PHYSIOLOGY OF A NERVOUS AND MUSCLE FIBERS.

A SYNAPSE.

QUESTIONS:

• Structure and physiology of nervous fibers

• Structure and physiology of synapse

• Structure and physiology of muscle fibers

NERVE

• 1. Epineuros

• 2. A fatty cell

• 3. A fascicle

• 4. Perineuros

• 5. Endoneuros

• 6. A cover of a Shvann cells

• 7. An axon

• 8. A nucleus of a Shvann cell

• 9. A myelin nervous fiber

NEURON is made up of a the cell body and cell processes.

I. The cell body:

contains a large central nucleus, no centrosome

indicating that nerve cells are not able for cell division

II. The cell processes:

a. The dendrites: conduct impulses towards the cell body

b. The axon or nerve fiber: conducts nerve impulses away

from the cell body.

TYPES OF NERVE FIBERS: ARE OF 2 TYPES.

a. Unmyelinated nerve fiber: has a neurilemma, which is essential for nerve regeneration.

b. Myelinated nerve fiber: the axon is surrounded with a sheath of protein-lipid complex called the myelin sheath, in addition to the outer neurilemma.

The myelin sheath is interrupted by constrictions called the nodes of Ranvier.

Unmyelinated Myelinated

• The myelin sheath in peripheral nerves consists of Schwann cells wrapped in many layers around the axon fibers. The myelin is a lipid substance (cells membrane consists of 70% lipids), which main role is isolation. Not all fibers in a nerve will be myelinated, but most of the voluntary fibers are. The Schwann cells are portrayed as arranged along the axon like sausages on a string. (A more apt analogy would be like jelly rolls!) Gaps between the Schwann cells are called nodes of Ranvier.

• The myelin sheath does several things:

• The myelin sheath provides for faster conduction.

• The myelin sheath provides for the possibility of repair of peripheral nerve fibers. Schwann cells help to maintain the micro-environments of the axons and their tunnel (the neurilemma tunnel) permits re-connection with an effector or receptor. CNS fibers, not having the same type of myelination accumulate scar tissue after damage, which prevents regeneration.

FORMATION OF AMYELIN COVER

• No-myelin fibers - the Shvann cell surrounds the axial cylinder

• Myelin fibers - the Shvanncell surrounds the axial cylinder as a spiral and its covers merge and form a myelin

STRUCTURE OF A MYELINATEDNERVOUS FIBER

CONDUCTION IN UNMYELINATEDFIBER

• a local circuit of current flows between the depolarized area of the

membrane and the adjacent membrane areas.

• the velocity of conduction in unmyelinated nerve fibers is 0.5-3 ms/sec.

• the local circuits produced in unmyelinated axon (continuous is where

impulse is transferred from a point to a point along a membrane)

CONDUCTION IN

UNMYELINATED FIBRE

Once the impulse is generated at a particular

point on the membrane, that point becomes

depolarized. The cations diffuse from the

electropositive depolarized region to the

electronegative polarized region through the

axoplasm (cytoplasm of the axon).

Simultaneously, the cations in the ECF

(extracellular fluid) diffuse from the

electropositive polarized region to the

electronegative depolarized region through

the membrane. Thus, the neighbouring

region becomes depolarized. In this manner,

the impulses travel all along the non-

myelinated fibres.

On a scene of action of a threshold stimulus

there comes to rest potential

• the myelin sheath is highly insulator.

• at nodes of Ranvier the myelin sheath is absent.

• depolarization occurs at node of Ranvier and conduction occurs by local current

flow between adjacent nodes.

• salutatory conduction means jumping of the A.P. from one node of Ranvier to the

next.

• the local circuits produced in myelinated axon (saltatory is where impulse jump

across distance)

• It increases the velocity of conduction of the nerve impulse (3-120 ms/sec).

• It saves the energy required.

Conduction in myelinated fibre

Conduction in

myelinated fibre

In myelinated fibers the

depolarization can take place

only at the nodes of Ranvier.

The diffusion of cations can

also take place only at the nodal

regions. So, the impulse that is

generated at one node 'jumps' to

the next node. This type of

transmission is called the

saltatory conduction. In this

type of transmission, the

impulses do not have to travel

all along the length of the fibers

and hence they are faster.

