Stimuli and Response in Organisms

Irritability

• Organisms can detect changes in the environment, including stimuli, and respond to them.
• Stimuli are any changes in the environment that can be detected, and responses are the reactions to those changes.
• Stimuli are detected by receptors in the body.
• Receptors consist of sensory cells.
• Receptors are connected to form a sense organ like eyes or ears.
• In response to stimuli, receptors send nerve impulses to the coordinator (the brain).
• The brain interprets the nerve impulse and produces a signal.
• The brain sends nerve impulses to the effector (muscle or glands).
• The muscle contracts or glands secrete hormones to give a response.
• Adrenal Glands secrete adrenaline, increasing heart rate.
• Different receptors detect different stimuli:
  • Photoreceptors in the eye detect light.
  • Mechanoreceptors in the ear detect sound.
  • Chemoreceptors in the nose detect chemicals in the air.
  • Chemoreceptors in the tongue detect chemicals in food.
  • Mechanoreceptors in the skin detect pressure.
  • Thermoreceptors in the skin detect temperature change.

Human Eye

• Structures around the eye include the eyebrow, eyelid, eyelash, tear gland, and tear duct.
• The eyebrow prevents sweat from running into the eye.
• The eyelash traps dust and prevents it from entering the eye.
• The eyelid closes to protect the eye from dirt and strong light.
• The eyelid spreads tear over the eye surface when we blink.
• The tear gland produces tears that contain sodium chloride and lysozyme, which kills bacteria.
• Tears keep the eye moisture and clean.
• The tear duct drains tear into the nasal cavity.
• The eye structures include the sclera, choroid, retina, cornea, iris, pupil, lens, suspensory ligament, ciliary body, optic nerve, blind spot, and yellow spot.
• The sclera is the outermost, tough and white layer of the eyeball and maintains its shape.
• The sclera is a surface for eye muscle attachment and protects the inner structure.
• The cornea is a transparent layer of tissue at the front part of the eye.
• The cornea has a curved surface and allows light to enter the eye.
• The cornea refracts and focuses light onto the retina and contains no capillaries.
• The cornea obtains nutrients and oxygen from the aqueous humour and obtains Oxygen directly from the air through diffusion.
• The choroid is black in color, rich in capillaries and removes waste and supply nutrients and oxygen to the retina and sclera.
• The Iris contains pigment; the amount of pigment determine the color of the iris.
• The iris is made up of muscle and control the size of pupil so as to regulate the amount of light entering the eye.
• The pupil is the opening at the centre of the iris to allow light to enter the eye, controlled by the size of the iris.
• The lens is transparent, elastic and biconvex in shape.
• It is made up of living cells with no nucleus and no capillaries.
• The lens obtains nutrients from aqueous humour.
• With its anterior curved surface, the lens refracts and focuses light onto the retina.
• Its thickness can be adjusted by ciliary body to focus light from objects at different distances onto the retina.
• The cells that make up most of the lens lose their internal membranous organelles early in life during differentiation, becoming transparent but losing the ability to replicate.
• The only lens cells that retain the capacity to divide are on the lens surface.
• Newer, older cells come to lie deeper within the lens.
• With a protein (crystallin), the human lens is very pale yellow to clear in color.
• With increasing age, the central part of the lens darken and coloration progresses from yellow to dark brown.
• Because of this, light rays refract and focus on to the retina and stimulate photoreceptors.
• With increasing age, the lens tends to lose elasticity (presbyopia) and the ability to assume a spherical shape decreases.
• The suspensory ligament holds the lens in position and are connected to the ciliary body.
• The ciliary body is a ring of muscle and controls the tension of the ligament, changing lens thickness.
• The innermost layer of the eyeball is the retina.
• Two types of photoreceptors/light sensitive cells exist (cone and rod).
• Cone cells contain a pigment sensitive to light of high intensity.
• Cone cells are important for vision in bright light and responsible for color vision.
• Rod cells contain a pigment sensitive to light of low intensity.
• Rod cells work best in dim light and help with black and white vision.
• Nerve fibre in the retina is grouped to from the optic nerve.
• Optic nerve transmits a nerve impulse generated from photoreceptors in the retina to the visual centre of the cerebrum of the brain to generate the sensation.
• The region on the retina where the optic nerve leaves the eyeball is the blind spot.
• The blind spot has no photoreceptors, so an image cannot form and nerve impulse cannot send to the cerebrum.
• The central region on the of retina is the yellow spot.
• The yellow spot has a high density of cone cells and does not contain rod cells.
• The anterior chamber between the cornea and the lens is filled with aqueous humour, a watery fluid, produced by the ciliary body.
• Aqueous Humour maintain the shape of the eyeball and helps to refract light onto the reina.
• Aqueous humour supplies nutrients and oxygen to the cornea and lens.
• It obtains them through diffusion from the capillaries in the choroid.
• The posterior chamber between the lens and the retina is filled with vitreous humour, which is jelly-like secreted by the ciliary body .
• Vitreous humour helps maintain the shape of the eyeball and refract light onto the retina.
• The wall of the eyeball consists of three layers:
  • Fibrous coat: consists of sclera and cornea and protected by a thin transparent layer, conjunctiva (a modified skin without dermis)
  • Vascular coat: consists of choroid, ciliary body and iris
  • Retina: contains nerve cells, nerve fibres and light sensitive cells
• The photoreceptor of the eye are sensitive only to the visible light spectrum.
• Each photoreceptor absorb light most effectively at a specific part of visible spectrum.
• Retina contains two types of photoreceptors:
  • Rod and Cone cells
  • Photoreceptors contain pigments to absorb light.
  • Rod cells contain rhodopsin and Cone cells contain iodopsin
• Rod cells are more numerous than cone cells and cannot detect colour.
• Some rod cells connect with a nerve and only one nerve impulse may be sent to the brain even if two or more adjacent rod cells are stimulated so the brain cannot distinguish between the separate source of light that stimulated the photoreceptors, giving rod cells low visual acuity.
• Cone cells are less numerous than rod cells.
• Cone cells are responsible for colour vision, with 3 types of cone cells, sensitive to red, blue and green light respectively.
• A cone cell connects its own individual nerve, if two adjacent cone cells are stimulated by two light source, two separate impulse will be sent yo the brain so the brain can distinguish the different sources to give high visual acuity.
• More neurones in the visual center are assigned to cone cells than other cells.
• Light passes through the pupil and enters the retina where photoreceptor cells are located.
• Light rays are refracted and focused onto the retina by the cornea, aqueous humour, lens, and the vitreous humour.
• The cornea is responsible for most of the refraction.
• The lens is responsible for the fine focusing of light rays.
• A real and inverted image is formed on the retina which is smaller than the object.
• The photoreceptors are stimulated once the image is formed on the retina.
• The nerve impulse will be transmitted by the neural cells.
• The signal then transmitted by the optic nerve to the visual centre in the cerebrum of the brain, processing an upright image that produce vision of the object.

