nerve physiology

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NERVE PHYSIOLOGY Dr Swapna Apte Asst Prof of Physiology GMC, Surat

Nervous system:

Nervous system

NERVOUS SYSTEM:

NERVOUS SYSTEM Central nervous system- Brain & Spinal cord Peripheral nervous system Neuron- functional unit Neuroglia

The Cells of the Nervous System:

The Cells of the Nervous System The human nervous system is comprised of two kinds of cells: Neurons Glia The human brain contains approximately 100 billion individual neurons. Behavior depends upon the communication between neurons.

STRUCTURE OF NEURON:

STRUCTURE OF NEURON Nerve cell with all it’s processes is neuron

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Fig. 2-7, p. 33

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III. Cell body:- Neurocyton or Soma i) Nucleus- pale, large, spherical, central ii) Neuroplasm- has neurofibrils, nissl granules, mitochondria, golgi apparatus, neurosecretory material

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Parts:- I. Axon - i) generally long ii) arises from axon hillock iii) axis cylinder has axoplasm, neurofibrils & mitochondria iv) axons end in terminal buttons v) carry impulses away from cell body II. Dendrite:- i) multiple & short ii) contain nissl granules iii) carry impulses towards soma

MYELINATION:

MYELINATION Nerve cells in grey matter are naked. As they enter white matter they acquire myelin sheath. As the nerve leaves CNS it acquires neurolemma (sheath of schwann) Myelin sheath- a protein-lipid complex Envelops the axon except at its ending & at nodes of ranvier

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Myelinogenesis - Inside CNS myelin is produced by oligodendroglia & outside CNS by schwann cells Schwann cell wraps around axon up to 100times. This is compacted by apposition of protein zero. Nodes of ranvier are periodic 1 μ m constrictions which are 1mm apart where there is no myelination

neurons:

neurons Apolar Unipolar-mesnchephalic nuc. 5thc.n. Bipolar-retina,vestibular and cochlear ganglion,olfactory epithelium. Pseudounipolar-spinal ganglia,ganglion of all c.n.except 8 th c.n. Multipolar-most common form purkinjie cells,pyramidal cells.

The Cells of the Nervous System:

The Cells of the Nervous System Glia are the other major component of the nervous system and include the following: Astrocytes helps synchronize the activity of the axon by wrapping around the presynaptic terminal and taking up chemicals released by the axon. Microglia - remove waste material and other microorganisms that could prove harmful to the neuron.

The Cells of the Nervous System:

The Cells of the Nervous System (Types of glia continued) Oligdendrocytes & Schwann cells - build the myelin sheath that surrounds the axon of some neurons. Radial glia - guide the migration of neurons and the growth of their axons and dendrites during embryonic development.

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Fig. 2-10, p. 35

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Terms used to describe the neuron include the following: Afferent axon - refers to bringing information into a structure. Efferent axon - refers to carrying information away from a structure. Interneurons or Intrinsic neurons are those whose dendrites and axons are completely contained within a structure.

The Cells of the Nervous System:

The Cells of the Nervous System The blood-brain barrier is a mechanism that surrounds the brain and blocks most chemicals from entering. The immune system destroys damaged or infected cells throughout the body. Because neurons in the brain generally do not regenerate, it is vitally important for the blood brain barrier to block incoming viruses, bacteria or other harmful material from entering.

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Fig. 2-12, p. 37

FUNCTION OF GLIAL CELLS:

FUNCTION OF GLIAL CELLS

Cell body site of energy generation and synthesis :

Cell body site of energy generation and synthesis Axonal transport Vesicles – Fast axonal transport to terminal Retrograde to cell body Electrical depolarizations

METABOLISM AND SYNTHESIS IN NEURONS:

METABOLISM AND SYNTHESIS IN NEURONS

CLASSIFICATION OF NEURONS:

CLASSIFICATION OF NEURONS (a) Golgi bottle type I (b) Golgi bottle type II II. Anatomic classification- a) Unipolar b) Pseudounipolar c) Bipolar d) Multipolar e) Apolar

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III. Physio-anatomic classification-functionally a) afferent (sensory) i) somatic ii) visceral b) efferent(motor) i) somatic ii) visceral IV. Depending on myelination a) myelinated b) unmyelinated

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CHEMICALLY:- cholinergic(ach) adrenergic(noradrenaline) DPENDING ON DIAMETER AND CONDUCTION VELOCITY.

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V. Erlanger-Gasser’s Classification:- Type Function diameter conduction ( μ m) velocity (m/s) A α proprioception, somatic 12-20 70-120 motor A β touch, pressure 5-12 30-70 A γ motor to muscle spindle 3-6 15-30 A δ pain, temperature, 2-5 12-30 touch B preganglionic autonomic <3 3-15 C i) Dorsal root- pain, touch, 0.4-1.2 0.5-2 ii) postganglionic 0.3-1.3 0.7-2.3 sympathetic

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VI. Numerical classification -sensory neurons Number origin fiber type Ia Muscle spindle, A α annulospiral ending Ib Golgi tendon organ A α II Muscle spindle, flower-spray A β ending, touch, pressure III Pain, temperature, touch A δ IV Pain dorsal root ‘C’ fibers

PROPERTIES OF NERVE:

PROPERTIES OF NERVE EXCITABILITY- it’s the ability of a cell to produce action potential in response to a stimulus. action potential- it’s a self-propagating change in potential across a cell membrane.

