Neurons. General neuron anatomy. BIOL 164 Human Biology Ch 7 Neurons. Nervous system:

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1 BIOL 164 Human Biology Ch 7 Neurons Nervous system: Neurons Integrates and coordinates the body s ac3vi3es Provides rapid and brief responses to s3muli Major divisions: Central nervous system (CNS) brain and spinal cord Peripheral nervous system (PNS) nerves (sensory and motor) Made up of nervous 3ssue: Neurons (nerve cells) excitable; can generate/ transmit electrochemical signals that carry informa3on Neuroglial (or glial) cells support/protect neurons General neuron anatomy Red arrows indicate direc.on of informa.on (electrochemical signal or nerve impulse) flow

2 Neurons vs. nerves Types of neurons: 1. Sensory (afferent) neurons carry info from sensory receptors towards CNS 2. Motor (efferent) neurons carry info away from CNS to an effector (muscle or gland) 3. Interneurons (associahon neurons) carry info within the CNS (between sensory and motor neurons) Nerves = bundles of neuron processes (axons and/or dendrites) in PNS May be sensory, motor, or mixed (which contain both sensory and motor neuron processes) Some glial glial cells Some glial cells of the CNS: 1. Ependymal cells help produce and circulate cerebrospinal fluid (CSF) 2. Astrocytes connect neurons to blood vessels; help form bloodbrain barrier 3. Microglia phagocytes that remove cell debris, wastes, pathogens 4. Oligodendrocytes myelinate (electrically insulate) axons in CNS Some glial cells of the PNS 1. Schwann cells myelinate (electrically insulate) axons in PNS (see next slide) Schwann cells and the myelin sheath Func3ons: electrically insulates axon, speeds up conduc3on of nerve impulse, protects, aids in axon repair

3 Ion flow across the neuron membrane The electrochemical signal (nerve impulse) depends on the flow of ions across the neuron membrane Ions may passively diffuse down their concentra3on gradients through open channels or be ac3vely pumped against their concentra3on gradients (a.k.a leak channels) (may open/close in response to electrical or chemical stimuli) (requires ATP) ResHng potenhal Res3ng Poten3al Membrane Poten3al Membrane potenhal (in general) a voltage difference across a plasma membrane due to a separa3on and unequal distribu3on of electrical charges The main charged par3cles involved: Na + and K + ions Usually reported in millivolts (mv) Membrane poten.al is an electrochemical gradient the ions have the poten3al to ( want to ) diffuse across the membrane due to concentra3on differences and electrical forces but are being prevented from diffusing across due to limited permeability (via channel proteins) The inside of the plasma membrane of most cells is more nega3ve compared to the outside of the membrane

4 Membrane potenhal of reshng neuron ResHng potenhal = voltage difference across plasma membrane for an uns3mulated ( res3ng ) neuron Na+ ions predominate outside, ions predominate (although not as strongly) inside, due to: ~ 70 mv for most neurons (inside is more nega3ve compared to outside) leak channels much more permeable than Na+ leak channels Na+ pump exchanges 3 Na+ for every 2, ejec3ng Na+ as quickly as it enters (Note: leak channels not shown below) Na+ Na+ Na+ Na+ Na+ Na+ Na Na+ Na+ Na+ + The achon potenhal (AP) (or nerve impulse) While most body cells have membrane poten3als only excitable cell types (such as neurons and muscle cells) are able to u3lize membrane poten3als to generate and transmit/conduct moving electrochemical signals that can spread/propagate like a wave along the membrane over long distances This electrochemical signal is called an achon potenhal (AP) or nerve impulse sudden/rapid reversal of the charge of the membrane poten3al (followed by repolariza3on) that travels along the membrane Once ini3ated, it is self regenera3ng does not diminish in intensity as it travels An AP in ac.on polarized (leak channels not shown) depolarization (threshold is reached) Gated Gated sodium channels close and are inactivated Gated Gated potassium channels close repolarization

5 An AP (as measured on a voltmeter) All or nothing either an AP happens all the way if threshold is reached or it doesn t happen at all if threshold isn t reached Refractory period brief 3me during which a new AP cannot be generated due to gated sodium channels being inac3vated un3l res3ng poten3al reestablished Depolarization Repolarization ^ always Refractory period Spread/propagaHon/conducHon of an achon potenhal An AP ini3ated at one part of the axon membrane depolarizes adjacent areas of the membrane to the threshold level causing nearby gated sodium channels to open and the AP to spread con3nuously down the membrane Wave of posi3vity enters the axon The refractory period keeps AP moving in one direc3on only Velocity of achon potenhal propagahon Influenced by: 1. Axon size (diameter) diameter, velocity 2. Presence of electrical insula3on (myelin) electrical insula3on, velocity See next slide: saltatory conduc.on

6 Saltatory conduchon Rather than opening and closing gated ion channels con3nuously along en3re length of axon it s quicker to do so only at the nodes of Ranvier (= unmyelinated axon segments between Schwann cells) The synapse Synapse = junc3on/connec3on between neuron and another cell typically another neuron or a cell of a muscle or gland Axon endings terminate in bulblike swellings called synaphc knobs Store chemicals called neurotransmiver in synap3c vesicles Release neurotransmicer in response to incoming AP Electron micrograph of a synapse

7 Transmission at an excitatory synapse (Slide 1 of 3) Transmission at an excitatory synapse Example of a neurotransmicer that may have an excitatory effect (Slide 2 of 3) acetylcholine (ACh) Transmission at an excitatory synapse Why excitatory? (Slide 3 of 3) Receptors are gated Na+ channels which when opened cause depolariza.on toward threshold If receptors are gated channels opening them would cause hyperpolariza.on away from threshold and the synapse would be inhibitory

8 InformaHon processing by neurons A neuron may have up to 10,000 synapses with other neurons from different sources some excitatory, some inhibitory First segment of axon of postsynap3c neuron sums up input: If net effect = threshold is reached, then it generates an AP If net effect = threshold is not reached, then it doesn t generate an AP (postsynaptic) of presynaptic neurons