Seeing In Bright & Dim Light

• In Bright light, the pupil constrict and circular muscle contract and the radial muscle relax, so less light is allowed to enter the eyes.
• In dim light, the pupil dilates and circular muscle relax whereas the radial muscle contract, so more light is allowed to enter the eyes.
• Importance - to prevent to photoreceptors cell in the retina from being damaged by the bright light and also prevent the photoreceptors being over stimulated by bright light so in dim light more light is allowed to enter the retina and stimulate the photoreceptors to generate nerve impulse to form a clear image.

Seeing Objects At Different Distances

• Structures used to see object are different distances are suspensory ligament+ ciliary body+ lens

• To focus on near object:

1. ciliary muscle contract
2. tension of suspensory ligament reduced → becomes slackened
3. Lens becomes thicker / more convex tor efract more light, so light rays from the near object can be focused onto the retina, causing eye strains.

• To focus on distant object:

1. ciliary muscle relax
2. Tension of suspensory ligament increases → becomes thinner
3. Lens becomes thinner / less convex tor efract less light, so light rays from the distant object can be focused onto the retina

Eye Defects

• Shortsightedness - People can see near objects clearly, but cannot see distance objects.
  • Cause:
    • Lens too thick to refract light, or the eyeball being too long (light rays converge too early)
    • Wearing concave lens as correction, which diverge light rays from distant objects before they reach the eye so image can be focused on the retina.
• Longsightedness - People can see distance objects clearly, but cannot see near objects.
  • Cause:
    • Lens being too thin to refract enough light, or the eyeball being too short (light rays converge after passing retina)
    • Wearing convex lens as correction, which converge light rays from near objects before they reach the eye so image can be focused on the retina. LASIK surgery uses a laser to modify the curvature of the cornea to improve the sharpness of the image formed on the retina.

Color Blindness

• Vision Problem is people cannot distinguish some or all the colors.
• The most Common type is red and green color blindness as people cannot distinguish between red and green.
• Cause: Inherited disease where there is a defect of one or more of three types of cells that detect color
• The deficiency or detect the red or green cone cell
• Correct:
  • It cannot be cured or corrected by wearing len

Cataract

• A cataract causes blurred vision.
• It occurs because the lens becomes cloudy, mostly due to aging and UV light exposure which induce chemical reactions.
• Cataracts can be corrected by replacing the lens with an artificial lens through surgery.

Macular Degeneration

• Macular degeneration causes a black or grey spot to appear in the center of the vision/ distorted vision.
• Macular degeneration
  • occurs because of degeneration of cone cell in the macula and when there is blood vessel growth
  • Vision defect cannot be cured
  • Corrected by wearing a lens.

Human Ear

• Structure includes:
  • Pinna
  • auditory canal
  • ear drum
  • oval window
  • ear bones
  • inner middl outer

Functions of Ear

Function:A flap cartilage covered by the skin, it collects the sound and directs it to the outer auditory canal .The outer Canal transmits the signal to the canal to inner chamber. The auditory transmits to the Eardrum the signals or transmits to the Eardrum.An elastic membrane at located at the auditory located at the inner channel it vibrates the signal from the outer part signal. It the converts to it transmits the bones. It amplifies the signal. membrane to the to transmits to the air. Pharnx to air. It equzlises the sound equaly.Bulging to vibate the siganl due to waves and clear transmits the cochlea.It stimulates signals as send the signals clearly to hear or clearly to senosroy sound to the inner or outer ear.. It transmits the siganl due to waves.

• Functions cartilage covered by skin It vibrates the signal from the outer part signal outer part signal. It the coverts to it transmits the bones signals of sounds.

Phototropism in Plants

• Plants exhibit phototropism, which is the directional growth movement of a plant in response to unilateral (one-sided) light.
• Shoots will grow towards the unilateral light, which enables leaves to reach a position where they obtain maximum amount of light for photosynthesis (positive phototropism).
• Roots grow away from the unilateral light (negative phototropism), which helps the plant to anchor into the soil for support
• Auxins are hormones that promote growth in plants.
• Auxins are produced in the apical meristem (shoot tip and root tip) and transferred to the region of elongation to promote cell elongation.
• Light destroys or inhibits the distribution of auxins.
• Light causes auxins to move away from the illuminated side to the shaded side.