Review and In-depth Signaling

1. Review and In-depth Signaling FUNCTIONAL ORGANIZATION OF NERVOUS TISSUE 1

2. Organization of the Nervous System • The nervous system controls all behaviors and many of the internal systems involved in homeostasis. • Sensory input system – information from environment • Integrative system – decision making • Motor output system – action • Central Nervous System (CNS) • Includes Brain and Spinal cord • Peripheral Nervous System (PNS) • Includes Cranial nerves and Spinal nerves • PNS further subdivided into: • Sensory division (input) and Motor division (output) • Motor division further subdivided into: • Somatic and Autonomic • Autonomic further subdivided into: • Sympathetic and Parasympathetic 2

3. Blue arrows: afferent signals Red arrows: efferent signals 3

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5. Histology of Nervous Tissue • Nervous tissue is primarily cellular in nature. • Only 20% of the CNS is extracellular space. • Between “Epithelial” &“Connective” Tissues in cellularity • Nervous tissue contains basically 2 types of cells: • Neurons • Described on the next few slides • Neuroglia • Also known as Glia or Glial cells • There are various types of neuroglia • We will describe two in particular after describing the neurons. • They all help support neurons in some way. • Neuroglia outnumber neurons 10 to 1. 5

6. Neurons • Neurons are the principal functional cells of the nervous system • They have a great longevity. • They are amitotic. • They maintain a high metabolic rate. • Neurons may be divided into two main regions: • Cell Body • Processes • Dendrites (incoming information) • Axon (outgoing information) 6

7. Cell Body • The cell body or soma contains: • the nucleus • the protein synthesizing components • extensive rough ER known as Nissl bodies. • also contains extensive Golgi. • large number of mitochondria • This part of the cell is the integration part of the neuron. • It can also function as a receptive or input region. • Nucleus (pl. nuclei) is a collection of cell bodies in the CNS. • Ganglion (pl. ganglia) is the same thing in the PNS. 7

8. Neuron Processes (extensions) • There are 2 types of processes that extend from neurons: • Dendrites – the main receptive region of the neuron. • Axon– the main signal transmission part of the neuron. • The axon hillock is the initial part of the axon, where the outgoing signal is triggered. • Although only one axon leaves the cell body, this can branch into collaterals ending in the axon terminals. • The terminals release neurotransmitters. • A tract is a collection of axons in the CNS. • A nerve is the same thing in the PNS. 8

9. Structural Classification of Neurons • Neurons can be grouped by their number of processes. • Multipolar: 3 or more processes; most (99%) neurons in CNS; all motor neurons • Bipolar: 2 processes; sensory in retina of the eye and nose • Unipolar: 1 process that divides into two branches. Part that extends to the periphery has dendrite-like sensory receptors. Found in ganglia of PNS. 9

10. Signaling in Layman’s Terms • The function of neurons is to receive and transmit signals. • When a neuron receives a signal (explained later), • the signal is converted to a form that can be passed along to the cell body (explained later). • If the signal is large enough, • the axon hillock is stimulated to pass the signal along the axon (explained later) • and to the next cell (explained later). • If it is not strong enough, the signal is not passed along. 10

11. Background to Understand Neuron Signaling • Earlier this semester we studied an example of active membrane transport: the sodium-potassium pump. • The effect of the sodium-potassium pump is to create a difference between the inside and the outside of a cell. • By actively pumping positively charged sodium ions (Na+) out of the cell, while leaving behind negatively charged proteins, the cell ends up with more positive charges on the outside and more negative charges on the inside. • Voltage is the measurement used to quantify such a difference in charge. • The basic unit of measure is the volt. For cells, the mV. • The difference in charge (voltage) between the inside and the outside of the cell membrane is also known as the membrane potential difference or just membrane potential. 11

12. Source of Resting Membrane Potential • The resting membrane potential results from the concentrations of ions that are inside & outside the cell and the permeability of the membrane to those ions. 12

13. Measuring Membrane Potential • If we measure the membrane potential (difference in charge) between the inside of a neuron and the outside we find that the neuron is more negative inside than outside. • The difference in charge is typically 70 mV. • Because the inside of the cell is more negative, by convention, we say that a cell’s resting membrane potential is –70 mV. 13

