Neurons – Structure and Functions (2) PDF

Nervous system About this Chapter Organization of the nervous system Electrical signals in neurons Cell-to-cell communication in the nervous system  The Nervous System (NS)  Functions  Sensory function : gathering information  Integrative function : processes & interprets...

Nervous system About this Chapter Organization of the nervous system Electrical signals in neurons Cell-to-cell communication in the nervous system  The Nervous System (NS)  Functions  Sensory function : gathering information  Integrative function : processes & interprets sensory input, and decides if action is needed  Motor function : response to integrated stimuli  effectors  The NS is classified by structure & function Structural Classification Functional Classification The Nervous System Cells ofthe Nervous System Neurons – Functional Classification 1. Afferent – Conduct APs to the CNS 2. Interneurons – Conduct APs within the CNS 3. Efferent – Conduct APs to the PNS  Nervous Tissue: The Neurons  Functional cells of the NS  Specialized to transmit stimuli  Consists of:  cell body: nucleus & cell organelles  cytoplasmic extensions from the cell body form  Dendrites  Axons The Nervous System Cells ofthe Nervous System Cells are grouped into two functional categories – Neurons Do all of the major functions on their own, are 1. Afferent 2. Interneurons 3. Efferent – Neuroglia Play a supporting role to the neurons Divided into CNS and PNS Neuroglia – CNS – PNS » Astrocytes » Neurolemmocytes » Oligodendrocytes (Schwann cells) » Microglia » Satellite cells » Ependymal cells Electrical Signals in Neurons  Neurons are electrically ex citable due to the voltage difference across their membrane. Electrical Signals in Neurons  Neurons communicate with two types of electric signals:  graded potentials that are used for short- distance communication only ( i.e.local membrane changes).  action potentials that can travel over both short and long distances within the body.  (Nerve action potential is called nerve impulse)  As you touch the pen, a graded potential develops in the sensory receptors in the sk in of the finger.  The graded potential trigger an action potential which travel to the CNS. Electrical Signals in Neurons  The production of graded potentials and action potentials depends on:  The resting membrane potential.  (an electrical voltage difference across the plasma membrane)  The presence of certain ion channels. Types of I on Channels  Leak age (nongated)channels  These channels are randomly alternate between open and closed positions ( always open).  Nerve cells have more K+ than Na+ leak age channels  Membrane permeability to K+ is higher which explains the resting membrane potential of -70mVin nerve tissue.  Gated channels open and close in response to a stimulus  results in neuron ex citability Gated I on Channels 1. Voltage-gated channels respond to a direct change in the membrane potential. They participate in the generation and conduction of action potentials. 2. Ligand (chemical)-gated channels respond to a specific chemical stimulus. Chemical ligands include neurotransmitters, hormones and ions. L igands may act directly ( e.g.acetylcholine)or indirectly ( e.g. hormones) 3. Mechanically gated ion channels respond to mechanical vibration ( sound), pressure or stretching Terminology Associated with Changes in Membrane Potential Polarized:+ve outside and –ve inside at rest Depolarization:tracing moves upwards from rest (becomes more +ve,e.g.Na+ enters cell). Repolarization:tracing moves back to rest (returns to rest,K+ exits cell). Hyperpolarization:tracing moves downwards from rest (becomes more -ve,e.g.K+ exits cell). Graded Potentials  Action Potential Action potential(AP)is a sequence ofelectrochemicalchanges that increase membrane potentialfrom resting value ofabout -70mV to a peak ofabout +30mV (depolarization),and then return it back to -70mV again (repolarization).  ActionPotential Chemicalor mechanicalstimulus caused a graded potentialto reach at least (-55mV or threshold) Ifgraded potentialreaches threshold AP occurs (Allor none principle). Voltage-gated Na+ channels open & Na+ rushes into cell(Depolarization). only a totalof20,000 Na+ actually enter the cell,but they change the membrane potentialconsiderably(up to +30mV). Positive feedback process. Voltage-gated K+ channels open but slowly & K+ rushes outside cell (Repolarization) The Action Potential:Summarized Potentials in Electrical Signaling The Process Action Potential Formation Membrane is at rest -70mV ECF Chemically gated Na+ channel ICF Potentials in Electrical Signaling Action Potentials – The process – Excitatory stimulus (mechanical, electrical, chemical) applied & activates corresponding Na+ gated channel ECF Chemically gated Na+ channel ICF Potentials in Electrical Signaling Action Potentials – The process – Na+ enters in causing slight depolarization Possibly to threshold ECF Chemically gated Na+ channel ICF Potentials in Electrical Signaling Action Potentials – The process – The Rising phase – If threshold is reached All of the voltage gated Na+ channels will open, increasing membrane permeability some 6000 fold! Causing further depolarization of the membrane to +30 mV ECF V-gated Na+ Channels V-gated Na+ Channels Chemically gated V-gated Na+ Channels V-gated Na+ Channels V-gated Na+ Channels Na+ channel ICF Potentials in Electrical Signaling