2 Nervous System Functions: Sensory Input – monitoring stimuli occurring inside and outside the bodyIntegration – interpretation of sensory inputMotor Output – response to stimuli by activating effector organs
3 Organization of the Nervous System CNSBrain and Spinal Cord (in dorsal body cavity)Integration and command center – interprets sensory input and responds to inputPNSPaired Spinal and Cranial nervesCarries messages to and from the spinal cord and brain – links parts of the body to the CNS
4 PNS - Two Functional Divisions Sensory (afferent) DivisionSomatic afferent nerves – carry impulses from skin, skeletal muscles, and joints to the CNSVisceral afferent nerves – transmit impulses from visceral organs to the CNSMotor (efferent) DivisionTransmits impulses from the CNS to effector organs, muscles and glands, to effect (bring about) a motor response
5 Motor Division: two subdivisions Somatic Nervous System (voluntary)Somatic motor nerve fibers (axons) that conduct impulses from CNS to Skeletal muscles – allows conscious control of skeletal musclesAutonomic Nervous System (ANS) (involuntary)Visceral motor nerve fibers that regulate smooth muscle, cardiac muscle, and glandsTwo functional divisions – sympathetic and parasympathetic
10 Membrane Potentials: Signals Two types of signals are produced by a change in membrane potential: graded potentials (short-distance) action potentials (long-distance)
11 Graded Potentials1-Short-lived, local changes in membrane potential (either depolarizations or hyperpolarizations)2-Cause currents that decreases in magnitude with distance3-Their magnitude varies directly with the strength of the stimulus – the stronger the stimulus the more the voltage changes and the farther the current goes4-Sufficiently strong graded potentials can initiate action potentials
12 Action Potentials (APs) An action potential in the axon of a neuron is called a nerve impulse and is the way neurons communicate.The AP is a brief reversal of membrane potential with a total amplitude of 100 mV (from -70mV to +30mVAPs do not decrease in strength with distanceThe depolarization phase is followed by a repolarization phase and often a short period of hyperpolarizationAll-or-None phenomenon – action potentials either happen completely, or not at all
13 Propagation of an Action Potential The action potential is self-propagating and moves away from the stimulus (point of origin)
14 Stimulus IntensityAll action potentials are alike and are independent of stimulus intensityHow can CNS determine if a stimulus intense or weak?Strong stimuli can generate an action potential more often than weaker stimuli and the CNS determines stimulus intensity by the frequency of impulse transmission
15 Axon Conduction Velocities Conduction velocities vary widely among neuronsDetermined mainly by:Axon Diameter – the larger the diameter, the faster the impulse (less resistance)Presence of a Myelin Sheath – myelination increases impulse speed (Continuous vs. Saltatory Conduction)
16 Saltatory ConductionCurrent passes through a myelinated axon only at the nodes of RanvierVoltage-gated Na+ channels are concentrated at these nodesAction potentials are triggered only at the nodes and jump from one node to the nextMuch faster than conduction along unmyelinated axons
17 Saltatory ConductionCurrent passes through a myelinated axon only at the nodes of Ranvier (Na+ channels concentrated at nodes)Action potentials occur only at the nodes and jump from node to node
18 Erlanger and Gasser divided mammalian nerve fibers into A, B, and C groups, further subdividing the A group into α, β, γ, and δ fibers.
19 Fiber Type Origin Number A α Ia Ib A β II A δ III Dorsal root C IV Muscle spindle, annulospinal ending.IaGolgi tendon organ.IbA βMuscle spindle, flower-spray ending; touch, pressure.IIA δPain and cold receptors; some touch receptors.IIIDorsal root CPain, temperature, and other receptors.IV
21 SynapseA junction that mediates information transfer from one neuron to another neuronPresynaptic neuron – conducts impulses toward the synapse (sender)Postsynaptic neuron – transmits impulses away from the synapse (receiver)
22 Types of SynapsesAxodendritic – synapse between the axon of one neuron and the dendrite of anotherAxosomatic – synapse between the axon of one neuron and the soma of anotherOther types:Axoaxonic (axon to axon)Dendrodendritic (dendrite to dendrite)Dendrosomatic (dendrites to soma)
25 Electrical Synapses Less common than chemical synapses Gap junctions allow neurons to be electrically coupled as ions can flow directly from neuron to neuron - provide a means to synchronize activity of neurons
26 Electrical Synapse Electrical synapses gap junctions (connexins) smooth and cardiac muscles, glial cellsOnly a few examples of GJ have been found in the central nervous system
28 Chemical Synapse One way conducton Functional connection between a neuron and another neuron (or effector cell such as muscle, gland).One way conducton
29 Chemical SynapsesSpecialized for the release and reception of chemical neurotransmittersTypically composed of two parts:Axon terminal of the presynaptic neuron containing membrane-bound synaptic vesiclesReceptor region on the dendrite(s) or soma of the postsynaptic neuron
30 The synaptic terminal contains numerous vesicles that enclose a neurotransmitter for which the postsynaptic neuron has membrane receptors. When an action potential enters the synaptic terminal of the presynaptic neuron, the vesicles dump their neurotransmitter into the gap between the neurons. The neurotransmitter diffuses rapidly across the space, binds to postsynaptic receptors, and causes ion channels to open. Ions flow through these open channels, causing a postsynaptic potential in the postsynaptic cell.
