Nerve Function PDF

Nerve and how it Functions nction of the Ner vous Syste sensory input motor input sensory receptor effector integration uron Interaction & Integrati sensory neuron interneu...

Nerve and how it Functions nction of the Ner vous Syste sensory input motor input sensory receptor effector integration uron Interaction & Integrati sensory neuron interneuron sensory receptors effector motor neuron Neuroglia  Typical Neuron dendrite cell body Myelin sheath axon Synapse  Myelin Sheath & Shwann Cell Nodes of Ranvier Schwann Axon Cells Types of Neurons unipolar bipolar multipolar Dorsal root eye, ear, & olfactory most abundant type in CNS ganglion cells Information must be transmitted  within each neuron  and between neurons  The membrane surrounds the neuron.  It is composed of lipid and protein.  There is an electrical charge across the membrane.  This is the membrane potential.  The resting potential (when the cell is not firing) is a 70mV difference between the inside and the outside. + + + + + outside - - - - - inside Resting potential of neuron = -70mV  Ions are electrically-charged molecules e.g. sodium (Na+), potassium (K+), chloride (Cl-).  The resting potential exists because ions are concentrated on different sides of the membrane. ◦ Na+ and Cl- outside the cell. ◦ K+ and organic anions inside the cell. Na + Na + Cl- Na+ Na+ Cl- outside Organic anions (-) inside K + Organic anions (-) K+ Organic anions (-)  Na+ ions are actively transported (this uses energy) to maintain the resting potential.  The sodium-potassium pump (a membrane protein) exchanges three Na+ ions for two K+ ions. Na+ Na+ Na+ outside inside K+ K+  The action potential is a rapid depolarization of the membrane.  It starts at the axon hillock and passes quickly along the axon.  The membrane is quickly repolarized to allow subsequent firing.  When partial depolarization reaches the activation threshold, voltage-gated sodium ion channels open.  Sodium ions rush in.  The membrane potential changes from -70mV to +40mV. Na+ + - - Na+ Na+ +  Sodium ion channels close and become refractory.  Depolarization triggers opening of voltage-gated potassium ion channels.  K+ ions rush out of the cell, repolarizing and then hyperpolarizing the membrane. Na+ K+ K+ + Na+ Na+ K+ -  The action potential is “all-or-none”.  It is always the same size.  Either it is not triggered at all - e.g. too little depolarization, or the membrane is “refractory”;  Or it is triggered completely.  The action potential begins with a partial depolarization (e.g. from firing of another neuron ) [A].  When the excitation threshold is reached there is a sudden large depolarization [B].  This is followed rapidly by repolarization [C] and a brief hyperpolarization [D].  There is a refractory period immediately after the action potential where no depolarization can occur [E] +40 Membrane [C] potential 0 [B] [E] (mV) [A] [D] excitation threshold -70 0 1 2 3 Time (msec)  Passive conduction will ensure that adjacent membrane depolarizes, so the action potential “travels” down the axon.  But transmission by continuous action potentials is relatively slow and energy- consuming (Na+/K+ pump).  A faster, more efficient mechanism has evolved: saltatory conduction.  Myelination provides saltatory conduction.  Most mammalian axons are myelinated.  The myelin sheath is provided by oligodendrocytes and Schwann cells.  Myelin is insulating, preventing passage of ions over the membrane.  Myelinated regions of axon are electrically insulated.  Electrical charge moves along the axon rather than across the membrane.  Action potentials occur only at unmyelinated regions: nodes of Ranvier. Myelin sheath Node of Ranvier  Information is transmitted from the presynaptic neuron to the postsynaptic cell.  Chemical neurotransmitters cross the synapse, from the terminal to the dendrite or soma.  The synapse is very narrow, so transmission is fast.   An action potential causes neurotransmitter release from the presynaptic membrane.  Neurotransmitters diffuse across the synaptic cleft.  They bind to receptors within the postsynaptic membrane, altering the membrane potential. extracellular fluid terminal synaptic cleft presynaptic membrane postsynaptic membrane dendritic spine  Ca2+ causes vesicle membrane to fuse with presynaptic membrane.  Vesicle contents empty into cleft: exocytosis.  Neurotransmitter diffuses across synaptic cleft. Ca2+  Synaptic activity at ionotropic receptors is fast and brief (milliseconds).  Acetylcholine (Ach) works in this way at nicotinic receptors.  Neurotransmitter binding changes the receptor’s shape to open an ion channel directly. ACh ACh  Opening of ion channels which leads to depolarization makes an action potential more likely, hence “excitatory PSPs”: EPSPs. ◦ Inside of post-synaptic cell becomes less negative. ◦ Na+ channels (NB remember the action potential) ◦ Ca2+. (Also activates structural intracellular changes -> learning.) + Na+ Ca2+ outside - inside  Opening of ion channels which leads to hyperpolarization makes an action potential less likely, hence “inhibitory PSPs”: IPSPs. ◦ Inside of post-synaptic cell becomes more negative. ◦ K+ (NB remember termination of the action potential) ◦ Cl- (if already depolarized) Cl- + outside - inside K+ For the synapse to work properly, six basic events need to happen:  Production of the Neurotransmitters ◦ Synaptic vesicles (SV)  Storage of Neurotransmitters ◦ SV  Release of Neurotransmitters  Binding of Neurotransmitters ◦ Lock and key  Generation of a New Action Potential  Removal of Neurotransmitters from the Synapse ◦ reuptake  PSPs are small. An individual EPSP will not produce enough depolarization to trigger an action potential.  IPSPs will counteract the effect of EPSPs at the same neuron.  Summation means the effect of many coincident IPSPs and EPSPs at one neuron.  If there is sufficient depolarization at the axon hillock, an action potential will be triggered. axon hillock