Muscle & Nerve II PDF

## Section 10: Nervous System ### Parts of Brain - Prosencephalon (forebrain) - Telencephalon - Cerebral hemispheres - Basal ganglia - Hippocampus - Amydaloid nucleus - Diencephalon - Thalamus - Hypothalamus - Metathalamus - S...

## Section 10: Nervous System ### Parts of Brain - Prosencephalon (forebrain) - Telencephalon - Cerebral hemispheres - Basal ganglia - Hippocampus - Amydaloid nucleus - Diencephalon - Thalamus - Hypothalamus - Metathalamus - Subthalamus - Mesencephalon (midbrain) - Metencephalon - Pons - Cerebellum - Rhombencephalon (hindbrain) - Myelencephalon (medulla oblongata) ### Types of Brain 1. **Prosencephalon** - also known as forebrain. It's divided into: - Telencephalon - Cerebral hemispheres - Basal ganglia - Hippocampus - Amygdaloid nucleus - Diencephalon - Thalamus - Hypothalamus - Metathalamus - Subthalamus 2. **Mesencephalon** - also known as midbrain 3. **Rhombencephalon** - also known as hindbrain. It's subdivided into: - Metencephalon - formed by pons and cerebellum - Myelencephalon - or medulla oblongata ### Organization of Nervous System _The nervous system is made of the central nervous sytem and the peripheral nervous system._ - **Central Nervous System** - consists of the brain and spinal chord - **Peripheral Nervous System** - made up of cranial and spinal nerves **The peripheral nervous system is subdivided into:** 1. **Somatic Nervous System** - Concerned with somatic functions. - Includes nerves supplying skeletal muscles. - Responsible for muscular activities and movements of the body. 2. **Autonomic Nervous System** - Concerned with regulation of visceral or vegetative functions. - Otherwise known as vegetative or involuntary nervous system. - Consists of two divisions: - Sympathetic division - Parasympathetic division ### Neuron - Structural and functional unit of the nervous system. - Similar to other cells, but with differences: - Neuron has branches or processes called axons and dendrites. - Neuron does not have centrosome and cannot undergo division. ### Classification of Neuron **Based on the number of poles:** - **Unipolar Neurons:** Have only one pole. - **Bipolar neurons:** Have two poles. - **Multipolar neurons:** Have many poles with one pole giving rise to an axon and the rest to dendrites. **Based on function:** - **Motor or Efferent neurons:** Carry the motor impulses from the central nervous system to peripheral effector organs. - **Sensory or Afferent neurons:** Carry the sensory impulses from periphery to central nervous system. **Based on the length of axon:** - **Golgi Type I Neurons:** Long axons with cell body in different parts of the central nervous system, reaching remote peripheral organs. - **Golgi Type II Neurons:** Short axons with cell body in the cerebral cortex and spinal chord. ### Structure of a Neuron - **Nerve Cell Body** - also known as soma or perikaryon. It's irregular in shape and is made up of a mass of cytoplasm called neuroplasm covered by a cell membrane. - Contains a large nucleus, Nissl bodies, neurofibrils, mitochondria and Golgi apparatus. - Nissl bodies and neurofibrils are found only in the nerve cell. - **Nucleus** - each neuron has one nucleus, centrally placed in the nerve cell body. - It contains one or two prominent nucleoli. - It does not contain centrosome, meaning the nerve cell cannot multiply like other cells. - **Nissl Bodies** - small basophilic granules found in the cytoplasm of neurons. - Are also called tigroid substances and are responsible for the tigroid or spotted appearance of soma. - Are present in soma and dendrites, not in the axon and axon hillock. - Are membranous organelles containing ribosomes, concerned with the synthesis of proteins in the neurons. - **Neurofibrils** - thread-like structures present in a network in the soma and nerve processes. - They are characteristic of neurons are made up of microtubules and microfilaments. - **Mitochondria** - present in soma and axon. - They are the powerhouse of the nerve cell, where ATP is produced. - **Golgi Apparatus** - similar to that of other cells. - It processes and packages proteins into granules. - **Dendrite** - branched process of neuron. It may be present or absent. - Has Nissl granules and neurofibrils. - Transmits impulses towards the nerve cell body. - Shorter than the axon. - **Axon** - longer process of the nerve cell. - There is only one axon per neuron. - Arises from the axon hillock of the nerve cell body. - Devoid of Nissl granules. - Extends for a long distance