1. Nervous Tissue
Dr. Michael P. Gillespie

2. Structures of the Nervous System
Brain
Spinal cord
Nerves
Cranial nerves
Ganglia
Sensory receptors

4. Functions of the Nervous System
Sensory function – afferent neurons
Integrative function - interneurons
Motor function – efferent neurons
The cells contacted by these neurons are called effectors

5. Organization of the Nervous System
Central nervous system
Brain
Spinal cord

6. Organization of the Nervous System
Peripheral nervous system
Cranial nerves and their branches
Spinal nerves and their branches
Ganglia
Sensory receptors
Somatic nervous system
Autonomic nervous system
Enteric nervous system

9. Somatic Nervous System (SNS)
Sensory neurons.
Motor neurons located in skeletal muscles.
The motor responses can be voluntarily controlled; therefore this part of the PNS is voluntary.

10. Autonomic Nervous System (ANS)
Sensory neurons from the autonomic sensory receptors in the viscera.
Motor neurons located in smooth muscle, cardiac muscle and glands.
These motor responses are NOT under conscious control; Therefore this part of the PNS is involuntary.

11. ANS Continued…
The motor portion of the ANS consists of sympathetic and parasympathetic divisions.
Both divisions typically have opposing actions.

13. Enteric Nervous System (ENS)
“The brain of the gut”.
Functions independently of the ANS and CNS, but communicates with it as well.
Enteric motor units govern contraction of the GI tract.
Involuntary.

15. Nervous Tissue Neurons. Neuroglia. Sensing. Thinking. Remembering.
Controlling muscular activity.
Regulating glandular secretions.
Neuroglia.
Support, nourish, and protect neurons.

16. Neurons
Have the ability to produce action potentials or impulses (electrical excitability).
Action potentials propagate from one point to the next along the plasma membrane.

17. Parts of a Neuron Cell body. Dendrites (= little trees). Axon.
Contains the nucleus surrounded by cytoplasm which contains the organelles.
Dendrites (= little trees).
The receiving (input) portion of a neuron.
Axon.
Each nerve contains a single axon.
The axon propagates impulses toward another neuron, muscle fiber, or gland cell.

19. Synapse
The site of communication between two neurons or between a neuron and an effector cell.
Synaptic end bulbs and varicosities contain synaptic vesicles that store a chemical neurotransmitter.

20. Axonal Transport Slow axonal transport. Fast axonal transport.
1-5 mm per day.
Travels in one direction only – from cell body toward axon terminals.
Fast axonal transport.
200 – 400 mm per day.
Uses proteins to move materials.
Travels in both directions.

21. Structural Classifications of Neurons
Multipolar neurons.
One axon and several dendrites.
Most neurons of the brain and spinal cord.

22. Structural Classifications of Neurons
Bipolar neurons.
One axon and one main dendrite.
Retina of the eye, inner ear, and the olfactory area of the brain.
Unipolar neurons.
The axon and the dendrite fuse into a single process that divides into two branches.
The dendrites monitor a sensory stimulus such as touch or stretching.

25. Neuroglia Half the volume of the CNS.
Generally, they are smaller than neurons, but 5 to 50 times more numerous.
They can multiply and divide.
Gliomas – brain tumors derived from glia.

26. Types of Neuroglia CNS PNS Astrocytes Oligodendrocytes Microglia
Ependymal cells
PNS
Schwann cells
Satellite cells

27. Myelination
The myelin sheath is a lipid and protein covering. It is produced by the neuroglia.
The sheath electrically insulates the axon of a neuron.
The sheath increases the speed of nerve impulse conduction.
Axons without a covering are unmyelinated. Axons with a covering are myelinated.

29. Myelination Continued…
Two types of neuroglial cells produce myelination.
Schwann cells – located in the PNS.
Oligodendrocytes – located in the CNS.

30. Gray and White Matter
The white matter consists of aggregations of myelinated and unmyelinated axons.
The gray matter consists of neuronal cell bodies, dendrites, unmyelinated axons, axon terminals, and neuroglia.

