Coordination in Humans

Nerve Impulses : Transmission of nerve impulses is an active process, that requires energy supplied by mitochondria.

Receptor Organs : They detect stimulus (changes in the environment around the organism). They respond by generating electrical impulse and pass this to CNS or brain via sensory neurones. For example :

·                skin

·                tongue

·                cochlea

·                semi circular canals

·                nose

·                eye

EYE

Iris : Iris controls the amount of light entering the eye. It adjusts the size of the pupil.

Lens : If it becomes hard and less flexible, changing the shape of the lens becomes more difficult. So, accommodation becomes more difficult. It is difficult to form sharp focus on distant / close objects.

If it becomes cloudy, light rays cannot penetrate the lens anymore. The rays are scattered, causing image to become blurred.

Lack of eye pigmentation : Internal reflection of light causes blurred vision. The retina is thus damaged.

Stereoscopic vision :

·                Retina is stimulated twice.

·                Image of an object is formed by both eyes simultaneously.

·                Each eye shows a different image.

·                The brain interprets and combines images from two eyes to form a three dimensional image of an object.

·                The brain needs information from two eyes to judge relative positions of objects (distances), and thereby speed of moving objects.

Accommodation :

·                Light rays are refracted (bent) at the cornea and at the lens.

·                Rays converge at the focal point of retina.

To view a distant object

·                Light rays are almost parallel to one another.

·                They enter the eye via the cornea and are refracted at the air / cornea boundary. This bending always remains constant.

·                Light rays pass unaltered through aqueous humour.

·                Light rays enter the lens via the pupil.

·                Ciliary muscles relax.

·                Pressure from the vitreous humour pushes on the sclera.

·                Suspensory ligaments are pulled taut.

·                This pulls on the lens.

·                The lens is elastic, and flexible (jelly like).

·                So, it becomes less convex, thinner and flatter.

·                The light rays are finally refracted by the lens.

·                The light rays pass unaltered through vitreous humour.

·                They focus on fovea.

·                The focal length of the lens increases to bend the light rays less.

To view a nearby object

·                Light rays are more diverging

·                They enter the eye via the cornea and are refracted at the air / cornea boundary. This bending always remains constant.

·                Light rays pass unaltered through aqueous humour.

·                Light rays enter the lens via the pupil.

·                Ciliary muscles contract.

·                Pressure from the vitreous humour pushes on the sclera.

·                This exerts less pulls on the ligaments.

·                Suspensory ligaments slacken.

·                The lens is elastic, and flexible (jelly like).

·                So, it becomes more convex, fatter / thicker and rounded.

·                The light rays undergo more refraction at the lens.

·                The light rays pass unaltered through vitreous humour.

·                They focus / light rays meet on fovea.

For instance,

·                Threading a needle

·                Reading fine print

·                Applying eye makup

Iris Reflex : Iris controls the amount of light entering the eye. It adjusts the size of the pupil.

If there is too much light, iris closes the pupil, to prevent damage to the retina. In bright light,

·                Circular muscles of iris contract

·                Radial muscles of iris relax

·                Pupil constricts / Iris is enlarged rapidly (if reflex).

·                Less light enters the eye

·                The radial and circular muscles work in antagonistic pairs.

If light intensity is too low, damage to retina is prevented by

·                Radial muscles of iris contract

·                Circular muscles of iris relax

·                Pupil dilates

·                Iris constricts

·                More light enters the eye, thus stimulating the rods and cones

·                The radial and circular muscles work in antagonistic pairs.

Blink Reflex :

·                Blinking occurs when an object approaches the eye and is about to touch the eye.

·                Blinking moves tear fluid over the surface of the eye to wash away particles that have entered the eye into the tear duct.

·                It also prevents the entry of light to the eye when light intensity is too high.

·                Physical damage to eye is prevented.

Note :

·                On entering a bright room from a dark room, the person is unable to see anything as not enough light reaches the retina to stimulate the rod cells.

EAR

Passageway of sound :

·                Sound waves / vibrations in the air are collected by the pinna.

·                They are transferred via the auditory canal to the ear drum.

·                This makes the ear ossicles in the middle ear vibrate.

·                The ear ossicles amplify the vibrations.

·                Vibrations are passed to the oval window.

·                They are transmitted across the air – filled cavity.

