Nervous System

1. Nervous System Part 3: Integration & Control

2. Nervous Tissue • One of the 4 basic tissue types in the human body. • 4 Functions: Known as the 4 C’s • Communicates • Commands • Controls • Coordinates

3. Types of Nervous Tissue Functions • Communication: Occurs through action potentials. • Action potentials: Electrical impulses triggered by ion flux. • Sensory Functions: Conveying information from external and internal stimuli to the CNS via afferent neurons. • Integrative Functions: Analyzing & storing sensory information; deciding on appropriate responses via interneurons. • Motor Functions: Responding to integrative decisions by carrying impulses from the CNS to the effectors in the organs via efferent neurons.

4. Divisions of the Nervous System • Central Nervous System: Consists of the brain and spinal cord. • Peripheral Nervous System: Consists of all nervous tissue outside the brain & spinal cord – 2 subdivisions: • Autonomic Nervous System (ANS) • Sympathetic Nervous System • Parasympathetic Nervous System • Enteric Nervous System • Somatic Nervous System (SNS)

6. Central Nervous System (CNS) • Structures: • Brain • Spinal Corn • Both will be talked about in detail in Chapter 11!

8. Peripheral Nervous System (PNS) • Consists of all nervous tissue located outside of the CNS. • Includes Ganglia, which are small masses of nervous tissue. • Subdivisions: • Autonomic Nervous System (ANS) • Sympathetic Nervous System • Parasympathetic Nervous System • Enteric Nervous System • Somatic Nervous System (SNS)

10. Autonomic Nervous System (ANS) • Consists of sensory neurons that pass information from the autonomic sensory receptors in the visceral organs & motor neurons to the CNS and back. • 2 Major Branches: • Sympathetic Nervous System: Triggers fight-or-flight response. • Parasympathetic Nervous System: Returns body to homeostasis when threat has passed.

11. Enteric Nervous System (ENS) • Considered a “small brain” since it has so many neurons. • Some debate about its classification is still continuing. Right now it is classified as a subdivision of the peripheral nervous system, but receives most of its innervations from the autonomic nervous system so is also classified as a subdivision of the ANS.

12. Somatic Nervous System (SNS) • Consists of the… • Sensory neurons passing information from receptors to the sensory areas of the CNS • Motor neurons passing impulses from the SNS to the skeletal muscles.

13. Types of Neurons • 3 Basic Types of Neurons: • Sensory Neurons aka Afferent Neurons: Detect stimuli & carry impulses from the cranial or spinal nerves to the CNS. • Motor Neurons aka Efferent Neurons: Carry impulses away from the CNS from the brain to the spinal cord and PNS. • Interneurons aka Association Neurons: Located inside the CNS and perform nervous system integrative functions.

15. Properties of Neurons • Properties shared by all neurons: • Excitability: Ability to respond to stimuli & changes in the environment that excites a sensory receptor, neuron, or muscle fiber. • Conductivity: The ability to produce and respond to electrical signals to aid in communication. • Secrete: The ability to secrete chemical neurotransmitters to aid in communication.

16. The Nerve Cell • 3 Basic Parts: • Dendrites • Soma • Axon

17. The Nerve Cell • Dendrites: Consists of many branches coming out of the soma of the cell. • Function to receive information from the synaptic clef. • Process: Fibers that run to dendrites and from axons.

18. The Nerve Cell • Soma: The cell body consisting of a central nucleus surrounded by cytoplasm & containing standard organelles. • Nissl Bodies: Clusters of endoplasmic reticulum found in the soma.

19. The Nerve Cell • Axon: The long, single-stranded portion of the neuron exiting from the opposite end of the soma from the dendrites. • Functions to transmit information to the synaptic clef. • Axon Hillock: The point of connection between the axon to the soma. • Axon Terminals: The tiny branches dividing off at the end of the axons. • Axoplasm: The cytoplasm of the axon.

20. The Nerve Cell • Myelin Sheath: Thin layer of fat cells around the axon that greatly increases the speed of neural transmission. • Produced by Schwann Cells and Oligodendrocytes. • Nodes of Ranvier: Gaps in the myelin sheath that occur at regular intervals to help impulses “skip” between sections & therefore travel faster. • Salutatory Conduction: The process of signals “skipping” across the nodes of Ranvier to increase transmission speed.

22. Classification of Neurons • Based on myelin: • Myelinated: Neurons that have a myelin sheath. • Unmyelinated: Neurons that do not have a myelin sheath. • Based on number of axons & dendrites: • Multipolar: Neurons that have a single axon and several dendrites – the most common! • Bipolar: Neurons with a single axon and single dendrite – found in the eye, inner ear, and olfactory region of the brain. • Unipolar: Neurons where the axon and dendrite have fused into a single process (axon) that branches in two directions. Primarily serves as a sensory neuron.