The Nerve Impulse:

The Nerve Impulse In a motor neuron, the action potential begins at the axon hillock (a swelling where the axon exits the soma). Propagation of the action potential is the term used to describe the transmission of the action potential down the axon. the action potential does not directly travel down the axon.

LOCAL RESPONSE:

LOCAL RESPONSE

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ELECTROTONIC POTENTIAL ACTION POTENTIAL Produced due to application of subthreshold stimulus Produced due application of threshold stimulus It is a local response Propagative type of response It is a graded response All or nothing response It has no latent period It has a latent period It has no refractory period It has a refractory period Not affected by hypoxia, anaesthesia Not produced during hypoxia, anaesthesia

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Stimulus- it’s a change in environment which brings about a change in potential across a membrane in an excitable tissue Types of stimuli- Electrical Chemical Thermal Mechanical Electromagnetic it can also be classified into subliminal, minimal (threshold), sub-maximal and maximal, depending on the strength of stimulus.

STRENGTH-DURATION CURVE:

STRENGTH-DURATION CURVE TIME UTILISATION TIME STRENGTH RHEOBASE 2 X RHEOBASE CHRONAXIE

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RHEOBASE - minimum current required to produce action potential. UTILIZATION TIME- time taken for response when rheobase current is applied. CHRONAXIE - time taken for response when twice rheobase current is applied. It is a measure of excitability of tissues.

Factors affecting excitability:

Factors affecting excitability Temperature Mechanical pressure Blood supply Chemicals- CO 2 & narcotics pH- increased excitability in alkaline and reduced excitability in acidic media. Ions- Na + , Mg ++ ,ca ++ are neuro-excitatory and k+ decreases. 7) Mylenation and diameter.

II. CONDUCTIVITY:

II. CONDUCTIVITY  Action potential is self-propagative  Conduction may orthodromic or antedromic  In axon, conduction is towards terminal buttons physiologically.  In myelinated nerves, conduction is saltatory type.

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Factors affecting conductivity:

Factors affecting conductivity Temperature Mechanical pressure Blood supply Chemicals pH Ions Size of the nerve Myelination

The Nerve Impulse:

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.

III. ALL OR NONE RESPONSE:

III. ALL OR NONE RESPONSE The action potential doesn’t occur in a nerve if the stimulus is sub-threshold. If the stimulus is threshold and above, the action potential produced will be of same amplitude, regardless of intensity of stimulus. *.

IV.REFRACTORY PERIOD:

IV.REFRACTORY PERIOD Absolute refractory period- it is the period during an action potential, during which a second stimulus can’t produce a second response. Relative refractory period- it is the period during an action potential, during which a stimulus of higher intensity can produce a second response

V.ACCOMODATION:

V.ACCOMODATION When a stimulus is applied very slowly, no matter however strong it might be, it fails to produce an action potential. Cause: a slowly applied stimulus causes slower opening of Na + channels with concomitant opening of K + channels. The influx Na + of is balanced by efflux of K + .

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Vi- indefatiguebility

COMPOUND ACTION POTENTIAL:

COMPOUND ACTION POTENTIAL Multi-peaked action potential recorded from a mixed nerve bundle is called a compound action potential.

Neurotrophins:

Neurotrophins Protiens produced by the structures that the neurons innervate and by astrocytes. They are essential for growth and survival of nerves. eg. 1.NGF-nerve growth factor 2.BDNF-brain derived neurotropic factor 3.Neurotropins3,4,5. 4.insulin like growth factors,fibroblast GF

Degeneration and regeneration in nerves:

Degeneration and regeneration in nerves causes- 1.transection- cut 2.Crushing 3.Injection of toxic subs. In nerve 4.Ischaemia 5.hyperpyrexia

Grading of injury:

Grading of injury 1 st degree-ischaemia 2 nd -prolonged pressure 3 rd -endoneural tube gets interrupted 4 th -fasciles becomes disorganised 5 th -complete transcetion

Changes at three levels:

Changes at three levels Nerve cell body In the distal stump(wallerian degeneration) At the site and proximal to the injury

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Nerve cell- 1. chromatolysis 2.fragmmentation of mitochondria, golgi bodies,neurofibrils 3.cell becomes rounded 4.nucleus degree of damage-neurons affected,site of Lesion,nature of injury.

2. Wallerian degeneration:

2. Wallerian degeneration Process starts within 24 hrs. Changes in action potential -2days, 5 th day no conduction. Axis cyclinder-breaks up into short lengths Myelin sheath-oily droplets 8-32 days (cholesterol esters) *if damage-cns-no further change. -pns

PNS:

PNS Nucleus of schwann cell-divide-cords of cell –fill endoneurial tube. Macrophages-remove debris Schwann cell cytoplasm-fills up the tube.

3. At the site and proximal:

3. At the site and proximal Schwann cells –differentiate into thin elongated cells . From cut distal end-proliferating schwann cells grow towards distal stump

Regenrative changes:

Regenrative changes seen outside the CNS where neurolema is present. Neurolemma is essential for regeneration. Changes in cell body. begin-20 days,completed in 80 days. Changes in distal stump axis cylinder grows from proximal stump fibrills grow towards distal endoneural tube(neurotropism) Mylein sheath appears in 15 days,completed by 1yr