14. Changes in Membrane Potential • A cell’s membrane potential is due to having different numbers of positively and negatively charged ions on the inside and outside of the cell. • Therefore, a cell’s membrane potential can be easily changed by allowing the movement of ions across the cell membrane. • Due to their charges, ions move across the membrane most easily through specific ion channels. • We will focus our attention on gated channels, which only open when they have received the appropriate signal. • The gated channels of interest here are • Chemically gated channels – only open (and thereby change membrane voltage) when the appropriate chemical is present. • Voltage gated channels – only open when the membrane voltage has already changed somewhat. Trend setters 14 Copy cats

15. Gated Ion Channels Chemically gated channel Found on dendrites and cell body Voltage gated channel Found on axon 15

16. Using Membrane Potentials • Neurons can use changes in membrane potentials to receive, integrate and send signals. • When a cell has a membrane potential, we say the membrane is polarized (has 2 poles, positive and negative). • Because membrane potentials are differences in electrical charges these signals can be described as electrical signals. • These electrical signals are not mysterious. They are simply the movement across the membrane of the charged ions described earlier. 16

17. Signaling in Layman’s Terms • The function of neurons is to receive and transmit signals. • When a neuron receives a signal (explained later), • the signal is converted to a form that can be passed along to the cell body (explained later). • If the signal is large enough, • the axon hillock is stimulated to pass the signal along the axon (explained later) • and to the next cell (explained later). • If it is not strong enough, the signal is not passed along. 17

18. Changes in Membrane Potential (Polarization) • All changes in electrical potential refer to changes from the resting membrane potential of -70mV. • Depolarization is a change toward 0 mV difference (less polarized). Achieved by allowing Na+ in. • Hyperpolarization is a change away from zero towards an even more negative resting potential (more polarized). Achieved by allowing Cl- in. 18

19. Signaling in Layman’s Terms • The function of neurons is to receive and transmit signals. • When a neuron receives a signal (explained later), • the signal is converted to a form that can be passed along to the cell body (explained later). • If the signal is large enough, • the axon hillock is stimulated to pass the signal along the axon (explained later) • and to the next cell (explained later). • If it is not strong enough, the signal is not passed along. 19

20. Size of signal • The changes in membrane potential that occur in the dendrites and cell body are of varying strength. • Because the strength varies from weak to strong, these signals are known as “graded” potentials. • If the graded potential is large enough, it can cause the voltage gated channels of the axon to open. Resulting in a cascade of opening voltage gated channels along the axon, known as a nerve impulse or action potential. 20

21. Signaling in Layman’s Terms • The function of neurons is to receive and transmit signals. • When a neuron receives a signal (explained later), • the signal is converted to a form that can be passed along to the cell body (explained later). • If the signal is large enough, • the axon hillock is stimulated to pass the signal along the axon (explained later) • and to the next cell (explained later). • If it is not strong enough, the signal is not passed along. 21

22. Action Potentials • Action potentials are simply large depolarizations of the axon made possible by voltage-gated (copy cat) ion channels. • An action potential will only occur if the preceding graded potential (caused by chemically-gated ion channels) was strong enough to cause the voltage-gated ion channels of the axon hillock to open. • Once the voltage-gated ion channels of the axon begin to open at the axon hillock, the wave of depolarization continues along the membrane until it reaches the axon terminal. • As the depolarization of the membrane continues down the axon, the strength of the depolarization remains the same. • The action potential is an all-or-none phenomenon. • The neuron either fires an action potential or not. 22

23. The Propagation of an Action Potential Along an Axon 23

24. Threshold and Coding Information • Unlike graded potentials which can vary in strength, action potentials are of the same size. • The larger the graded potential (stimulation) that led to the action potential, the more action potentials are fired. • Information is coded by the frequency of action potentials. • In other words, the strength of stimulation will determine the frequency. 24

25. View of the Neuron 25

26. Myelin Sheath is formed by 2 types of Glia • The myelin sheath is the wrapping seen around the axons of some neurons. • Oligodendrocytes – form myelin sheaths on axons in the CNS • Schwann cells – form myelin sheaths on axons in the PNS Schwann cells are vital to PNS nerve repair and regrowth of axons of damaged but living neurons • Myelinated fibers can conduct electrical signals much faster than unmyelinated fibers (about 150 times faster). • Gaps between myelin sheath cells are the Nodes of Ranvier. • Voltage gated ion channels only occur at these nodes. • Multiple Sclerosis (MS) leads to demyelination of CNS neurons resulting in numbness and paralysis. 26

