introduction to nervous system
TRANSCRIPT
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Introduction To Nervous System
Dr. Kanwal Abbasi
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Nervous System Function
• The nervous system has three functions.• 1- monitors the internal and external
environments• 2- integrates sensory information • 3- coordinates voluntary and involuntary
responses.
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Division of Nervous system
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Neuron and NeurogliaTwo principal cell types
Neurons
Excitable nerve cells that transmit electrical signals– Supporting cells
Smaller cells surrounding and wrapping neurons“Neuroglia”
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Neurons
• Cells of the nervous system, called nerve cells or neurons, are specialized to carry "messages" through an electrochemical process. The human brain has about 100 billion neurons that carry out the nerve impulses through a process called action potential.
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Neurons are similar to other cells in the body because:
1. Neurons are surrounded by a cell membrane. 2. Neurons have a nucleus that contains genes. 3. Neurons contain cytoplasm, mitochondria and other "organelles". 4. Neurons carry out basic cellular processes such as protein synthesis and energy production. Neurons differ from other cells in the body because: 1. Neurons have specialized extensions called dendrites and axons. Dendrites bring information to the cell body and axons take information away from the cell body. 2. Neurons communicate with each other through an electrochemical process. 3. Neurons contain some specialized structures (for example, synapses) and chemicals (for example, neurotransmitters).
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The Cells of the Nervous System
• All neurons have the following major components:– Dendrites.– Soma/ cell body.– Axon.– Presynaptic terminals.
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The Cells of the Nervous System
• Dendrites- branching fibers with a surface lined with synaptic receptors responsible for bringing in information from other neurons.
• Some dendrites also contain dendritic spines that further branch out and increase the surface area of the dendrite.
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Fig. 2-7, p. 33
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The Cells of the Nervous System
• Soma - contains the nucleus, mitochondria, ribosomes, and other structures found in other cells.– Also responsible for the metabolic work of the
neuron.
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The Cells of the Nervous System
• Axon - thin fiber of a neuron responsible for transmitting nerve impulses away to other neurons, glands, or muscles.
• Some neurons are covered with an insulating material called the myelin sheath with interruptions in the sheath known as nodes of Ranvier.
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The Cells of the Nervous System
• Presynaptic terminals refer to the end points of an axon responsible for releasing chemicals to communicate with other neurons.
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Axons
• Take information away from the cell body
• Smooth Surface • Generally only 1 axon per
cell • No ribosomes • Can have myelin • Branch further from the cell
body
Dendrites
• Bring information to the cell body
• Rough Surface (dendritic spines)
• Usually many dendrites per cell
• Have ribosomes • No myelin insulation • Branch near the cell body
There are several differences between axons and dendrites
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Organization of Nerve
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Nerve
• Each nerve is a cable-like structure that contains many axons that are sometimes referred to as "fibers. " Within a nerve, each axon is surrounded by a layer of connective tissue called the endoneurium. The axons are bundled together into groups called fascicles. Each fascicle is wrapped in a layer of connective tissue called the perineurium. Finally, the entire nerve is wrapped in a layer of connective tissue called the epineurium
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Myelin Sheath
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Myelin is an insulating layer, or sheath, that forms around nerves, including those in the brain and spinal cord. It is made up of protein and Lipids e.g Cholesterol, lecithin and cerebroside (spingomyelin). Functions of Myelin Sheath1)Faster Conduction The purpose of the myelin sheath is to allow electrical impulses to transmit quickly and efficiently along the nerve cells. If myelin is damaged, the impulses slow down.
2)Insulating Capacity Myelin sheath has a high insulating capacity means its restricts the nerve impulses within the signal never fiber and prevents the stimulation of neighboring nerve fibers.
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Neurilemma• Surrounding the myelin sheath, there is a thin membrane called
neurilemmal sheath. This is also called neurilemma or sheath of Schwann.
