Chapter 7 - Nervous System Overview; Neurons Nervous system: • Provides rapid, brief responses to stimuli Endocrine system: • Adjusts metabolic operations and directs longterm changes Nervous system includes: • All the neural tissues of the body Brain, spinal cord, nerves • Basic unit = the neuron 1!
Divisions of the Nervous system CNS (Central Nervous system) • Brain and spinal cord PNS (Peripheral Nervous system) • Neural tissues outside CNS • Afferent division brings sensory information to CNS from receptors • Efferent division carries motor commands to effectors from CNS
2!
1
Neurons - Schematic Neuron
3!
Neuron Anatomy
Figure 7.2
4!
2
Parts of a Neuron - Cell Membrane Membranes contain different types of channels
Dendrites (many) • Receive information from other neurons • Carry information towards cell body • Do not typically generate nerve impulses (action potentials) Axon (one only) • Connects to cell body at axon hillock • Carries information away from cell body • Initial segment generates action potentials
6!
3
Parts of a Neuron
Telodendria • Small branches at the end of an axon Synaptic terminals - ends of the telodendria • a.k.a. axon endings, synaptic end bulbs, boutons, synaptic knobs • Store neurotransmitter • Release neurotransmitter in response to nerve impulses 7!
Neuron Classification Schemes Neurons are classified: • Structurally (based upon anatomy) • Functionally (based upon physiology)
8!
4
Structural Classification - Unipolar Neurons • Unipolar = one process attached to cell body • Are sensory neurons • Cell bodies in dorsal root ganglion • Up to 1 meter long!!
Direction of information flow
9!
Bipolar neuron • Bipolar = two short processes attached to cell body • Relatively rare Direction • Sensory neurons of information • Eye, ear, nose flow
10!
5
Multipolar Neuron Multipolar • More than two processes attached to cell body • Most common type of neuron • e.g. motor neurons innervating skeletal muscles
Direction of information flow
11!
Functional Classification of Neurons Sensory = afferent neurons • Carry information towards CNS Motor = efferent neurons • Carry information away from CNS Interneuron = association = internuncial neurons • Carry information within the CNS
12!
6
Neuroglia or Glial Cells (that you should know about) Neuroglia = “nerve glue” A. Glial cells of the CNS: 1. Ependymal cells 2. Astrocytes 3. Oligodendrocytes 4. Microglia B. Glial cells of the PNS Schwann cells 13!
Ependymal Cells and Astrocytes 1. Ependymal cells: • Assist in cerebrospinal fluid (CSF) production • Assist in CSF circulation 2. Astrocytes - “star cells” • Maintain blood-brain barrier • Support neurons physically, physiologically • Repair damage → scar 14!
7
Oligodendrocytes and Microglia 3. Oligodendrocytes = “few branches” compared to astrocytes • Myelinate CNS axons (speeds up nerve impulses) • Myelinate more than one axon at a time (in contrast to Schwann cells in PNS) 4. Microglia - “small” • Phagocytic Remove cell debris, wastes, pathogens
15!
PNS Glial Cells - 1
(Figure 7.3a)
Schwann cells (neurilemmocytes) • Myelinate peripheral axons (myelin sheath) • Myelination speeds up action potential • Myelin sheath also important in axon repair
16!
8
Schwann Cells - 2
(Figure 7.3b, c)
17!
millivolts
Membrane Potentials
Action Potential (“nerve impulse”) 18!
9
The Transmembrane Potential (1 of 2) • Transmembrane potential = Electrochemical gradient • “Potential” = voltage difference across a membrane • Arises from chemical and electrical forces acting across the cell membrane • Usually reported in millivolts (mV) • Inside of membrane is negative compared to the outside of the membrane
19!
The Transmembrane Potential (2 of 2) Factors determining transmembrane potential Flux = P • ΔC a. Ion concentration differences [Na+] and [K+], inside vs. outside cell b. Membrane permeability differences for ions Membrane channel types: • Leak channels • Gated channels
20!
10
Types of Potentials (1 of 2) 1. Resting (membrane) potential • Voltage difference across the cell membrane for an unstimulated (“resting”) cell 2. Equilibrium potential (for a particular ion) • Membrane voltage at which electrical and concentration difference forces acting on an ion are equal • No net diffusion of the ion occurs at this membrane potential 21!
