Nervous Tissue Histology for Veterinary Medicine PDF

NERVOUS TISSUE Histology course for Veterinary Medicine students DVM, Lect. Arnoldas Pautienius  Nervous tissue The nervous system enables the body to respond to changes in its external environment and controls functions of internal organs and systems....

NERVOUS TISSUE Histology course for Veterinary Medicine students DVM, Lect. Arnoldas Pautienius  Nervous tissue The nervous system enables the body to respond to changes in its external environment and controls functions of internal organs and systems. Nerve tissue consists of two principal types of cells: neurons (specialized cells that conduct impulses) and supporting cells (nonconducting cells in close proximity to nerve cells and their processes). The neuron is the structural and functional unit of the nervous system.  Nervous system The central nervous system (CNS) consists of the brain and spinal cord. It contains gray and white matter made up of neurons and supporting cells. The peripheral nervous system (PNS) consists of all the nervous tissue outside the CNS, including cranial nerves, spinal nerves, and ganglia.  Neurons Each neuron consists of: a cell body, an axon (usually the longest process of the cell body; transmits impulses away from the cell body), and several dendrites (shorter processes that transmit impulses toward the cell body). Neurons communicate with other neurons and with effector cells by specialized junctions called synapses.  Neurons Neurons are grouped into three categories: Motor neurons (carry impulses from the CNS or ganglia to effector cells). Sensory neurons (carry impulses from receptors to the CNS). Interneurons (communicate between sensory and motor neurons).  Neurons Multipolar neurons, such as motor neurons and interneurons, have numerous dendrites and a single axon. Bipolar neurons, such as in the retina and other special sense organs, have a single axon and a single dendrite that extend from the cell body. Unipolar neurons (pseudounipolar neurons) are sensory neurons that have one short process that arises from the cell body and bifurcates.  Neurons Many neurons are very long and not easy to see in a single section. However, smaller neurons in some part of CNS can be seen. The cell bodies of most neurons are located in the CNS. A few lie just outside the spinal cord in the ganglia, which are part of the peripheral nervous system (PNS). Rough endoplasmic reticulum (ER) in neurons are also known as Nissl bodies  Supporting cells: neuroglia (Central neuroglia) There are four types of central neuroglia: 1. Astrocytes are the most common type of supporting cell They make connections called ‘end- feet’ with capillaries, and they facilitate metabolic exchange between neurons and blood. They are star-shaped in appearance, when stains are used that show up the cell body and all its processes.  Supporting cells: neuroglia (Central neuroglia) There are four types of central neuroglia: 2. Oligodendrocytes generate the myelin sheath for up to 50 axons. The myelin sheath insulates the nerve and increases the rate at which the axon conducts its action potential. In the disease, multiple sclerosis, the myelin sheath is damaged, and axons become demyelinated.  Supporting cells: neuroglia (Central neuroglia) There are four types of central neuroglia: 3. Microglia (less common) play a role in immune defense, and can become phagocytic. These three types of cells are difficult to distinguish in routine histological sections  Supporting cells: neuroglia (Central neuroglia) There are four types of central neuroglia: 4. Ependymal cells make up the ependyma, which lines the ventricles of the brain and the central canal of the spinal cord. They contain microvilli and one or more cilia on their apical (lumenal) surface Left ventricle of the brain  Gray and white A section through the spinal cord or brain shows the ‘gray’ matter and ‘white’ matter Gray matter contains all the cell bodies of neurons, their unmyelinated dendrites, and all the cells that support the neurons (neuroglia). White matter contains myelinated processes of neurons, and a small number of cell bodies (from supporting cells only).  Supporting cells: neuroglia (Peripheral neuroglia) There are two types of peripheral neuroglia: 1. Schwann cells are the main type of supporting cell in the PNS Schwann cells wrap their plasma membrane concentrically around the axon, forming a segment of myelin sheath about 1 mm long. One Schwann cell generates the myelin sheath around the axon between each node of Ranvier. Small gaps between each segment of sheath (nodes of Ranvier) allow saltatory conduction (the propagation of nerve impulses from one node to the next).  Supporting cells: neuroglia (Peripheral neuroglia) There are two types of peripheral neuroglia: 2. Satellite cells maintain a controlled microenvironment around the nerve cell bodies in ganglia of the PNS.  Peripheral nerve structure In myelinated axons, an individual Schwann cell envelops a single axon multiple times In unmyelinated axons, the axons are myelinated, but a single Schwann cell envelopes several axons, and does not form tightly wrapped layers around the axon. Instead the axons sit in grooves in the surfaces of the cells. Both types of nerve fiber are present in nerves  Peripheral nerve structure Individual nerve fibers are held together by connective tissue organized into: Endoneurium (surrounds each individual nerve fiber and associated Schwann cell), Perineurium (surrounds each nerve fascicle), Epineurium (surrounds a peripheral nerve and fills the spaces between nerve fascicles).  Mechanoreceptor Lamellar corpuscles – the nerve endings responsible for sensitivity to vibration and pressure Approximately oval-cylindrical-shaped and 1 mm in length. The entire corpuscle is wrapped by a layer of connective tissue. Its capsule consists of 20 to 60 concentric lamellae  Neuroregeneration Neurons do not divide; However it can partially regenerate after injury (regeneration starts 20 days after injury)  Neuroregeneration After injury macrophages phagocytizes debris and stimulates Schwann cells to secrete substances that promote axon growth Schwann cell starts to form myelin sheath around the axonal sprout. It takes one year to complete Axon growth rates can reach 2 mm/day in small nerves and 5 mm/day in large nerves