pons – regulates breathing centers in medulla Transmission of information - sensory axons to and motor from higher brain Coordination of large-scale body movements – walking, changing of sides of axons Midbrain – centers of auditory and visual systems In mammals – visual reflexes (e.g. turning head automatically) The vertebrate nervous system is regionally specialized The brainstem – arousal and sleep Arousal – awareness of the external word; Sleep – receiving external stimuli without awareness Reticular formation (RF) – network of neurons in brainstem Part of RF – reticular activating system (RAS) – regulates arousal x sleep, Stimulation of centers in pons and medulla => sleep neurotransmitter - serotonin (consolidation of learning and memory?) Midbrain – stimulation => arousal Sensory filter – always same stimuli can be ignored The vertebrate nervous system is regionally specialized The cerebellum – from part of metencephalon Integrates sensory information - from organs of equilibrium, visual system, motor commands, length of muscles Coordinates movement and balance Involved in learning and remembering motor skills The vertebrate nervous system is regionally specialized The diencephalon – epithalamus, thalamus, hypothalamus Epithalamus – pineal glad (melatonin) and choroid plexus (capillaries producing cerebrospinal fluid from blood) Thalamus – input center for sensory information, sorting, sending to cerebrum output center for motor information from cerebrum Hypothalamus – homeostatic regulation neurosecretory cells, body thermostat, centers regulating hunger, thirst, sexual, mating behavior, centers for pleasure, rage The vertebrate nervous system is regionally specialized Circadian rhythms – sleep/wake, hormone release, sensitivity, hunger Biological clock - in mammals in suprachiasmatic nucleus (SCN), in Drosophila on wings In human – 24 hours 11 minutes Synchronization with natural light/dark cycles The vertebrate nervous system is regionally specialized The cerebrum – in vertebrate evolution a region for, above all, olfactory and also auditory and visual information processing Basal nuclei – centers for planning and learning movement sequences Cortex – sensory, motor, association centers In mammals – isocortex (neocortex) evolved in ancestors (mammal like reptiles) In human – about 5 mm thick, 0.5 m2 Convolutions – allow by 6 parallel layers of neurons a large surface Large cortex in primates, cetaceans Hemispheres – connected by corpus callosum The vertebrate nervous system is regionally specialized – summary The peripheral nervous system The PNS consists of paired cranial and spinal nerves and associated ganglia. Functionally, the PNS is divided into the somatic nervous system, which carries signals to skeletal muscles, and the autonomic nervous system, which regulates the primarily automatic, visceral functions of smooth and cardiac muscles. The autonomic nervous system has three divisions: the sympathetic and parasympathetic divisions, which usually have antagonistic effects on target organs, and the enteric division, which controls the activity of the digestive tract, pancreas, and gallbladder. Embryonic development of the brain The vertebrate brain develops from three embryonic regions: the forebrain, the midbrain, and the hindbrain. In humans, the most expansive growth occurs in the part of the forebrain that gives rise to the cerebrum. The vertebrate nervous system is regionally specialized – summary The brainstem The medulla oblongata, pons, and midbrain make up the brainstem, which controls homeostatic functions such as breathing rate, conducts sensory and motor signals between the spinal cord and higher brain centers, and regulates arousal and sleep. The cerebellum The cerebellum helps coordinate motor, perceptual, and cognitive functions. It also is involved in learning and remembering motor skills. The diencephalons The thalamus is the main center through which sensory and motor information passes to and from the cerebrum. The hypothalamus regulates homeostasis; basic survival behaviors such as feeding, fighting, fleeing, and reproduction; and circadian rhythms. The cerebrum The cerebrum has two hemispheres, each of which consists of cerebral cortex overlying white matter and basal nuclei, which are important in planning and learning movements. In mammals, the cerebral cortex has a convoluted surface called the neocortex. A thick band of axons, the corpus callosum, provides communication between the right and left cerebral cortices. The cerebral cortex controls voluntary movement and cognitive functions 4 lobes Association centers dominate in human The cerebral cortex controls voluntary movement and cognitive functions Information processing in the cerebral cortex Via thalamus to primary sensory areas – processed parameters of objects In association areas – integrated, processed information from sensory areas (complex images, faces) Based on information from association areas primary motor cortex generate commands Action potentials travel along axons through brainstem, spinal cord, to motor neurons – excite skeletal muscle Neuron number, distribution according to the body part and skills needed The cerebral cortex controls voluntary movement and cognitive functions Lateralization of cortical function After birth – competing functions segregate and displace each other in the cortex = lateralization of functions Left hemisphere – language, math, logical operations, serial processing, bias for detail Right – pattern and face recognition, spatial relations, emotional processing, simultaneous processing of many kinds of information, bias for context Language and speech Left frontal lobe – Broca’s area – speaking Posterior portion, temporal lobe – Wernicke’s area – hearing The cerebral cortex controls voluntary movement and cognitive functions Emotions – results of interplay of many brain regions – the limbic system (ring around brainstem) Cerebral cortex – amygdala, hippocampus, olfactory bulb Thalamus Hypothalamus Emotions manifested as laughing, crying – interaction with sensory areas Feelings to functions controlled by brainstem – aggression, feeding, and sexuality Amygdala – recognition of emotional content of facial expressions emotional memories The cerebral cortex controls voluntary movement and cognitive functions Memory and learning Short-term memory Activation of hippocampus => long-term memory Influence of positive or negative emotions (mediated by amygdala) Mechanism in Aplysia Interneuron => serotonin => closes K+ channels => prolonged depolarization => more Ca2+ diffuse into terminals => more neurotransmitter released The cerebral cortex controls voluntary movement and cognitive functions Long-term potentiation – increase in the strength of synaptic transmission Positive feedback > enforcing synaptic connection The cerebral cortex controls voluntary movement and cognitive functions – summary Each side of the cerebral cortex has four lobes – frontal, temporal, occipital, and parietal – which contain primary sensory areas and association areas. Information processing in the cerebral cortex Specific types of sensory input enter the primary sensory areas. Adjacent association areas process particular features in the sensory input and integrate information from different sensory areas. In the somatosensory cortex and the motor cortex, neurons are distributed according to the part of the body that generates sensory input or receives motor commands. Lateralization of cortical function The left hemisphere is normally specialized for high-speed serial information processing essential to language and logic operations. The right hemisphere is stronger at pattern recognition, nonverbal ideation, and emotional processing. The cerebral cortex controls voluntary movement and cognitive functions – summary Language and speech Portions of the frontal and temporal lobes, including Broca’s area and Wernicke’s area, are essential for generating and understanding language. Emotions The limbic system, a ring of cortical and noncortical centers around the brainstem, mediates primary emotions and attaches emotional “feelings” to survival-related functions. The association of primary emotions with different situations during human development requires parts of the neocortex, especially the prefrontal cortex. Memory and Learning The frontal lobes are a site of short/term memory and can interact with the hippocampus and amygdale in consolidating long-term memory. Experiments on invertebrates and vertebrates have revealed the cellular basis of some simple forms of learning, including sensitization and long-term potentiation. Consciousness Modern brain-imaging techniques suggest that consciousness may be an emergent property of the brain based on activity in many areas of the cortex.