Human Sensory System, Skeleton, and Muscles

Posted on Nov 23, 2024 in Biology

Sensory Perception

Nerve stimulation is perceived by specialized cells called receptors, distributed throughout the body. There are different types:

  1. Thermoreceptors: Detect temperature. Located under the skin.
  2. Chemoreceptors: Detect chemicals. Found in the nose and mouth (smell and taste).
  3. Photoreceptors: Detect light. Located in the eyes.
  4. Pain Receptors: Located throughout the body, irregularly shaped.
  5. Pressure Receptors: Located in the skin, enabling detection of objects through touch.

Sense organs perceive external stimuli and communicate them to the nervous system for response. These include:

A. Touch

Located in the skin. Detects pressure, temperature, heat, and pain through tactile cells.

B. Smell

Located behind the nostrils in the pituitary region. Detects gaseous substances through olfactory cells.

C. Taste

Located in the mouth, particularly on the tongue and palate. Detects sweet, sour, salty, bitter, and umami (sodium glutamate) tastes through taste buds, symmetrically and irregularly distributed on the tongue.

D. Vision

Located in the eyes. Eye parts:

  • Sclera: Outermost, stiff, white membrane. Transforms into the transparent cornea at the front, allowing light passage.
  • Choroid: Lies beneath the sclera. Thin membrane with blood vessels, forming the iris at the front. The iris is a pigmented membrane with a central hole called the pupil, which controls light entry.
  • Retina: Inner layer containing light-detecting cells (cones and rods). The fovea has the highest concentration of cones and rods. The eye contains vitreous humor (gelatinous fluid) and aqueous humor (clear fluid between lens and cornea). Images are inverted and smaller when projected onto the retina.

E. Hearing

Divided into three parts:

  • External Ear: Includes the ear (pinna) with recesses for sound localization, the auditory canal (with wax-secreting cells), and the eardrum (tympanic membrane).
  • Middle Ear: Starts at the eardrum, which vibrates with sound. Contains ossicles (malleus, incus, stapes) transmitting vibrations to the inner ear, and the Eustachian tube connecting to the pharynx.
  • Inner Ear: Located in the skull cavity, starting with the oval and round windows where the ossicles connect. Includes the cochlea (fluid-filled tube for sound) and the vestibular system (fluid-filled canals for balance). The cochlea communicates with the auditory nerve.

Skeletal System

The skeleton, formed by bones, provides body shape and movement. It’s divided into:

  1. Axial Skeleton: Bones of the head, spine, ribs, and sternum.
  2. Appendicular Skeleton: Bones of the shoulder girdle (arms) and pelvic girdle (legs).

Bone Functions:

  • Protect vital organs
  • Support body weight
  • Produce blood cells
  • Store minerals
  • Enable body movement

Bone Structure:

Bones are made of bone cells:

  • Osteocytes: Mature bone cells, located in groups within bone gaps.
  • Osteoblasts: Located in the periosteum (outer membrane), responsible for bone growth in thickness.
  • Osteoclasts: Located in the endosteum (inner membrane), responsible for bone tissue reabsorption.

Bones also contain mineral salts (Ca, P) for hardness and collagen for elasticity.

Internal Bone Structure:

Two types of bone tissue:

A. Spongy Bone

Found in the heads of long bones and inside flat and short bones. Contains red bone marrow for blood cell production.

B. Compact Bone

Found in the shafts of long bones. Covered by the periosteum (outer) and endosteum (inner). Contains yellow bone marrow in the medullary cavity. The functional unit is the osteon, with:

  • Haversian canals: Vertical ducts for blood vessels and nerves.
  • Bone lacunae: Cavities containing osteocytes, arranged concentrically around Haversian canals, connected by canaliculi.
  • Volkmann’s canals: Connect Haversian canals horizontally.

Joints

Joints are classified by mobility:

  • Mobile Joints: Allow extensive movement (elbow, knee, wrist).
  • Semi-mobile Joints: Allow limited movement.
  • Fixed (Immobile) Joints: Allow no movement (skull bones).

Muscle Tissue

Muscles are attached to bones by tendons. They generally work in antagonistic pairs to enable body movements. Muscle fibers contain actin and myosin proteins.