Motor Response: Skeletal System, Bones, Muscles and Endocrine Regulation in Animals and Plants

Motor Response

The locomotor apparatus is the system that carries out movement. It is composed of two elements: the skeleton (passive elements, bones) and the muscles (active elements).

Types of Skeletons

• Hydrostatic skeletons: fluid maintained under pressure, found in cnidarians and worms.
• Exoskeletons: made of chitin in arthropods, and shells in molluscs.
• Endoskeletons: found in sponges (spicules) and in vertebrates (bones).

Bone Tissue

Bones are formed by organic substances (ossein, which gives elasticity) and mineral substances (phosphate and calcium carbonate, which give resistance).

Classification of Bones

According to their shape, bones are classified as:

• Long bones: made of an epiphysis, a medullary cavity, a shaft, marrow, and a surrounding periosteum (e.g., humerus).
• Short bones: wrist and heel bones.
• Flat bones: ribs.
• Irregular bones: skull and shoulder blades.

Osteogenesis

Bone formation occurs during growth. Cartilage ossifies longitudinally, and bone grows in thickness as ossification takes place beneath the periosteum, where new cells are formed. In turn, the spongy bone is formed in the marrow cavity.

The Skeleton of Vertebrates

The skeleton of vertebrates is divided into the axial skeleton (head, skull, face, and spine) and the appendicular skeleton (shoulder girdle, pelvic girdle, and limbs).

Axial Skeleton

Skull bones: 1 frontal, 2 parietals, 2 temporals (sides), 1 occipital, 1 sphenoid, 1 ethmoid.

Facial bones: 2 nasals, 2 lacrimals, 2 zygomatics (cheekbones), 2 superior maxillae, 2 palatines, 2 inferior turbinates, 1 vomer, 1 lower maxilla (mandible).

Vertebral and Costal Column

Vertebrae are separated by intervertebral discs. The column is divided into regions:

• Cervical region (neck): 7 vertebrae.
• Dorsal region (chest): 12 vertebrae.
• Lumbar region (lower back): 5 vertebrae.
• Sacral region: 5 vertebrae that fuse into a single bone called the sacrum.
• Coccygeal region: 4 or 5 vertebrae that form the small tailbone (coccyx).

Ribs: There are 12 pairs of ribs that articulate with the dorsal vertebrae and form the ribcage.

• True ribs: the first 7 pairs, which join the sternum directly through cartilage.
• False ribs: the next 3 pairs, which join the sternum indirectly through a common cartilage.
• Floating ribs: the last 2 pairs, which are not attached to the sternum.

The sternum is a flat bone in the middle of the chest that joins the ribs.

Appendicular Skeleton

Shoulder girdle: connects the upper limbs to the trunk of the skeleton. It is formed by the clavicle (collarbone) and the scapula (shoulder blade, triangular and flat).

Pelvic girdle: formed by the coxal bones (hip bones), each of which has an ilium, ischium, and pubis.

Limbs:

• Upper limbs: arm: humerus; forearm: radius and ulna; hand: 8 carpal bones (wrist), 5 metacarpal bones (palm), and phalanges (fingers).
• Lower limbs: thigh: femur; knee: patella; leg: tibia and fibula; foot and ankle: 7 tarsal bones, metatarsals (sole), and phalanges (toes).

Joints

• Movable joints: joined by tendons and synovial fluid (which acts as a cushion and prevents friction).
• Semimovable joints: fibrocartilaginous discs with ligaments.
• Immovable joints: bones of the skull.

Muscles

Muscles move the skeletal organs. They are attached to bones by tendons.

• Fusiform muscles: for example, the biceps.
• Flat muscles: for example, the pectorals.
• Orbicular muscles: circular in shape, such as the muscles around the mouth.

Head Muscles

These are the muscles of facial expression. The frontal muscle raises the eyebrows, the nasal muscles move the nose, the risorius muscle draws the cheek back for laughter, the buccinator muscles are used for whistling and blowing, the orbicularis muscles move the lips and eyelids, the masseter and temporal muscles move the jaw, and the sternocleidomastoid muscle moves the neck and head.

Trunk Muscles

Anterior: the pectoralis major and minor, serratus, intercostals (which raise the ribs), rectus abdominis (belly), and obliques.

Posterior: the trapezius, and the diaphragm, which separates the chest from the abdomen.

Limb Muscles

Upper limbs: deltoids, biceps, and forearm flexors and extensors.

Lower limbs: gluteals, sartorius, the gastrocnemius (calf) and Achilles tendon, and leg and foot flexors and extensors.

Endocrine System

Endocrine glands produce chemical substances called hormones, which are of different chemical nature.

Hormonal System in Invertebrates

In insects, the molting hormone (ecdysone) controls growth and development. Juvenile hormone delays molting so that the larva keeps growing, and its absence allows metamorphosis. These two hormones act antagonistically.

Hormonal System in Vertebrates

Hypothalamus (diencephalon): secretes activating and inhibiting factors that act on the pituitary gland.

Pituitary Gland

Anterior lobe: secretes important hormones: adrenocorticotropic hormone (ACTH), thyroid-stimulating hormone (TSH), sex hormones (FSH and LH), prolactin (milk secretion), and growth hormone (GH). Melanocyte-stimulating hormone (MSH) gives skin color.

Posterior lobe: releases vasopressin (regulates blood pressure and has an antidiuretic effect) and oxytocin (stimulates uterine contractions).

Thyroid

Hypothyroidism (too little hormone) and hyperthyroidism (too much hormone) affect metabolism and growth. The thyroid secretes thyroxine, which regulates metabolism and development, and calcitonin, which promotes calcium resorption.

Parathyroids

They regulate calcium levels through parathormone.

Pancreas

Insulin reduces blood sugar, while glucagon increases blood sugar.

Adrenal Glands

• Adrenal cortex: mineralocorticoids regulate mineral salts (aldosterone), glucocorticoids (cortisol), and sex corticoids.
• Adrenal medulla: regulates emotions and stress responses (epinephrine and norepinephrine).

Sex Glands

• Ovaries: produce estrogens and progesterone, which regulates ovulation.
• Testes: produce testosterone.

Pineal Gland

Melatonin influences aging and biological rhythms, and is regulated by light and darkness.

Hormonal Regulation

Hypothalamus → TRH → pituitary → TSH → thyroid → thyroxine. The pituitary and hypothalamus regulate blood sugar levels.

Hormones in Plants

Plant hormones are called phytohormones.

• Auxins: produced in the embryo and apical meristems. They are distributed by diffusion and stimulate growth in fruits and flowers. Light has a modifying effect on them.
• Ethylene: a gas (ethene) that occurs in ripe fruits. It accelerates growth and maturation, and can be applied artificially.

Plant Responses to Stimuli

Tropisms: directional growth movements of a plant in response to an external stimulus. The main types are:

• Phototropism: response to light.
• Geotropism: response to gravity.
• Thigmotropism: response to mechanical stimulus (touch).
• Hydrotropism: response to water.

Nasties: temporary movements that do not depend on the direction of the stimulus. Examples include photonasties (response to light) and seismonasties (produced by a mechanical stimulus).