31 Synaptic CleftFluid-filled space separating the presynaptic and postsynaptic neurons, prevents nerve impulses from directly passing from one neuron to the nextTransmission across the synaptic cleft:Is a chemical event (as opposed to an electrical one)Ensures unidirectional communication between neurons
34 Synapse AP comes down the axon. At the synapses, VG Ca++ channels let in calcium.This triggers the release (exocytosis!) of the contentsof vesicles in the axonal bouton.The contents are: NEUROTRANSMITTERS.
35 SynapseNeurotransmitters (NT) cross the narrow synaptic space and bind to receptors on the dendrite (or other cell).This causes a response in the postsynaptic cell.The whole cycle starts again in this second cell!
36 Postsynaptic Potentials EPSP (excitatory postsynaptic potential):Depolarization.Brings cell closer to threshold for an AP.Often Na+ channels.IPSP (inhibitory postsynaptic potential):Hyperpolarization.Takes cell further away from threshold for an AP.Often Cl- and K+ channels.
37 Excitatory Postsynaptic Potentials EPSPs are local graded depolarization events that can initiate an action potential in an axonPostsynaptic membranes do not generate action potentials. The currents created by EPSPs decline with distance, but can spread to the axon hillock and depolarize the axon to threshold leading to an action potential
38 Inhibitory Postsynaptic Potentials Neurotransmitter binding to a receptor at inhibitory synapses reduces a postsynaptic neuron’s ability to generate an action potentialPostsynaptic membrane is hyperpolarized due to increased permeability to K+ and/or Cl- ions. Leaves the charge on the inner membrane face more negative and the neuron becomes less likely to “fire”.
40 SummationA single EPSP cannot induce an action potential EPSPs must summate (add together) to induce an APTemporal Summation – presynaptic neurons transmit impulses in quick successionSpatial Summation – postsynaptic neuron is stimulated by a large number of terminals at the same timeIPSPs also summate and can summate with EPSPs.
44 So… AP flies down axon of first neuron. NT are released at synapse. Receptors bind NT and produce an EPSP or IPSP in postsynaptic neuron.The sum of the inputs -> AP in this second neuron.
45 NeurotransmittersChemicals used for neuron communication with the body and the brainMore than 50 different neurotransmitters have been identifiedClassified chemically and functionally
46 Neurotransmitters Small molecules, Rapidly acting Cause most acute responses of the nervous system such as:Transmission of sensory signals to the brainTransmission of motor signals to the muscles
47 Small molecules Rapidly acting Synthesized in presynaptic terminalsAbsorbed by means active transport to the vesicle
48 Neurotransmitters – Chemical classification Acetylcholine (ACh)Biogenic aminesAmino acidsPeptidesNovel messengers: ATP and dissolved gases NO and CO
49 Neurotransmitters: Acetylcholine Released at the neuromuscular junctionEnclosed in synaptic vesiclesDegraded by the acetylcholinesterase (AChE)Released by:All neurons that stimulate skeletal muscleSome neurons in the autonomic nervous system
50 Neurotransmitters: Biogenic Amines Include: Catecholamines – dopamine, norepinephrine, and epinephrine Indolamines – serotonin and histamine Broadly distributed in the brain Play roles in emotional behaviors and our biological clock
51 Neurotransmitter Receptor Mechanisms Direct: neurotransmitters that open ion channelsPromote rapid responsesExamples: ACh and amino acidsIndirect: neurotransmitters that act through second messengersPromote long-lasting effectsExamples: biogenic amines, peptides, and dissolved gases
54 Termination of Neurotransmitter Effects Neurotransmitter bound to a postsynaptic neuron produces a continuous postsynaptic effect and also blocks reception of additional “messages”Terminating Mechanisms:1- Degradation by enzymes2- Uptake by astrocytes or the presynaptic terminals3- Diffusion away from the synaptic cleft
55 Neuropeptides Large molecules, slowly acting Cause more prolonged actions such as:Long term changes in number of neuronal receptorsLong term opening / closure or of certain ion channelsLong term changes in numbers of synapses or sizes of synapses
56 Neuropeptides:Are generally thousand or more times as potent as small molecules
57 Neuropeptides Are synthesized by ribosome in cell body The vesicles are transported to the terminalMuch smaller quantities released than small molecules
59 Synaptic DelayNeurotransmitter must be released, diffuse across the synapse, and bind to receptors ( ms)Synaptic delay is the rate-limiting step of neural transmission
60 Fatigue of synaptic transmission Exhaustion or partially exhaustion of neurotransmitter storesProgressive inactivation of postsynaptic receptorsSlow development of abnormal concentrations of ions inside the postsynaptic neuron
61 Effect of acidosis and alkalosis on synaptic transmission Alkalosis increases neuronal excitability-Overbreathing can precipitate an epileptic attackAcidosis depresses the neuronal activity-in very sever diabetic acidosis, coma always develops
62 Effect hypoxia on synaptic transmission Neuronal excitability is highly dependent on adequate supply of oxygenCessation of oxygen for only a few seconds can cause inexcitability of some neuronsWhen brain blood flow interrupted the person becomes unconscious
63 Effect drugs on synaptic transmission Caffeine (found in the coffee), theophylline( tea) and theobromine(cocoa) increase neuronal excitability-By reducing threshold for excitation of neuronsAnesthetics increase threshold