MYELINATION

most neuron axons are surrounded by a myelin sheath

protein lipid covering produced by neuroglia

1) electrically insulates axon
2) speeds up the transmission of nerve impulse through the axon

WHITE MATTER

the major component of a cell membrane is the phospholipid bilayer
many layers of membrane stacked on top of one another creates a fatty appearance due to the presence of this phospholipid
lipid has a glistening white appearance

such as fat found on meats

myelinated axons have a glistening white appearance

GRAY MATTER

areas containing mainly cell bodies tend to lack myelin

NEUROLEMMA

neurolemmocytes (Schwann cells) wrap several times around a small portion of the PNS axon
myelin sheath is called a neurolemma
aids regeneration of an axon if it is injured

forms a regeneration tube that guides and stimulates regrowth of the axon

NODES OF RANVIER

Intervals along the axon where there are gaps between the myelin sheath
Neurolemmocytes wrap (neurolemma) the axon segment between the two nodes
oligodendrocytes myelinate many cells of the CNS in much the same manner as a neurolemmocyte myelinates parts of a single PNS axon

CNS LACKS NEUROLEMMA

many broad flat processes spiral about CNS axons and deposit a myelin sheath

neurolemma is not formed

axons in the CNS display little regrowth after injury

due to absence of neurolemma and inhibitory influence exerted by CNS neuroglia

MYELIN AND NERVE REGENERATION

possible only if the axon is myelinated

REGENERATION OF NERVOUS TISSUE

neurons have very limited powers of regeneration neurons lose the ability to divide at 6 months of age any neuron destroyed is permanently lost only certain types of damage to neuron cells can be repaired

PNS REGENERATION

PNS dendrites and axons may be repaired

1) if cell body remains intact
2) if Schwann cells remain active

CNS REGENERATION

	CNS shows little repair of damage to neurons injury to brain or spinal cord is usually permanent
	1) axons of CNS are myelinated by oligodendrocytes and do not form neurolemmas 2) neuroglia of CNS inhibit axon regrowth possibly the same mechanism that inhibits axonal growth during development once a target region has been reached 3) astrocytes invade area forming scar tissues which act as physical barriers to regeneration

NEURONAL REGROWTH

neuronal tumor cells
brains of some songbirds

nerve tissue appears and disappears every year

lack of mammalian CNS regeneration

1) inhibitory influences from neuroglia
2) absence of growth cues present during development

developmental cues are electrical and chemical in nature use electrical or chemical stimulation to promote axon regrowth EGF (1992) used to trigger mitosis

MYELIN DEVELOPMENT

amount of myelin increases from birth to maturity
myelin presence greatly increases the speed of nerve impulse conduction
infant response to stimuli are not as rapid or coordinated as those of older children or adults

myelination is still in progress

DISEASE OF MYELIN SHEATH

Tay-Sachs disease, diabetes mellitus and multiple sclerosis cause destruction of the myelin sheaths
Results in slowed action potential and impaired control of skeletal and smooth muscle

MULTIPLE SCLEROSIS (MS)

Progressive destruction of myelin sheath in CNS neurons
Chronic, disabling disease affection over 2.5 million people world wide
Myelin sheaths deteriorate to scleroses (hardened scars or plaques) in multiple regions

characteristics of the disease

1) progressive loss of muscle strength
2) strange sensations
3) double vision occurs periodically
4) "attacks" every year or two with periods of remission

implications of a viral cause which precipitates the activation of killer T-cells to destroy myelin producing oligodendrocytes
1993 FDA approved use of Betaseron (a form of interferon)

note greyish plaques around the ventricles

ENDONEURIUM

a connective tissue wrapping enveloping individual axons

PERINEURIUM

a connective tissue wrapping bundles or fascicles of axons

EPINEURIUM

a connective tissue sheath enveloping the nerve as a whole
These connective tissue sheaths help to give peripheral nerves a certain toughness and resistance to tearing