1. Functional Organization of Nervous Tissue
Chapter 10
Functional Organization of Nervous Tissue
Neuron Network
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

2. Functions of the Nervous System
The master controlling and communicating system of the body
Functions
Sensory input: detects external and internal stimuli
Integration: processes and responds to sensory input
Control of Muscles and Glands
Homeostasis is maintained by regulating other systems
5. Center for Mental Activities

3. Fig. 10.1

4. Parts of the Nervous System
Two anatomical divisions
Central Nervous System (CNS)
Brain and spinal cord
Encased in bone
Peripheral Nervous System (PNS)
Nervous tissue outside of the CNS
Consists of sensory receptors and nerves
The anatomical divisions perform different functions
PNS detects stimuli and transmits information to the CNS and receives information from the CNS
CNS processes, integrates, stores, and responds to information from the PNS

5. Parts of the Nervous System
PNS has two divisions
Sensory division transmits action potentials from sensory receptors to the CNS
Motor division carries action potentials away from the CNS in cranial or spinal nerves (two subdivisions)
Somatic nervous system innervates skeletal muscle
Autonomic nervous system (ANS) innervates cardiac muscle, smooth muscle, and glands (three subdivisions)
Sympathetic division is most active during physical activity (fight or flight division)
Parasympathetic division regulates resting functions (rest and digest division)
Enteric nervous system controls the digestive system

6. Parts of the Nervous System
Central Nervous System (CNS)
Brain and Spinal Cord
Peripheral Nervous System (PNS)
Nervous tissue outside the CNS
Sensory receptors and nerves
Sensory Division
Transmits action potentials from sensory receptors to the CNS
Motor Division
Carries action potentials away from the CNS in cranial nerves or spinal nerves
Sympathetic Division
Most active during physical activity
Autonomic Nervous System (ANS)
Innervates cardiac muscle, smooth muscle, and glands
Somatic Nervous System
Innervates skeletal muscle
Parasympathetic Division
Regulates resting functions
Enteric Nervous System
Controls the Digestive System

7. Fig. 10.2

8. Cells of the Nervous System
The two principal cell types of the nervous system are:
Neurons: excitable cells that transmit electrical signals
Non-neural cells (Glial cells): cells that surround neurons. Account for over half of the brain’s weight
Less than 20% is extracellular space

9. Neurons Receive stimuli and transmit action potentials
Have three components:
The cell body (soma) is the primary site of protein synthesis
Dendrites are short, branched cytoplasmic extensions of the cell body that usually conduct electric signals toward the cell body
An axon is a cytoplasmic extension of the cell body that transmits action potentials to other cells

10. Fig. 10.3

11. Neuron Structure Cell Body (Soma) Contains the nucleus and a nucleolus
Nissl substance is an aggregate of rough ER and free ribosomes
Primary site of protein synthesis
Golgi apparatus, mitochondria, and other organelles are present
Has no centrioles (hence its amitotic nature)
Clusters of cell bodies in the CNS are called nuclei and in the PNS ganglia
Fig. 10.3

12. Neuron Structure Axons (Nerve Fibers)
Trigger zone is the part of the neuron where the axon originates
Action potential is generated from the trigger zone
Slender processes of uniform diameter and may vary in length from a few millimeters to more than a meter
Usually, there is only one unbranched axon per neuron
Rare branches, if present, are called collateral axons
Presynaptic terminal: branched terminus of an axon (10,000 or more)
Synapse: junction between a nerve cell and another cell
Bundles of processes are called nerve tracts in the CNS and nerves in the PNS
Fig. 10.3

13. Types of Neurons
Multipolar neurons have several dendrites and a single axon
Interneurons and motor neurons
Bipolar neurons have a single axon and dendrite
Components of sensory organs
Unipolar neurons have a single axon
Most sensory neurons

14. Fig. 10.4

15. Glial Cells Glial Cells (Supporting Cells): Glial Cells of the CNS
Provide a supportive scaffolding for neurons
Segregate and insulate neurons
Guide young neurons to the proper connections
Promote health and growth
Glial Cells of the CNS
Astrocytes
Microglial
Ependymal cells
Oligodendrocytes
Glial Cells of the PNS
Satellite cells
Schwann cells

16. Glial Cells of the CNS Astrocytes
Most abundant, versatile, and highly branched
They cling to neurons and their synaptic endings, and cover capillaries
Functions:
Support and brace neurons and blood vessels
Anchor neurons to their nutrient supplies
Influence the functioning of the blood-brain barrier
Guide migration of young neurons
Process substances
mopping up leaked potassium ions
recycling neurotransmitters
Isolate damaged tissue and limit the spread of inflammation

17. Astrocytes
Fig. 10.5

18. Glial Cells of the CNS
Ependymal cells: range in shape from squamous to columnar and many are ciliated
They line the ventricles of the brain and the central canal of the spinal cord
Some are specialized (choroid plexuses) to produce cerebrospinal fluid (CSF)
Help to circulate CSF using their cilia
Fig. 10.6

