Ch 44: Nervous Systems

1Bio 215 Chapter 48 Nervous systems

Ch 48: Nervous SystemsBio 1B Chapter 44 Nervous systems

ch 44 5

Lecture notesSome of the goals for this chapter include:

1) Draw and describe the functions of the nervous system cells2) define and describe impulses and action potentials3) describe the chemical and electrical communications that occur at synapses4) Describe the different types of nervous systems that invertebrates have5) describe the nervous system of the human 6) outline all the structures and their function of the human brain

The endocrine and nervous systems of animals often cooperate and interact to maintain homeostasis and control behavior. The roles these two systems play are as follows

Nervous SystemEndocrine SystemComplexityMore structurally complex:can integrate vast amounts of information and stimulate a wide range of responsesLess Structurally complexStructureSystem of neurons that branch throughout the bodyendocrine glands secrete hormones into the bloodstream where they are carried to the target organcommunicationNeurons conduct electrical signals directly to and form specific targets: allows fine pinpoint controlAs chemical messengers, hormones circulate throughout the body in the bloodstream: exposes most body cells to the hormone and only target cells with receptors respondResponse TimeFast transmission of nerve impulses up to 100m/secMay take minutes, hours or days time for hormones to be produced, carried by blood to target organ, and for response to occur

1 Nervous systems functions: an overview3 overlapping functions:

A. Sensory input:conduction of signals form sensory receptors to integration centers of the nervous systemB. Integration: information form sensory receptors is interpreted and associated with appropriate responses of the bodyC. Motor output: conduction of signals form processing center to effector cells that actually carry out the body's response to stimuli(muscle cells, gland cells)D. These functions in both parts of the nervous system1. Central nervous system (CNS): brain and spinal cord: responsible for integration of sensory input and associating stimuli with appropriate motor output.2. Peripheral nervous system (PNS): consists of the network of nerves extending into different parts of the body that carry sensory input to the CNS and motor output away form the CNS

2 Neurons and supporting cellsA. Neurons: cells specialized for transmitting chemical and electrical signals form one location in the body to another1. large cell body:contains cytoplasm and other organellesa. mainly cell body are in CNS2. two types of fiberlike extensions(processes) that increase the distance over which the cells can conduct messagesa. Dendrites convey signals to the cell body, short, numerous and extensively branchedb. Axons: conduct impulses away form the cell body, long single processes(*) wrapped in insulating layers of Schwann cells, forms myelin sheath(*) impulses that are generated move away form the cell body3. Synapse: gap between a synaptic terminal and target cell4. Neurotransmitters: chemicals that cross the synapse to relay the impulses5. Three major classes of neurons:a. Sensory neurons: convey information about the external and internal env form sensory receptors to cns. most synapse with interneuronsb. Interneurons: integrate sensory input and motor output: located within the CNS, synapse only with other neuronsc. Motor neurons: convey impulses form the CNS to effector cellsB. Supporting cells:1. Structurally reinforce, protect, insulate and generally assist neuronsa. The do not conduct impulsesb. outnumber neurons 10 to 50 fold2. Glial cells: supporting cells of CNSa. astrocytes: encircle capillaries in brain, contribute to blood brain barrierb. Oligodendrocyte: form myelin sheaths that insulate nerve impulses3. Schwann cells: insulators of the PNS, myelination

3 Impulses are action potentials, electrical signals propagated along neuronal membranes1. signal transmission along the length of a neuron depends on voltages created by ionic fluxes across neuron plasma membranesA. Origin of the Electrical Membrane potential1. All cells have electrical membrane potential or voltage across their plasma membranesa. -50 to -100 mV in animalsb. a resting charge is slightly negativec. resting neuron is -70mV2. Membrane potential arise becausea. Differences in the ionic composition of the intracellular and extracellular fluids(*) cells tend to accumulate more negative charges on the inside of the cellb. selective permeability of the plasma membrane(*) Ion channels selectively transport certain ions across the membrane, and keep the gradient up.B. Membrane Potential changes and the Action Potential1. While all cells exhibit a membrane potential neurons and muscles can change their membrane potentials in response to stimulia. called excitable cellsb. membrane potential of an excitable cell at rest is called a resting potential2. The presence of special gated ion channels in neurons permits these cells to changes the plasma membranes permeability and to alter its membrane potential3. Action potential is the rapid change in the membrane potential of an excitable cell, caused by stimulus triggered selective opening and closing of voltage gated ion channelsa. four phases(*) resting state, no channels are open(*) large depolarizing phase: membrane briefly reverses polarity.(*) Steep repolaring phase: quickly returning potential to resting level(*) undershoot phase: when membrane potential is temporarily more negative than the resting state4. refractory period: during undershoot phase, neuron is insensitive to depolarizing stimuli. is maximum rate at which action potentials can be stimulated in a neuron