• This contains Schwann cells, which have flattened and elongated nuclei. The cytoplasm is thin and modified to form the thin sheath of neurilemma enclosing the myelin sheath.
• One nucleus is present in each internode of the axon. The nucleus is situated between myelin sheath . At the node of Ranvier (where myelin sheath is absent), the neurilemma invaginates and runs up to axolemma in the form of a finger like process.
• In nonmyelinated nerve fiber, the neurilemma continuously surrounds axolemma. Neurilemma is absent in central nervous system. Neurilemma is necessary for the formation of myelin sheath (myelinogeneis).
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Neurotrophins
• Promotes neuron growth and Proteins in nature
• Nerve growth factors include:• Nerve growth factor (NGF), brain-derived
neurotrophic factor (BDNF), glial-derived neurotrophic factor (GDNF), neurotrophin-3, and neurotrophin-4/5
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Structural classification of neurons
1. A multipolar neuron has multiple processes extending away from the cell body. These are very common in the CNS.
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2. A unipolar neuron, the dendrites and axon are continuous, and the cell body lies off to one side. In a unipolar neuron, the action potential begins at the base of the dendrites and the rest of the process is considered an axon
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3. Bipolar neurons have two processes, one dendrites and one axon, with the cell body between them. Bipolar neurons are rare but occur in special sense organs such as the eye and the ear.
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Neurons can also be classified by the direction that they send information:
• Sensory (or afferent) neurons: send information from sensory receptors (e.g., in skin, eyes, nose, tongue, ears) TOWARD the central nervous system.
• Motor (or efferent) neurons: send information AWAY from the central nervous system to muscles or glands.
• Interneurons: send information between sensory neurons and motor neurons. Most interneurons are located in the central nervous system.
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INTRODUCTION ABOUT NERVE FIBER
Dr. kanwal Abbasi
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INTRODUCTION ABOUT NERVE FIBER• A nerve fiber is a thread like
extension of a nerve cell and consists of an axon and myelin sheath (if present) in the nervous system.
RAJ NIDHI 28
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BASIS OF CLASSIFICATION
DEPENDING UPON
DISTRIBUTION
DEPENDING UPON ORIGIN
DEPENDING UPON
SECRETION OF
NEUROTRAN-SMETTER
DEPENDING UPON
FUNCTION
ERLANGER AND
GRASSER’S CLASSIFICATI
ON
DEPENDING UPON
STRUCTURE
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• NERVE FIBERS THOSE ARE COVERED BY MYELIN SHEATH
MYELINATED NERVE FIBERS
• THOSE ARE NOT COVERED BY MYELIN SHEATH
UNMYELINATED NERVE FIBERS
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Depending upon STRUCTURE
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MYE
LIN
SH
EATH
In peripheral nervous system it is formed byschwann’s cell. While in case of central nervous system it is formed by oligodendroglia.
COMPOSITION
PROTEINSLIPIDS(CHOLESTEROL,
LECITHIN & SPHINGOMYELIN)+
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• Supply the skeletal muscles of the body.
SOMATIC NERVE FIBERS
• Supply the various internal organs of body.
VISCERAL OR AUTONOMIC NERVE FIBERS
Depending upon DISTRIBUTION
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• Arising from brain.
CARNIAL NERVE FIBERS
• Supply the various internal organs of body.
SPINAL NERVE FIBERS
Depending upon ORIGIN
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• Afferent nerve fibers carry sensory impulses from different part of body to the CNS
SENSORYNERVE FIBERS
• Efferent nerve fibers carry motor impulses from CNS to different part of body
MOTOR NERVE FIBERS
Depending upon Function
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• Secrete noradrenalineADRENERGIC
NERVE FIBERS
• Secrete AcetylcholineCHOLINERGIC
NERVE FIBERS
Depending upon Secretion Of Neurotransmitter
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ERLANGER AND GRASSER’S CLASSIFICATION
• Erlanger and Grasser studied the action potential of mixed nerve trunk by means of cathode ray oscilloscope and they obtained the compounded spike. So they divided nerve fibers into 3 groups. They observed that the main cause of difference in nerve fibers is diameter
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• A GROUP
• B GROUP
• C GROUP
GROUPS OF NERVE FIBERS Depending of Diameter
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PROPERTIES CORELATED WITH DIAMETER
AS Diameter increases• Velocity of conduction increases.• Magnitude of electrical response increases.• Threshold of excitation decreases.• Duration of response decreases.• Refractory period decreases.
