1. Functional Organization
Chapter 11
Functional Organization of
Nervous Tissue

2. Nervous System
The master controlling and communicating system of the body
Functions
Sensory input – monitor internal and external stimuli
Integration – interpretation of sensory input
Motor output – response to stimuli by activating effector organs

3. Divisions of the Nervous System
Central nervous system (CNS)
Consists of Brain and spinal cord
Controls entire organism
Integrates incoming information and responses
Peripheral nervous system (PNS)
Link between CNS, body and environment
Consists of Spinal and cranial nerves
Carries messages to and from the spinal cord and brain

4. Peripheral Nervous System (PNS) Two Functional Divisions
Sensory (afferent) division
Sensory afferent fibers – carry impulses from skin, skeletal muscles, and joints (sensory receptors) to the brain
Visceral afferent fibers – transmit impulses from visceral organs to the brain
Motor (efferent) division
Transmits impulses from the CNS to effector organs (muscles, glands)

5. Motor Divisions of PNS Two Divisions:
Somatic nervous system (=Voluntary)
Conscious control of skeletal muscles
Conducts impulses from the CNS to skeletal muscles
Autonomic nervous system (ANS= involuntary)
Regulates smooth muscle, cardiac muscle, and glands
Subconscious or involuntary control

6. Divisions of ANS Sympathetic Nervous System (Thoraco-lumbar outflow)
“Flight or fright system”
Most active during physical activity
Parasympathetic Nervous System (Cranial-sacral outflow)
Regulates resting or vegetative functions such as digesting food or emptying of the urinary bladder

7. Histology of Nerve Tissue
The two principal cell types of the nervous system are:
Neurons – excitable cells that transmit electrical signals
Supporting (glial) cells – cells that surround and wrap neurons

8. Neurons (Nerve Cells) Structural units of the nervous system
Receive stimuli and transmit action potentials
Long-life
Amitotic
Have a high metabolic rate
Each neuron consists of:
Body
Axon
dendrites

9. Structure of Neuron Cell Body (soma or perikaryon)
Contains the nucleus and a nucleolus & usual organelles
Has no centrioles (amitotic nature)
Has well-developed Nissl bodies (rough ER)
Nissl bodies -primary site of protein synthesis
Contains an axon hillock – cone-shaped area from which axons arise

10. Dendrites Short, branched cytoplasmic extensions
They are the receptive, or input, regions of the neuron
Electrical signals are conveyed toward the cell body

11. Axons
Slender processes of uniform diameter arising from the axon hillock
Initial segment: beginning of axon
Axoplasm : Cytoplasm of axon
Axolemma : Plasma membrane of axon
Long axons are called nerve fibers
Usually there is only one unbranched axon per neuron

12. Axons Rare branches, if present, are called axon collaterals
Presynaptic (Axon) terminal – branched terminus of an axon
Trigger zone: site where action potentials are generated; axon hillock and part of axon nearest to cell body
AP are conducted along the axons to axonal terminals and release neurotransmitters
AP conduction away from cell body

13. Neuron Classification
Neurons can be classified by structure:
Multipolar
Most common in both CNS & PNS
Single axon, many dendrites (motor neurons and interneurons of CNS)
Bipolar
two processes (one axon and one dendrite)
Are sensory neurons found in the retina, olfactory nerve
Unipolar
single short process extending from cell body
Divides into two branches and functions as both dendrite and axon (sensory neurons , dorsal root ganglia)

14. Neuron Classification
Neurons can be classified by function:
Sensory (afferent) — transmit impulses from receptors toward the CNS
Motor (efferent) — carry impulses from CNS to muscles and glands
Interneurons (association neurons) Link sensory and motor neurons within CNS
Make up 99% of neurons in body

15. NERVOUS SYSTEM CELL TYPES
NEUROGLIA (Glial cells)
Supporting cells
Surround neurons
Non-conducting
6 types
2 in PNS
4 in CNS
Astrocyte
Oligodendrocytes
Microglia
Ependymal Cells
Satellite Cells
Schwann Cells