19. Glial Cells of the CNS Microglia
Small, ovoid cells with spiny processes
Phagocytes that monitor the health of neurons
Fig. 10.7

20. Glial Cells of the CNS
Oligodendrocytes: form myelin sheaths around the axons of several CNS neurons
Fig. 10.8

21. Glial Cells of the PNS
Schwann cells: form a myelin sheath around part of the axon of a PNS neuron
Satellite cells: support and nourish neuron cell bodies within ganglia
Fig. 10.9

22. Myelinated and Unmyelinated Axons
Plasma membrane of Schwann cells or Oligodendrocytes repeatedly wraps around a segment of an axon to form the myelin sheath
Myelin is a whitish, fatty (protein-lipid), segmented sheath around most long axons
It functions to:
Protect the axon
Electrically insulate
fibers from one another
Increase the speed of
nerve impulse transmission
Node of Ranvier
Gaps in the myelin sheath
Fig

23. Myelinated and Unmyelinated Axons
Rest in invaginations of Schwann cell (PNS) or Oligodendrocytes (CNS)
Conduct action potentials slowly
Fig

24. Fig

25. Organization of Nervous Tissue
Nervous tissue can be grouped into white matter and gray matter
White matter
Consists of myelinated axons
Propagates action potentials
Forms nerve tracts in the CNS and nerves in the PNS
Gray Matter
Collections of neuron cell bodies or unmyelinated axons
Forms cortex and nuclei in the CNS and ganglia in the PNS
Axons synapse with neuron cell bodies, which are functionally the site of integration in the nervous system

26. Electric Signals
Electric signals produced by cells are called action potentials
When action potentials are received from sensory cells it can result in the sensations of sight, hearing, and touch
Complex mental activities, such as conscious thought, memory, and emotions, result from action potentials
Contraction of muscles and the secretion of certain glands occur in response to action potentials
Electrical properties of cells result from
Ionic concentration differences across the plasma membrane
Permeability characteristics of the plasma membrane

27. Electric Signals Concentration Differences Across the Plasma Membrane
Sodium ions (Na+), calcium ions (Ca2+), and chloride ions (Cl-) are in much greater concentration outside the cell than inside
Potassium ions (K+) and negatively charged molecules, such as proteins, are in much greater concentration inside the cell than outside
Negatively charged proteins are synthesized inside the cell and cannot diffuse out of it

28. Electric Signals Concentration gradients of ions result mainly from
The Na+-K+ pump
Moves ions by active transport
Potassium ions are moved into the cell, and Na+ are moved out of it
Permeability characteristics of the plasma membrane are determined by
Leak channels (always open)
Potassium ion leak channels are more numerous than Na+ leak channels; thus, the plasma membrane is more permeable to K+ than to Na+ when at rest
Gated ion channels
Include ligand-gated ion channels, voltage-gated ion channels, and other gated ion channels

29. Electric Signals Gated Ion Channels
Open and close in response to stimuli
Ligand-gated ion channels
Open or close with the binding of a specific ligand (neurotransmitter)
Ligand is a molecule that binds to a receptor
A receptor is a protein or glycoprotein that has a receptor site to which a ligand can bind
Common in tissues such as nervous and muscle tissue, as well as glands
Voltage-gated ion channels
Open and close in response to small voltage changes across the plasma membrane
Common in tissues such as nervous and muscle tissues
Other gated ion channels
Open and close in response to physical deformation of receptors
Touch receptors (mechanical stimulation) and temperature receptors (temperature changes) of the skin

30. Electric Signals Establishing the Resting Membrane Potential
Charge difference across the plasma membrane when the cell is not being stimulated
The inside of the cell is negatively charged, compared with the outside of the cell
Due mainly to the tendency of positively charged K+ to diffuse out of the cell
Opposed by the negative charge that develops inside the plasma membrane

31. Electric Signals Changing the Resting Membrane Potential
Depolarization is a decrease in the resting membrane potential caused by
A decrease in the K+ concentration gradient
A decrease in membrane permeability to K+
An increase in membrane permeability to Na+ or Ca2+
A decrease in extracellular Ca2+ concentrations
Hyperpolarization is an increase in the resting membrane potential caused by
An increase in the K+ concentration gradient
An increase in membrane permeability to K+
An increase in membrane permeability to Cl-
A decrease in membrane permeability to Na+
An increase in extracellular Ca2+ concentrations

32. Graded Potentials Are small changes in the resting membrane potential
Confined to a small area of the plasma membrane
An increase in membrane permeability to Na+ can cause graded depolarization
An increase in membrane permeability to K+ or Cl- can result in graded hyperpolarization
Decreases in magnitude as the distance from the stimulation increases
The term graded potential is used because a stronger stimulus produces a greater potential change than a weaker stimulus
Graded potentials can summate, or add together
Fig

