Introductory Lecture on Nervous System Physiology by Dr. Roomi 18-11-13

7/27/2019 Introductory Lecture on Nervous System Physiology by Dr. Roomi 18-11-13

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INTRODUCTION OF EXCITATORY TISSUES

Dr. Mudassar Ali Roomi (MBBS, M.Phil.)Assistant Professor Physiology

BLUE PNS

RED CNS

Divisions of nervous system:

CNS & PNS

Central nervous system (CNS):

• Brain (in skull) & Spinal cord (in vertebral canal):

Divided into gray matter & white matter.

• Formed by neurons & supporting cells (neuroglia)

• Gray matter: nerve cell bodies, proximal parts of axons &dendrites.

• White matter: nerve fibers / axons.

Major parts of Brain:

1. Prosencephalon: Forebrain: (cerebralhemispheres, thalamus,

hypothalamus).2. Mesencephalon: Midbrain

3. Rhombencephalon: Hindbrain: (Pons,cerebellum, medulla

oblongata)Brain stem: Midbrain,

Pons, Medulla oblongata.

Peripheral nervous system (PNS)

• Formed by neurons & their processes outside CNS.

• Cranial nerves (from brain)

• Spinal nerves (from spinal cord)

• 2 divisions of PNS:

SOMATIC / VOLUNTARY NERVOUS SYSTEM:Nerves supplying skeletal muscles

Controls movements of body by acting on skeletal

muscles.

AUTONOMIC / VISCERAL / VEGETATIVE/ INVOLUNTARY NERVOUS SYSTEM:Function: Controls viscera

2 sub-divisions:

SYMPATHETIC

PARASYMPATHETIC

Cells of nervous tissue

TWO TYPES OF CELLS ARE

PRESENT IN THE NERVOUS

TISSUE:

• Nerve cells or Neurons:

which are excitable and

conduct electrical

impulses

• Glial (neuroglial) cells or

supporting cells: which

support, nurture, and

protect the neurons.

18 November 2013 6

The Neuron

• Structural and

functional unit of

nervous system.

• Features: – Nucleus & cytoplasmic

organelles.

– No centrosome

cannot divide.

– Branches Axon /

nerve fiber & Dendrites.

18 November 2013 7

Nerve cell body (perikaryon or soma)

• It contains nucleus,cytoplasmic organelles,inclusions and cytoskeletalcomponents.

• Nucleus: it is large sphericaland pale staining (vesicular)and centrally placed in mostof the cases. It contains

abundant euchromatin(chromatin is finelydispersed) and a largenucleolus (owl-eye nucleus).

18 November 2013 8

Nerve cell body (perikaryon or soma)

• (a) Nissl bodies/granules/substance( chromatophilic substance) - basophilic

patches, cytoplasmic structures are

concentrations of RER. ***

• The amount of chromatophilic substance

varies according to the type and

functional state of the neuron and isparticularly abundant in large nerve cells

such as motor neurons

• (b) Neurofilaments - a variety of

intermediate filament.

• (c) Surrounding the nucleus are elements

of the Golgi apparatus, mitochondria,

lysosomes, and microtubules.

• (d) Pigment is sometimes present, e.g.,

melanin in substantia nigra neurons

(midbrain), and brown granules of

lipofuscin pigment (wear and tear

pigment) in old neurons. ***

18 November 2013 9

dendrites

(a) Definition: These are theneuronal processes that receiveinformation and transmit it tothe cell body.

(b) Contain mitochondria,

microtubules, and granular ER.(c) Nissl granules are present in the

proximal part of the dendrites.

(d) Lack Golgi apparatus***

(e) dendrites have spine-like sideprocesses.

function: Dendrites integrate theexcitatory influences and increasethe receptive area of a neuron.

18 November 2013 10

axon•

(a) Contains axoplasm flowingcentrifugally from the pyramidal shape

starting-point of the axon - the axon

hillock . ***

• initial segment is the site b/w the apex of

the axon hillock and the start of the

myelin sheath. Action potential is

generated here.***

• (b) Has mitochondria, neurofilaments,

microtubules, travelling vesicles, and, in

some neurons, secretion droplets, in the

axoplasm.

• (c) Membrane of the axon is called as

axolemma, swelling out into a bag at itsends (synaptic boutons) which holds

vesicles.

• Axonal transport: Kinesin, are

responsible for anterograde transport.

Dynein allows retrograde transport .***

18 November 2013 11

Axon structure

3 LAYERS: (outside inside)

• Epineurium: sheath

covering whole nerve that

contains many fasiculi ofnerve fibers.

• Perineurium: covers each

fasciculus of the nerve

• Endoneurium: coverseach nerve fiber within

the fasiculus.

