Nerve Tissue

1.         The nervous system is composed primarily of two cell types:

a.         support cells known as glial cells or neuroglia, which

i.          have short processes.

ii.          support and protect neurons.

iii.            participate in neural activity, neural nutrition and the defense processes of the central nervous system.

b.            neurons, the basic signaling units of the nervous system.

2.         Role of neurons in the nervous system

a.            Neurons form an intercommunicating network of specialised cells in the nervous system which constitute:

i.          most sensory receptors.

ii.          the conducting pathways.

iii.         the sites of integration and analysis.

b.            Neurons respond to change in their environment by altering electrical potential that exist between the inner and outer surfaces of their membrane.

c.         Cells with this property are called excitable, or irritable.

d.         The sudden change in membrane potential causes an action potential, or nerve iimpulse to be propagated throughout the neuron.

e.         Upon reaching the synapse, this impulse will cause the release of neurotransmitters into the synaptic cleft which will initiate an action potential in the adjacent neurone.

f.          In this way, the vast network formed by neurons enable information to be transmitted smoothly from the central control system in the brain to the effectors and from the sensory receptors to the brain.

3.            Development of Nerve Tissue

a.         The nervous tissue is derived from the embryonic ectoderm.

b.         During the third week of embryonic development, the dorsal midline ectoderm undergoes thickening to form the neural plate.

c.         The lateral margins of the neural plate become elevated forming neural folds on either side of a longitudinal, midline depression, the neural groove.

d.         The neural folds then become apposed and fuse together, hence sealing the neural groove and creating the neural tube.

e.         Some cells from the apices of the neural folds become separated to form groups lying dorsalateral to the neural tube, which are called neural crests.

f.          The formation of the neural tube is complete by about the middle of the fourth week of embryonic development.

g.         The central cavity within the neural tube becomes the central canal of the spinal cord and the ventricles of the brain.

h.         The neural crests form the sensory ganglia of spinal and cranial nerves, and also the autonomic ganglia.

4.         Most neurons consist of three parts:

a.            Dendrites:

i.          multiple elongated thin processes that are specialized in receiving stimuli from the environment, sensory epithelial cells or other neurons.

ii.          they increase the surface area of the neuron so that it can communicate with multiple other neurons.

iii.         simple neurons have only one dendrite while cells in the brain have multiple dendrites with complex branching.

iv.         signals reaching the dendrites are transferred to an integrating region within the neuron.

b.         Cell body:

i.          in general, the cell bodies of all neurones are located in the central nervous system.

ii.            exceptions are the cell bodies of most primary sensory neurons and the terminal effector neurones of the autonomic nervous system where the cell bodies lie in peripheral ganglia.

iii.         has a nucleus and all organelles needed to direct cellular activity.

iv.         an extensive cytoskeleton extends outward into the axon and dendrites.

v.         the cell body occupies only one-tenth or less of the total cell volume.

vi.         the cell body is essential for the functioning of the neuron which derived all its nutrients from it.

vii.            severance of the cell body from the neuron will cause it to degenerate slowly and die.

c.         Axon:

i.          the axon is a single process projecting from the cell body at the axon hillock.

ii.          it allows for the generation and transmission of nerve impulses to other cells.

iii.         they may branch sparsely along their length, forming collaterals.

iv.         the distal portion of the axon is usually branched and constitutes the terminal arborization.

v.         each branch of this arborization terminates on the next cell in dilations called end bulbs (boutons), which interact with other neurons to form synapses.

5.         Axonal Transport

a.         The cytoplasm of axons is filled with many types of fibers and filaments but lack ribosomes and endoplasmic reticulum.

b.         Any proteins destined for the axon or the axon terminal must be synthesized on the rough endoplasmic reticulum in the cell body.

c.         The proteins are then packaged into vesicles and moved down the axon by axonal transport.

d.         Slow axonal transport:

i.          moves material by axoplasmic (cytoplasmic) flow,

ii.          it carries materials that are not consumed rapidly by the cell, such as enzymes and cytoskeleton proteins.

iii.         moves material from the cell body to the axon terminal at a rate of 0.2-2.5mm/day.

e.         Fast axonal transport:

i.          takes place along stationary microtubules that run the length of the axon from cell body to terminal.

ii.          the microtubules act as tracks along which transported particles ‘walk’ with the aid of attached footlike proteins.

iii.         these foot proteins, called kinesins, alternately bind and unbind to attached organelles down the axon in a stop-and-go manner.

iv.         use for transport of vesicles and mitochondria, which accumulate in the end terminals.

f           Fast axonal transport can occur in two directions:

i.            anterograde or forward: transport takes synaptic and secretory vesicles and mitochondria from the cell body to the axon terminal.

ii.            retrograde or backward: returns old membrane components from the axon terminal to the cell body for recycling.

6.         Types of neurons

a.            Sensory neurons: carry information about temperature, light, pressure from sensory stimuli from sensory receptors to the central nervous system.

b.            Interneurons: they lie entirely in the central nervous system and they have complex branching processes that allow them to form synapses with many other neurones.

c.         Efferent neurons:

i.          carry information from the central nervous system to the effector muscles and cells.  

ii.          in the autonomic division, some neurons have enlarged regions along the axon called varicosities which store and release neurotransmitter.