Download MACROSCOPIC NEUROLOGY:- The execution of the command

MACROSCOPIC NEUROLOGY:The execution of the command from the brain to lift a glass of water consists of transmission of
nerve impulse through motor neurons and from motor neurons to the muscle fibres via the
neuromuscular junction.
The brain sends signals to the motor neurons in the form of nerve impulses. These nerve
impulses travel from neuron to neuron in a fashion similar to the domino effect. The impulse is
received by one neuron and sent to another neighboring neuron and hence the signal is
transmitted. The impulse is picked up by the dendrites of a neuron, it then moves through the
body of the neuron to axon where is transmitted to the dendron of the next neuron through a
synapse by means of a cascade of chemical reactions.
The transmission of nerve impulse from the motor nerves to the muscles takes place across the
neuro-muscular junction (NMJ). Here the axon of the motor neurons emerging from the spinal
cord form a synapse with the muscle fibres and transmission across the NMJ takes place with the
help of chemical neurotransmitters.
Thus, the nerve impulse is received by the muscle fibres and the muscle fibres act accordingly to
lift the glass.
MICROSCOPIC NEUROLOGY:Transmission through neurons:
The transmission of nerve impulse through a neuron takes place with the help of Na+/K+ ions.
When the neuron is inactive, i.e. it is not stimulated; there exist Na+ ions on the outer side of the
cell membrane and K+ on the inner side of the cell membrane. The potential difference in this
state is called the resting potential. At this resting stage, the charge on an ion inhibits its
permeability across the membrane. The neuron remains in this resting state until the arrival of a
nerve impulse.
The arrival of an impulse at the dendron of a neuron opens the gated channels and Na+ moves
into the inner side of the membrane leading to depolarization of the neuron. As the stimulus
reaches the threshold level, there is no more resistance to the movement of Na+. More gated
channels open for Na+ and all the Na+ move into the cell leading to complete depolarization and
generation of an action potential. The complete depolarization leads to the transmission of
stimulus to the neuron. As the impulse moves through the axon, it moves through the Nodes of
Ranvier existing between each Schwann cell.
Subsequently, K+ from the inside to the outside of the membranes through the K+ gated ion
channels present on the inside of the membranes which are now open while the Na+ ion channels
get closed. Thus there now exist Na+ on the inside and K+ on the outside of membrane. In this
manner, the membrane is re-polarized and electrical balance of opposite polarity as that of the
initial one is achieved.
Now the K+ ion channels get closed. The K+ concentration outside is slightly greater than Na+
concentration inside making the membrane potential lower than the resting potential and the
membrane is said to be hyper-polarized.
Following the transmission of the impulse along the neuron, resumes its resting state by the help
of Na+/K+ pumps which restore the ions to their initial position. This state is known as the
refractory period and during this period, the neuron does not react to any external stimulus.
The entire process of transmittal of signal across the neuron occurs in only seven milliseconds.
Transmission through synapse:A synapse is the gap between the axon of the one neuron and the dendron of another which has
to tranversed by a nerve impulse in order to be transmitted through the nervous system. Chemical
messengers called neurotransmitters help in this process. Signal is transmitted across the synapse
in the following manner:
The membrane of the axon of the pre-synaptic neuron gets depolarized, which makes the ion
channels gates open, thereby allowing Ca2+ to enter the neuron. This in turn signals the neuron to
release neurotransmitters into the synapse. The neurotransmitter then traverses the synapse and
binds to receptor proteins present on the membrane of the post-synaptic neuron.
The subsequent inhibition or excitation of the post-synaptic membrane depends on nature of the
neurotransmitter and action potential it generates. If Na+ gated ion channels are opened, the
neuron is excited. Depolarization of the membrane occurs and the signal is transmitted through
the neuron. Inhibition occurs when K+ ion channels get opened and hyper-polarization occurs.
The impulse is not transmitted when there is no generation of action potential.
Following the transmission of nerve impulse through the synapse, the used-up neurotransmitter
is recycled. It is released from the receptor protein and moves back to the pre-synaptic membrane
to be released for transmission during the next nerve signal.
Transmission through NMJ:A motor unit refers to the smallest contractile element that can be acted upon by the nervous
system. The muscle fibre forms a synapse with a single motor neuron only. The innervated
fibres of the motor unit contract when action potential traverses through the axon of the neuron.
Acetylcholine acts as the neurotransmitter and nicotine as receptors at the NMJ.
Neurotransmitter is released into the NMJ when nerve impulse reaches it. The acetylcholine then
goes on to bind to the receptor proteins at the motor-end plate of the muscle fibre causing the
receptor ion channels to open and thereby exciting the muscle fibres by the entrance of Na+ into
the fibre. Following the depolarization of the membrane, when the resting potential is overcome,
the action potential generated first travels along the sarcolemma. The sarcolemma is the excitable
membrane surrounding the myofibrils. The myofibrils are cylindrical, contractile structure vital
to the contraction of muscle fibres. The transverse tubule system begins at the sarcolemma and
goes deep into the fibre and helps the action potential to reach myofibrils situated deep within the
fibre.
The sarcosplamic reticulum situated in the muscle fibre contains calcium ions essential for
contraction. When the action potential reaches the sarcoplasmic reticulum, coupling occurs
between a protein sensitive to the potential of the tranverse tubule and calcium channels in the
reticulum causing the release of calcium which then triggers off biochemical reactions of
contractile proteins in myofibrils.