16. SUPPORTING CELLS OF THE CNS
1) Astrocytes
In CNS only
Anchor neurons to capillaries
Regulate what substances reach the CNS from the blood (blood-brain barrier)
Regulate extracellular brain fluid composition
Pick up excess K+
Recapture released (recycle) neurotransmitters
Astrocytes:
Most numerous of all glial cells.
In CNS only.
Radiating processes attach neurons to nearby capillaries, anchoring the neuron to its nutrient supply.
Take up glucose and deliver it to neurons in the form of lactic acid.
Also take up excess potassium, sodium, calcium and excitotoxic neurotransmitters such as glutamate. (possibly also take up other neuropeptides)
Help guide the formation of synapses in young neurons.
Astrocyte processes wrapped around brain capillaries help to form the blood-brain barrier, making the capillaries in the brain less permeable to blood-borne metabolic
wastes, proteins, some toxins and drugs.
Secrete pro-inflammatory chemicals that may contribute to magnification of immune responses within the CNS (which may lead to neuronal dysfunction in Alzheimer’s, autism, HIV dementia, multiple sclerosis) 16

17. SUPPORTING CELLS OF THE CNS
2) Ependymal Cells
CNS only
Line the cavities of the brain and spinal cord
Ciliated
Circulate the cerebrospinal fluid (CSF)
Ependymal Cells:
Only in the CNS.
Squamous to columnar cells that are often ciliated.
Line the central cavity of the spinal cord and the brain ventricles.
Cilia help to move CSF through the CNS.
Form a permeable barrier between the CSF and the tissue fluid bathing the CNS cells. 17

18. SUPPORTING CELLS OF THE CNS
3) Microglia
CNS only
Migrate toward injured neurons
Specialized macrophages
Phagocytize necrotic tissue, microorganisms, and foreign substances that invade the CNS
Microglial Cells:
In CNS only.
Extensive processes reach out to nearby neurons, monitoring the health and status of the nerve cells.
When injury to neurons is detected, the microglial cells become active macrophages that migrate towards the injured cells and phagocytize the microorganisms involved or the debris of the damaged neuron.
Microglial cells have been found to be activated in neurodegenerative immune disorders such as Alzheimer’s disease, Parkinson’s, Amyotrophic lateral sclerosis, HIV dementia and autism. (Cells of the immune system are denied access to the CNS)
Microglial activation in CNS immune-triggered disorders may contribute to neuronal damage and dysfunction. 18

19. SUPPORTING CELLS OF THE CNS
4) Oligodendrocytes
CNS only
Wrap extensions around neuron fibers (axons)
Form myelin sheath
Oligodendrocytes:
Found only in the CNS.
Wrap insulating processes around neuron axons to form a myelin sheath.
Nodes of Ranvier are present but not as numerous as in PNS. (also no neurilemma)
No neurilemma since oligodendrocyte processes form the sheath rather than the entire cell wrapping around the axon (as with Schwann cells)
Destruction of this sheath (as in disorders such as multiple sclerosis) causes short circuiting of the neurons so that neurons are excited more slowly or conduction ceases all together.
Destruction of the myelin sheath in MS is believed to be caused by an attack on the sheath by the immune system.
MS results in visual disturbances, muscle weakness, clumsiness,
paralysis, speech disturbances and urinary incontinence. 19

20. SUPPORTING CELLS OF THE PNS
1) Schwann Cells or Neurolemmocytes
PNS only
Wrap around axons of neurons in the PNS
Forms myelin sheath
Schwann Cells:
Found only in the PNS.
The entire Schwann cell wraps around the axon of a neuron in the PNS forming a myelin sheath similar to that of the oligodendrocyte in the CNS.
In the event that a PNS neuron is injured, if the cell body is still intact regeneration of the nerve fiber can occur due to the presence of Schwann cells.
Surviving Schwann cells migrate to the injury site and release chemicals such as growth factors which stimulate re-growth.
They also form a regeneration tube which guides the re-growth of the axon. They protect, support and remyelinate the axon as it regenerates.
The lack of these cells in the CNS may help explain the fact the fact that CNS neurons do not regenerate 20

21. SUPPORTING CELLS OF THE PNS
2) Satellite Cells
PNS only
Surround neuron cell bodies
Provide support and nutrients to neuronal cell bodies
Protect neurons from heavy metal poisons (lead, mercury) by absorbing them
Satellite Cells:
Surround neuron cells bodies (within ganglia) of the PNS.
Function is unknown. 21

22. Myelinated and Unmyelinated Axons
Myelinated Axons: Whitish, fatty (protein-lipid), segmented sheath around most long axons
Functions:
Protect the axon
Electrically insulate fibers from one another
Increase the speed of nerve impulse transmission
Formed by Schwann cells in the PNS
In CNS formed by oligodendrocytes
Nodes of Ranvier : Gaps in the myelin sheath between adjacent Schwann cells
Unmyelinated Axons : Schwann cell surrounds nerve fibers but coiling does not take place

