Spinal Cord, Invertebrate & Vertebrate Hormones, and Nervous System
Spinal Cord
The spinal cord extends from the base of the brain to the second lumbar vertebra. The bone presents a very narrow cavity (ependyma). Inside the spinal cord, nerve cell bodies are clustered, bearing gray matter on its collaboration. Axons originate from the white matter surrounding the gray matter, forming longitudinal beam currents leading to the brain and sensory-motor currents from the brain to the muscles and glands. The gray matter is shaped like a butterfly, and their ends are called horns, which are divided into before and after. The spinal cord has the following functions:
- Transmitting impulses to higher centers.
- Controlling reflex activities that do not require orders from higher centers.
Invertebrate Hormones
- The first group of invertebrates are neurohormones, such as those of the annelids, which are produced at each node cephalad and control the processes of regeneration and growth.
- Mollusks secrete gonadotropic hormones, which stimulate body growth and maturation of the gonads.
- Insects have neurosecretory and endocrine organs. The hormonal control of insect development is as follows:
- Neurosecretory cells secrete brain hormone, which is discharged to corpora cardiaca and corpora allata, two endocrine glands located on both sides of the protocerebrum.
- After receiving the brain hormone, corpora allata secrete juvenile hormone, whose mission is to preserve the insect at the larval stage during a specified number of molts.
- The corpora cardiaca stored brain hormone and, when activated, the hormone is expelled and stimulates another endocrine gland called the prothoracic gland, which secretes molting hormone or ecdysone, which causes growth. Thus, during successive molts, juvenile hormone is secreted along with molting hormone. When the concentration of juvenile hormone is low, the passage from larva to pupa occurs, and then, when only ecdysone is secreted, the insect becomes an adult or imago.
- In crustaceans, the shifts are controlled by the equilibrium of the molt-inhibiting hormone and molting hormone or ecdysone. Also, the discoloration is due to the action of chromatophorotropin.
Vertebrate Hormones
Most of the hormones are transported by the blood along with the transport proteins of blood plasma. To reach the target organ, the free hormone exerts its action in different ways depending on the type in question. Steroid hormones (lipophilic) penetrate cell membranes and bind to the receptor in the cytoplasm, through which they reach the core. Protein hormones are large molecules that cannot penetrate inside target cells, so they join receptor molecules on the surface of their membranes, triggering the activation of a set of cellular enzymes that alter the operation of the cell: modification of its membrane permeability and protein synthesis, increased enzyme activity that act in certain processes of cell responses, and so on.
Endocrine System
The endocrine system is the set of all the ductless glands of an animal. Major hormone-secreting glands are the pituitary, pancreas, gonads, and thyroid glands, parathyroid, and adrenal glands.
Effectors
Effectors are the active part of the movement, and through their contractions, perform behavioral motor responses. These organs, together with the skeleton, form the musculoskeletal system.
- Striated muscles: Contraction is rapid and is regulated by the central nervous system.
- Smooth muscles: Internal organs are part of the contraction is slow and involuntary.
Peripheral Nervous System
The peripheral nervous system (PNS) originates from the CNS, whose mission is to connect all the receptors and effectors with the nerve centers. It is formed via afferent neurons that carry information from receptors to the CNS, and the efferent pathway leads from the CNS response to effectors.
Somatic System
The somatic system motor neurons innervate somatic skeletal muscles, which are under voluntary control. The cranial nerves enter and leave the brain and innervate the head, upper trunk, and certain internal organs. Spinal nerves exit the spinal cord through intervertebral spaces and join to form the dorsal and ventral roots, so all are mixed.