Sense Organs

Sensory adaptation refers to the phenomenon where the sensitivity of sensory receptors decreases over time when exposed to a constant stimulus. Initially, the response to a stimulus is strong, but with continuous exposure, the receptor's response becomes weaker and may eventually cease.

This process helps organisms focus on more significant changes in their environment, preventing overstimulation from constant, non-threatening stimuli.

Example: When a person is injured (cut or bruised), they initially feel sharp pain. Over time, the pain may lessen as part of the body's sensory adaptation to the injury.

Humans have various sensory receptors located in different parts of the body.

These receptors are responsible for detecting specific types of stimuli such as:

1. Touch: Located in the skin
2. Sight: Located in the eyes
3. Hearing: Located in the ears
4. Smell: Located in the nostrils
5. Taste: Located in the tongue
6. Balance and Proprioception: Located in muscles, tendons, and the inner ear

Human beings have different types of sensory receptors which are located in different parts of the body. They are found in the skin, eyes, ears, blood vessels, muscles, tendon, nostril and tongue.

Ear

There are three types of sensory receptors in human ear

1. That concerned with hearing.
2. Equilibrium of the body (balance)
3. With acceleration ( structure concerned with detection).

The mammalian ear is divided into three organs.

i. The outer ear. ii. The middle ear. iii. The inner ear

The human ear contains specialized receptors that are responsible for:

1. Hearing
2. Balance (Equilibrium)
3. Acceleration Detection

The ear is divided into three parts: the outer ear, middle ear, and inner ear.

Outer ear

1. Pinna: The visible, outer part of the ear made of cartilage. It captures sound waves and directs them into the ear canal.
2. Ear Canal: A tube that conducts sound waves from the pinna to the eardrum. It contains hairs and earwax to trap dust and debris.
3. Eardrum (Tympanic Membrane): A thin membrane that vibrates when sound waves hit it, transferring vibrations to the middle ear.

Middle ear

The middle ear is an air-filled cavity behind the eardrum. It contains three small bones known as the ossicles:

1. Malleus (Hammer)
2. Incus (Anvil)
3. Stapes (Stirrup) These bones amplify the vibrations from the eardrum and transmit them to the inner ear.
4. Eustachian Tube: Connects the middle ear to the throat and equalizes pressure between the middle ear and the external environment. Blockage (e.g., due to mucus from a cold) can impair hearing.
5. Oval Window and Round Window: These are openings leading to the inner ear. The oval window receives vibrations from the stapes, transmitting them to the fluid-filled inner ear.

Inner ear

1. The inner ear contains structures involved in both hearing and balance.
2. Cochlea: A coiled tube that detects sound vibrations through its fluid. It contains hair cells that convert these vibrations into electrical impulses for the brain to process sound.
3. Semicircular Canals: Three fluid-filled canals responsible for detecting changes in the body's position, thus helping maintain balance and posture.
4. Sacculus and Utriculus: These structures work with the semicircular canals to detect balance and acceleration.

The sense of vision is facilitated by receptors located in the eyes, which respond to light and allow us to perceive our surroundings.

The mammalian eye is a highly specialized organ designed to capture light and convert it into neural signals that can be interpreted by the brain. The receptors in the eye are located in the retina, the light-sensitive layer at the back of the eyeball.

From the front view of the eye, there are three primary regions that are clearly visible:

Pupil

The pupil is a small, dark, central portion of the eye.

Function:

1. It controls the amount of light entering the eye by adjusting its size.
2. The pupil appears black because light enters it and is absorbed by the inner parts of the eye.
3. The size of the pupil is controlled by the iris, adjusting based on the lighting conditions.

Iris

Surrounding the pupil is the iris, the colored part of the eye.

Function:

1. The iris is responsible for controlling the size of the pupil.
2. It contains muscles that contract or relax to adjust the pupil's size, allowing more or less light to enter the eye.
3. The iris also determines the eye color, which varies from person to person.

Sclera (Sclerotic Layer)

The sclera is the white, outer part of the eyeball, surrounding the iris.

Function:

1. It provides structural support and protection to the inner components of the eye.
2. The sclera maintains the shape of the eyeball and serves as an attachment point for the muscles that control eye movement.

Cornea

The cornea is a transparent, dome-shaped layer in front of the eyeball that covers the iris and the pupil.

Function:

1. The cornea plays a key role in focusing light onto the retina by bending or refracting the light as it enters the eye.
2. It acts as the eye's first line of defense against dirt, germs, and other particles, as it is an important part of the eye's protective barrier.

