Human Sensory System and Locomotion: A Comprehensive Guide
Receiver Sensitivity
Stimulus
Environmental changes detectable by our body are called stimuli. These stimuli are detected by sensitive nerve cells called receptors.
Types of Receptors
There are two types of receptors: internal receptors (interoreceptors) and external receptors (exteroreceptors).
Internal receptors inform us about the state and function of our internal organs. External receptors, located on the body’s surface, detect external stimuli.
Receptor Classification
- Mechanoreceptors: React to pressure changes.
- Thermoreceptors: Detect temperature changes.
- Chemoreceptors: Detect chemicals.
- Photoreceptors: Sensitive to light.
Senses
Senses are formed by exteroreceptors scattered throughout the skin. The sensory receptors forming sense organs are sensitive to different stimuli.
Sense of Touch
Skin
The skin, which coats and protects the body, houses the touch receptors. It consists of two layers: the superficial epidermis and the deeper dermis.
The epidermis is composed of several tissue layers. The cells of the superficial layers are dead and filled with keratin. These cells continuously divide to renew lost cells. This layer also contains melanocytes, which produce the skin pigment melanin.
Epidermal structures include nails, hair, sebaceous glands, and sweat glands.
Touch Receptors
Touch receptors are enclosed or free nerve endings dispersed throughout the skin. They detect various tactile stimuli: pressure, cold, heat, and pain.
- Meissner’s corpuscles: Light touch and pressure.
- Pacinian corpuscles: Deep pressure and vibrations.
- Ruffini corpuscles: Touch, pressure, and heat.
- Krause’s end bulbs: Cold.
- Free nerve endings: Pain, heat, and temperature.
Sense of Smell and Taste
The senses of smell and taste work together to help us recognize and appreciate food flavors.
Smell
The sense of smell is located in the nasal passages, lined by the pituitary membrane. This membrane has two regions:
- The red pituitary contains blood capillaries to warm inhaled air.
- The yellow pituitary contains olfactory receptors.
Taste
Taste receptors are in the taste buds of the tongue and palate papillae. They are sensitive to substances dissolved in saliva. There are four basic tastes: sweet, salty, sour, and bitter. The flavor of food is a combination of smells and tastes.
Sense of Hearing
The ears are the organs of hearing and balance. They perform two functions: perceiving vibrations and transmitting nerve impulses through the auditory nerve to the brain.
Sense of Sight
The eyes are the organs of sight, the most complex sense. The eye comprises several structures attached to the eyeball, housed in a cavity formed by skull and facial bones.
How Vision Works
Light enters through the pupil and lens, forming images on the retina. Retinal receptors capture light reflected by objects, producing nerve impulses carried by the optic nerve to the brain. There are two types of receptors:
- Rods: Perceive low-intensity light but not colors.
- Cones: Sensitive to color but not low-intensity light.
The eye performs two processes for correct vision:
- Accommodation: Allows images of objects at different distances to focus on the retina.
- Light Adjustment: The pupil constricts or dilates depending on light intensity.
Vision Anomalies
- Myopia: Excessive eyeball length causes images to focus in front of the retina, resulting in blurred distance vision.
- Hyperopia: Short eyeball length causes images to focus behind the retina, resulting in blurred near vision.
- Astigmatism: Irregular cornea curvature causes distorted vision.
Eye Care
- Wash eyes daily with water.
- Use good lighting for reading and studying.
- Maintain an appropriate distance from screens.
- Avoid reading in moving vehicles.
Response Preparation
Cerebral Cortex
The cerebral cortex contains areas that receive sensory impulses and motor areas that produce nerve stimuli. These stimuli are carried by motor neurons to effector organs.
Voluntary Actions
Conscious acts, like walking and thinking, are controlled voluntarily. Voluntary acts are also triggered by stimuli, such as the smell of food reminding us we are hungry.
Reflex Actions
Reflex actions are rapid, automatic responses to stimuli without voluntary control, such as coughing or blinking. These actions are controlled by motor neurons in the spinal cord, without cerebral cortex involvement.
Reflex actions occur through a reflex arc, involving:
- A receptor sensitive to stimulation.
- A sensory neuron carrying the impulse to the spinal cord.
- Association neurons connecting sensory and motor neurons.
- A motor neuron transmitting the message to the muscle.
- An effector muscle performing the response.
Locomotor System
The locomotor system, composed of bones and skeletal muscles, carries out motor responses directed by the nervous system.
Skeleton
The skeleton is an internal framework supporting the body, protecting delicate parts, and providing attachment points for muscles. It consists of 208 bones.
Skeletal Muscle
Skeletal muscles act on the skeleton, performing voluntary movements in response to stimulation. The body contains over 650 skeletal muscles.
Joints and Movements
Joints connect bones and allow movement. Movable joints, like the knee, have:
- Ligaments: Strong fibers attaching bones.
- Cartilages: Cover and protect bone surfaces.
- Synovial capsule: Bag filled with synovial fluid.
- Menisci: Distribute load across the joint.
Locomotor System Injuries
Common injuries include muscle contractures, bone fractures, dislocations, sprains, and cartilage tears.
Muscle Action on the Skeleton
Skeletal muscles attach to bones via tendons and contract or relax to produce movement. Muscles work in pairs to perform coordinated movements.
Levers in the Human Body
Bones and joints form lever systems acted upon by muscles. A lever has three elements:
- Power (P): Point where force is applied.
- Support (A): Point supporting the lever.
- Resistance (R): Point where opposing force is located.
There are three types of levers:
- First-class: Allows a wide range of action, mobility, and power.
- Second-class: Produces large displacements but overcomes significant resistance.
- Third-class: Allows long movements but cannot overcome significant resistance.