32. Electrical Signals in Neurons
Neurons are electrically excitable and communicate with one another using 2 types of electrical signals.
Action potentials.
Graded potentials.
The plasma membrane exhibits a membrane potential. The membrane potential is an electrical voltage difference across the membrane.

33. Electrical Signals in Neurons
The voltage is termed the resting membrane potential.
The flow of ions produces the electrical current.

34. Ion Channels
The plasma membrane contains many different kinds of ion channels.
The lipid bilayer of the plasma membrane is a good electrical insulator.

35. Ion Channels
The main paths for flow of current across the membrane are ion channels.

36. Ion Channels
When ion channels are open, they allow specific ions to move across the plasma membrane down their electrochemical gradient.
Ions move from greater areas of concentration to lesser areas of concentration.
Positively charged cations move towards negatively charged area and negatively charged anions move towards a positively charged area.
As they move, they change the membrane potential.

37. Ion Channel “Gates”
Ion channels open and close due to the presence of “gates”.
The gate is part of a channel protein that can seal the channel pore shut or move aside to open the pore.

38. Types of Ion Channels There are 4 types of ion channels.
Leakage channels – gates randomly alternate between open and closed positions.
Voltage-gated channels – opens in response to c change in membrane potential (voltage).
Ligand-gated channels – opens and closes in response to a specific chemical stimulus.
Mechanically gated channels – opens or closes in response to mechanical stimulation.

40. Resting Membrane Potential
The resting membrane potential occurs due to a buildup of negative ions in the cytosol along the inside of the membrane and positive ions in the extracellular fluid along the outside of the membrane.
The potential energy is measured in millivolts (mV).

41. Resting Membrane Potential
In neurons, the resting membrane potential ranges from –40 to –90 mV. Typically –70 mV.
The minus sign indicates that the inside of the cell is negative compared to the outside.
A cell that exhibits a membrane potential is polarized.

43. Electrochemical Gradient
An electrical difference and a concentration difference across the membrane.

44. Graded Potentials
A graded potential is a small deviation from the resting membrane potential.
It makes the membrane either more polarized (more negative inside) or less polarized (less negative inside).
Most graded potentials occur in the dendrites or cell body.

45. Graded Potentials Hyperpolarizing graded potential.
Depolarizing graded potential.
Graded potentials occur when ligand-gated or mechanically gated channels open or close.
Mehcanically gated channels are present in sensory neurons.
Ligand-gated channels are present in interneurons and motor neurons.

47. Action Potentials An action potential is known as an impulse.
Depolarizing phase – the resting membrane potential decreased towards zero.
Repolarizing phase – restores the resting membrane potential.

48. Action Potentials
Threshold – depolarization reaches a certain level (about –55 mV), voltage gated channels open.
Action potentials arise according to an all or none principal.

49. Comparison of Graded Potentials and Action Potentials
See table 12.2 p. 404

51. Depolarizing Phase
A depolarizing graded potential or some other stimulus causes the membrane to reach threshold.
Voltage-gated ion channels open rapidly.
The inflow of positive Na+ ions changes the membrane potential from –55mv to +30 mV.
About 20,000 Na+ enter through the gates. Millions are present in the surrounding fluid.
Na-k pumps bail them out.

52. Repolarizing Phase
While Na+ channels are opening during depolarization, K+ channels are opening, although slowly.
K+ channels allow outflow of K+ ions.
The closing of Na+ channels and the slow opening of K+ channels allows for repolarization.

54. Refractory Period
The period of time after an action potential begins during which an excitable cell cannot generate another action potential.
Absolute refractory period – a second action potential cannot be initiated, even with a very strong stimulus.
Relative refractory period – an action potential can be initiated, but only with a larger than normal stimulus.

55. Propagation of Nerve Impulses
The impulse must travel from the trigger zone to the axon terminals.
This process is known as propagation or conduction.
As Na+ ions flow in, they trigger depolarization which opens Na+ channels in adjacent segments of the membrane.