·                Vibrations are transmitted via fluid to cochlea.

·                Sensory hair cells in the organs of Corti are stimulated by the movements of the fluid.

·                Electrical impulses are generated.

·                They are passed along the sensory auditory nerve to the brain.

·                The cerebral hemispheres of the brain interpret the impulses.

·                Memory / Coordination with brain*

Overexposure to loud sound (near the runway of an airport) :

·                Loud noise causes large vibrations of the eardrum, causing large vibrations of the middle earbone (malleus).

·                Ligaments to malleus slackens.

·                The person suffers from temporary deafness.

·                Extensive overexposure can damage cochlea and cause deafness.

Blockage of Eustachian tube (when suffering from a common cold) :

·                Eustachian tube is a passage to air to the throat and the middle ear.

·                Blockage causes pressure to be unequal, causing the shape of the eardrum to be distorted.

·                The eardrum does not vibrate against the ossicles anymore.

·                Hence, sound waves are not transmitted to the middle ear.

Note : If the eardrum is burst by pressure from a very loud noise, there will be no membrane to vibrate with sound.

If the cochlea is destroyed by an infection, sensory hairs are not stimulated. Impulses are not generated in sensory neurones.

REFLEX ACTION

Definition : A reflex action is a rapid, involuntary, automatic response to a stimulus, where the conscious part of the brain is not involved. Response is initiated from the spinal cord. It is usually protective in against further damage. For example – A sharp or hot object can cause limb to be pulled away instantly to prevent skin from damage.

Reflex Pathway of Iris Reflex :

·                Bright light is detected by the retina

·                Impulses pass along the sensory optic nerve to the grey matter of the CNS / brain

·                Impulses are passed via motor neurone to iris muscles

·                Circular muscles contract

·                Radial muscles relax

Note : Voluntary actions are

·                under conscious thought

·                slower

·                are controlled by the brain.

SYNAPSE

·                A synapse is a gap between two neurones.

·                The electrical impulse arrives at the axon terminal of the presynaptic membrane of the neurone.

·                The permeability of calcium ions to the presynaptic membrane increases.

·                This causes vesicles to form.

·                The synaptic knob causes the release of neurotransmitters, like acetylcholine.

·                The chemical diffuses the gap and fuses with the second neurone (post synaptic membrane).

·                An electrical impulse is generated in the second neurone, allowing the impulse to pass.

·                Enzyme from post synaptic membrane destroys neurotransmitter / deactivates acetylcholine in the synapse.

Note : Secretion of neurotransmitter is an active process. Hence, this required energy from ATP released by respiration in mitochondria. Energy is needed for restitution of membrane when the impulse has passed.

Axon = Node of Ranvier

Fatty Myelin Sheath = Schwann cell

Sensory neurone = afferent neurone

Relay neurone = association / intermediate / multipolar neurone

Adaptation of neurones can be presence of axon / Dendron / extension of cell body to transmit impulses.

BRAIN :

Cerebral Hemispheres : control

·                intelligence

·                reasoning

·                learning

·                voluntary muscle

·                memory

·                vision

·                hearing

·                personality

Cerebellum :

·                balance

·                involuntary control of skeletal muscles

·                reflexes

Medulla Oblongata :

·                The medulla oblongata is also known as the brain stem.

·                It controls involuntary muscles, the heart beat, breathing and gut.

·                It regulates many homeostatic mechanisms.

If the medulla is damaged or destroyed in an accident, loss of control of these processes lead to death.

Spinal Cord :

A damage to the vertebral column in the neck region can lead to loss of feeling and paralysis below break, even if no damage is done to the leg muscles.

·                Damage to the bones may damage the nerves and the spinal cord.

·                Leg muscles are controlled by nerves that run via the spinal cord.

·                Connection to the brain could be lost.

·                Nerve impulses from the legs do not reach the brain leading to loss of feeling.

·                Feeling and movement are controlled from the brain.

·                There is no way to send nerve impulses to the leg muscles from the brain leading to paralysis.

·                Nerve pathway between body and brain gets severed

Why can broken nerves not be cured?

·                Broken nerve cells are almost impossible to reunite.

·                They do not regenerate or repair themselves.

The spinal cord does not grow new nerve cells, and so, cannot repair itself.