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A GROUP• A group is composed of largest fibers.• The fibers of this group are myelinated.• Both sensory and motor in function.• It is found in somatic nerves It is further classified into 4 sub groups.• Aα (afferent and efferent fibers)• Aβ (afferent and efferent fibers) • Aγ (efferent fibers)• Aδ (afferent fibers)
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B GROUP
• The fibers of this group are myelinated.• The B fibers are found solely in preganglionic
autonomic nerve.
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C GROUP
• It is composed of smallest fibers.• All the fibers of this group are unmyelinated.• Mostly found in visceral and cutaneous nerve.• They have high threshold i.e. 30 folds that of A group.• Generally they are found in postganglionic
sympathetic nerve.
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Type Of Nerve Fibers
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Preganglionic and postganglionic neurons
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Properties of Nerve Fibers
1) Excitability:It is the ability of generating electrochemical
impulse (action potentials) at the cell membrane in response to any stimulus.
Stimulus*The stimulus is defined as an external agent
that produce excitabilty in tissues.
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Types of Stimulus
-Chemical transmitters
- Hormones.
- Drugs.
-Ions (Na+, K+, .... etc).
- Gases (O2 and CO2).
-Chemical transmitters
- Hormones.
- Drugs.
-Ions (Na+, K+, .... etc).
- Gases (O2 and CO2).
-Thermal. e.g. cooling or warming.- Mechanical.e.g. stretch, touch, pressure and injury.- Electromagnetic.e.g. light rays
-Thermal. e.g. cooling or warming.- Mechanical.e.g. stretch, touch, pressure and injury.- Electromagnetic.e.g. light rays
Chemical Physical1 2 Electrical3
- Galvanic Current:
Low intensity
Long Duration
- Faradic Current:
High intensity
Short duration
- Galvanic Current:
Low intensity
Long Duration
- Faradic Current:
High intensity
Short duration
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Resting membrane potentialA voltage exists across the plasma membrane– Due to separation of oppositely charged ions
Potential difference in a resting membrane is termed its “resting membrane potential”~ -70 mV in a resting neuronMembrane is “polarized”
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Action Potential Or Nerve Impulus
The synchronized opening and closing of Na+ and K+ gates result in the movement of electrical charges that generates a nerve impulse or action potential.Action potentials reach the end of each neuron where these electrical signals are either transmitted directly to the next cell in the sequence via gap junctions, or are responsible for activating the release of specialized neurotransmitter chemicals
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A nerve impulse is “all-or-none:” it either goes or not, and there’s no halfway. A neuron needs a threshold stimulus, the minimum level of stimulus needed, to generate action potential to go and the impulse to travel. A neuron cannot immediately fire again; it needs time for the sodium and potassium to return to their places and everything to return to normal. This time is called the refractory period.
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Conductivity
• Conductivity:• It is the ability to propagate action potential
from the point of generation to the rest of the membrane.
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The Nerve Impulse
• The myelin sheath of axons are interrupted by short unmyelinated sections called nodes of Ranvier.
• At each node of Ranvier, the action potential is regenerated by a chain of positively charged ion pushed along by the previous segment.
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The Nerve Impulse
• Saltatory conduction is the word used to describe this “jumping” of the action potential from node to node.– Provides rapid conduction of impulses– Conserves energy for the cell
• Multiple sclerosis is disease in which the myelin sheath is destroyed and associated with poor muscle coordination.
