THE NERVE - neurons, structure, division
- 2. Neuron or nerve cell is defined as the structural and functional unit of nervous system. Neuron is similar to any other cell in the body, having nucleus and all the organelles in cytoplasm. Neuron has branches or processes called axon and dendrites. Neuron does not have centrosome. So, it cannot undergo division.
- 3. CLASSIFICATION OF NEURON • 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: I. Unipolar neurons - Unipolar neurons are the neurons that have only one pole. II. Bipolar neurons- Neurons with two poles are known as bipolar neurons. III. Multipolar neurons- Multipolar neurons are the neurons which have many poles.
- 5. • DEPENDING UPON THE FUNCTION: On the basis of function, nerve cells are classified into two types: a) Motor or efferent neurons b) Sensory or afferent neurons. Motor or efferent neurons: Motor or efferent neurons are the neurons which carry the motor impulses from central nervous system to peripheral effector organs like muscles, glands, blood vessels, etc. Sensory or afferent neurons: Sensory or afferent neurons are the neurons which carry the sensory impulses from periphery to central nervous system.
- 7. • DEPENDING UPON THE LENGTH OF AXON: • Depending upon the length of axon, neurons are divided into two types: I. Golgi type I neurons II. Golgi type II neurons I. Golgi Type I Neurons: Golgi type I neurons have long axons. Cell body of these neurons is in different parts of central nervous system and their axons reach the remote peripheral organs. II. Golgi Type II Neurons: Neurons of this type have short axons. These neurons are present in cerebral cortex and spinal cord.
- 8. STRUCTURE OF NEURON • Neuron is made up of three parts: 1. Nerve cell body/Soma I. Nissl granules II. Nucleus III. Neurofibrils 2. Dendrite 3. Axon. 4. Synaptic process
- 9. I. Nissl bodies: Nissl bodies or Nissl granules are small basophilic granules found in cytoplasm of neuron. Composed of many thin, parallelly arranged membrane bound cavities which are covered by many minute particles consisting of Ribose , nucleoproteins. • Functions: Synthesis of proteins II. NUCLEUS Each neuron has one nucleus, which is centrally placed in the nerve cell body. Nucleus has one or two prominent nucleoli. Nucleus does not contain centrosome. So, the nerve cell cannot multiply like other cells.
- 10. II. Neurofibrils Neurofibrils are thread-like structures present in the form of network in the soma and the nerve processes. 2. DENDRITES: 5-7 processes Extends out from cell body They also contain Nissl granules, mitochondria, and neurofibrils FUNCTIONS: Receptine process of neuron (receives impulses).
- 11. 3. AXON: Axon is the longer process of nerve cell. Each neuron has only one axon. FUNCTIONS: Conducts impulse away from cell body. Axon hillock: Thickened area of cell body from where axon originates. Initial segment of axon First proper part of axon. Initiation of action potential in motor neurons.
- 12. Internal Structure of Axon – Axis Cylinder: Axon has a long central core of cytoplasm called axoplasm. Axoplasm is covered by the tubular sheath like membrane called axolemma. Axolemma is the continuation of the cell membrane of nerve cell body. Axoplasm along with axolemma is called the axis cylinder of the nerve fiber Axis cylinder of the nerve fiber is covered by a membrane called neurilemma. Conduction in axon can be slow or fast: If myelin present- conduction fast If myelin absent – conduction slow
- 15. Nodes of Ranvier: Area of axon devoid of myelin Maximum number of Na+ channels are present here Initiation of action potential occurs at 1st node of Ranvier in sensory neurons. • MYELIN SHEATH: Myelin sheath is a thick lipoprotein sheath that insulates the myelinated nerve fiber. Myelin sheath is not a continuous sheath. It is absent at regular intervals. Chemistry of Myelin Sheath: Myelin sheath is formed by concentric layers of proteins, alternating with lipids. Lipid(80%)- Sphingomyelin Protein (20%)- Myelin basic protein (MEP)
- 16. FUNCTIONS: 1. Faster conduction • Myelin sheath is responsible for faster conduction of impulse through the nerve fibers. 2. Insulating capacity Myelin sheath has a high insulating capacity. Because of this quality, myelin sheath restricts the nerve impulse within single nerve fiber and prevents the stimulation of neighboring nerve fibers. • Synaptic Knob: Axon divides into terminal branches , each ending into a number of Synaptic knobs. They contain granules or vesicles in which synaptic transmitter secreted by the nerve is stored.
- 18. Neuroglia or glia (glia = glue) Neuroglia or glia (glia = glue) is the supporting cell of the nervous system. Neuroglial cells are non-excitable and do not transmit nerve impulse (action potential). Neuron: Glial cells= 1: 10 • Functions: Neuroglial cells play an important role in the reaction of nerve during infection. CLASSIFICATION OF NEUROGLIAL CELLS: The neuroglial cells are classified into two types: A. Central neuroglial cells B. Peripheral neuroglial cells.
- 19. CENTRAL NEUROGLIAL CELLS Neuroglial cells in CNS are of three types: 1. Astrocytes 2. Microglia 3. Oligodendrocytes • ASTROCYTES 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 -Fibrous astrocytes occupy mainly the white matte of CNS. ii. Protoplasmic astrocytes - Protoplasmic astrocytes are present mainly in gray matter.
