Neurons/Nerves

Faiq ejaz

INTRODUCTION

• The human CNS contains about 10¹¹(100 billion) neurons
• Also contains 10–50 times this number of glial cells
• About 40% of the human genes participate, at least to a degree, in formation of CNS
• The neurons are the basic building blocks of the�nervous system
• Neurons perform the specialized function of integration & transmission of nerve impulse

• Neurons and glial cells along with brain capillaries form a functional unit that is required for normal brain function, including synaptic activity, ECL fluid homeostasis, energy metabolism, and neural protection
• Disturbances in the interaction of these elements are the pathophysiological basis for many neurological disorders (eg, cerebral ischemia,�seizures, neurodegenerative diseases, and cerebral edema)

GLIAL CELLS

• Previously, glial cells (or glia) were viewed as CNS connective/ supporting tissue; also called neuroglia or glia (glia = glue)
• Today theses cells are recognized for their role in communication within the CNS in partnership�with neurons
• non-excitable and do not transmit nerve impulse (action potential)
• Unlike neurons, glial cells continue to undergo�cell division in adulthood and their ability to proliferate is particularly noticeable after brain injury (eg, stroke)

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• Most commonly, neuroglial cells constitute the�site of tumors in nervous system
• Two major types of glial cells in the nervous system; microglia and macroglia
• Microglia; smallest neuroglial cells; derived from macrophages outside of the nervous system; often called the macrophages of CNS
• Scavenger cells that remove debris resulting from injury, infection, and disease (eg, multiple sclerosis, AIDS-related dementia, Parkinson disease, and�Alzheimer disease)

• Three types of macroglia: oligodendrocytes,�Schwann cells, and astrocytes
• Oligodendrocytes and Schwann cells are involved in myelin formation around axons in the CNS and peripheral nervous system, respectively
• Astrocytes are star-shaped neuroglial cells present�in all the parts of the brain; Two types of astrocytes are found in human brain:
• i. Fibrous astrocytes- occupy mainly the white matter�ii. Protoplasmic astrocytes- found in gray matter and have a granular cytoplasm

• Both send processes to blood vessels of brain, particularly, the capillaries, forming tight junction with capillary membrane; Tight junction in turn forms the blood-brain barrier
• Also send processes that envelop synapses and the surface of nerve cells
• Protoplasmic astrocytes have a membrane potential that varies with the external K+ concentration but do not generate propagated potentials
• Produce substances that are tropic to neurons, and help maintain the appropriate conc. of ions and neurotransmitters by taking up K+ and the neurotransmitters glutamate and γ-aminobutyrate

Neurones

• Neurons occur in a variety of sizes and shapes
• Most of them contain four parts: (1) a cell body, (2) dendrites, (3) an axon, and (4) axon terminals
• The cell body(soma) contains the nucleus & ribosome and is the metabolic center of the neuron
• The dendrites form a series of highly branched outgrowths from the cell body; receive most of the inputs from other neurons
• The branching dendrites (some neurons may have as many as 400,000!) increase the cell’s receptive surface area and thereby increase its capacity to receive signals from a myriad of other neurons

• Particularly in the cerebral and cerebellar cortex, the dendrites have small knobby projections called dendritic spines
• The axon, sometimes also called a nerve fiber, is�a single long process that extends from the cell body to its target cells (from μm to m in length)
• The part of the cell body where the axon is joined is known as the initial segment, or axon hillock
• The initial segment is the “trigger zone” where, in most neurons, the electric signals are generated & are then propagated away from the cell body along the axon or, sometimes, back along the dendrites

• The main axon may have branches, called collaterals, along its course
• Near the ends both the main axon and its collaterals undergo further branching; The greater the degree of branching of the axon and axon collaterals, the greater the cell’s sphere of influence
• Each branch ends in an axon presynaptic terminal; composed of a number of synaptic knobs which are also called terminal buttons or boutons
• They contain granules or vesicles in which the synaptic transmitters secreted by the nerves are stored