Types of Potentials (2 of 2) 3. Graded potentials • Local changes in membrane potential due to chemical or physical changes in the membrane • Do not self-regenerate or spread over long distances 4. Action potentials • Self-regenerating changes in membrane potential due to chemical or physical changes in the membrane • Spread over long distances (along axons) 22!
Equilibrium Potential – Important, but not difficult • Membrane potential at which electrical and concentration difference forces on a particular ion are equal (Nernst equation) • This results in NO NET MOVEMENT of the ion across the membrane. Eion= (-58mV) log [ion]inside = ? mV [ion]outside Equilibrium potential for K+ (EK+) = -90 mV Equilibrium potential for Na+ (ENa+) = +66 mV 28!
14
Changes in the Transmembrane Potential Membrane at rest = “polarized” Ions cross the membrane through: 1. Leak channels - always open 2. Gated channels - open or closed a. Chemically-gated (ligand-gated) channels b. Voltage-gated channels c. Mechanically-gated channels
29!
1. Leak (Passive) Channels Always open • Ions “leak” down their electrochemical gradients e.g. K+ leak channels, Na+ leak channels • Size, charge, etc. determine which ion(s) can pass through a channel • Determine resting permeabilities for membrane • PK at rest 50 - 100x greater than PNa+ • There are 50 – 100x as many K+ leak channels
30!
15
2A. Chemically-gated Channels Open after binding a specific chemical (ligand) E.g. Acetylcholine receptor Binding of ACh changes shape of receptor. • Channel becomes permeable to both Na+ and K+. • Which ion will move most rapidly through this channel??? • What will be the effect of this ion’s movement on the membrane potential??? • Why??? 31!
2B. Voltage-gated Channels • Channel opens and/or closes in response to changes in membrane potential • Important in action potential conduction, neurotransmitter release from end bulbs • E.g. voltage-gated K+, Na+ and Ca2+ channels
2C. Mechanically-gated Channels • Open or close in response to physical distortion • e.g. touch, pressure receptors
32!
16
Depolarization and Hyperpolarization
Depolarization Inside more positive than at rest e.g. Na+ enters cell
Hyperpolarization Inside more negative than at rest e.g. K+ leaves cell e.g. Cl- enters cell 33!
Action Potential - Introduction A sudden major change in membrane potential Is an All-or-none phenomenon: Either an action potential happens or it doesn’t! • Occurs when membrane reaches a specific membrane voltage called threshold • Does not degrade over long distances • Depends upon the presence of voltage-gated Na+ and K+ channels • At threshold, voltage-gated Na+ channels open 34!
Leak channels for both ions are open and are unaffected by the voltage changes to come. 36!
18
Action Potential - “Step #1” Chemically-gated Na+ channels open. Local current flow depolarizes membrane towards threshold, but threshold has not been reached. #1
Membrane depolarizes to about +30mV Voltage-gated Na+ channels • Closed Voltage-gated K+ channels • Change in voltage causes these to open • K+ efflux begins repolarization
39!
Action Potential - “Step 4” Re- and Hyperpolarization Membrane potential greater (more negative) than at rest Voltage-gated Na+ channels • Closed #4
Voltage-gated K+ channels • Open until voltage reaches about -90 mV
40!
20
Action Potential - Back to Rest Back to resting conditions Voltage-gated Na+ channels • Closed Voltage-gated K+ channels • Closed Note that leak channels have remained open throughout this process Back to resting potential 41!
Action potential travels along an axon Information passes from presynaptic neuron to postsynaptic neuron
45!
Information Processing (2)
Post-synaptic cell receives many inputs Effect of presynaptic cell activity on postsynaptic cell’s membrane = postsynaptic potential 46!
23
Postsynaptic Potentials Excitatory Postsynaptic Potential - EPSP • Postsynaptic cell moved closer to threshold • More likely to fire action potential Inhibitory Postsynaptic Potential - IPSP • Postsynaptic cell moved further from threshold • Less likely to fire action potential
47!
Summation Postsynaptic potentials are added together • If initial segment reaches threshold → action potential • If initial segment does not reach threshold → no action potential Temporal summation • Single synapse active repeatedly Spatial summation • Different synapses active at same time 48!
24
Summation Scheme
Temporal summation: Neuron A fires repeatedly Spatial summation: Neurons A and B fire at the same time 49!