23. Organization of Nervous Tissue
White matter – dense collections of myelinated fibers
Gray matter – mostly nerve cell bodies and unmyelinated fibers
In brain: gray is outer cortex as well as inner nuclei; white is deeper
In spinal cord: white is outer, gray is deeper

24. NEURON FUNCTION: The Synapse
Junction between one neuron and another
Where two cells communicate with each other
Presynaptic neuron – conducts impulses toward the synapse
Postsynaptic neuron – Cell that receive the impulse
Most are axo-dendritic or
axo-somatic
Electrical Synapses:
Pass voltage changes directly from one cell to another across low resistance paths (connexon channels of gap junctions link the cytoplasm of adjacent cells allowing ions to flow directly from one neuron to the next).
The speed of transmission is instantaneous.
Found in regions of the brain responsible for sterotyped movements (such as jerky eye movements).
Transmission can be unidirectional or bidirectional
Chemical Synapses-
Adjacent cells are separated by a much larger gap (30 to 50nm).
Synaptic vesicles on the presynaptic side store neurotransmitter.
The chemical neurotransmitter must diffuse across the gap and bind to a specific receptor protein on the postsynaptic neuron in order to
Transmit the electrical change. Transmission is unidirectional.
Takes much longer than an electrical synapse ( 1 msec. up to a minute).
Most synapses are chemical. 24

25. Types of Synapses Electrical Synapses:
Are gap junctions that allow ion flow between adjacent cells by protein channels called Connexons
Not common in CNS
Found in cardiac muscle and many types of smooth muscle Action potential of one cell causes action potential in next cell

26. Chemical Synapses Chemical Synapses Most are this type
Neurotransmitter released from synaptic vesicles of presynaptic neuron
Neurotransmitter binds to receptors on postsynaptic membrane
Binding of neurotransmitter to receptor  permeability change in postsynaptic membrane
Electrical Synapses:
Pass voltage changes directly from one cell to another across low resistance paths (connexon channels of gap junctions link the cytoplasm of adjacent cells allowing ions to flow directly from one neuron to the next).
The speed of transmission is instantaneous.
Found in regions of the brain responsible for sterotyped movements (such as jerky eye movements).
Transmission can be unidirectional or bidirectional
Chemical Synapses-
Adjacent cells are separated by a much larger gap (30 to 50nm).
Synaptic vesicles on the presynaptic side store neurotransmitter.
The chemical neurotransmitter must diffuse across the gap and bind to a specific receptor protein on the postsynaptic neuron in order to
Transmit the electrical change. Transmission is unidirectional.
Takes much longer than an electrical synapse ( 1 msec. up to a minute).
Most synapses are chemical. 26

27. NEUROTRANSMITTERS Released at chemical synapses
In response to AP Voltage-regulated calcium channels open
Ca2+ diffuse into presynaptic terminal
And causes synaptic vesicles to fuse with presynaptic membrane
This fusion releases neurotransmitter into the synaptic cleft via exocytosis
Neurotransmitters
The means by which each neuron communicates with another (when chemical synapses are involved).
More than 50 neurotransmitters have been identified.
Some neurons produce and release only one transmitter while many make two or more.
Often synthesized and enclosed in vesicles in the axonal terminals (enzymes for production of neurotransmitter are transported down the axon from the soma while mitochondria in the knob supply the energy needed to produce the neurotransmitter)
A nerve impulse traveling down the axon causes voltage-gated calcium ion channels to open in the presynaptic terminal.
The increase in intracellular calcium triggers fusion to
the synaptic vesicles with the presynaptic membrane. As the vesicles rupture, transmitter molecules are dumped into the synaptic cleft.
The neurotransmitter will bind to receptors on the postsynaptic neuron membrane.
The result can be either excitatory or inhibitory.
The process is terminated by enzymatic destruction of the neurotransmitter, by uptake of the transmitter into the presynaptic terminal or by diffusion of the transmitter away from the synapse. 27

28. NEUROTRANSMITTERS
When bound to receptors on postsynaptic neuron, the neurotransmitter can either excite or inhibit the postsynaptic neuron
Neurotransmitters can be either excitatory (bring the postsynaptic neuron closing to firing and impulse) Or
Inhibitory (Take the postsynaptic neuron farther away from firing) 28