33. Action Potentials
Are larger changes in the resting membrane potential that spread over the entire surface of the cell
Occurs when a graded potential causes depolarization of the plasma membrane to a level called threshold
Occur in an all-or-none fashion and are of the same magnitude, no matter how strong the stimulus
Occurs in three phases
Depolarization phase
Repolarization phase
Afterpotential

34. Action Potentials Depolarization Phase Repolarization Phase
Inside of the membrane becomes more positive
Na+ diffuses into the cell through voltage-gated ion channels
Repolarization Phase
Return of the membrane potential toward the resting membrane potential
Voltage-gated Na+ channels close
Voltage-gated K+ channels open and K+ diffuses out of the cell
Afterpotential
Brief period of hyperpolarization following repolarization
Fig

35. Refractory Periods Absolute refractory period
Time during an action potential when a second stimulus (no matter how strong) cannot initiate another action potential
Relative refractory period
Time during which a stronger-than-threshold stimulus can evoke another action potential

36. Action Potential Frequency
The number of action potentials produced per unit of time in response to stimuli
It is directly proportional to stimulus strength and to the size of the graded potential
Subthreshold stimulus: graded potential
Threshold stimulus: a single action potential
Submaximal stimulus: action potential frequency increases as the strength of the stimulus increases
Maximal or a supramaximal stimulus: produces a maximum frequency of action potentials

37. Propagation of Action Potentials
An action potential generates ionic currents
Currents stimulate voltage-gated Na+ channels in adjacent regions of the plasma membrane to open
Producing new action potentials
Reversal of the direction of action potential propagation is prevented by the absolute refractory period
Occurs most rapidly in myelinated, large-diameter axons

38. Propagation of Action Potentials
In an unmyelinated axon, action potentials are generated immediately adjacent to previous action potentials
In a myelinated axon, action potentials are generated at successive Nodes of Ranvier
Fig

39. The Synapse
The synapse is the junction between two cells where communication takes place
Presynaptic cell: transmits signal towards a synapse
Postsynaptic cell: receives the signal
Two types of synapses
Electrical synapse
Chemical synapse

40. Electrical Synapses
Gap junctions in which tubular proteins called connexons allow ionic currents to move between cells
An action potential in one cell generates an ionic current that causes an action potential in an adjacent cell
Action potentials are conducted rapidly between cells allowing for synchronized activity
Common in cardiac muscle and in many types of smooth muscle where coordinated contractions are essential

41. Chemical Synapses Have three anatomical components
The enlarged ends of the axon are the presynaptic terminals containing synaptic vesicles
The postsynaptic membranes contain receptors for the neurotransmitter
The synaptic cleft, a space, separates the presynaptic and postsynaptic membrane
Fig

42. Chemical Synapse Activity
Action potentials arriving at the presynaptic terminal cause voltage-gated Ca2+ channels to open
Calcium ions diffuse into the cell and cause synaptic vesicles to release neurotransmitters
Neurotransmitters diffuse from the presynaptic terminal across the synaptic cleft
Neurotransmitters combine with their receptor sites and cause ligand-gated ion channels to open. Ions diffuse into the cell (shown) or out of the cell (not shown) and cause a change in membrane potential
Fig

43. Chemical Synapse Activity
The effect of the neurotransmitter on the postsynaptic membrane is stopped in two ways
The neurotransmitter is broken down by an enzyme
The neurotransmitter is taken up by the presynaptic terminal
A list of neurotransmitters and neuromodulators is presented in Table 10.4
Neuromodulators are substances released from neurons that can presynaptically or postsynaptically influence the likelihood that an action potential will be generated

44. Chemical Synapse Activity
Excitatory and inhibitory postsynaptic potentials
An excitatory postsynaptic potential (EPSP) is a depolarizing graded potential of the postsynaptic membrane
An inhibitory postsynaptic potential (IPSP) is a hyperpolarizing graded potential of the postsynaptic membrane
Presynaptic inhibition decreases neurotransmitter release
Presynaptic facilitation increases neurotransmitter release

45. Spatial and Temporal Summation
Presynaptic action potentials through neurotransmitters produce graded potentials in postsynaptic neurons. The graded potential can summate to produce an action potential at the trigger zone
Spatial summation occurs when two or more presynaptic terminals simultaneously stimulate a postsynaptic neuron
Temporal summation occurs when two or more action potentials arrive in succession at a single presynaptic terminal
Inhibitory and excitatory presynaptic neurons can converge on a postsynaptic neuron
An action potential is produced at the trigger zone when the graded potential is produced as a result of the sum of the EPSPs and IPSPs reaching threshold

46. Neuronal Pathways and Circuits
Convergent pathways have many neurons synapsing with a few neurons
Divergent pathways have a few neurons synapsing with many neurons
Oscillating circuits have collateral branches of postsynaptic neurons synapsing with presynaptic neurons