Nervous Tissue and Nervous System

Nervous Tissue Overview

• The nervous system is divided anatomically into the Central Nervous System (CNS) and Peripheral Nervous System (PNS).
• The CNS consists of the brain and spinal cord.
• The PNS consists of cranial, spinal nerves, and ganglia.

Neurons

• Neurons are the main cells of the nervous tissue and are stellate-shaped with long processes.
• A neuron has three parts: dendrites, soma (cell body), and axon.
• Dendrites receive impulses and have the same structure as the soma.
• The soma contains a central nucleus, basophilic cytoplasm, and neurofilaments, but no centrioles.
• The axon transmits nerve impulses and is surrounded by axolemma, containing neurofilaments and mitochondria.

Classification of Neurons

• Pseudounipolar neurons have a common stem that divides into two branches, found in sensory ganglia.
• Bipolar neurons have two processes, found in the retina and olfactory cells of olfactory epithelium.
• Multipolar neurons have more than two processes and are the most common type, including motor neurons.

Classification by Shape

• Pyramidal neurons are found in the cerebral cortex.
• Pyriform neurons are found in Purkinje cells of the cerebellum.
• Stellate neurons are found in the spinal cord and autonomic ganglia.

Classification by Function

• Sensory neurons receive stimuli from the environment and conduct them to the CNS.
• Motor neurons originate in the CNS and conduct impulses to effecter organs like muscles and glands.
• Associated neurons (interneurons) connect sensory and motor neurons.

Nerve Fibers

• Nerve fibers are made up of axons of neurons.
• Most axons are enveloped by myelin sheaths formed by oligodendrocytes in the CNS and Schwann cells in the PNS.
• Myelin sheath increases nerve impulse conduction through salutatory conduction.

Connective Tissue Coats

• Epineurium is a connective tissue that surrounds the nerve trunk.
• Perineurium is a connective tissue that surrounds nerve bundles.
• Endoneurium is a connective tissue that surrounds individual nerve fibers.

Neuroglial Cells

• Neuroglial cells support and protect neurons.
• PNS neuroglial cells include Schwann cells and satellite cells.
• CNS neuroglial cells include astrocytes, oligodendroglia, microglia, and ependymal cells.

Ganglia

• Ganglia are ovoid structures containing neuronal cell bodies and glial cells supported by connective tissue.
• Sensory (spinal) ganglia receive peripheral afferent impulses that go to the CNS and have pseudounipolar neurons.
• Autonomic ganglia receive efferent impulses to effecter organs and have multipolar neurons.