29. Resting Membrane Potential
Resting neurons maintain a difference in electrical charge inside and outside cell membrane = RESTING MEMBRANE POTENTIAL (RMP)
The inside of the resting neuron is negatively charged, the outside is positively charged.
Concentration of K+ higher inside than outside cell
Na+ higher outside than inside
RMPs vary from -40 to
-90mV in different neuron types 29

30. EXCITATORY NEUROTRANSMITTERS
When bound to receptors on the postsynpatic neuron membrane:
Causes the opening of positive ion channels
Sodium ions enter rapidly
RMP becomes more positive
This positive change in the RMP is called depolarization
This brings the neuron closer to firing
Excitatory Neurotransmitters:
Bind to the postsynaptic neuron membrane and cause opening of a positive ion channel (usually sodium).
As the positive ions enter the cell, the inside of the membrane becomes more positive.
This result is a positive change in the RMP which is called
Depolarization.
If the depolarization is large enough (threshold) it will result in firing of the postsynaptic neuron. 30

31. DEPOLARIZATION A positive change in the RMP
Caused by influx of positive ions
Causes the inside of the cell membrane to become less negative
Depolarization spreads to adjacent areas

32. INHIBITORY NEUROTRANSMITTERS
When bound to receptors on the postsynaptic membrane:
Make the membrane more permeable to negative ions (usually Cl-)
As negative ions rush into the neuron, the RMP becomes more negative
The negative change in the RMP = hyperpolarization
Brings the neuron farther from firing
Inhibitory Neurotransmitters
Bind to the postsynaptic neuron and cause the opening of negative ion channels such as chlorine.
The influx of negative ions causes the RMP to become more negative (hyperpolarized).
The more negative the RMP, the farther the neuron is from firing. 32

33. Hyperpolarization A negative change in RMP
Usually caused by influx of chloride ions
Decreases the likelihood of the neuron firing

34. Graded or Local Potential
Short changes in the RMP in small regions of the membrane
Can be positive or negative (depolarize or hyperpolarize the membrane)
Alone, not strong enough to initiate an impulse
summate or add onto each other
Together, can trigger a nerve impulse (action potential)

35. POSTSYNAPTIC POTENTIALS
EPSP (Excitatory Postsynaptic Potential)
When depolarization occurs, response is stimulatory
& graded potential is called EPSP
Binding of a neurotransmitter on the postsynaptic membrane more positive RMP, reaches threshold (depolarization occurs)
producing an action potential and cell response
EPSP’s-
Small, local, positive changes in the RMP.
Individually, these small areas of depolarization are not enough
to be considered an impulse or action potential.
Postsynaptic membranes do not generate action potentials.
EPSP’s last a few milliseconds and help to trigger an action potential distally at the axon hillock.
If the current of the EPSP’s is great enough and they spread to the axon hillock, they may stimulate the generation of an action potential down the axon.
A neuron will not fire an impulse (action potential) unless the membrane is depolarized above a certain threshold voltage. (Usually 20 mV in the positive direction—from -70mV to -50mV) 35

36. POSTSYNAPTIC POTENTIALS
IPSP (Inhibitory Postsynaptic Potential)
When hyperpolarization occurs, response is inhibitory
& graded potential is called IPSP
Binding of the neurotransmitter on the postsynaptic membrane more negative RMP (hyperpolarization)
Decrease action potentials by moving membrane potential farther from threshold
IPSP’s-
Small, local, negative changes in the RMP.
Hyperpolarize the membrane (becomes more negative inside).
Usually caused by opening Cl- channels, allowing negative ions to enter or by opening K+ channels—allowing positive ions to leave.
The more negative the RMP becomes, the farther the neuron is from firing. The neuron must still reach threshold before it will fire (20 mV in a positive direction from the original RMP). 36

37. Types of Neurotransmitters
40 to 50 Known Neurotransmitters
Acetylcholine (ACh)
Norepinephrine (NE)
GABA
Dopamine
Serotonin
GABA (gamma aminobutyric acid)-
Inhibitory neurotransmitter in CNS: hypothalamus, cerebellum, spinal cord.
Lack of it (as induced by blockers) causes convulsions.
Dopamine-
Released from neurons in the CNS. Released from cells of the substantia nigra in the midbrain which project axons to the basal ganglia ( nucleus).
Facilitate initiation of voluntary movement. Degeneration of the dopamine containing cells produces the progessively worsening motor disorders associated with Parkinson’s disease. Lack of dopamine causes the basal nuclei to become overactive resulting in persistant tremors at rest (mostly in head and fingers). L-Dopa is administered to enhance release of dopamine.
Serotonin producing neurons may be involved in sleep/wake cycles and
control of mood and emotional behavior. May be involved in clinical depression. Many drugs used to treat depression are blockers of serotonin reuptake, prolonging its action in the brain ( prozac) 37

