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Unit 3: Control Systems of the Human Body

Unit 3: Control Systems of the Human Body . Dr. Achilly. Part 1: Nervous Tissue. “Concepts” chapter 14. Nervous System--overview. One of the smallest, but most complex body systems. Made of: Brain Cranial nerves—12 pairs emerge from base of brain.

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Unit 3: Control Systems of the Human Body

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  1. Unit 3: Control Systems of the Human Body Dr. Achilly

  2. Part 1: Nervous Tissue “Concepts” chapter 14

  3. Nervous System--overview • One of the smallest, but most complex body systems. • Made of: • Brain • Cranial nerves—12 pairs emerge from base of brain. • Spinal cord—connects to the brain thru foramen magnum in skull & is encircled by vertebrae.

  4. Nervous System--overview • Spinal nerves—emerge from spinal cord & serve specific regions of body. • Ganglia—masses of nerve tissue (mainly neuron cell bodies) outside brain or spinal cord. • Enteric plexuses—networks of neurons that regulate the digestive system. • Sensory receptors—ends of sensory neurons that monitor internal or external environmental changes.

  5. Nervous System--overview

  6. Nervous System--overview • Nervous system has 3 basic fxns: • Sensory—detects internal & external stimuli. Carries this info to brain & spinal cord. • Integrative—the analyzing, storing & responding to sensory info. • Motor—carry out appropriate response like myo contraction or gland secretion. Info is carried from brain or spinal cord to effectors.

  7. Nervous System--overview • The two main divisions of the nervous system are: • Central nervous system (CNS)—consisting of brain and spinal cord • Peripheral nervous system (PNS)—all the nervous tissue outside of CNS

  8. Nervous System--overview • PNS can be further divided: • Somatic nervous system—carries sensory fibers from head, body wall, limbs, special senses, etc. Also carries motor neurons to skeletal myos.

  9. Nervous System--overview • Autonomic nervous system—carries sensory neurons from most of the organs and motor neurons to smooth & cardiac myo and glands. • The ANS can be further divided into the sympathetic division which handles “flight or fight” responses and the parasympathetic division for “rest and digest” responses.

  10. Nervous Tissue • The functional unit of the nervous system is the neuron. • It has electrical excitability & can propagate an electrical signal called an action potential. • Various sizes, but all contain similar parts.

  11. Nervous Tissue • Cell body—contains nucleus, cytoplasm, all other cellular organelles. • Dendrites—these are the receiving fibers of the neuron. Usually many of them. • Axon—propagates action potential away from cell body toward another neuron. Usually singular. Place where the axon joins the cell body is an important area called axon hillock.

  12. Nervous Tissue

  13. Nervous Tissue

  14. Nervous Tissue • The end of each axon contains many fine projections called axon terminals. • From here a neuron can communicate with another thru the synapse (the gap between neurons). • The axon terminal contains many membrane-enclosed sacs called synaptic vesicles. They store many types of neurotransmitters which are chemicals that help the electrical impulse cross the synaptic gap btwn neurons.

  15. Nervous Tissue

  16. Nervous Tissue • Many axons are surrounded by a lipid & protein covering called a myelin sheath. • The sheath electrically insulates an axon & speeds conduction. Also aides in regeneration of injured neurons. • In the PNS the myelin is produced by a support cell called a Schwann cell. • Gaps in Schwann cells are called nodes of Ranvier.

  17. Nervous Tissue

  18. Nervous Tissue • In CNS it’s the oligodendrocytes that myelinate the axons. • Little re-growth after injury. • Amount of myelin increases from birth to maturity.

  19. Nervous Tissue • The areas of the nervous system that have myelinated axons appear white (white matter). • Areas of neuronal cell bodies, dendrites & unmyelinated axons appear gray (gray matter).

  20. Nervous Tissue • In addition to neurons, about ½ the nervous system consists of support cells called neuroglia. • These cells do not propagate electrical impulses. • Can divide & “fill in” areas of injury.

