LE NERVOUS SYSTEM

Reflexes: Sensory nuerons: trasmit info from sensors that detect external stimuli/internal conditionsInfor sent to CNS where interneurons analyze/interpret sensory inputMotor output leaves CNS via motor nuerons which communicate w/ effector cellsLe Neuron Structure

Neurons organelles located in cell body. Dendrites are highly branched extensions that receive signals from other neurons. The axon is a loner extension that transmits signals to other cells (other neurons or effector cells). The conical region of axon where it joins cell body is axon hillock, region where signals that travel down axon are generated. Many axons are enclosed in myelin sheath layer. Near the end, the axon divides into several branches, each ending in a synaptic terminal. The site of communication between a synaptic terminal and another cell is a synapse. Info can be passed by neurotransmitters. Gilia are supporting cells that are essential for sturctural integrity of nervous system: 1)Astrocytes: structural support for neurons/regulate extracellular concentrations of ions/neurotransmitters, can facilitate info transfer at synapse between neighboring neurons (cellular mechanism: learning/memory). Astrocytes next to active neurons also cause nearby blood vessels to dilate, increasing blood flow and enabling neurons to obtain O2 and glucose more quickly. During development, astrocytes induce formation of tight junctions between cells that line capillaries in brain/spinal cord resulting in the blood-brain barrier-which restricts passage of certain stuff into CNS 2)Radial Glia: (embryo) form tracks along which newly formed neurons migrate from neural tubegiving rise to CNS, can act as stem cell. So can astrocytes. 3)Oligodendrocytes (CNS) and Schwann cells (PNS): glia that form the myelin sheaths around axons. Neurons become myelinated when Schwann cells or oligodendrocytes wrap around axons in my layers of membrane-mostly lipid so it is poor conductor of electrical currents & provides for electrical insulation of axon. Gaps between adjacent Schwann cells are nodes of Ranvier. Le Resting Potential/Gated Ion ChannelsResting potential-membrane potential of neuron not transmitting a signal, depends on ionic gradients across membrane. Concentrations of Na+ and Cl- are higher in extracellular fluid than cytosol. The reverse is true for K+. Neurons have gated ion channels which open/close in response to 1)stretch-gated ion channels: found in cells that sense stretch when membrane is mechanically deformed 2)ligand-gated channel: found at synapses and open/clsoe when chemical like a neurotransmitter binds to channel 3)voltage-gated ion channels-found in axons and open/close when membrane potential changes.

Dem Action PotentialsIf cell has gated ion channels, its membrane potential may change in response to stimuli. Stimuli can trigger a hyperpolarization, an increase in magnitude of membrane potential (inside of membrane becomes more negative, may be caused by opening of gated K+ channels). Other stimuli trigger depolarization, reduction in magnitude of membrance potential (may be due to opening of gated Na+ channels)Depolarizations are graded only up to a certain membrane voltage, called a threshold. The action potential [all or none] is strong enough to produce a depolarization that reaches the threshold. Voltage-gated Na+ and voltage-gated K+ channels produce action potential. Na+ channel opens first. Na+ channel has an activation gate/inactivation gate whereas K+ has one activation gate. 1)Resting State: Activation gates of Na+ and K+ channels are closed. 2)Depolarization: Stimulus opens activation gates on some Na+ channels. Na+ influx thru channels depolarizes membrane; if depolarization reaches threshold, it triggers an action potential. 3)Rising Phase of Action Potential: Depolarization opens more Na+ channels, while K+ channels remain closed. Na+ influx makes inside of membrane positive w/ respect to outside. 4)Falling Phase of Action Potential: The Inactivation gates close, blocking Na+ influx. K+ channels open permitting K+ efflux, making inside of cell negative. 5)Undershoot: Both Na+ gates close, K+ channels are still open, but eventually closeresting state. The refractory period sets a limit on the max frequency at which action potentials can be generated. The action potential functions as a long-distance signal by not diminishing from the cell body to the synaptic terminals; it does so by regenerating itself along axon. 1)Action potential is generated as Na+ flows inward across membrane at one location. 2)Depolarization of action potential spreads to neighboring region of membrane, re-initiating action potential there. To the left of this region, the membrane is repolarizing a K+ flows outward. 3)Depolarization/repolarization repeats in next region so action potential is propagated along axon. The larger the axons diameter, the faster the conduction. Myelin increases the conduction speed of action potentials by insulating the axon membrane. Insulation causes the depolarizing current to spread further along interior of axon, bringing distant regions of membrane to threshold sooner. Action potentials are not generated in regions between the nodes of Ranvier-Saltatory conduction. Dem Neurons Communicate at Dem SynapsesMany synapses are chemical synapses which involve release of chemical neurotransmitter by presynaptic neuron. Presynaptic neuron synthesizes the neurotransmitter and packages it in synaptic vesicles (stored in synaptic terminals which interact with postsynaptic neuron). 1)When action potential depolarizes plasma membrane of synaptic terminal, it 2)opens voltage-gated Ca2+ channels in membrane. 3)Elevated Ca concentration in terminal causes synaptic vessicles to fuse w/ presynaptic membrane. 4)Vesicles release neurotransmitter into synaptic cleft. 5)Neurotransmitter binds to receptor portion of ligand-ion channels in postsynaptic membrane, opening them and allowing ions to diffuse across-direct synaptic transmission and result is a postsynaptic potential. 6)Synpatic transmission ends when neurotransmitter diffuses out of synaptic cleft, taken by another cell, or degraded by enzyme. Indirect Synpatic Transmission (slower but longer onset):1)Neurotransmitter binds to receptor not part of an ion channel, activating a signal transduction pathway involving a 2nd messenger in postsynaptic cell.Dem Neurotransmitters