Internal structure of axon:

• Axoplasm:

(long central core of cytoplasm in the axon)

Receives synthesized proteins & neurotransmitters

from cell body by axonal flow• Axolemma:

(tubular sheath like membrane covering the

axoplasm)

• Axis cylinder of nerve fiber:

(axoplasm + axolemma)

CLASSIFICATION OF NEURONS

Neurotransmitter Released

Cholinergic GABAergic

Cell Processes

MultipolarMotor, pyramidal, purkinje

BipolarSensory, retina, olfactory mucosa,

cochlear, vest. ganglia

Unipolar

rods & cones

Pseudo-unipolarSensory, dorsal roots & cranial

ganglia

Cell Size

Golgi I

Motor paramidal cell,

Golgi IIInterneuron spinal cord

Function

Motor Interneurons Sensory

Sympathatic Parasympathatic

Adrenergic Noradrenergic

18 November 2013 14

Classification of neurons on the basis of structure

• Myelinated covered by myelin sheath

• Unmyelinated not covered by myelin

sheath

Myelinated

Fibers

• myelin is a whitish

lipoprotein complex.

Sphingmyelin is the

main lipid in it• These are axons of large

diameter grow in the

11/18/2013 16

Myelinated Fibers (cont.)

• Membranes of Schwann cellshave a higher proportion oflipids (80%) than do other cellmembranes.

• Between adjacent Schwann

cells the myelin sheath showssmall nodal gaps along theaxon, also called nodes ofRanvier.

• The length of axon covered byone Schwann cell is called the

internodal segment and maybe more than 1 millimeter.

11/18/2013 17

Process of myelination

(Myelinogenesis)

11/18/2013 18

Process of myelination (cont.)

11/18/2013 19

Myelination of nerve fibers

In PNS In CNS

11/18/2013 20

Morphological Classification of neurons-according to the number of cell processes

• Unipolar neurons : rare in humans.Found in the mesencephalic nucleus of

the 5th cranial nerve

• pseudounipolar neurons, which have a

single process that bifurcates close to the

perikaryon, with the longer branch

extending to a peripheral ending and theother toward the CNS. Examples: found

in the dorsal root ganglia of spinal nerves

• Bipolar neurons, with one dendrite and

one axon. These neurons are found in

cochlear and vestibular ganglia, retina and

olfactory epithelium.

• Multipolar neurons, which have one axon

and two or many dendrites. Most human

neurons are of this type. Some examples

are: pyramidal cells of cerebral cortex,

Purkinje cells of cerebellar cortex, and

anterior horn cells of the spinal cord.

18 November 2013 21

18 November 2013 22

Classification of Neurons by

Functional Role

Motor (efferent) neurons

• (CNS Periphery)

• Motor neurons control effector organs and

muscle fibers.Sensory (afferent) neurons

• (periphery CNS)

Sensory neurons receive sensory stimuli from theinternal or external environment and relay themto the CNS.

18 November 2013 23

Classification of Neurons-according to

the length of their axons Golgi type I neurons: possess many dendrites and a very long

axon that leaves the grey matter in which its cell body ispresent.

Examples:

pyramidal cells of the cerebral cortex• anterior horn cells of the spinal cord.

Golgi type II neurons: possess many dendrites and a relativelyshort axon that does not leave the part of grey matter in

which the cell body of the neuron is present.Example:

• Interneurons.

18 November 2013 24

Classification On the basis of distribution:

• Somatic supply skeletal muscles

• Visceral / Autonomic supply internal organs

Classification on the basis of source of origin:

• Cranial from the brain

• Spinal from the spinal cord

COMPOUND ACTION POTENTIAL

basis for physiological classification of nerve fibers.

• Action potential recordingfrom a nerve trunk.

• 3 main peaks: A, B, C.

• A peak is further divided

into alpha, beta, gamma,delta.

• A & B types are myelinated& C type is unmyelinated.

November 18, 2013 27

Electrophysiologic

classification of nerve

fibers

Sensory classification of nerve fibers

Sensory classification Vs electrophysiological

classification

• Type A (myelinated): A-alpha (Group Ia & Ib), A-beta(Group II), A-gamma (Group II), A-delta (Group III).

• Type B (myelinated)

• Type C (unmyelinated): Group IV

Diameter is directly proportional to conductionvelocity.

Range of velocity: 120m/sec (in large myelinated fibers)to 0.5 m/sec (in smallest unmyelinated fibers).

Sensory classification Vs

electrophysiological classification

Ia / A

Ib / A

II / A beta III / A

IV / C

Fibers from

annulospiral

endings of

muscle spindles.

17 micron dia

Fibers from

golgi tendon

organs.

16 µm in dia.

Fibers from

most discrete

cutaneous tactile

receptors &

flower spray

endings of

muscle spindle.

8 µm in dia.

Fibers carrying

temperature,

crude touch &

pricking pain.

3 µm in dia.

Unmyelinated

fibers carrying

pain, itch,

temperature &

crude touch.

0.5-2 µm in dia

Introductory Lecture on Nervous System Physiology by Dr. Roomi 18-11-13

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