  21. Nervous Tissue • Neuroglia of CNS • Astrocytes—give structural support, wrap around capillaries of brain to form blood-brain barrier • Oligodendrocytes—form myelin • Microglia—fxn as phagocytes to remove cellular debris • Ependymal cells—produce & circulate cerebral spinal fluid which nourishes the brain and spinal cord.

  22. Nervous Tissue

  23. Nervous Tissue • Neuroglia of PNS • Schwann cells—form myelin • Satellite cells—surround neuron cell bodies. Regulate exchange of materials btwn them & interstitial fluid (the fluid found surrounding all cells).

  24. Electrical Signals • Production of nerve impulses depends on two features of the plasma membrane. • Membrane potential—the separation of ions across the membrane leading to an electrical voltage difference. • Specific ion channels.

  25. Electrical Signals • When ion channels are open, ions will move down their concentration gradient thru the channels & according to charge (+ towards -, and vice versa) • The opening or closing of ion channels is due to presence of “gates.” • Four types of channels:

  26. Electrical Signals • Leakage channels randomly alternate btwn opened & closed. • Usually there are more K+ channels than Na+. Also K+ ones are “leakier.” Result is higher membrane permeability to K+. • Voltage gated channels open in response to change in membrane potential (voltage).

  27. Electrical Signals • Ligand-gated channels open/close in response to specific molecules that bind to the channels. • E.g. neurotransmitter binding to it

  28. Electrical Signals • Mechanically gated channel opens/closes in response to mechanical stimulation: • Vibration • Pressure • Stretch

  29. Electrical Signals • Resting membrane potential • Exists b/c of build up of (-) ions just inside the neuron cell membrane & (+) ions outside. • Separation of charges is a form of potential energy. About -70mV in a typical cell. • Dominant cation inside is K+, many anions (phosphates, amino acids) also. K+ can leak out, anions can’t.

  30. Electrical Signals • Negative ions inside cell work to attract K+ back in. • Eventually an equal # of K+ ions enter & leave. • Na+ leaks inward a little. • Na+/K+ pump maintains charge difference.

  31. At rest, the inside of a neuron's membrane has a negative charge. As the figure shows, a Na+ / K+ pump in the cell membrane pumps sodium out of the cell and potassium into it. However, because the cell membrane is a bit leakier to potassium than it is to sodium, more potassium ions leak out of the cell. As a result, the inside of the membrane builds up a net negative charge relative to the outside.

  32. Electrical Signals • Graded potentials • Arises when a stimulus causes a ligand or mechanically gated channel to open or close. • Depending on the type of ion channel opened, the membrane can become more negative (hyperpolarized) or more positive (depolarized).

  33. Electrical Signals • These signals are “graded” b/c they vary in size depending on the strength of the stimulus. • Large stimulus  more gates open • Ion flow is localized, so it’s only useful for communication over short distances. • Usually present in dendrites.

  34. Electrical Signals • Action Potentials • Has depolarization & repolarization phases • All or nothing response once threshold is reached. • Before an AP begins the membrane is at its resting potential. • The only movement of ions is thru leakage gates.

  35. Electrical Signals • At rest both Na+ & K+ gated channels are closed; membrane is at -70mV resting potential.

  36. Electrical Signals • A stimulus opens some Na+ channels. The # of channels opened depends on the strength of the stimulus. If enough channels are opened, the inside of the neuron becomes slightly positively charged b/c of all the Na+ flowing in.

  37. Electrical Signals • Depolarization—so many Na+ channels are open that the inside of cell becomes very positive. Positive feedback is involved here. In other words, the more positive the inside becomes, the more Na+ gates that open and so on.

  38. Electrical Signals • Repolarization—finally K+ gates open & K+ rushes out. At the same time Na+ gates close.

  39. Electrical Signals • Undershoot—so much K+ leaves the cell that it becomes more negative than the original resting potential. K+ gates close. This phase is also called a refractory period. Another AP cannot occur in this portion of the membrane until Na/K pumps can restore the original ion concentration gradient & resting potential.

  40. Electrical Signals • In order to relay information the AP must travel all the way down the axon. Called propagation. • When one segment of the membrane depolarizes the flood of Na+ causes gated channels in the next section to open, etc. • Called continuous conduction.

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