the nervous system ii. i. myelination and action potential conduction a. axons are myelinated by the...

THE NERVOUS SYSTEM II

I. Myelination and action potential conduction

A. Axons are myelinated by the activities of oligodendrocytes in the central nervous system and Schwann cells in the peripheral nervous system.

B. Perhaps the most important reason for this is that myelination allows for higher velocities of nervous impulse or action potential conduction.

C. Action potentials are generated by ion permiability changes in the axon membrane. Once an action potential is initiated, it is self-propagating. In nonmyelinated axons, the permiability changes must be induced along the entire length of the axon. The action potential can only be conducted as fast as the permiability changes can occur.

D. Because myelinated axons are well insulated by their myelin coverings, the action potential can skip the myelinated lengths of the axon and it is only necessary for the permiability changes to occur at the nodes of Ranvier (between myelinated portions of the axon). So, myelination shortens the length of axon membrane in which permiability changes must be induced and thereby speeds up transmission of the nervous impulse.

E. The fewer the nodes of Ranvier for a given length of axon, the faster the transmission.

II. Involuntary component of the nervous system

A. The voluntary component of the nervous system is important in effecting the movement of skeletal muscle and sensing changes in our external environment.

1. modulation of smooth muscle contraction - particularly the digestive and reproductive tracts.

C. Thus, the autonomic component of the nervous system relieves us of the necessity of tying up large parts of our voluntary circuitry simply to maintain vital body functions. If conscious thought were necessary every time our heart beat and every time the muscles of our digestive tract contracted, we would not have much time to think about anything else.

B. There is also an involuntary component to the nervous system called the autonomic nervous system. The neurons of this part of the central and peripheral nervous systems are important in,

2. secretion of some glands

3. modulation of cardiac rhythm

D. Autonomic nervous system is divided into two parts

II. Involuntary component of the nervous system

1. Sympathetic - neurotransmitter is norepinephrine (noradrenaline) for postganglionic axons, has adrenergic synapses - stimulates (enhances) activity. Preganglionic axon synapses use acetyl choline.

http://faculty.washington.edu/chudler/auto.html

2. parasympathetic - neurotransmitter is acetylcholine, has cholinergic synapses - generally slows activity

E. These two components of autonomic nervous system are generally antagonistic, so in a given organ that they innervate, one will stimulate and the other will inhibit the activities of the organ [like an accelerator (sympathetic) and a brake (parasympathetic)] . There are exceptions where both stimulate such as in some salivary glands.

http://faculty.washington.edu/chudler/auto.html

II. Involuntary component of the nervous system

III. Ganglion structure

A. Ganglion (pl. - ganglia) - an aggregation of neurons and associated glial cells outside the central nervous system.

http://millette.med.sc.edu/Lab%209&10/F5%20image%20page.htm

spinal ganglia

medulla oblongata

dorsal rootlets of spinal nerves

1. Dorsal root (spinal) ganglia and cranial ganglia (craniospinal ganglia) - neurons are purely sensory, considered part of voluntary nervous system. Connective tissue capsule surrounds ganglion.

B. Types of ganglia,

a. Cranial ganglia - nerves running through them may have both motor and sensory components - sensory components (axons/dendrites) from neurons in ganglia, motor components (axons) from neurons in brain.

III. Ganglion structure

http://education.vetmed.vt.edu/Curriculum/VM8054/Labs/Lab9/Examples/exsomarc.htm

III. Ganglion structure

a. Neurons mainly cortical, small (15-30 um) and large (120 um) neurons. Ganglion medula is mainly axonal processes and a few glial and neuronal cells.

http://www.lab.anhb.uwa.edu.au/mb140/CorePages/Nervous/Nervous.htm#labganglia

1. Dorsal root (spinal) ganglia and cranial ganglia (craniospinal ganglia)

c. Neuron perikarya are surrounded by glial cells called satellite cells. Form a sort of capsule around the perikaryon

d. Fibroblasts are present between the satellite cell surrounded perikarya.

1. Dorsal root ganglia and cranial ganglia

b. If properly stained, neurons of these ganglia usually show fine Nissl bodies and may have lipofuscin deposits.

http://www.lab.anhb.uwa.edu.au/mb140/CorePages/Nervous/Nervous.htm#labganglia

e. Neurons of dorsal root and cranial ganglia are generally pseudounipolar. Sensory stimulation of dendritic end causes nerve impulse that bypasses the neuron perikaryon.

f. An exception are neurons of the vestibular ganglion of the inner ear (associated with cranial nerve #8) which are bipolar

1. Dorsal root ganglia and cranial ganglia

http://www.csus.edu/org/nrg/carter/NeurosylActive/histology/neuron/bipolar.htm

2. Autonomic ganglia - associated with nerves of the autonomic nervous system. Both sensory and motor cell bodies. Connective tissue capsule may surround autonomic ganglia.

III. Ganglion structure

B. Different types of ganglia,

a. intramural ganglia

• generally considered part of parasympathetic system

(recent work suggests both parasym-pathetic and sympathetic neurons)

• small (just a few neurons)

• located within viscera particularly in the walls of digestive tract

• no connective tissue capsule surrounding intramural ganglia

• A lot fewer satellite cells than in dorsal root and cranial ganglia

III. Ganglion structure

B. Different types of ganglia,

2. Autonomic ganglia

b. Other autonomic ganglia

• may have connective tissue capsule

• neuron perikarya not just cortical

• both sympathetic and parasympathetic neurons

• mainly multipolar neurons with fine Nissl bodies

• innervation to both smooth muscle and secretory components of mucosa.