Cross-section of the mammalian eye

The human eye is a highly adapted organ, with each part playing a vital role in vision.

Vitreous humour

Location: Found in the large, posterior portion of the eyeball, between the lens and the retina.

Function:

1. The vitreous humour is a jelly-like substance that helps maintain the shape of the eyeball.
2. It also reflects light and is transparent, allowing light to pass through without obstruction.

Aqueous humour

Location: Found in the small anterior portion of the eye, between the cornea and the lens.

Function:

1. The aqueous humour is a watery substance that also helps maintain the shape of the eyeball.
2. It is transparent and allows light to pass through, while also reflecting light.

The process of image formation in the eye is essential for vision, and it involves the refraction of light through the eye's various parts.

Refraction of light

Refraction occurs when light passes from one medium to another, causing its velocity to change. For example, when light enters the eye from the air (a less dense medium) into the cornea and lens (denser media), it bends. This bending is called refraction.

Path of light through the eye

1. Light enters the eye through the cornea.
2. It then passes through the aqueous humour, followed by the pupil.
3. The light then travels through the lens, where further refraction occurs, and passes through the vitreous humour before reaching the retina.

Image formation on the retina

The light is focused on the retina, where it forms an image that is real, upside down, and smaller than the actual object.

Signal transmission to the brain

1. When the light reaches the retina, it stimulates photoreceptors (rods and cones).
2. These photoreceptors send electrical impulses to the brain through the optic nerve.
3. The cerebrum in the brain interprets these impulses, allowing a person to see the object in its correct orientation and size.

Accommodation refers to the ability of the eye to adjust its lens to focus on objects at different distances, both near and far, by altering the focal length of the lens. This process allows clear vision of objects at varying distances.

Mechanism: The process of accommodation is controlled by the ciliary muscles and the elasticity of the lens.

1. When focusing on a distant object, the ciliary muscles relax, and the suspensory ligaments become tighter, pulling on the lens. This causes the lens to become thinner, helping to focus the light from the distant object on the retina.
2. When focusing on a near object, the tension in the suspensory ligaments decreases or relaxes, allowing the lens to become thicker and more convex. This increases the focusing power of the lens to focus light rays from nearby objects onto the retina.

The common eye defects are due to structural issues in the eye that affect its ability to focus properly.

Hypermetropia (Long-sightedness)

Hypermetropia, or long-sightedness, occurs when a person has difficulty focusing on nearby objects, while distant objects are clear. This happens because the eyeball is too short or the lens is too thin.

Cause: In this condition, light rays from nearby objects do not converge on the retina, but instead, they focus behind the retina. Light rays from distant objects focus properly on the retina.

Symptoms:

1. A person cannot see nearby objects clearly.
2. Images of near objects are focused behind the retina.

Correction: Hypermetropia is corrected by using convex lenses. These lenses converge the light rays before they enter the eye, ensuring they focus correctly on the retina.

Myopia (Short-sightedness)

Myopia is a condition where a person can focus on nearby objects clearly but has difficulty seeing distant objects.

Cause: In myopia, the eyeball is usually too long, or the cornea is too curved. This causes light rays from distant objects to converge at a point in front of the retina, rather than directly on it.

Effect:

1. When the light rays from a distant object enter the eye, they don't focus on the retina as they should, leading to blurry vision for far-off objects.
2. A person with myopia can clearly see objects that are close to them, but distant objects appear blurry.

Correction: The condition can be corrected using concave lenses (biconcave lenses), which diverge the light rays before they enter the eye, allowing them to focus correctly on the retina.

This may be due to eyeball being too large. This may be corrected by a biconcave lens. This help to diverge the light rays from distance objects so that they can be focused on the retina.

Colour vision

There are three kinds of cone in a human retina, all three respond to more than one color, but each particular cone is sensitive either to blue, green or yellow.

Types of Cones: The human retina contains three types of cone cells, each sensitive to a specific range of colors:

1. Blue-sensitive cones
2. Green-sensitive cones
3. Yellow-sensitive cones

Response to Colors:

1. Yellow light stimulates both green and yellow-sensitive cones.
2. Red light primarily affects yellow-sensitive cones.

White Light Sensation: When all three types of cones are equally stimulated, the brain interprets the sensation as white light.

Astigmatism

This is a condition in which the cornea or lens is uneven such that is not focused properly on the retina. The defect can be corrected by using spectacles with special cylindrical lenses.

