NERVOUS TISSUE

NERVOUS TISSUE

• Main component of the nervous system (brain, spinal cord, peripheral nerves) which regulates and controls bodily functions

• Composed of neurons which transmit impulses, and neuroglial cells which assist in the propagation of nerve impulses as well as providing nutrients to the neurons

• All types of nerve cells have an axon that sends action potential signal to the next cell

• specialized to react to stimuli and conduct impulses to

  various organs in the body which bring about response to stimuli

• responds to stimuli and transmits impulses from one

  body part to another

glia

Greek word meaning glue

1. Sensory input

2. Integration

3. Control of muscles and glands

4. Homeostasis

5. Mental activity

Functions of the nervous system

cell body

enclosed by a cell/plasma membrane and has a central nucleus

Nissl bodies

granules found in the cytoplasm of the cell body

Neurofibrils

extremely fine fibers within the cell body from the dendrites to the axon

Myelin sheath

whitish, non-cellular fatty layer that surrounds the axon

Neurilemma

• “sheath of Schwann cells”

• cellular layer outside of the myelin sheath

Medullary sheath

Myelin sheath together with the neurilemma

Nodes of Ranvier

interrupted intervals along the medullary sheath

Nervous

easily stimulated and transmit impulses very rapidly

Nerve

made up of nerve fibers bound together by connective tissue

Epineurium sheath of dense connective tissue

surrounds the nerve and penetrates the nerve to form the perineurium which surrounds bundles of nerve fibers

blood vessels

found in the epineurium

endoneurium

a thin layer of loose CT which surrounds individual nerve

fibers

neuron (nerve cell)

• Majority generated before birth

• Persisting stem cells give rise to new neurons throughout lifetime

• Additional neurons important for maintenance and plasticity but insufficient to replace neurons that die

• Mature neurons do not mitotically divide

• functionally meet at the synapse but normally without direct contact

• may occasionally make synapse-like contact with glial cells

1. Shape of the neuron and its processes

2. Chemicals the neurons uses to communicate (neurotransmitters)

3. Ways in which neurons may react to the neurotransmitters released by other neurons

Key to understanding function of neuron lies in:

neurotransmitters

• Chemicals the neurons uses to communicate

• transmit information across gap

dendrites

• part of receptive surface of neuron

• One or several primary dendrites which divide into secondary and tertiary dendrites

• Maybe smooth or studded with small, mushroom-shaped appendages called spines

axon

• never more than one

• Point of origin from perikaryon is axon hillock

• May branch like dendrites

• “Transmitting” process of the neuron

• Small bulb-shaped swellings called boutons at the end (terminal boutons) or along the course (boutons en passant)

synapse

specialized contacts between a bouton formed by one neuron (presynaptic neuron) and the cell surface of another neuron (postsynaptic neuron)

synaptic vesicls

accumulate close to point of contact between bouton and postsynaptic neuron

impulses

Messages carried by nervous system are electrical signals called __

Transmitters

• Excite or inhibit postsynaptic neurons

• L-glutamate

• gamma- amino butyric acid (GABA)

• Other main neurotransmitters are dopamine, serotonin, acetylcholine, noradrenaline and glycine

• Each neuron uses only one main transmitter

• One or more minor transmitters like cholecystokinin, endogenous opiods, somatostatin, maybe used together with main transmitter

L-glutamate

Most prominent excitatory transmitter

gamma- amino butyric acid (GABA)

Most prominent inhibitory transmitter

Glial Cells

• Several types of non-neuronal, supporting cells (neuroglia) in CNS tissue

• many express neurotransmitter receptors

• may communicate with each other via the gap junctions

Astrocytes (astroglia)

• Star-shaped cells which processes are often in contact with blood vessels (perivascular foot processes)

• Provide mechanical and metabolic support to the neurons of the CNS

• Participate in maintenance of composition of ECF

• Maybe involved in removal of transmitters from synapses and metabolism of transmitters

• Scar-forming cells of CNS

Oligodendrocytes (oligoglia)

• Fewer and shorter processes

• Form myelin sheath around axons in the CNS, homologue of peripheral Schwann cells

• Unlike Schwann cells, may form parts of myelin sheath of several axons

Microglia

• Small cells with complex shapes Of mesodermal origin

• Derived from cell lines which gives rise to monocytes

• In case of tissue damage, microglia can proliferate and differentiate into phagocytic cells

Ependymal cells

• Line the ventricles of the brain and central canal of the spinal cord

• Often ciliated and of simple cuboidal or low columnar epithelium

• Lack of tight junction between cells allow free exchange between CSF and nervous tissue

• Can specialize into tanycytes which are rarely ciliated with long basal processes; form ventricular lining over a few CNS regions in which the (blood-brain barrier) BBB is incomplete

• Form tight junctions and control exchange of substances between these regions and surrounding nervous tissue or CSF

Neuronal activity

may regulate glial functions by spillover of transmitter from synaptic sites surrounded by fine processes of glial cells

Amphycytes or satellite cells (gliocytes of the PNS)

• Oval or spindle-shaped cells with scanty cytoplasm and round nucleus at expanded part of the cell

• Form rosary-like capsule around individual cell bodies of neurons located in cranial and spinal ganglia

Neurolemmocytes of Schwann cells

• Cells enclosing both the unmyelinated and myelinated axons in the PNS

• Single neurolemmocyte can invest one to several nerve fibers

• Produce myelin sheath of myelinated fibers in the PNS

Unipolar neuron type of neuron

• only one protoplasmic process (neurite) extends from the cell body

• pseudounipolar neurons

• has only one process, the axon

• Present in developing or embryonic nervous system and is also common in insects

pseudounipolar neurons

• Unipolar neurons that begins as bipolar neurons during development

• has 2 processes, one axon and one dendrite that fuse close to the cell body but separate at some distance from it

• Soma is large with large, round, centrally located nucleus

• Both processes appear structurally as axons but functionally, afferent and efferent

Bipolar neuron

• has 2 processes, one axon and one dendrite that originate at opposite poles of the soma

• Ex. Photoreceptor cells of the retina and olfactory hair cells of the olfactory epithelium

Multipolar neuron

has many processes, one axon and several dendrites that arise from an oval, pyramidal or stellate soma