23. Types of Matter • Gray Matter: Contains mainly nerve cell bodies and unmyelinated fibers. • Found in the outer layer of the brain. • Found in the inner layer of the spinal cord. • White Matter: Contains mainly myelinated axons and appears to be white. • Found on the inside of the brain. • Found on in the outer later of the spinal cord. Cerebeluum

25. Synapses • Synapse: The point at which one neuron communicates with another. • Synaptic Cleft: The slight gap between two neurons where the synapse occurs. • Presynaptic Neuron: The neuron transmitting the signal through its axon. • Postsynaptic Neuron: The neuron receiving the signal through its dendrites.

27. Synapses • Two types of synapses exist, named for their method of communication transmission. • Electrical Synapses: Electrical signals are directly transmitted between cells. • Gap Junctions convey electrical signals directly between adjacent cells. • Provide faster communication & can be produced in unison by a large number of neurons & muscle fibers. • More common in smooth muscle, cardiac muscle, and in embryos.

28. Synapses • Chemical Synapses: Membranes do not touch, so signals do not pass directly. • Cells respond to neurotransmitters released from synaptic knobs into the synaptic cleft binding to receptor sites. • Steps: • The presynaptic cell translates the electrical impulse into a chemical signal. • The postsynaptic cell translates the chemical signal back into an electrical impulse. • Steps cause a synaptic delay close to 0.5 microseconds.

29. Neuraglia • Neuroglia: Support cells of the nervous system. • Cells smaller than neurons and outnumbering them 50 to 1. They do not generate impulses but can divide & multiply. • Types of Neuroglia: • Schwann Cells: Cells located in the PNS that produce myelin. • Neurolemma aka Sheath of Schwann: Outer cytoplasmic layer of the Schwann cells containing its own nucleus. • Oligodendrocytes: Cells located in the CNS that produce myelin. • Astrocytes: Star-shaped cells that... • Provide nutrients • Help regulate the chemical environment • Absorb excess neurotransmitters • Assist in forming the blood-brain barrier

30. Neuroglia • Types of Neuroglia: • Microglia: Macrophage cells that act as an immune response to protect the CNS from disease. • Ependymal Cells: Cells that line the brain. • Help circulate the cerebrospinal fluid in the CNS using cilia on the outside of the cells. • Satellite Cells: Help support the other cells of the PNS.

31. Electrophysiology • Electrical Potential: The gradient difference in the concentration of charged particles between points. • Polarized: The term used to describe cells that have an electrical potential. • Depolarized: The term used to describe cells that do not have an electrical potential.

32. Electrophysiology • Membrane Potential: The potential electrical voltage difference stored in the resting membrane. • Works the same way a car battery does! • Resting Membrane Potential: The amount of electrical charge built up as a result of…. • Negative ions lined up along the inside of the cell membrane • Positive ions lined up along the outside of the • Potassium (K+) ions leaking outward at a faster rate than Sodium (Na+) ions permeating inward • Resting Membrane Potential of a polarized neuron is -70mV (minivolts).

33. Neural Communication • Neurons communicate through action potentials brought on by polarization & depolarization. • Ion Channels: Channels in the cell membrane that allow ions to move in and out of the cell to create the resting membrane potential. • Gates: The method of controlling ion passage.

34. Neural Communication • 4 Types of Ion Channels: • Leakage Channels: Gates open and close randomly – regulated by normal permeability. • Voltage-Gated Channels: Open in response to a change in the membrane potential. • Voltage-gated sodium channels are blocked with local anesthetics, which prevents the pain signal from reaching the CNS. • Ligand-Gated Channels: Open and close in response to chemical stimuli – primarily regulated by neurotransmitters and hormones. • Mechanically Gated Channels: Open and close in response to outside mechanical stimulation. • Example: Auditory receptors in the ears stimulated by sound waves or physical pressure.

35. Neural Communication • Local Potential or Graded Potential: Occurs when only a small deviation is found in the resting potential. • Only occurs due to the opening of a ligand-gated or mechanically-gated channel. • Potentials vary in size, are only effective for a short distance, and can be excitatory, inhibitory, & reversible.

36. Action Potentials • Action Potentials aka Impulses: Generated along voltage-gated ion channels. • Consist of rapid changes in membrane voltage. • Triggered by depolarization of the cellular membrane.

37. Action Potentials • Stimuli triggers the start of the neural process. • Depolarization occurs as the resting membrane voltage changes toward threshold level. • Threshold is reached when the voltage rises enough to open the voltage-gated ion channels. • Typically occurs at -55mV (minivolts) in neurons.