C. Propagation of the action potential:1. A neuron is stimulated at its dendrites of cell body and the action potential travels along the axon to the other end of the neuron2. Signal travels in a perpendicular direction along the axon regenerating the action potentiala. Na+ influx in the are of the action potential results in depolarization of the membrane just ahead of the impulse, surpassing the thresholdb. the voltage sensitive channels in the new location will go through the same sequence previously described regenerating the action potentialc. subsequent portions of the axons are depolarized in the same mannerd. the action potential moves in only one direction (down the axon) once each action potential is followed by a refractory period when sodium channels inactivation gates are closed and no action potential can be generated

D. Action potential Transmission Speed1. the larger the diameter of the axon, the faster the rate of transmission since resistance to the flow of electrical current is inversely proportional the the cross section area of the wire conducting the current2. saltatory conduction: faster transmission because action potential jumps form one node of ranvier to the next.

4 Chemical or electrical communication between cells occurs at synapses1. Synapse= tiny gap between a synaptic terminal of an axon and a signal receiving portion of another neuron or effector cella. presynaptic: transmitting cellb. postsynaptic: receiving cell2. Two types of synapses existA. Electrical synapses: electrical synapses allow action potentials to spread directly form pre to postsynaptic cells via gap junctions (intercellular channels) is very rareB. Chemical synapses: neurotransmitters are used to travel across a cleft and it excites the postsynaptic cell

C. Summation: Neural integration at the cellular level1. one neuron may receive information form thousands of synapses, these synapses can be excitatory or inhibitory, when the excitatory messages reach a certain level above the inhibitory levels the neuron will fire.

D. Neurotransmitters and Receptors1.Criteria for neurotransmitters:a. present in and discharged form synaptic vesicles in the presynaptic cell when stimulated and affect the postsynaptic cell's membrane potentialb. Can see an inhibition of excitation of synaptic potential when injected into the synapsec. must be rapidly removed form the synapse by an enzyme of uptake by a cell permitting the postsynaptic membrane to return to resting potential2. Types of Neurotransmittersa. Acetylcholine: most common neurotransmitter in vertebrates and invertebrates, controls muscle movementb. Biogenic amines: derived from amino acids, dopamine and serotonin, is involved in CNS, imbalances in dopamine and serotonin are associated with mental illnessc. Neuropeptides: short chains of amino acids(substance P for pain)

E. Gaseous signals of the Nervous system1. during sexual arousal neurons release nitrous oxide that causes dilation of blood vessels

F. Neural Circuits and Clusters1. Neurons are arranged in groups referred as circuitsa. convergent circuits= Neural circuits in which information form several presynaptic neurons come together at a singe postsynaptic neuronb. Divergent circuits= Neural circuit in which information from a single neuron spreads out to several postsynaptic neuronsc. Reverberating circuits= circular circuits in which the signal returns to its source2. clusters of nerve cells bodies are called ganglia allow for activity by only part of the nervous system (reflexes)

5 Invertebrate nervous systems:examplesA. hydra a cnidarian has a nerve net loosely organized system of nerves with no central controlB. Cephalization= evolutionary trend for concentration of sensory and feeding organs on the anterior end of a moving animal1. found in most bilaterally symmetrical animalsC. Flatworms have a simple brain, with two nerve trunks traveling posteriorlyD. Annelids and arthropods have well defined ventral nerve cord and prominent brain, each segment has its own gangliaE. Cephalopods have the most sophisticated invertebrate nervous system containing large brain and giant axons

F. cephalization depends on what the animals do. Sessile organisms do not exhibit this trait

6 Vertebrate nervous system:composed of two functional componentsA. Peripheral nervous system PNS1. further subdivided into a. sensory (afferent) nervous system receptors to CNSb. motor (efferent) nervous system CNS to effector cells(*) somatic nervous systems (voluntary skeletal muscle(*) autonomic nervous system( involuntary visceral functions

B. Central Nervous Systems (CNS)1. bridges the sensory and motor functions of the peripheral nervous system2. consists of thea. spinal cord,b. brainc. covered in cerebrospinal fluid3. simple tasks like the patellar reflex is in the spinal cord4. hither tasks go to the brainC. Evolution of the Vertebrate Brain1. vertebrate brain shows and evolutionary trend toward greater complexity and more complex behavioral patterns2. complex brain consists ofa. rhombencephalon hind brainb. mesencephalon midbrainc. prosencephalon forebrain3. trends in the evolution of the vertebrate brain area. relative brain size increases in certain evolutionary lineagesb. increased compartmentalization of function with certain areas of the brain assuming specific responsibilitiesc. increasing complexity and sophistication of the forebrain: increased complexity of behaviors parallels an increased in growth of the cerebrum