38. Action Potential Action Potential = Nerve Impulse Consists of:
Depolarization
Propagation
Repolarization
Action potentials can only be generated in cells with excitable membranes (muscle and neurons)
Action Potential= a brief reversal of membrane potential with a total change in voltage of about 100 mV (in a positive direction—i.e. from -70 mV to a +30mV)
Only axons can generate an action potential.
Will only occur if the neuron is adequately stimulated. 38

39. Action Potential
If depolarization reaches threshold (usually a positive change of 15 to 20 mV or more), an action potential is triggered
The positive RMP change causes electrical gates in the axon hillock to open
Sudden large influx of sodium ions causes a reversal in the membrane potential (becomes approx. 100mV more positive)
Begins at the axon hillock and travels down the axon
Action Potential
The sodium ion channels on the dendrites and the soma are chemically gated. They will only open with the binding of a neurotransmitter. For this reason, they will only produce graded potentials and are incapable of generating an action potential.
In contrast, the sodium gates in the axon hillock region and along the axon are voltage/electrically gated. They will open in response to a positive change in the RMP. They will only open when the RMP moves approximately 20 mV in a positive direction (threshold).
Once threshold is reached, the sodium gates in the axon hillock will open and large amounts of sodium will enter—causing a rapid positive change in the RMP of 100mV or more---this is the beginning of the action potential. 39

40. (on axon hillock and axon)
Types of Ion Channels
Chemically gated channels – open with binding of a specific neurotransmitter
Voltage-gated channels – open and close in response to membrane potential
Chemically Gated
(on dendrite or soma)
Voltage Gated
(on axon hillock and axon)

41. Propagation Movement of the action potential down the axolemma
voltage-gated sodium channels open in response to positive RMP change
Propagation-
As sodium ions enter the axon hillock, the positive voltage change causes more Na channels to open. This continues until the Na permeability is about 1000 greater than in a resting neuron.
The Na influx causes Na channels in adjacent areas to open (in response to the positive voltage change) and sodium influx increases in the surrounding areas.
This phenomenon continues down the length of the axon. Propagation of the action potential is caused by opening of voltage gated sodium channels in the axolemma. 41

42. REPOLARIZATION Restoration of the RMP back to it’s negative state
A repolarization wave follows the depolarization wave
3 factors contribute to restoring the negative RMP:
Sodium (Na+) gates close (it no longer enters)
Potassium (K+) gates open, potassium rushes out
Sodium/potassium pump kicks in
Repolarization-
Before the neuron can fire again, the negative RMP must be restored. This is accomplished by repolarization.
As the RMP becomes more positive (over 0mV) the positive intracellular charge will resist the entry of more positively charged sodium ions.
The inactivation (Na) gates close after a few msecs. of depolarization allowing no further entry of sodium ions.
Voltage sensitive potassium channels open, allowing potassium ions to rush out of the cell.
Ion redistribution is accomplished by the sodium/potassium pump.
Three sodium ions are pumped outward while two potassiums are pumped inward (the potassium continues to leak out). 42

43. THE SODIUM/POTASSIUM PUMP
An active process: requires cellular energy
Actively pumps 3 sodium (Na+) ions out of the cell and 2 potassium (K+) ions in
Potassium leaks back out 43

44. ABSOLUTE REFRACTORY PERIOD
Period of time when electrical sodium gates are open
From beginning of action potential until near end of repolarization
No matter how large the stimulus, a second action potential cannot be produced
Absolute Refractory Period
During the period of time when the Na channels are open, a neuron is incapable of responding to another stimulus—no matter how strong.
Repolarization must occur to a certain extent before a neuron can respond to a stimulus and fire again. 44

45. Relative Refractory Period
The interval following the absolute refractory period when:
Sodium gates are closed
Potassium gates are open
Repolarization is occurring
A stronger-than-threshold stimulus can initiate another action potential

