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Chapter 9

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  1. Chapter 9 Nervous System

  2. 9.1 Introduction • Neurons = nerve cells • Neurons transmit information in the form of electrochemical changes nerve impulses, along nerve fibers. • Nerves = bundles of nerve fibers • Neuroglial cells = provide neurons with physiological requirements; functions like connective tissue cells in other systems.

  3. 9.1 Introduction • Two groups in the nervous system: • Central nervous system = brain, spinal cord • Peripheral nervous system = nerves, connect central nervous system to the rest of the body • Functions: • Sensory • Integrative • Motor

  4. 9.2 General Functions • The sensory function of the nervous system: • Derives from sensory receptors that are located at the ends of peripheral neurons. • Sensory receptors = • Detect changes from inside and outside of the body • Monitor external environmental factors (light, sound) • Monitor internal environmental factors (temp, oxygen concentration) • Convert environmental info. into nerve impulses

  5. 9.2 General Functions • After the sensory receptors…. • Nerve impulse is transmitted over peripheral nerves to the CNS • CNS brings impulses together create sensations (integrative function) • We make conscious or subconscious decisions and then act on them using motor functions • Motor functions employ neurons and carry impulses from CNS to effectors • Effectors = outside the nervous system • Ex: muscles that contract, glands that secrete

  6. 9.3 Neuron Structure • Neurons vary in size and shape but have certain features in common. • Every neuron has: • cell body • Granular cytoplasm • Cell membrane • Organelles: mitochondria, lysosomes, golgi apparatus • Nucleus • Neurofibirls = fine threads that extend into nerve fiber • tubular, cytoplasm-filled nerve fibers which conduct impulses to or from the cell body. • Chromatophilic substance • scattered throughout the cytoplasm • Membranous sacs • Similar to ribosomes, transport system

  7. 9.3 Neuron Structure • Two kinds of nerve fibers: • Dendrites • Can have multiple per cell • Short and highly branched • Reception area, communicates with other neurons • Receives information • Axons • only one per cell • Conducts impulse away from the nerve cell body • Organelles are found in the cytoplasm of axons

  8. 9.3 Neuron Structures • Schwann cells • wind tightly around axons • Axons are enclosed in sheaths of these • Myelin • Lipid-protein that protects axon (myelin sheath) • Myelinated fibers are within myelin sheaths • Unmyelinated are fibers that lack sheaths • Neurilemma • Portion of schwann cell that contains most of the cytoplasm, surrounds the myeline sheath • Nodes of Ranvier • Narrow gaps in the myeline sheath between schwann cells

  9. 9.4 Classifications of Neurons

  10. 9.4 Classifications of Neurons (structurally) • 1. Biopolar = cell body has two nerve fibers, one axon one dendrite • Found in: eyes, nose, ears • 2. Unipolar = single nerve fiber that divides into two branches, one connects to peripheral body (dendrite), and other connects to brain or spinal cord (axon) • 3. Multipolar = many nerve fibers arise from cell body. Only one fiber is an axon • Found in: brain or spinal cord

  11. 9.4 Classification of Neurons (functionally) • 1. Sensory neurons = from body to brain or spinal cord • 2. Interneuron = link other neurons • 3. motor neurons = out of brain and spinal cord to effectors

  12. 9.4 Classifications of Neuroglial Cells • 1. Microglial cells = scattered through out, digest dibris or bacteria • 2. Oligodendrocytes = provide insulation around the axons • 3. Astrocytes = connect blood vessels to neurons • 4. Ependymal = form membrane that covers brain-like parts

  13. 9.5 Cell Membrane Potential • The surface of the cell membrane is polarized. • Important to muscle and nerve impulses • Potassium ions can pass through cell membranes much more easily than sodium ions • This makes potassium ions a major contributor to membrane polarization

  14. 9.5 Resting Potential • At resting position, potassium diffuses out of the cell more rapidly than sodium ions can diffuse in. • Result: more positive charges leaving the cell than entering • Outside of the cell = + • Inside of the cell = - • Passive transport (diffusion) • Na/K pumps send sodium ions out of the cell and K ions into the cell. • Active transport • Resting membrane potential = the differences in the charges of the inside and outside of the cell


  16. 9.5 Potential Changes • Nerve cells respond to change in their surroundings. • These changes can effect the resting potential • Decrease…inside of the membrane becomes less negative when compared to outside the membrane is depolarizing • Changes in resting potential are graded • The amount of change is proportional to the intensity of stimulation • Threshold stimulus = stimulation level that must be exceeded to elicit nerve impulse or muscle contraction • Action potential = electrical changes when a nerve cell is exposed to a stimulus that exceeds threshold

  17. 9.5 Action Potential • Sodium channels open and allow sodium to diffuse freely inward • As sodium ions diffuse inward, the membrane loses its negative charge = depolarized • At the same time, potassium channels open as well • As these positive ions diffuse out the inside becomes negative again = repolarized • The balance change between repolarization and depolarization takes about 1/1,000th