• perikarya with incomplete layer of satellite cells

• nuclei of neurons offset to one side of perikaryon

http://millette.med.sc.edu/Lab%209&10/F7%20image%20page.htm

Autonomic ganglion in the wall of the ureter.

http://www.med.mun.ca/anatomyts/digest/gut97a.htmhttp://www.lab.anhb.uwa.edu.au/mb140/CorePages/Muscle/Muscle.htm#LABSMOOTH

CNS consists of ganglia with characteristic structural organization

Neurons typically unipolar.

All neuron perikarya in cortex, just under connective tissue sheath

Medulla of ganglion, the neuropil, consists of closely packed dendrites/axons and synapses

Nerve tracts extend from ganglia to peripheral regions of body. Both motor and sensory function.

Ganglia exhibit highly specific organization, often possible to identify the same specific neuron in different individuals of same species (identifiable neurons)

Invertebrate ganglia

http://slugsite.us/bow/nudiwk76.html

IV. CNS, Gray Matter, White Matter

A. White matter - cortically located (except in cerebrum and cerebellum). Composed of

1. myelinated and unmyelinated axons,

2. oligodendrocytes,

3. astrocytes, mainly fibrous

4. microglial cells

5. White color due to presence of large amounts of myelin.

6. no neuron perikarya!

cortical - peripheral

medullary - middle

Review+

B. Gray matter - usually medullary (except cerebrum and cerebellum). Composed of

cortical - peripheral

medullary - middle

Review+

1. neuron perikarya,

3. mostly unmyelinated axons

4. some myelinated axons,

5. astrocytes, mainly protoplasmic

6. oligodendrocytes,

7. microglial cells.

2. dendrites, some lightly myelinated

* ventral (anterior) horns - ventral roots, motor function

C. Consider spinal cord structure versus cerebellum structure.

1. Spinal cord

a. cortical white matter

b. medullary gray matter in a butterfly shape

c. central canal lined with ependymal cells

d. gray matter divided into

* dorsal (posterior) horns - dorsal roots, sensory function

http://www.lab.anhb.uwa.edu.au/mb140/

Posterior median sulcus

Anterior median fissure

cortical - peripheral

medullary - middle

IV. CNS, Gray Matter, White Matter

2. Cerebellum

a. composed of many folds called folia (sometimes gyri).

b. these folds have cortical gray matter, medullary white matter

* Most external - molecular layer with few perikarya and many unmyelinated nerve fibers.

c. Cerebellum - the cortical gray matter is divided into 3 regions.

* Middle - Purkinje neuron/cell layer - single layer of neurons with perikarya lying between molecular layer and granular layer. Highly branched dendritic tree that extends into molecular layer. Axon extends into granular layer. Dendritic branching in one plane (fan-shaped).

* Inner - granular layer - Smallest neurons in human body (5 um diameter). Neurons are multipolar with 3 - 6 dendrites and one axon.

http://www.histol.chuvashia.com/atlas-en/nerv-04-en.htm

Cerebellum - hematoxylin

Cerebellum - silver nitrate

IV. CNS, Gray Matter, White Matter

3. Cerebrum also has cortical gray matter and medullary white matter.

http://www.histology.wisc.edu/histo/uw/histo.htm

http://www.kumc.edu/instruction/medicine/anatomy/histoweb/nervous/nerve11.htm

b. Perikarya of neurons in cerebrum are mostly pyramidal, stellate (multipolar), or spindle shaped (bipolar).

a. General gross structure similar to that of cerebellum (folds called gyri), but neurons are different and the gray matter only consists of one layer.

V. Sensory structures

A. There are 5 principle types of receptors.

1. nociceptors (somatic and visceral receptors) that are sensitive to pain, pressure, vibration and temperature.

B. Your slide set in lab only has an example of Pacinian corpuscles (proprioceptor). We will only discuss Pacinian corpuscles, the end=bulb of Krause (nociceptor), and Meissner's corpuscles (nociceptor) here.

2. proprioceptors that are sensitive to ones position in space. Inner ear, Pacinian corpuscles, and receptors of muscular info (muscle spindles).

3. chemoreceptors - taste and smell

4. audioreceptors - hearing

5. photoreceptors - sight.

Somatic receptors - receptors associated with the skin, i.e. involved in reception of cutaneous senses.

Visceral receptors - receptors associated with the internal tissues and organs, i.e. the viscera.

V. Sensory structures

1. TOUCH - Meissner's corpuscles

a. the most numerous sensory structure in your body.

b. Common in hairless skin such as tips of fingers, palms, soles, nipples, and lips.

d. Naked (unmyelinated) dendrite/axon enters one end and zigzags up through stack. This structure is encapsulated in connective tissue and is usually located in a dermal papilla just below the epidermis. The naked dendritic endings are what perceive touch.

http://www.udel.edu/Biology/Wags/histopage/colorpage/cin/cinmcs.gif

c. Elongated structures consisting of fibroblasts and/or modified Schwann cells. that form a multilayered stack.

Meissner's corpuscles

http://www.udel.edu/Biology/Wags/histopage/colorpage/cin/cintpmc.GIF

V. Sensory structures Meissner's corpuscle

V. Sensory structures

2. PRESSURE - Pacinian corpuscles

a. Central unmyelinated nerve ending (dendrite/axon) surrounded by 30 or more concentric layers of connective tissue composed of epithelioid fibroblasts.

b. Found in deep layer of dermis, in loose connective tissue, in mesenteries, and in external genitalia of both males and females.

d. Respond to vibrations and pressure

c. Function in propriocetion, i.e. sense/signal mechanical deformations of limbs and other body parts.

http://www.udel.edu/Biology/Wags/histopage/colorpage/cne/cnepc.GIF

V. Sensory structures

http://www.udel.edu/Biology/Wags/histopage/colorpage/cne/cnepc.GIF