from the nerve cell body. - Length of the longest axon is about 1 meter. - Transmits impulses away from the nerve cell body. ### Coverings of Nerve - **Epineurium** - a areolar membrane covering the whole nerve. - **Perineurium** - a areolar membrane covering each fasciculus of the nerve - **Endoneurium** - a areolar membrane covering each nerve fiber. ### Internal Structure of Axon - **Axoplasm** - a long central core of cytoplasm covered by a tubular sheath-like membrane called the - **Axolemma** - continuation of the cell membrane of the neuron. - **Cylinder of the nerve fiber** - is made up of the axoplasm and axolemma. It contains mitochondria, neurofibrils and axoplasmic vesicles. - **Axonal Flow** - proteins necessary for the nerve fibers are synthesized in the soma and transported to the axon by axonal flow. - **Myelin sheath** - covers the axon; it’s responsible for faster conduction. It is not a continuous sheath and is absent at regular intervals called **Nodes of Ranvier**. - The segment between two nodes is called an **Internode**. ### Formation of the Myelin Sheath - **Myelinogenesis** - formation of the myelin sheath by Schwann cells. - **Myelin sheath of the peripheral nerve** - begins at the 4th month of intrauterine life and is completed in the second year after birth. - **Schwann cells** - are very close to the axolemma and have a double layered membrane, wrapping around the axis cylinder in many concentric layers, forming the myelin sheath. - **Neurilemma** - is the outermost membrane of the Schwann cell. The nucleus of the Schwann cells remains between the myelin sheath and the neurilemma. ### Function of Myelin Sheath - **Faster Conduction** - myelin sheath is responsible for faster conduction of nerve impulses, allowing the impulses to jump from one node to another - **saltatory conduction**. - **Insulating Capacity** - myelin sheath restricts nerve impulses to a single nerve fiber and prevents the stimulation of neighboring nerve cell fibers. ### Neurilemma - A thin membrane surrounding the axis cylinder. - Also called the neurilemmal sheath or sheath of Schwann. - Contains Schwann cells with flattened and elongated nuclei. - Cytoplasm is thin and modified to form a thin sheath of neurilemma. - One nucleus is present in each internode. - It sits between the myelin sheath and neurilemma. - Covers the axon. - At the Node of Ranvier, the neurilemma invaginates and runs up to the axolemma. - Important for formation of myelin sheath. - It is absent in the central nervous system. ### Neurotrophins - Also known as **neurotrophic factors**. - Protein substances important in the growth and functioning of nervous tissue. ### Nerve Growth Factor (NGF) - A neurotrophin found in many peripheral tissues. - A peptide with 118 amino acids, each molecule having two alpha-subunits, two beta-subunits and two gamma-subunits. - The beta-subunits are responsible for nerve-growth stimulating activity. ### Functions of NGF - Promotes early growth and development of neurons, mainly on sympathetic and sensory neurons. - Promotes growth of cholinergic neurons in cerebral hemispheres. - Used to treat sympathetic neuron diseases. ### Brain-derived neurotrophic growth factor (BDGF) - Found in pig brains and in human brain and sperm. - Promotes the survival of sensory and motor neurons arising from the embryonic neural crest. - Protects sensory and motor neurons and enhances the growth of cholinergic, dopaminergic and optic nerves. - Used to treat motor neuron diseases. ### Ciliary neurotrophic factor (CNTF) - Secreted in peripheral nerves, ocular muscles and cardiac muscle. - Protects neurons of ciliary ganglion and motor neurons. ### Glial cell line-derived neurotrophic factor (GDNF) - Found in neuroglial cells. - Acts as a potent protector of dopaminergic neurons. - Used to treat Parkinson disease. ### Fibroblast growth factor (FGF) - First discovered for its role in promoting fibroblastic growth. - Also known to protect neurons. ### Neurotrophin-3 (NT-3) - Acts on gamma motor neurons, sympathetic neurons - Found in sensory organs; it also regulates the release of neurotransmitter from neuromuscular junction. - Used to treat motor axonal neuropathy and diabetic neuropathy. - There are several additional neurotrophins, such as NT-4 and NT-5 which act on sympathetic, sensory and motor neurons. ## Classification of Nerve Fibers **Classification of Nerve Fibers** - **Based on Structure:** - **Myelinated Nerve Fibers** - are covered by myelin sheath. - **Non-myelinated Nerve