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• The axons of many neurons are myelinated, that is, they acquire a sheath of myelin, a protein–lipid complex that is wrapped around the axon
• In the PNS, myelin forms when a Schwann cell wraps its membrane around an axon up to 100 times
• The myelin is compacted when the extracellular portions of a membrane protein called protein zero (P₀) lock to the extracellular portions of P₀ in the apposing membrane
• Various mutations in the gene for P₀ cause peripheral neuropathies; 29 different mutations have been described that cause symptoms ranging from mild to severe

• Myelin sheath is not a continuous sheath; absent at regular intervals (1-μm constrictions that are about 1 mm apart); the area where myelin sheath is absent is called node of Ranvier
• Segment of the nerve fiber between two nodes�is called internode
• Myelin sheath is responsible for white color of nerve fibers
• Unmyelinated neurones are simply surrounded by Schwann cells without the wrapping of the Schwann cell membrane that produces myelin around the axon
• In the CNS, most neurons are myelinated; the myelin-forming cells are the oligodendroglia

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• Here oligodendrocytes emit multiple processes that�form myelin on many neighboring axons
• In multiple sclerosis, a crippling autoimmune disease, patchy destruction of myelin occurs in the CNS
• The loss of myelin is associated with delayed or blocked conduction in the demyelinated axons
• Neurons are classified by three different methods.�A. Depending upon the number of poles�B. Depending upon the function�C. Depending upon the length of axon

• Depending Upon The Number Of Poles:
• Based on the number of poles from which the nerve�fibers arise, neurons are divided into three types:
• 1. Unipolar Neurons: Neurons that have only one pole; from a single pole, both axon and dendrite arise; this type of nerve cells is present only in embryonic stage in human beings
• 2. Bipolar Neurons: Neurons with two poles are known as bipolar neurons; Axon arises from one pole and dendrites arise from the other pole
• 3. Multipolar Neurons: Neurons which have many poles; one of the poles gives rise to axon and all other poles give rise to dendrites

• Depending Upon The Function
• On the basis of function, nerve cells are classified into two types:�1. Motor or efferent neurons�2. Sensory or afferent neurons
• 1. Motor or Efferent Neurons: Neurons which carry�the motor impulses from CNS to peripheral effector organs like muscles, glands, blood vessels, etc. Generally, each motor neuron has a long axon and short dendrites
• 2. Sensory or Afferent Neurons: Neurons which�carry the sensory impulses from periphery to CNS; generally, each sensory neuron has a short axon and long dendrites

• Depending Upon The Length Of Axon
• Depending upon the length of axon, neurons are divided into two types:�1. Golgi type I neurons�2. Golgi type II neurons
• 1. Golgi Type I Neurons: They have long axons; cell body of these neurons is in different parts of CNS�and their axons reach the remote peripheral organs
• 2. Golgi Type II Neurons: Such neurons have short axons; present in cerebral cortex and spinal cord

AXONAL TRANSPORT

• Neurons-secretory cell; differ from other secretory cells in that the secretory zone is generally at the end of the axon, far away from the cell body
• Protein synthesis occurs in cell body & transported to the axonal ending by axoplasmic flow
• The functional and anatomic integrity of the axon is very important; if the axon is cut, the part distal to the cut degenerates (wallerian degeneration)
• Orthograde transport occurs along microtubules that run along the length of the axon and requires two molecular motors, dynein and kinesin

• Orthograde transport moves from the cell body toward the axon terminals
• Has both fast and slow components; fast axonal transport occurs at about 400 mm/day, and slow axonal transport occurs at 0.5 to 10 mm/day
• Retrograde transport, which is in the opposite direction (from the nerve ending to the cell body), occurs along microtubules at about 200 mm/day
• Synaptic vesicles recycle in the membrane, but some used vesicles are carried back to the cell body and deposited in lysosomes
• Some materials taken up at the ending by endocytosis, including nerve growth factor (NGF) and some viruses, are also transported back to the cell body