Le Specialized Nervous SystemVertebrate CNS: narrow central canal of spinal cord and four ventricles of the brainfilled w/ cerebrospinal fluid, which circulates slowly thru central canal/ventricles and drains into veins, assisting in supply of nutrients/hormones to diff parts of brain and in removal of wastes. In mammals, the fluid cushions the brain & spinal cord. In cross sections of the brain and spinal cord, there is the white matter (myelin sheaths give axons whitish appearance) and the gray mattermainly dendrites, unmyelinated axons, & neuron cell bodies. PNS: transmits info to and from CNS, regulates movement/internal environ. Vertebrate PNS consists of light-right pairs of cranial and spinal nerves and associated ganglia. Somatic Nervous System: carries signals to and from skeletal muscles (response to external stimuli and is voluntary) Autonomic Nervous System: regulates internal environ by controlling smooth and cardiac muscles and organs of digestive, cardiovascular, excretory, and endocrine systems Sympathetic: fight or flight response Parasympathetic: promote calming and a return to self-maintenance functions Enteric: consists of network of neurons in digestive tract, pancreas, and gallbladder that control organs secretions as well as activity in smooth muscles

Le Brain Stuff

Forebraintelencephalon [cerebrum(cerebral hemispheres: cerebral cortex, white matter, basal nuclei)]and diencephalon [Diencephalon(thalamus, hypothalamus, epithalamus)] Midbrainmesencephalon [Midbrain(part of brainstem)]Hindbrainmetencephalon [pons(part of brainstem), cerebellum)] and myelencephalon [medualla oblongata (part of brainstem)]BrainstemMedulla Oblongata, Pons, Midbrain-homeostasis, coordination of movement, conduction of info to higher brain centersMedulla Oblongata: (automatic,homeostatic) breathing, heart and blood vessel activity, swallowing, vomiting, digestion Pons: regulates breathing centers in medulla Information transmission is one of the most important functions of medulla dn pons since all axons carrying sensory info to and motor instructions from higher brain regions pass thru brainstem. The midbrain contains centers for receipt and integration of several types of sensory info. It also sends coded sensory info along neurons to specific regions of forebrain.

The reticular formation(RAS) is a network of neurons present in the core of the brainstem. Part of the RAS regulates sleep and arousal. RAS acts as a sensory filter, selecting which info reaches cerebral cortex; the more info that reaches cerebral cortex, the more alert/aware a person is. The pons and medulla contain centers that cause sleep when stimulated, and the midbrain has a center that causes arousal.

CerebellumDevelops from part of metencephalon, important for coordination and error checking during motor, perceptual, and cognitive functions (learning, decision making, consciousness), recieves sensory info about position of joints and length of muscles and auditory/visual systems. Cerebellum also integrates sensory/motor info to coordinate movement and balance.

DiencephalonEpithalamus: pineal gland and choroid plexus cluster of capillaries that produce cerebrospinal fluidThalamus: main input/output center for motor info leaving cerebrum. Incoming info from senses is sorted in thalamus and sent to proper cerebral centers for processing, also receives imput that regulate emotion/arousal. Hypothalamus: regulates hunger, thirst, sexual/mating behaviors, fight-flight responses, pleasure

Biological clocks-maintain circadian rhythms, regulate physiological phenomena, including hormone release, hunger, heightened sensitivity to external stimuli. In mammals, biological clock is the suprachiasmatic nuclei (SCN). Bio clocks require external cues to remain synchronized w/ environmental cycles.

CerebrumDevelops from telencephalon, divided into right and left cerebral hemispheres and each hemisphere consists of basal nuclei located deep within white matter-basal nuclei are important centers for planning and learning movement sequences. In humans, cerebral cortex is the most complex part of brain; sensory info is analyzed, motor commands are issues, and language is generated in there. Neocortext forms the outermost part of mammalian cerebrum, consisting of 6 parallel layers of neurons arranged tangential to brain surface. The left side of cortext recieves info from, and controls movement of. The corpus callosum enables communication between right/left cerebral cortices.