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Nervous tissue

 

What is nervous tissue?

 
Complete neuron diagram

Public domain: Kunstenaar - LadyofHats
  • Coetzee et al p151-166
  • Stevens and Lowe p77-98
  • Wheater p117-142

Classification of nervous tissue

  Nervous tissue is histologically divided into neurons and neuroglia.
  1. Neurons are basic structural and functional units of the nervous system.
  2. Neuroglia perform a support and nutritive function.

Neurons

Slide 26:
Spinal Cord
Slide 82:
Spinal Cord
Slide 60:
Cerebellum


Neurons are basic structural and functional units of the nervous system. The cell consists of a cell body or perikaryon containing the nucleus and organelle and one or more cytoplasmic processes. These may be one or more dendrite and a single axon. Axons conduct impulses to other neurons or effectors, while dendrites receive impulses from receptors or other neurons.

Neurons may be subdivided by:

  1. Morphology:
    • Unipolar - one process a single axon
    • Bipolar - one axon and one dendrite
    • Multipolar - one axon and numerous dendrites
    • Pseudo-unipolar - an axon and a dendrite is fused for a small distance before splitting

  2. Function:
    • Afferent neurons conduct impulses to the CNS
    • Efferent neurons conduct impulses from the CNS
    • Interneurons connect afferent and efferent neurons in the CNS
    • Excitatory neurons stimulate the next neuron
    • Inhibitory neurons suppress the next neuron

  3. Length of the axon
    • Golgi type I neurons have a long axon that leaves the CNS
    • Golgi type II neurons have a short axon that does not leave the CNS

Neuroglia

  Neuroglia perform a support and nutritive function

  • Astrocytes give physical support and are subdivided into:
    • Protoplasmic astrocytes
    • Fibrous astrocytes

  • Oligodendroglia form the myelin sheath in the CNS
  • Microglia are the macrophages of the CNS
  • Ependyma line the ventricles and spinal canal.
It should be kept in mind that the different glial types have different embryological origins, different locations and different functions.

Central nervous system and peripheral nervous system

  The nervous system is anatomically divided into the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS is encased in a bony cavity and consists of the brain in the cranium and the spinal cord in the spinal column.

The PNS consists of:

  1. Receptors that detect the stimulus and turns it into an electrical impulse. Receptors are specialized nerve endings or specialized nerve cells and may be sub classified by:
    • Location eg muscle spindle inside skeletal muscle, etc
    • Type of sense organ eg skin, visceral, etc
    • Type of stimulus eg chemoreceptors, photoreceptors, etc

  2. Afferent division, which conducts impulses from the receptors to the CNS.

  3. Efferent division which consists of a single motor neuron and conducts impulses away from the CNS and which is subdivided into:
    • Somatic efferent division which supplies the skeletal muscles
    • Autonomic division consisting of two neurons, a preganglionic and a postganglionic neuron. This division supplies the viscera and is further divided into the sympathetic and parasympathetic divisions.

Synapse

  There is no physical contact between two neurons, only a physiological connection known as a synapse.

The presynaptic axon ends in a bulb-like structure, the end bulb or "boutons terminaux". There is a space, the synaptic cleft, between the end bulb and the cell membrane or post-synaptic membrane of the postsynaptic neuron. The end bulb contains synaptic vesicles that release neurotransmitters. These can either stimulate or inhibit the postsynaptic neuron.


Myelin

Slide 32:
Neuromuscular Bundle
The myelin sheath is formed by Schwann cells in the PNS and by oligodendroglia in the CNS.

The myelin sheath serves as a biological insulation to facilitate impulse conduction. As it consists of consecutive layers of cell membrane, it is rich in phospholipids and cholesterol, which affects its staining characteristics. The junction where two adjacent Schwann cells meet and where the axon is not covered with myelin, is known as the node of Ranvier.


Nerves

  A thin, delicate connective tissue layer, the endoneurium, surrounds the individual nerve fibres (axons). Several nerve fibres are bundled together as nerve bundles or fasciculi and are surrounded by the perineurium. The nerve bundles are arranged together as a nerve that is encapsulated by a dense connective tissue layer, the epineurium.

Ganglia

  Ganglia are neuronal relay stations in the PNS and consists of:
  1. Neuron cell bodies
  2. Support cells (Schwann cells and satellite cells)
  3. Loose connective tissue.

Ganglia are subdivided into:

  1. Sensory ganglia
    1. Cranial ganglia associated with cranial nerves
    2. Spinal ganglia - associated with the dorsal roots of the spinal nerves
  2. Autonomic ganglia may be located inside organs eg wall of GI tract

Diseases of nervous tissues

 
  • Motor neuron disease spontaneous dying of motor neurons cause skeletal muscle weakness and eventual death
  • Multiple sclerosis is an autoimmune disease that attacks the myelin sheath and has a tendency to strike young adults.

Senuweeweefsel plaatjies

Slide 24 Wagner-Meissner body
 

Slide 26 Multipolar neurons: cross section of the spinal cord
 

Slide 32 Cross section of peripheral nerve
 

Slide 44 Motor endplates
 

Slide 59 Peripheral nerve
 

Slide 61 Astrocytes: section through cerebrum
 

Slide 82 Multipolar neurons: cross section of the spinal cord stained with silver
 

Slide 93 Pacini corpuscle: section through skin from the palm, illustrating sensory nerve endings of the peripheral nervous system.
 

Slide 98 Ependyme: section through choroïed plexus
 

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© aug 2002 -2006 marius loots