27. Action Potential Speed • The speed with which an action potential travels depends on 2 factors: • Diameter of axon – the larger the faster • Degree of myelination • Myelinated – faster • Saltatory conduction allows depolarization to skip from node to node 27

28. Signaling in Layman’s Terms • The function of neurons is to receive and transmit signals. • When a neuron receives a signal (explained later), • the signal is converted to a form that can be passed along to the cell body (explained later). • If the signal is large enough, • the axon hillock is stimulated to pass the signal along the axon (explained later) • and to the next cell (explained later). • If it is not strong enough, the signal is not passed along. 28

29. Presynaptic Neuron Postsynaptic Neuron Synapses • Synapses are the connections between a neuron and another neuron or effector cell (muscle cell or gland cell). • Usually these are formed between axon terminals and cell dendrites or body. • The connection may be chemical if the cell membranes are not joined or electrical if they are. 29

30. Chemical and Electrical Synapses • The connection made at a chemical synapse contains a small synaptic cleft (20-50 nm) between the connected neurons. • The action potential may not be conveyed across the cleft as there is no membrane in the cleft. • This arrangement has two consequences: • 1) the axon terminal must transmit the signal across the cleft as a chemical (neurotransmitter). • 2) the postsynaptic cell is unable to “talk back” as the neurotransmitter is released by only the presynaptic neuron and then only interpreted by the postsynaptic neuron • Electrical synapses (syncitia) are formed when membranes of two neurons are connected by gap junctions. • These are very rare in mammalian adult nervous systems but more common in the developing fetal nervous system. 30

31. View of a Chemical Synapse 31

32. Signaling in Layman’s Terms • The function of neurons is to receive and transmit signals. • When a neuron receives a signal (explained later), • the signal is converted to a form that can be passed along to the cell body (explained later). • If the signal is large enough, • the axon hillock is stimulated to pass the signal along the axon (explained later) • and to the next cell (explained later). • If it is not strong enough, the signal is not passed along. 32

33. Postsynaptic Potentials • The interaction of neurotransmitters with their receptors can cause changes in the membrane potential. • The response to a given neurotransmitter depends on the type of receptor present on the postsynaptic site. • Excitatory postsynaptic potentials (EPSP) cause a depolarization of the membrane bringing a cell closer to achieving the threshold voltage required to fire an action potential. • Inhibitory postsynaptic potentials (IPSP) cause a hyperpolarization of the membrane making it less likely for the cell to achieve threshold voltage and fire an action potential. 33

34. Practice Questions 34

35. Neurons can be classified structurally by the number of processes extending from their cell body. Which is the most common neuron type in humans?    A All neurons have the same number of processes  B Tripolar  C Bipolar  D Unipolar  E Multiplolar 35

36. The interior of a nerve cell has a slight excess of negative charge because    A potassium is actively pumped into the cell.  B potassium diffuses out of the cell.  C sodium is actively pumped into the cell.  D sodium is actively pumped out.  E potassium is actively pumped out. 36

37. The Nissl bodies seen in the neuron cell body represents which cellular organelle?    A Mitochondria  B Microtubules  C Nucleus  D Rough endoplasmic reticulium  E Centrioles 37

38. Tracts in the CNS correspond to ____________ in the PNS.    A neuron cell bodies  B ganglia  C nerves  D myelin sheaths  E dendrites 38

39. A potential of –90 mV would be considered to be    A a normal resting potential.  B hyperpolarized.  C a graded potential.  D depolarized. 39

40. The sodium potassium pump provides energy for neurons, but does not affect resting membrane potential.  True False 40

41. Excitatory synapses can occur anywhere on a dendrite or soma, but it is at the axon hillock where an action potential is generated.  True False 41

42. The presence of the myelin sheath and the Nodes of Ranvier speed up the velocity of conduction along the axon.  True False 42

43. Unipolar neurons have a single short process that is an axon.  True False 43