1. Cause: An uneven cornea or lens, causing improper focusing on the retina.
2. Effect: Blurred or distorted vision.
3. Correction: Use of spectacles with special cylindrical lenses to compensate for the irregular shape.

Presbyopia

This condition, the lens cannot change its shape. It is brought about by loss in elasticity of lens and ciliary muscle due to old age can be corrected by the use of convex lenses

1. Cause: Loss of elasticity in the lens and ciliary muscles due to aging, leading to difficulty focusing on close objects.
2. Effect: Reduced ability to see near objects clearly.
3. Correction: Use of convex lenses to aid in focusing.

Cataract

The lens gradually becomes cloudy so that light cannot pass through easily and the person cannot see properly. It may become gradually worse. The lens may have to be removed by operation and can be replaced by a plastic lens inside the eye.

1. Cause: The lens becomes cloudy, blocking light from passing through effectively.
2. Effect: Gradual vision impairment and, in severe cases, blindness.
3. Treatment: Surgical removal of the lens, often replaced by a plastic lens.

Glaucoma

This defect is common in old people, glaucoma is caused by pressure in the eye.

1. Cause: Increased pressure inside the eye.
2. Effect: Damage to the optic nerve, leading to vision loss, commonly in older adults.

Colour blindness

This is the genetic disorder in which a certain colour cannot be distinguished by man. A common type is red green blindness, individual is not in position to determine/distinguish between red and green colour.

1. Cause: A genetic disorder affecting cone cells, limiting the ability to distinguish certain colors.
2. Effect: Inability to differentiate between colors like red and green (most common type).

Trachoma

These are a viral disease which affects the lighting of the eyelids. If not treated, trachoma can cause blindness

1. Cause: A viral infection affecting the inner lining of the eyelids.
2. Effect: Can cause blindness if untreated.

The skin is equipped with specialized receptors that detect different stimuli:

Touch receptors

1. Function: Detect light touch and texture differences (e.g., roughness, smoothness).
2. Distribution: Concentrated in sensitive areas like fingertips; also attached to hair bases.

Pain receptors

1. Function: Detect pain and potential harm.
2. Distribution: Found in the skin, muscles, tendons, ligaments, and digestive system, but not in the brain.

Heat receptors

• Function: Sensitive to temperature increases.

Cold receptors

• Function: Detect drops in temperature.

Proprioceptive and visceral senses

Proprioceptive Receptors:

1. Found in muscles and tendons.
2. Provide information about muscle tension and joint angles, allowing the brain to be aware of body movements.

Visceral Senses:

• Detect conditions within the body, such as internal organ status.

Transverse section of a mammalian skin.

Tongue (Taste Sensory Receptors)

1. The taste receptors are located on the upper surface of the tongue, and their distribution extends to the surface of the pharynx.
2. These receptors are sensitive to solutions of chemical substances that interact with them to produce the sensation of taste.
3. Taste Buds: Found in structures called papillae on the tongue, each taste bud consists of sensory cells that detect specific taste sensations.

Taste sensations are influenced by the four basic tastes:

1. Sweet: Detected at the tip of the tongue.
2. Sour: Detected on the sides of the tongue.
3. Salty: Detected on the front and sides of the tongue.
4. Bitter: Detected at the back of the tongue.

Nostrils (Smell Sensory Receptors)

1. The olfactory receptors are located in the upper parts of the nasal passages.
2. These receptors are sensitive to airborne chemical substances.
3. Smell is closely linked to taste, as the combined activity of taste and smell receptors gives the sensation of flavor.

Function:

1. Helps in identifying and distinguishing various odors.
2. Enhances the perception of taste by detecting aroma.

Significance of sensory receptors in the tongue and nostrils

1. Aid in distinguishing edible substances from harmful ones.
2. Stimulate salivary glands to produce saliva for digestion.
3. Enhance the ability to enjoy food by combining taste and smell sensations.

1. The sensation of flavor is a combination of taste and smell.
2. Taste buds and olfactory receptors (located in the nasal passages) work together to enhance food perception.
3. Many elements of what people consider "taste" are actually due to the sense of smell.

Distinguishing Edible Substances:

1. Helps animals differentiate between safe and harmful substances.
2. Encourages ingestion of nutritious food while avoiding toxins.

Stimulating Digestion:

1. Taste triggers salivary glands to produce saliva, which contains digestive enzymes.
2. Enhances the breakdown of food in the mouth.