Neuronal Tissue And Peripheral Nerve Structure Notes

Neuronal Tissue and Peripheral Nerve Structure

Neurons

Neurons are one type of cell that makes up neural tissue. They are highly specialised cells, and when fully mature lack the property of cell division. They do however have a limited ability to regenerate processes in the PNS.

Neurons are large cells that consist of a cell body, also called a perikaryon, and several processes extending out of the perikaryon. One of these processes will always be an axon, whilst the rest are dendrites.

The cell body has a large nucleus and nucleolus for production of ribosomes, and large amounts of endoplasmic reticulum, which contain Nissl substance. There is also a prominent Golgi body. This signs show that neurons are active protein producing cells.

Each neuron may have several dendrites that branch in a tree-like manner, and will contain Nissl substance. Dendrites are a cytoplasm-like extension of the cell body, and are the receptive area for impulse transmission.

However, each neuron has only one axon that is without Nissl substance and emerges from the axon hillock, also without Nissl substance, and is surrounded by Schwann cells. The axon is responsible for transmission of nerve impulses away from the cell body. The axon remains unbranched until it reaches its axon terminal.

Multipolar neurons are those with one axon and multiple dendrites. Groups of such neurons located in the CNS are called nuclei.

Pseudounipolar neurons have one process that leaves the cell body and quickly divides into two. Structurally, both processes resemble axons, but only one functionally acts as an axon. Unipolar neurons are often general sensory neurones. The cell bodies of unipolar neurons are often grouped together in ganglia in the PNS.

Bipolar neurons have two processes, one at each end. One is a dendritic, whilst the other is the axon.

Afferent or sensory neurons convey information towards the CNS. Efferent or motor neurons convey information away from the CNS.

Association or interneurons connect or associate one point in the CNS with another but never leave the CNS. They may bridge between sensory and motor neurons.

Neurotransmitters and proteins are synthesied in the cell body, in Nissl bodies, and transported via either: the axoplasmic flow – contractions that push the cytoplasm from the axon hillock to nerve endings; or the axonal forward/retrograde transport via neurotubules and kinesins.

Nerves

A nerve is composed of several bundles of nerve axons (called nerve fibres) held together by connective tissue. Most nerves contain sensory and motor fibres.

Nerves are surrounded by an outer epineurium, which is a protective layer of dense connection tissue, rich in collagen fibres.

The perineurium surrounds each individual fascicle. This is an epithelial layer that isolates the bundles of axons from the surrounding connective tissue. It encloses a fluid filled space between the perineurium and the underlying nerve axons.

The connective tissue within the perineural sheath is called endoneurium. This includes seams of loose connective tissue to provide pathways for small arterioles, venules and axons.

Neuronal Junctions

Neuronal junctions are of three types …

• Synapses (neurone/neurone) – can be excitatory or inhibitory in type.

• Neuromuscular junctions (neurone/muscle cells) – always excitatory in the case of skeletal muscle.

• Neuroglandular junctions (neurone/glandular cells).

Other neural cells

Other neural cells are called neuroglial cells. In the PNS, these include Schwann cells and satellite cells, whilst in the CNS they include oligodendrocytes, astrocytes, microglia and ependymal cells.

In the PNS, axons are surrounded by a myelin sheath formed by Schwann cells. Nodes of Ranvier exist between the Schwann cells that allow impulses to be propagated along the...

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