- 20. • Functions of Astrocytes Astrocytes: i. Twist around the nerve cells and form the supporting network in brain and spinal cord ii. Form the blood-brain barrier and thereby regulate the entry of substances from blood into brain tissues iii. Maintain the chemical environment of ECF around CNS neurons iv. Provide calcium and potassium and regulate neurotransmitter level in synapses v. Regulate recycling of neurotransmitter during synaptic transmission. MICROGLIA Microglia are the smallest neuroglial cells. These cells are derived from monocytes and enter the tissues of nervous system from blood. These phagocytic cells migrate to the site of infection or injury and are often called the macrophages of CNS. Functions of Microglia: Engulf and destroy the microorganisms and cellular debris by means of phagocytosis . Migrate to the injured or infected area of CNS and act as miniature macrophages.
- 21. OLIGODENDROCYTES Oligodendrocytes are the neuroglial cells, which produce myelin sheath around the nerve fibers in CNS. Oligodentrocytes are also called oligodendroglia. Oligodendrocytes have only few processes, which are short. Functions I. Provide myelination around the nerve fibers in CNS where Schwann cells are absent II. Provide support to the CNS neurons by forming a semi-stiff connective tissue between the neurons
- 22. • PERIPHERAL NEUROGLIAL CELLS Neuroglial cells in PNS are of two types: • 1. Schwann cells • 2. Satellite cells SCHWANN CELLS Schwann cells are the major glial cells in PNS. Functions of Schwann Cells: I. Provide myelination (insulation) around the nerve fibers in PNS II. Play important role in nerve regeneration III. Remove cellular debris during regeneration by their phagocytic activity.
- 23. • SATELLITE CELLS Satellite cells are the glial cells present on the exterior surface of PNS neurons. Functions: I. Provide physical support to the PNS neurons II. Help in regulation of chemical environment of ECF around the PNS neurons
- 25. SYNAPSE Synapse is the junction between two neurons. It is not an anatomical continuation. It is only a physiological continuity between two nerve cells. • CLASSIFICATION OF SYNAPSE Synapse is classified by two methods: A. Anatomical classification B. Functional classification. A. ANATOMICAL CLASSIFICATION: Depending upon ending of axon, synapse is classified into three types: i. Axoaxonic synapse in which axon of one neuron terminates on axon of another neuron. ii. Axodendritic synapse in which the axon of one neuron terminates on dendrite of another neuron. iii. Axosomatic synapse in which axon of one neuron ends on soma (cell body) of another neuron.
- 28. FUNCTIONAL CLASSIFICATION • Functional classification of synapse is on the basis of mode of impulse transmission. According to this, synapse is classified into two categories: 1. Electrical synapse 2. Chemical synapse. However, generally the word synapse refers to a chemical synapse. • ELECTRICAL SYNAPSE: Electrical synapse is the synapse in which the physiological continuity between the presynaptic and the post synaptic neurons is provided by gap junction between the two neurons.
- 29. CHEMICAL SYNAPSE Chemical synapse is the junction between a nerve fiber and a muscle fiber or between two nerve fibers, through which the signals are transmitted by the release of chemical transmitter. In the chemical synapse, there is no continuity between the two neurons because of the presence of a space called synaptic cleft between the two neurons.
- 31. • FUNCTIONALANATOMY OF CHEMICAL SYNAPSE: Neuron from which the axon arises is called the presynaptic neuron and the neuron on which the axon ends is called postsynaptic neuron. Axon of the presynaptic neuron divides into many small branches before forming the synapse. These branches are known as presynaptic axon terminals. TYPES OF AXON TERMINALS: Terminal knobs Some of the terminals are enlarged slightly like knobs called terminal knobs. Terminal knobs are concerned with excitatory function of the synapse. • Terminal coils or free endings: Other terminals are wavy or coiled with free ending with out the knob. These terminals are concerned with inhibitory function.
- 32. Structures of Axon Terminals and Presynaptic Membrane: Presynaptic axon terminal has a definite intact mem brane known as presynaptic membrane. Axon terminal has two important structures: i. Mitochondria, which help in the synthesis of neurotransmitter substance ii. Synaptic vesicles, which store neurotransmitter substance. • Synaptic Cleft and Postsynaptic Membrane: Membrane of the postsynaptic neuron is called post synaptic membrane. It contains some receptor proteins. Small space in between the presynaptic membrane and the postsynaptic membrane is called synaptic cleft. The basal lamina of this cleft contains cholinesterase, which destroys acetylcholine.
- 34. • FUNCTIONS OF SYNAPSE: Main function of the synapse is to transmit the impulses, i.e. action potential from one neuron to another. However, some of the synapses inhibit these impulses. So, The impulses are not transmitted to the postsynaptic neuron. On the basis of functions, synapses are divided into two types: 1. Excitatory synapses, which transmit the impulses (excitatory function) 2. Inhibitory synapses, which inhibit the transmission of impulses (inhibitory function).
- 35. Reflex Activity Reflex activity is the response to a peripheral nervous stimulation that occurs without our consciousness. It is a type of protective mechanism and it protects the body from irreparable damages. For example, when hand is placed on a hot object, it is withdrawn immediately. When a bright light is thrown into the eyes, eyelids are closed and pupil is constricted to prevent the damage of retina. • REFLEX ARC: Reflex arc is the anatomical nervous pathway for a reflex action. A simple reflex arc includes five components: 1. Receptor Receptor is the end organ, which receives the stimulus. When receptor is stimulated, impulses are generated in afferent nerve.
- 37. 2. Afferent Nerve Afferent or sensory nerve transmits sensory impulses from the receptor to center. 3. Center Center receives the sensory impulses via afferent nerve fibers and in turn, it generates appropriate motor impulses. 4. Efferent Nerve Efferent or motor nerve transmits motor impulses from the center to the effector organ. 5. Effector Organ Effector organ is the structure such as muscle or gland where the activity occurs in response to stimulus. Afferent and efferent nerve fibers may be connected directly to the center.