  18. Events leading to Nerve Impulse • 1. nerve fiber membrane maintains resting potential • 2. threshold stimulus is received • 3. sodium channels in a local region of the membrane open • 4. sodium ions diffuse inward, depolarizing the membrane • 5. potassium ions diffuse outward, repolarizing the membrane • 6. the resulting action potential causes a local bioelectric current that stimulates adjacent portions of the membrane • 7. Wave of action potentials travels the length of the nerve fiber as a nerve impulse

  19. 9.6 Nerve Impulse • Nerve impulse = one action potential sets off a wave of action potentials down a nerve fiber • Speed of the impulse is proportional to the diameter of the axon • Greater diameter = faster speed • Myelinated axons conduct faster than unmyelinated

  20. 9.7 The Synapse • Synapse = junction between two communicating neurons • The synaptic cleft separates these two neurons • Within the presynaptic neuron an impulse travels from a dendrite to cell body and then to axon. There lies the synaptic cleft separating it from the postsynaptic neuron • Synaptic transmission = process of crossing the synapse • one way transmission because of synaptic knobs on axons • Knobs contain synaptic vesicles which release neurotransmitter • Neurotransmitter diffuses across the synaptic cleft and reacts with specific receptors on the postsynaptic neuron.


  22. 9.7 Excitatory and Inhibitory Actions • Excitatory Neurotransmitters= increases membrane permeability, increases chance for threshold to be achieved • Inhibitory Neurotransmitters= decrease membrane permeability, decrease chance for threshold to be achieved • The effect on the postsynaptic neuron depends on which presynaptic knobs are activated • More excitatory = threshold reached, impulse initiated • More inhibitory = no impulse initiated

  23. 9.7 Neurotransmitters • There are about 50 types of neurotransmitters in the nervous system • Some neurons can release only one, while others can release multiple • Acetylcholine and norepinephrine are excitatory; dopamine and GABA are inhibitory • Neurotransmitters are synthesized in the cytoplasm of synaptic knobs and stored in the synaptic vesicles

  24. 9.7 Neurotransmitters • Synaptic vesicles release their neurotransmitters when Ca+ has been diffused inward due to the action potential • The vesicles get more neurotransmitters by breaking away and going to the cytoplasm • After the neurotransmitters have been released they are typically decomposed by enzymes or taken back into the synaptic knob

  25. 9.7 Events leading to the release of a Neurotransmitter • 1. action potential passes along a nerve fiber and over the surface of its synaptic knob • 2. synaptic knob membrane becomes more permeable to calcium ions, and they diffuse inward. • 3. In the presence of calcium ions, synaptic vesicles fuse to synaptic knob membrane. • 4. Synaptic vesicles release their neurotransmitter into synaptic cleft. • 5. Synaptic vesicles reenter cytoplasm of nerve fiber and pick up more neurotransmitter

  26. 9.7 Types of Neurotransmitters • Ach = skeletal muscle contraction • Monoamines = sense of feeling good, low levels = depression • Dopamine, norepinephrine • Serotonin = sleepiness and mood • Endorphins = reduce pain, inhibit receptors

  27. 9.8 Neuronal Pools • Group of neurons that make hundreds of synaptic connections and work together to preform a common function • These pools help us remember sequential tasks like riding a bike or tying your shoe • Facilitation = when a neuron in a neuron pool gets excited but not excited enough to reach threshold • Convergence = when a single neuron receives impulses from two or more incoming fibers. • Impulses arriving from different sources have an additive effect on a neuron.

  28. 9.9 Types of Nerves • Nerve = cordlike bundle of nerve fibers held together by layers of connective tissue • Sensory nerve – conduct impulses into the brain or spinal cord • Motor nerve – carry impulses to muscles or glands • Mixed nerve – contains both sensory and motor nerves

  29. 9.10 Nerve Pathways • Nerve pathways are the routes that nerves follow through the nervous system • Reflex arc = simplest pathway • Reflex behavior = automatic, subconscious responses • Types of reflexes - • Knee jerk reflex = maintains upright posture • Withdraw reflex = avoidance of painful stimuli

  30. Drugs Effect Neurotransmitters • Strychnine poison =antagonist of ach receptors. Effects the motor neurons in the spinal cord which control muscle contraction…death by asphyxia • Antidepressants (Zoloft) • “selective serotonin reuptake inhibitors”

  31. Drugs Effect Neurotransmitters • Ecstasy = serotonin release (feel good feeling) • Our brain cells are continually working to bring it back into the cells and out of synapse using reuptake receptors. Ecstasy reverses these receptors causing a flood of serotonin to the synapse. • Common deaths = hyperthermia (over-heating) because the drug effects the body’s ability to regulate it’s temperature… you over heat without discomfort • “Especially in hot clubs dancing for hours”

  32. Drugs Effect Neurotransmitters • LSD = alters the action of serotonin, norepinephrine, and dopamine, • Triggers brain function changes • Physically – increase body temperature, heart rate, and blood pressure • Physiologically – delusions, hallucinations, rapid mood swings • Cocaine = blocks the reuptake of dopamine