38. Action Potentials • Gates Open: • Sodium Gates open quickly, allowing an inrush of sodium ions. • Potassium Gates open slowly, causing a trickle of potassium ions into the cell. • Sodium Influx only lasts a few 10,000’s of a second and causes: • Membrane to reach +30mV (minivolts) • Sodium gates to close • Potassium gates to fully open • Repolarization occurs as the potassium levels rapidly drop, causing the voltage to drop back to -55mV (minivolts).

39. Action Potentials • Refractory Period: The amount of time necessary for the cell to return to its resting state via repolarization. • Absolute Refractory Period: The period when the repolarization is less than one-third complete and cannot generate another action potential, no mater what stimuli occur. • Relative Refractory Period: Once repolarization is one-third complete, stimuli considerably stronger than the threshold stimulus can produce an action potential.

40. Action Potentials • All-Or-Nothing Law: An action potential will occur once the voltage level reaches threshold, no matter what. • It will always be the same size • The signal will always travel down the entire length of the neuron • The action potential will not fire until it reaches threshold, and will always fire once it does.

42. Methods of Conduction • Continuous Conduction: When an impulse travels around 2m/sec (meters per second) down an unmyelinated neuron. • Saltatory Conduction: When an impulse travels around 120m/sec down a myelinated neuron. • The impulse “jumps” from new action potentials generated at each node of Ranvier.

43. Neurotransmitters • Neurotransmitters: Chemical signals transmitted and interpreted by the nervous system. • Neuroreceptors: Sites located on the postsynaptic neuron that binds to neurotransmitters. • Binding causes an influx of Sodium (Na+) which depolarizes the neuron and fires of an action potential. • Synaptic Vesicles: Sacs on the knobs on the end of the axons that are used to store neurotransmitters.

44. Neural Integration • Excitatory Postsynaptic Potential (EPSP) aka Excitatory Neurotransmitters: Brings the charge closer to threshold being reached by depolarizing the membrane. • The larger the number of EPSPs the higher the change that the threshold will be reached. • Effect of EPSPs must be grater than the effect of the IPSPs. • Inhibitory Postsynaptic Potential (IPSP) aka Inhibitory Neurotransmitters: Takes the charge farther away from threshold by hyperpolarizing the membrane.

45. Neural Integration • Summation: The process of a neuron integrating the information from thousands of synapses providing incoming information at a time. • Trigger Zone: The point of reception for the summation of incoming signals. • Two forms of summation: • Temporal Summation: A single presynaptic neuron generates enough EPSPs in short intervals, allowing signals to build up before the first ones decay. • Spatial Summation: EPSPs from several different postsynaptic neurons are added up to reach the threshold. • Facilitation: When neurons enhance the effects of other neurons. • Inhibition: When neurons inhibit or suppress the effects of other neurons.

46. Neurotransmitters • Over 100 neurotransmitters found in the body & categorized into 3 basic categories of small-molecule neurotransmitters: • Acetylcholine • Amino acids • Biogenic amines

47. Acetylcholine • ACh is the most studied neurtransmitter • Acts as an excitatory or inhibitory neurotransmitter depending on the synapse it is in. • Ionotropif Effect: ACh binding to ligand-gated channelscauses enough of a local action potential to fire the neuron. • Acetylcholinesterase (AChE): A second neurotransmitter released to deactivate Ach when it is no longer needed.

48. Amino Acids • Act as neurotransmitters in the CNS. • Two important amino acids: • Glutamate: Excitatory neurotransmitter. • Excitotoxisity: Occurs when too high a level of glutamate is present in the NS – leads to neural death. • Due to lack of oxygen or inadequate blood supply. • Seen in stroke victims. • Aspartate: Another excitatory neurotransmitter linked to glutamate.

49. Biogenic Amines • Modified amino acids that are both excitatory & inhibitory. • Catecholamine: An amino acid with a catechol ring (6 carbons and 2 hydroxyl groups). • Biogenic amines are typically made of these. • This category includes: • Norepinephrine: Catecholamine responsible for arousal, dreaming & mood. • Dopamine: Catecholamine that contributes to emotions, addictive behaviors, pleasures, & muscle tone. • Serotonin: NOT a catecholamine – involved in sensory perception, mood control, appetite, temperature regulation, and sleep.

50. Prozac Nation • Selective Serotonin Reuptake Inhibiter (SSRI): Stops the uptake of the neurotransmitter serotonin & increases the “good mood” by allowing the serotonin to stick around the synapse longer. • Prozac!