Introduction To Nervous System

Dr. Kanwal Abbasi

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.

Division of Nervous system

Neuron and NeurogliaTwo principal cell types

Neurons

Excitable nerve cells that transmit electrical signals– Supporting cells

Smaller cells surrounding and wrapping neurons“Neuroglia”

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.

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).

The Cells of the Nervous System

• All neurons have the following major components:– Dendrites.– Soma/ cell body.– Axon.– Presynaptic terminals.

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.

Fig. 2-7, p. 33

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.

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.

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.

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

Organization of Nerve

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

Myelin Sheath

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.

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).

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.

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.

INTRODUCTION ABOUT NERVE FIBER

Dr. kanwal Abbasi

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

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

• 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

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)+

• 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

• Arising from brain.

CARNIAL NERVE FIBERS

• Supply the various internal organs of body.

SPINAL NERVE FIBERS

Depending upon ORIGIN

• 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

• Secrete noradrenalineADRENERGIC

NERVE FIBERS

• Secrete AcetylcholineCHOLINERGIC

NERVE FIBERS

Depending upon Secretion Of Neurotransmitter

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

• A GROUP

• B GROUP

• C GROUP

GROUPS OF NERVE FIBERS Depending of Diameter

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.

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.

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.

Type Of Nerve Fibers

Preganglionic and postganglionic neurons

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.

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

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”

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

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.

Conductivity

• Conductivity:• It is the ability to propagate action potential

from the point of generation to the rest of the membrane.

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.

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.

Fig. 2-19, p. 46

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

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

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 .

Transneuronal Degeneration

The degenerative changes occur in the neuron with which the afferent nerve fiber synapses it is called transneuronal degeneration

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

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

CNS NEUROGLIA

AstrocytesMicrogliaEpendymal cellsOligodendrocytes

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

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)

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

CNS NEUROGLIAOligodendrocytesFewer processes than astrocytesWrap processes tightly around thicker neuron

fibers in CNS– “Myelin sheath”– Insulating covering

PNS NEUROGLIA

Satellite cellsSchwann cells

PNS NEUROGLIASatellite cellsSurround neuron cell bodies within ganglia– (A ganglion is a collection of nerve cell bodies

outside of the CNS)Function poorly understood

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