Transmission along nervous fibre

Conduction of excitation in myelinated fibers is more

faster and more economical then conduction of

excitation in non-myelinated fibers

LAWS OF CONDUCTION OF EXCITATION ON NERVOUS FIBERS

1. Anatomical and physiological integrity of nervous fiber – conduction of excitation is possible under condition of anatomical and physiological integrity of a fiber as nervous fibers can exist only in connection with a neuron’s body.

2. Two-sided conduction of excitation – theexcitation, having arisen in any site of a fiber, extends in two directions: centrifugal and centripetal

3. Isolated conduction of excitation – the excitation which has arisen in a nervous fiber, can't pass to other nervous fibers which are in structure of one nerve

SYNAPSEis a place of functional contact between two excitable cells, one of which is nervous

CLASSIFICATION OF SYNAPSES

1) According to the location and an accessory to corresponding structures:

• peripheral (neuro-muscular, neuro-secretory, receptor-neuronalic);• central (axo-somatic, axo-dendritic, axo-axonal, somato-dendritic, somato-somatic);

2) According to the final effect - excitatory and inhibitory

3) According to a way of signaling - chemical, electric, admixed

4) According to a neurotransmitter - cholinergic, adrenergic, serotoninergic, glycinergic, etc.

CHEMICAL SYNAPSE ELECTRIC SYNAPSE

• There are mainly two types of synapses. Electrical and Chemical depending upon the nature of transfer of information across the synapse

a)In electrical synapses, the cells are separated by a gap, the synaptic cleft of only 0.2 nm and there are specialised for rapid signal transmission.

b)Chemical synapses, the commonest type of synapse consist of a bulbous expansion of a nerve terminal, called synaptic knob. The cells are separated by a gap of 20 nm.

• In this manner, the message is transmitted as a wave of impulse along the lengths of connecting neurons.

MECHANISM OF CONDUCTION OF EXCITATION THROUGH A CHEMICAL SYNAPSE

1. Impulse arrives at the end bulb.

2. The end bulb membrane becomes more permeable to calcium.

3. Calcium diffuses into the end bulb and activates enzymes that cause the synaptic vesicles to move toward the synaptic cleft.

4. Some vesicles fuse with the membrane and release their neurotransmitter (a good example of exocytosis).

5. The neurotransmitter molecules diffuse across the cleft and fit into receptor sites in the postsynaptic membrane.

6. When these sites are filled, sodium channels open and permit an inward diffusion of sodium ions.

7. This causes the membrane potential to become less negative (or to approach the threshold potential). If enough neurotransmitter is released, and enough sodium channels are opened, then the membrane potential will reach threshold.

8. An action potential occurs and spreads along the membrane of the post-synaptic neuron (the impulse will be transmitted).

9. The neurotransmitter molecules are then either quickly pumped back into the presynaptic nerve terminal via transporters, are destroyed by enzymes near the receptors (e.g. breakdown of acetylcholine by cholinesterase), or diffuse into the surrounding area.

TYPES OF NEUROTRANSMITTERS:

1- Excitatory - neurotransmitters that make membrane potential less negative

(via increased permeability of the membrane to sodium) and, therefore, tend to

'excite' or stimulate the postsynaptic membrane

2 - Inhibitory -

neurotransmitters that make

membrane potential more

negative (via increased

permeability of the membrane to

potassium) and, therefore, tend to

'inhibit' (or make less likely) the

transmission of an impulse. One

example of an inhibitory

neurotransmitter is gamma

aminobutyric acid (GABA),

beta-endorphin, which results in

decreased pain perception by the

CNS, glycin.

PHYSIOLOGY OF THE MUSCLE

muscles constitute 45 to 50% of the body weight and are of

3 types:

1. Skeletal musclesStriated or voluntary muscles

2. Smooth musclesPlain or involuntary muscles

3. Cardiac muscle:

located only in the heart.

myogenic in action and regulated by aut. n.s.

STRUCTURE OF SKELETAL MUSCLE FIBER

Skeletal muscles consist of numerous subunits or bundles called fascicles. Fascicles are also surrounded perimysium and each fascicle is composed of numerous muscle fibers (or muscle cells).

Muscle cells are long, cylindrical structures that are bound by a plasma membrane – the sarcolemma. Muscle cells, ensheated by endomiosin, consist of many myofibrils, which are made up of long protein molecules called myofilaments. There are two types of myofilaments in myofibrils: thick myofilaments and thin myofilaments.