Action Potentials – The process – The falling phase next, slow voltage gated K+ channels open K+ flows down its concentration gradient… Membrane potential falls ECF V-gated Na+ Channels V-gated Na+ Channels V-gated Na+ Channels V-gated Na+ Channels V-gated Na+ Channels Slow V-gated K+ Slow V-gated K+ Slow V-gated K+ Slow V-gated K+ Channels Channels Channels Channels ICF Potentials in Electrical Signaling Action Potentials – The process – In the meantime… – The voltage gated Na+ channels have closed (both gates) – Membrane potential continues to fall as K+ continues its outward flow ECF V-gated Na+ Channels V-gated Na+ Channels V-gated Na+ Channels V-gated Na+ Channels V-gated Na+ Channels Slow V-gated K+ Slow V-gated K+ Slow V-gated K+ Slow V-gated K+ Channels Channels Channels Channels ICF Potentials in Electrical Signaling Action Potentials – The process – Next the slow voltage gated K+ channels start to close – There is additional K+ that diffuses through during the closing, causing membrane potential to hyperpolarize slightly ECF V-gated Na+ Channels V-gated Na+ Channels V-gated Na+ Channels V-gated Na+ Channels V-gated Na+ Channels Slow V-gated K+ Slow V-gated K+ Slow V-gated K+ Slow V-gated K+ Channels Channels Channels Channels ICF  Potentials in Electrical Signaling Action Potentials – The process – The Na+/K+ ATPase restores the resting membrane potential ECF V-gated Na+ Channels V-gated Na+ Channels V-gated Na+ Channels Slow V-gated K+ Slow V-gated K+ Channels Channels ICF  Potentials in Electrical Signaling Action Potentials – The process – The Na+/K+ ATPase restores the resting membrane potential ECF V-gated Na+ Channels V-gated Na+ Channels V-gated Na+ Channels Slow V-gated K+ Slow V-gated K+ Channels Channels ICF ATP  Potentials in Electrical Signaling Action Potentials – The process – The Na+/K+ ATPase restores the resting membrane potential ECF V-gated Na+ Channels V-gated Na+ Channels V-gated Na+ Channels Slow V-gated K+ Slow V-gated K+ Channels Channels ICF ATP ADP  Potentials in Electrical Signaling Action Potentials – The process – The Na+/K+ ATPase restores the resting membrane potential ECF V-gated Na+ Channels V-gated Na+ Channels V-gated Na+ Channels Slow V-gated K+ Slow V-gated K+ Channels Channels ICF  Potentials in Electrical Signaling Action Potentials – The process – The Na+/K+ ATPase restores the resting membrane potential ECF V-gated Na+ Channels V-gated Na+ Channels V-gated Na+ Channels Slow V-gated K+ Slow V-gated K+ Channels Channels ICF  Potentials in Electrical Signaling Action Potentials – The process – The Na+/K+ ATPase restores the resting membrane potential ECF V-gated Na+ Channels V-gated Na+ Channels V-gated Na+ Channels Slow V-gated K+ Slow V-gated K+ Channels Channels ICF  W hy does the AP only travelin ONE direction (towards the synapse)? Because the voltage gated Na+ and K+ channels undergo a refractory period  Refractory Period ofAction Potential Period oftime during which neuron can not generate another action potential. Absolute refractory period even very strong stimulus will not begin another AP inactivated Na+ channels must return to the resting state before they can be reopened Relative refractory period a suprathreshold stimulus willbe able to start an AP K+ channels are stillopen,but Na+ inactivation channels have return to resting state. Propagation ofAction Potential  SignalTransmission at Synapses El ectrical Chemical ChemicalSynapses Structure ofneurotransmitter receptors 1-Ionotropic receptor  I onotropic receptor Ionotropic receptors form an ion channel pore. When an ionotropic receptor is activated, it opens a channel that allows ions such as Na+, K+, or Cl- to flow 2-Metabotropic receptor  Excitatory & Inhibitory Potentials The effect ofa neurotransmitter can be either excitatory or inhibitory EPSP it results from the opening ofligand-gated Na+ channels the postsynaptic cellis more likely to reach threshold to give AP IPSP it results from the opening ofligand-gated Cl-or K+ channels it causes the postsynaptic cellto become more negative or hyperpolarized  RemovalofNeurotransmitter Diffusion move down concentration gradient Enzymatic degradation acetylcholinesterase Uptake by neurons or glia cells neurotransmitter transporter Summation SpatialSummation Summation ofeffects of neurotransmitters released from several presynaptic end bulbs onto one neuron TemporalSummation Summation ofeffect ofneurotransmitters released from 2 or more firings ofthe same end bulb in rapid succession onto a second neuron  Neurotransmitters Small-Molecule Neurotransmitters Acetylcholine (ACh) released by many PNS neurons & some CNS excitatory on NMJ but inhibitory at others inactivated by acetylcholinesterase Amino Acids Glutamate is excitatory neurons in the brain inactivated by reuptake. GABA is inhibitory neurotransmitter for 1/3 ofall brain synapses  Small-Molecule Neurotransmitters Biogenic Amines modified amino acids (decarboxylation) catecholamine norepinephrine epinephrine dopamine serotonin  Small-Molecule Neurotransmitters ATP and other purines (ADP,AMP & adenosine) excitatory in both CNS & PNS released with other neurotransmitters (ACh & NE) Gases (nitric oxide or NO) formed from amino acid arginine by NO synthase  Neuropeptides 3-40 amino acids linked by peptide bonds Substance P --enhances our perception ofpain Pain relief enkephalins --pain-relieving effect by blocking the release ofsubstance P