Presentation on theme: "Neurons The two principal cell types of the nervous system are:"— Presentation transcript:
1 Neurons The two principal cell types of the nervous system are: Neurons – excitable cells that transmit electrical signals.There are 150 different types.Vary in function and size.Supporting cells (neural glia)– cells that surround and wrap neurons.Oligodendrocytes, Schwann cell, Astrocytes
2 Supporting Cells: Neuroglia The supporting cellsProvide a supportive scaffolding for neuronsSegregate and insulate neuronsGuide young neurons to the proper connectionsPromote health and growth
4 Fundamental Types of Neurons Sensory (afferent) neuronsdetect changes in body and external environmentinformation transmitted to brain or spinal cordInterneurons (association neurons)lie between sensory and motor pathways in CNS90% of our neurons are interneuronsIntegration and retrieving informationMotor (efferent) neuronsend signals out to muscles and gland cellsorgans that carry out responses called effectors
6 Structure of a Neuron Cell body (soma) Axon single, central nucleus contains many multi-branched dendritesWhich receive signals from other neurons.Axon(nerve fiber) arising from axon hillock for rapid conductionAxon terminals release will neurotransmitters that communicate a chemical message to another nerve or muscle
7 Neuron Physiology Living nerve cells are polarized The inside of the cell (Intracellular fluid) ICF is negatively charged compared to the outside of the cell extracellular fluid (ECF)The cell is able to maintain a resting membrane potential of -70 mV (negative charge on the inside of membrane by active transport and specific voltage gated channels.
9 Resting Membrane Potential The cell membrane is considered a semi-permeable membrane which selectively allows things in and out of the cell.Large negatively charged molecules found in the ICF such as proteins and phosphates are confined to the inside of the cell. The membrane is impermeable to these molecules which contributes to the ICF maintaining RMP of -70mV
10 Ionic Basis of Resting Membrane Potential Na+ concentrated outside of cell (ECF)K+ concentrated inside cell (ICF)
11 Basis of the Resting Membrane Potential Since Na+ ion are more concentrated in the ECF when a specific voltage gated Na+ channel opens Na+ will always rush into the cell by diffusion.Since K+ ion channels are more concentrated in the ICF when a specific voltage gated K+ channel opens K+ will always rush out of the cell by diffusionIn order to keep the resting membrane potential at –70 mV the cell is constantly hydrolyzing ATP with the Na+,K+-ATPase pump.
18 Action Potentials (APs) There are 3 phases to an AP:Depolarizationa reduction in the polarity of the membrane potential by allowing Na+ to enter the cell.Repolarizationmembrane potential returns towards the resting value closing Na channels and opening K+ channels. K+ travels along its concentration gradient out of the cell returning the inside of the cell to a negative value.HyperpolarizationSlow closing K+ channels cause the inside of the cell to be more negative than the resting valueAll APs have the same magnitude regardless of the size of the stimulus
20 The Refractory Period Absolute refractory period as long as Na+ gates are openno stimulus will trigger APRelative refractory periodas long as K+ gates are openonly especially strong stimulus will trigger new APRefractory period is occurring only to a small patch of membrane at one time (quickly recovers)
21 Impulse Conduction in Unmyelinated Fibers The action potential in trigger zone begins impulseNerve signal (impulse) - a chain reaction of sequential opening of voltage-gated Na+ channels down entire length of axonThis is a very slow process. (2 m/sec)We need something to speed the process up.
23 Saltatory Conduction - Myelinated Fibers The velocity of an action potential propagates along the length of the axon depends on:axon diameterThe larger the diameter of the axon the greater the velocity of the action potential travels along the axon to the axon terminal.Myelin sheath increases the diameter of sections of the axon which dramatically increases impulse speed. (120 m/sec)
24 Myelin SheathMyelin is a white, fatty insulating covering around most of the long axons. It plays an important role in both conduction velocity and protection of the axon.In the CNS Oligodendrocytes can myelinate many different neurons.In the PNS Schwann cells are can only myelinate a portion of one axon.
25 Diseases of the Nervous System What would be worse?A disease that attacks neurons of the CNSA disease that attacks neurons of the PNSWhat are the deficits one might expect to see if the neurons loose their Myelination?
27 Saltatory ConductionNotice how the action potentials jump from node of Ranvier to node of Ranvier.
28 Presynaptic Neurons Presynaptic neurons Nerve signal(AP) opens voltage-gated calcium channels allowing it to diffuse into the synaptic knob.Calcium triggers the release of a neurotransmitter such as acetylcholine (Ach) from the (synaptic vesicles).The neurotransmitters are released into the synaptic cleft.
29 Postsynaptic NeuronNeurotransmitters diffuse across the synaptic cleft binding to ligand-gated channels on the postsynaptic neuron.Graded PotentialSpecific neurotransmitters can influence the permeability of the voltage gated channels.This influences the post synaptic neuron to become more likely to generate an AP (depolarization) or less likely (hyperpolarization).
30 Postsynaptic Potentials- EPSP Excitatory postsynaptic potentials (EPSP)a positive voltage change causing postsynaptic cell to be more likely to fireresult from Na+ flowing into the cellglutamate and aspartate are excitatory neurotransmitters
31 Postsynaptic Potentials- IPSP Inhibitory postsynaptic potentials (IPSP)a negative voltage change causing postsynaptic cell to be less likely to fire (hyperpolarize)result of Cl- flowing into the cell or K+ leaving the cellglycine and GABA are inhibitory neurotransmitters
32 The membrane potential of a real neuron typically undergoes many EPSPs (A) and IPSPs (B), since it constantly receives excitatory and inhibitory input from the axons terminals that reach it.
33 Summation - Postsynaptic Potentials Net postsynaptic potentials in trigger zonefiring depends on net input of other cellsThe trigger zone takes in both EPSP and IPSPs. If there are more EPSPs threshold will be reached.temporal summationsingle synapse receives many EPSPs in short timespatial summationsingle synapse receives many EPSPs from many cells
34 Summation of EPSP’sDoes this represent spatial or temporal summation?
35 Action Potentials vs. Graded Potentials Action Potentials (All-or-none phenomenon)action potentials either completely, or not at allGraded Potentials ( sub threshold)EPSP and IPSPs areare graded (vary in magnitude with stimulus strength)are decremental (get weaker the farther they spread)are reversible as K+ diffuses out of cell
36 Clinical Applications Many drugs work by altering neuronal functions.Block the receptor siteBeta blockers: prevent sympathetic input to the heart and various organsBlock the reabsorption of the neurotransmitterSSRI: Prevent the reuptake of serotonin so it stays in the synaptic cleft longer and continue to stimulate nerveBind to the receptor siteCurare blocks the (ACh) acetylcholine receptors by binding to the same position on the receptor