Fibers** - are not covered by myelin sheath. - **Based on Distribution:** - **Somatic Nerve Fibers** - supply skeletal muscles. - **Visceral or Autonomic Nerve Fibers** - supply various internal organs of the body. - **Based on Origin:** - **Cranial Nerve Fibers** - arise from the brain. - **Spinal Nerve Fibers** - arise from the spinal cord. - **Based on Function:** - **Sensory Nerve Fibers** - carry sensory impulses from various locations via different parts of the body to the central nervous system. - **Motor Nerve Fibers** - carry motor impulses from the central nervous system to different parts of the body. - **Based on Secretion of Neurotransmitters:** - **Adrenergic Nerve Fibers** - secrete noradrenaline. - **Cholinergic Nerve Fibers** - secrete acetylcholine. - **Based on Diameter & Conduction of Impulse:** - **Type A Nerve Fibers** - the thickest fibers and are subdivided into Type A alpha, Type A beta, Type A gamma and Type A delta. - **Type B Nerve Fibers** - are thinner than Type A fibers. - **Type C Nerve Fibers** - are the thinnest fibers and are also known as Type IV fibers. All fibers, except type C, are myelinated. ## Properties of Nerve Fibers - **Excitability** - the physiochemical change that occurs in a tissue when stimulus is applied - **Action Potential** - or nerve impulse, developing when a nerve fiber is stimulated by a stimulus with adequate strength - threshold or minimal stimulus. - **Electrotonic Potential** - or local potential develops when a subliminal or sub-threshold stimulus is applied. - **Conduction** - the ability of nerve fibers to transmit the impulse from the area of stimulation to the other areas. - **Mechanism of Conduction** - depolarization occurs first at the site of stimulation causing depolarization of the neighboring areas; this depolarization travels throughout the nerve fiber. - **Saltatory Conduction** - the form of conduction of nerve impulses in a myelinated nerve fiber where impulses jump from one node to another. - **Refractory Period** - the period at which the nerve does not give any response to a stimulus; there are two types: - **Absolute Refractory Period** - the nerve does not show any response at all. - **Relative Refractory Period** - the nerve will show a response if the strength of the stimulus is increased. - **Summation** - When one subliminal stimulus is applied, it does not produce any response in the nerve fiber, but two or more subliminal stimuli are combined within a short time interval, then the response is produced. - **Adaptation** - The excitability of a nerve fiber is higher in the beginning, when stimulated, however, it decreases slowly and finally, the nerve fiber does not show any response. - **Cause for Adaptation** - Continuous depolarization inactivates the sodium pump and increases the efflux of potassium ions. - **Infatigability** - Despite continuous stimulation, the nerve fiber does not become fatigued. - **All-or-None Law** - When a nerve is stimulated, it either gives a maximum response or no response at all. ## Degeneration and Regeneration of Nerve Fibers - **Degenerative Changes**- occur when a nerve fiber is injured. - **Causes for Nerve Injury:** - Obstruction of blood flow - Local injection of toxic substances - Crushing of nerve fiber - Transection of nerve fiber - **Degrees of Nerve Fiber Injury:** - **First Degree** - Mild pressure leading to occlusion of blood flow and hypoxia. Only demyelination occurs. It's not a true degeneration. The function returns within a few hours to few weeks. Also known as Seddon neuropraxia. - **Second Degree** - Prolonged pressure causing Wallerian degeneration. The endoneurium is intact. Repair takes about 18 months. Also known as axonotmesis. - **Third Degree** - The endoneurium is interrupted but the epineurium and perineurium are intact. The recovery is slow. Also known as neurotmesis. - **Fourth Degree** - The epineurium is interrupted but the perineurium is intact. - **Fifth Degree**- All coverings of the nerve are interrupted. - **Wallerian Degeneration:** Occurs in the second degree. - **Retrograde Degeneration:** Occurs in the neuron cell body. - **Transneuronal Degeneration:** Occurs in the neuron which makes a synapse with an injured neuron. - **Criteria for Regeneration:** - The cell body must be alive - The sheath tube must be intact - The Schwann cell must be alive - **Stages of Regeneration:** - **Stage 1:** Degeneration of the nerve fiber - **Stage 2:** Formation of a new axon - **Stage 3:** Myelination of the new axon