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Fig. 2-19, p. 46
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DEGENERATION OF NERVE FIBERS
When a peripheral nerve is injured, the degenerative changes occurs in the nerve cell body and in the nerve fibres.
Degenerative Changes are Classified in to Three Types.Wallarian Degeneration Retrograde Degeneration Transneural Degeneration
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Wallerian Degeneration The degenerative changes in the distal cut end of nerve fiber ( AXON) is known as Wallerian degeneration or orthograde degeneration1) In the part of nerve fiber distal to injury, the degenerative changes occur
within 24 hours of injury. 2) Axis cylinder swells and neurofibrils and axis cylinder breaks up into small
pieces. After few days the broken pieces appear as debris in the space occupied by axis cylinder.
3) The myelin sheath is slowly disintegrated into fat droplets. The changes in myelin sheath occur from 8th to 35th day.
4) The region is invaded by macrophages that remove degenerating axons, myelin and cellular debris. These macrophages probably secrete substances that causes proliferation of Schwann cells and also produce nerve growth factors. All these changes takes place for about 2 months from the day of injury. The schwann cells of distal side increase in size and proliferate to form series of tubes. When one of the regenerating axonal branch succeeds in reaching tube, it enters and grows rapidly inside it
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Retrograde Degeneration
The Retrograde degeneration changes in the nerve cell body and part of axon attached to nerve cell body, axon proximal to the cut are together known as RETROGRADE DEGENERATION .
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Transneuronal Degeneration
The degenerative changes occur in the neuron with which the afferent nerve fiber synapses it is called transneuronal degeneration
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Regeneration Of Nerve Fiber The injured and degenerated nerve fiber can regenerated, but regeneration is possible only If degenerated nerve fiber meets with following criteria1)The between the cut ends of the nerve should not exceed 3mm2) The neurilemma should be present 3) The nucleus must be intact4) The cut ends should remain in same line
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NEUROGLIA
“Nerve glue”Six types of small cells associated with neurons– 4 in CNS– 2 in PNS
Most have central cell body and branching processesSeveral functions– e.g., Supportive scaffolding for neurons– e.g., Electrical isolation of neurons– e.g., Neuron health and growth
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CNS NEUROGLIA
AstrocytesMicrogliaEpendymal cellsOligodendrocytes
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CNS NEUROGLIAAstrocytesMost abundant and versatile glial cellsNumerous processes support branching
neurons– Anchor neurons to capillary blood supply
Guide migration of young neuronsFacilitate nutrient delivery to neurons– (blood glial cell neuron)
Control chemical environment around neurons– Uptake of K+, neurotransmitters
Communicate with astrocytes & neurons– Gap junctions
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CNS NEUROGLIAMicrogliaSmall ovoid cellsRelatively long “thorny”
processes– Processes touch nearby neurons
Migrate toward injured neuronsTransform into macrophage– Phagocytize microorganisms, debris– (Cells of immune system cannot enter the CNS)
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CNS NEUROGLIAEpendymal CellsLine central cavities of brain and spinal cord– Form permeable barrier between cerebrospinal fluid inside
these cavities and tissue fluid of CNS tissueShapes range from squamous to columnarMany are ciliated– Beating helps circulate cerebrospinal fluid cushioning brain
and spinal cord
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CNS NEUROGLIAOligodendrocytesFewer processes than astrocytesWrap processes tightly around thicker neuron
fibers in CNS– “Myelin sheath”– Insulating covering
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PNS NEUROGLIA
Satellite cellsSchwann cells
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PNS NEUROGLIASatellite cellsSurround neuron cell bodies within ganglia– (A ganglion is a collection of nerve cell bodies
outside of the CNS)Function poorly understood
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PNS NEUROGLIASchwann cells “Neurolemmocytes”Surround and form myelin sheaths around larger nerve
fibers of PNS– Functionally similar to oligodendrocytes
Vital to regeneration of peripheral nerve fibers