The sarcoplasm is the specialized cytoplasm of a muscle cell that contains with the Golgi apparatus, myofibrils, a modified endoplasmic reticulum known as the sarcoplasmic reticulum (SR), myoglobin and mitochondria

STRUCTURE OF SKELETAL MUSCLE FIBER

The sarcolemma has a unique feature: it

has holes in it. These "holes" lead into

tubes called transverse tubules (T-tubules).

These tubules pass down into the muscle

cell and go around the myofibrils. These

tubules don’t open into the interior of the

muscle cell; they pass completely through

and open somewhere else on the sarcolem-

ma. The function of T- tubules is to con-

duct impulses from the surface of the cell

(sarcolemma) down into the cell and, spe-

cifically, to another structure in the cell

called the sarcoplasmic reticulum

The I-bands (the light areas) are made up of thin filaments and the A-bands (the darker

areas) are made up of thick filaments . Near the center of each I-bands is a thin dark line

called the Z-line. The Z-line is where sarcomeres come together and the thin

myofilaments of sarcomeres overlap slightly. Thus, a sarcomere can be defined as the

area between Z-lines.

THIN AND THICK FILAMENTS STRUCTURE

The actin molecules are spherical and form long chains. Each thin myofilament contains two such chains that coil around each other. Tropomyosinmolecules are lone, thin molecules that wrap around the chain of actin. At the end of each tro-pomyosin is an troponin molecule. The tropomyosin and troponin molecules are connected to each other.

Thick myofilaments are com-posed of a protein called myosin. Each myosin molecule has a tail which forms the core of the thick myofilament plus a head that projects out from the core of the filament.

MUSCLE CONTRACTION

• Contraction of skeletal muscle requires a nervous impulse. So, step 1 in contraction is when the impulse is transferred from a neu-ron to the sarcolemma of a muscle cell.

• The impulse travels along the sarcolemma and down the T-tu-bules. From the T-tubules, the impulse passes to the sarcoplas-mic reticulum.

• As the impulse travels along the Sarcoplasmic Reticulum (SR), the calcium gates in the membrane of the SR open. As a result, calcium diffuses out of the SR and among the myofilaments.

• Calcium fills the binding sites in the troponin molecules. This alters the shape and position of the troponin which in turn causes movement of the attached tropomyosin molecule.

• Movement of tropomyosin permits the myosin head to contact actin.

• During the swivel, the myosin head is firmly attached to actin. So, when the head swivels it pulls the actin forward. (one myosin head can not pull the entire thin myofilament. Many myosin heads are swivelling simultaneously and their collective efforts are enough to pull the entire thin myofilament).

• Contact with actin causes the myosin head to swivel.

• At the end of the swivel, ATP fits into the binding site on the cross-bridge and this breaks the bond between the myosin and actin. The myosin head then swivels back. As it swivels back, the ATP breaks down to ADP and P and the cross-bridge again binds to an actin molecule.

• As a result, the head is once again bound firmly to actin. However, because the head was not attached to actin when it swivelled back, the head will bind to a different actin molecule. Once the head is attached to actin, the cross-bridge again swivels.

Thus, the thick and thin myofilaments are actually sliding past each other. As this occurs, the distance between the Z-lines of the sarcomere decreases. As sarcomeres get shorter, the myofibril, of course, gets shorter. And, obviously, the muscle fibers get shorter.

Skeletal muscle relaxes when the nervous impulse stops. No impulse means that the membrane of the SR is no permeable to calcium – the calcium gates close. So, calcium no longer diffuses out. The calcium pump in the membrane will transport the calcium back into the SR. Calcium ions leave the binding sites on the troponin molecules.

Without calcium, troponin returns to its original shape and position. This means that tropomyosin is back in position, in contact with the myosin head. So, the myosin head is no longer in contact with actin and, therefore, the muscle stops contracting –relaxes.

STAGES OF CONTRACTIONS

TWITCH - the response of a skeletal muscle to a single stimulation (or action potential):

- latent period - no change in length; time during which impulse is traveling along sarcolemma and down T-tubules to sarcoplasmic reticulum, calcium is being released, and so on (muscle cannot contract instantaneously)

- contraction period - tension increases (cross-bridges are swivelling)

- relaxation period - muscle relaxes (tension decreases) and tends to return to its original length

If a muscle fiber is stimulated so rapidlythat it does not relax at all between stimuli,a smooth, sustained contraction calledtetanus occurs (illustrated by the straightline in c above and in the diagram below).

LITERATURE

• Anatomy and physiology. - The McGraw−Hill,

Companies, 2003

• www.mhhe.com/seeley6