SLIDING FILAMENT THEORY:When the stimulus arrives at the sarcolemma, the Na+ gated ion channels open and Na+ enters
the fibre making the membrane depolarized. Thus action potential is generated which reaches the
triad through the transverse tubule system. The nerve signal then stimulates the sarcoplasm to
expel the calcium ions which reversibly bind with the accessory protein - troponin. In result, the
attachment site gets exposed and cross-bridge is formed. The myosin head at the cross bridge
contains ATPase which hydrolyses ATP to liberate energy. The length of the sarcomere gets
reduced leading to contraction of the muscle fibre. Following excitation, calcium ions are
pumped back into the triad. Myosin head binds with ATP and consequently the cross-bridge is
broken. The actin and myosin filaments are restored to their resting state conformation.
ASSOCIATED MUSCLES, BONES AND JOINTS:The shoulder is a ball and socket type synovial joint. Here, the long head of biceps brachii
muscle acts as flexors. They contract and pull the humerus causing flexion at the joint, thus
helping in reaching out to the glass.
The hinge joint of the elbow allows only planar movement of the radius and ulna along a single
plane. During elbow flexion, eccentric tension occurs in elbow flexors which allow them to
extend and reach towards the glass.The brachilais and biceps brachii serve as primary agonists
during forearm curling . The brachioradialis, extensor radialis longus, and pronator teres aid the
primary agonists.
The intrinsic muscle group is composed of the thumb muscles and muscles of small finger,
interosseus muscles present between the metacarpal bones and lumbrical muscles. These
muscles, arising from the deep flexor and ending on the dorsal extensor hood mechanism
contribute to the grasping ability of the human hand.
The flexors regulate the grasping movement. The underside of the forearm has two long flexors.
The tendons attach the deep flexor to the distal phalanx while the superficial flexor is inserted
into the middle phalanx by the tendons. The thenar group of the thumb muscles is composed of
opponens and abductor muscles alongwith a long and short muscle which moves the thumb to
assist in grasping the glass of water.
There are a number of other muscles in other parts of the body which help in maintaining
balance while lifting the glass.
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The back muscles - latissimus and longissimus dorsi help in lifting.
The trunk is balanced by the rectus abdominus while bending backwards. This also
prevents the ribcage from folding forward.
Additional lift is provided by trapezius and deltoid. The triceps helps balance the biceps
while biceps help in keeping the glass in front of the body.
PATH OF WATER IN HUMAN BODY:Water enters the body through the mouth and is removed in several forms- sweat from skin,
faeces from the large intestine, water vapor from lungs and urine from kidneys. It also absorbed
and reabsorbed in various parts of the body.
Path of water in digestive system:Water is ingested into the body through the mouth which passes onto the food pipe. While water
enters the oesophagus, the trachea gets covered with the epiglottis to prevent the water from
entering the trachea. From the oesophagus, water moves into the stomach where a small quantity
of it gets absorbed. It further moves onto the small intestine where most of the water along with
salt is absorbed into the bloodstream. The leftover water with the undigested food goes into the
large intestine. A muscular valve prevents this slurry of undigested food and water from
returning into the small intestine. Some of the water is absorbed in the colon, the rest removed
along with undigested matter in the form of faeces.
Path of water in circulatory system:Most of water ingested is absorbed into the bloodstream in the small intestine alongwith salts
which provides the body with the essential fluid. The blood containing salt and water and several
other compounds circulates throughout the body and then goes to kidney for filtration. Some
quantity of water is removed from the body in the form of water vapor from the lungs.
Path of water in the kidney and urinary system:The glomerulus filters blood from the renal artery by the process of ultra-filtration and most of
the water along with salts and urea enters the kidney via the Bowman’s capsule. Then the
glomerular filtrate moves through the renal tubule where water is reabsorbed into the blood
stream by osmosis and this re-absorption is regulated by anti-deuretic hormone (ADH),
aldosterone and atrial natriuretic peptide. Re-absorption of water in the renal tubule of the
nephron occurs in the PCT (proximal convoluted tubule), descending limb and ascending limb of
Henle’s loop, DCT(distal convoluted tubule) and collecting duct.
The remaining water in the form of urine moves through the collecting duct and to the urinary
bladder present in the pelvis via the ureters. The bladder stores the urine and it can easily hold
300 mL of urine for up to five hours in absence of any bladder infection or disease.
The urge to urinate is initiated by stretch receptors in the bladder wall which signal the brain that
the bladder is full. Thus, micturition (or urination) i.e. the process of release of urine through the
external orifice , the urethra occurs. The urinary flow is regulated by sphincters and detrosal
muscle. The sphincter relaxes whereas the detrosal muscles contract to create pressure leading to
urinary flow through the urethra.
References:1. http://www.dummies.com/how-to/content/understanding-the-transmission-of-nerveimpulses.html
2. http://resources.edb.gov.hk/biology/english/teaching_source/ppt/support/E_Support_
Move_2.ppt
3. http://www.vivo.colostate.edu/hbooks/pathphys/digestion/smallgut/absorb_water.htm
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4. http://thebrain.mcgill.ca
5. http:// kinesiology.boisestate.edu