46. SUMMATION BY POSTSYNAPTIC NEURON
A single EPSP cannot induce an action potential
EPSP’s can add together or SUMMATE to initiate an action potential
Spatial Summation
Large numbers of axon terminals stimulate the postsynaptic neurons simultaneously
Summation
EPSP’s acting alone cannot stimulate an action potential.
Takes 40 to 80 presynaptic terminals discharging at onces to elicit and action potential.
Large numbers of EPSP’s can cause a large enough change in the RMP that threshold is reached and an action potential is generated.
Two Ways to Summate EPSP’s
Spatial summation-
The post synaptic membrane is stimulated by a large number of terminals at the same time (neurotransmitter being released in large amounts from one neuron or from many neurons).
Temporal summation-
When one or more presynaptic neurons transmit impulses in rapid fire succession and bursts of neurotransmitter are released one right after another. 46

47. SUMMATION BY POSTSYNAPTIC NEURON
Temporal Summation
One or more presynaptic neurons transmit impulses in rapid fire succession
Summation
EPSP’s acting alone cannot stimulate an action potential.
Takes 40 to 80 presynaptic terminals discharging at onces to elicit and action potential.
Large numbers of EPSP’s can cause a large enough change in the RMP that threshold is reached and an action potential is generated.
Two Ways to Summate EPSP’s
Spatial summation-
The post synaptic membrane is stimulated by a large number of terminals at the same time (neurotransmitter being released in large amounts from one neuron or from many neurons).
Temporal summation-
When one or more presynaptic neurons transmit impulses in rapid fire succession and bursts of neurotransmitter are released one right after another. 47

48. ALL-OR-NONE RESPONSE
An action potential is an “all or none” phenomenon
When threshold is reached, the action potential will occur completely
If threshold is not reached, the action potential will not occur at all 48

49. SALTATORY CONDUCTION Occurs only in myelinated axons
Depolarization wave jumps from one node of Ranvier to the next
Results in faster nerve impulse transmission
Saltatory Conduction
Occurs only in myelinated axons.
The presence of a myelin sheath drastically increases the rate of impulse transmission.
Current can pass through the membrane only at the nodes of Ranvier. The current cannot dissipate through the membrane regions covered with myelin---it moves on to the next node where it immediately triggers opening of the Na channels and continuation of the action potential.
This triggering of the action potential only at the nodes causes an increase in the speed of transmission and is called saltatory conduction. The electrical signal jumps from node to node. 49

50. SUMMARY OF EVENTS
A nerve impulse in the presynaptic neuron causes release of neurotransmitter into synaptic cleft
Neurotransmitter binding to receptors on postsynaptic neuron dendrite or soma cause certain chemically gated ion channels to open
If Na+ channels open:
Rapid influx of Na+ ions (depolarization)
A small positive graded potential occurs (EPSP)
If RMP changes in a positive direction by 20mV (or reaches the threshold), voltage gated sodium channels in the axon hillock open
Sodium rushes in at the axon hillock resulting in an action potential
As the positive ions get pushed down the axon, more voltage gated sodium channels open and the depolarization continues down the axon (propagation)
The process of restoring the negative RMP begins immediately following the depolarization wave (repolarization) 50

51. NERVE FIBER TYPES
The larger the axon diameter, the faster the impulse travels
Myelinated axons conduct impulses more rapidly
Fiber Types:
Type A fibers
Large diameter axon with thick myelin sheath
Impulse travels at 15 to 150 m/sec.
Sensory and motor fibers serving skin, muscles, joints
Type B fibers
Intermediate diameter axon, lightly myelinated
Impulse travels at 3 to 15 m/sec, Part of ANS
Type C fibers
Small axon diameter, unmyelinated
Slow impulse conduction (1 m/sec. or less)
Part of ANS
Type A fibers:
Somatic sensory (from skin and muscle) and somatic motor (to skin, muscle and joints). Large diameter, thick myelin m/s or over 300 mph.
Type B (40mph) and C fibers (2mph):
ANS motor fibers to viscera and visceral sensory fibers. 51

52. Neuronal Pathways and Circuits
Organization of neurons in CNS varies in complexity
Convergent pathways: many converge and synapse with smaller number of neurons. E.g., synthesis of data in brain
Divergent pathways: small number of presynaptic neurons synapse with large number of postsynaptic neurons. E.g., important information can be transmitted to many parts of the brain
Oscillating circuit: outputs cause reciprocal activation