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:
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
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
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 perform a support and nutritive function
Astrocytes give physical support and are subdivided into:
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:
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:
Type of stimulus – eg chemoreceptors, photoreceptors, etc
Afferent division, which conducts impulses from the receptors to the CNS.
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.
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.
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.
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 are neuronal relay stations in the PNS and consists of:
Neuron cell bodies
Support cells (Schwann cells and satellite cells)
Loose connective tissue.
Ganglia are subdivided into:
Cranial ganglia – associated with cranial nerves
Spinal ganglia - associated with the dorsal roots of the spinal nerves
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.