56. Neurotoxins & Local Anesthetics
Neurotoxins produce poisonous effects upon the nervous system.
Local anesthetics are drugs that block pain and other somatic sensations.
These both act by blocking the opening of voltage-gated Na+ channels and preventing propagation of nerve impulses.

57. Continuous and Saltatory Conduction
Continuous conduction – step-by-step depolarization and repolarization of adjacent segments of the plasma membrane.
Saltatory conduction – a special mode of impulse propagation along myelinated axons.

58. Continuous and Saltatory Conduction
Few ion channels are present where there is myelin.
Nodes of Ranvier – areas where there is no myelin – contain many ion channels.
The impulse “jumps” from node to node.
This speeds up the propagation of the impulse.
This is a more energy efficient mode of conduction.

60. Effect of Axon Diameter & Myelination
Larger diameter axons propagate impulses faster than smaller ones.
Myelinated axons conduct impulses faster than unmyelinated ones.

61. Effect of Axon Diameter & Myelination
A fibers.
Largest diameter.
Myelinated.
Convey touch, pressure, position, thermal sensation.

62. Effect of Axon Diameter & Myelination
B fibers.
Smaller diameter than A fibers.
Myelinated.
Conduct impulses from the viscera to the brain and spinal cord (part of the ANS).

63. Effect of Axon Diameter & Myelination
C fibers.
Smallest diameter.
Unmyelinated.
Conduct some sensory impulses and pain impulses from the viscera.
Stimulate the heart, smooth muscle, and glands (part of ANS).

64. Encoding Intensity of a Stimulus
A light touch feels different than a firmer touch because of the frequency of impulses.
The number of sensory neurons recruited (activated) also determines the intensity of the stimulus.

65. Signal Transmission at Synapses
Presynaptic neuron – the neuron sending the signal.
Postsynaptic neuron – the neuron receiving the message.
Axodendritic – from axon to dendrite.
Axosomatic – from axon to soma.
Axoaxonic – from axon to axon.

66. Types of Synapses
Electrical synapse
Chemical synapse

67. Electrical Synapses
Action potentials conduct directly between adjacent cells through gap junctions.

68. Electrical Synapses
Tubular connexons act as tunnels to connect the cytosol of the two cells.
Advantages.
Faster communication than a chemical synapse.
Synchronization – they can synchronize the activity of a group of neurons or muscle fibers. In the heart and visceral smooth muscle this results in coordinated contraction of these muscle fibers.

69. Chemical Synapses
The plasma membranes of a presynaptic and postsynaptic neuron in a chemical synapse do not touch one another directly.
The space between the neurons is called a synaptic cleft which is filled with interstitial fluid.
A neurotransmitter must diffuse through the interstitial fluid in the cleft and bind to receptors on the postsynaptic neuron.
The synaptic delay is about 0.5 msec.

71. Removal of Neurotransmitter
Diffusion.
Enzymatic degradation.
Uptake by cells.
Into the cells that released them (reuptake).
Into neighboring glial cells (uptake).

72. Spatial and Temporal Summation of Postsynaptic Potentials
A typical neuron in the CNS receives input from 1000 to 10,000 synapses.
Integration of these inputs is known as summation.

73. Spatial and Temporal Summation of Postsynaptic Potentials
Spatial summation – summation results from buildup of neurotransmitter released by several presynaptic end bulbs.
Temporal summation – summation results from buildup of neurotransmitter released by a single presynaptic end bulb 2 or more times in rapid succession.

74. Summary of Neuronal Structure and Function
Table 12.3
P. 408

75. Neural Circuits

76. Neurogenesis in the CNS
Birth of new neurons.
From undifferentiated stem cells.
Epidermal growth factor stimulates growth of neurons and astrocytes.
Minimal new growth occurs in the CNS.
Inhibition from glial cells.
Myelin in the CNS.

77. Damage and Repair in the PNS
Axons and dendrites may undergo repair if the cell body is intact, if the Schwann cells are functional, and if scar tissue does not form too quickly.
Wallerian degeneration.
Schwann cells adjacent to the site of injury grow torwards one another and form a regeneration tube.