Nervous and Endocrine Systems: Structure and Function
Nervous System Organization
Diffuse Network
The diffuse network, a very primitive nervous system found in polyps and jellyfish, is characterized by a network of star-shaped nerve cells distributed throughout the body. There is no cephalic region, but they have sensory organs such as statocysts for balance and ocelli for detecting light and shadow.
Cordlike Nervous System (SN Cordes)
Planaria possess a pair of nodes in the anterior region, forming a primitive brain. From this, two nerve cords run centrally through the body, with ramifications.
Ringlike Nervous System (SN Unsubscribe)
Echinoderms have a periesophageal nerve ring that connects to radial nerve cords.
Ganglionic Nervous System (SN Lymph Node)
Arthropods exhibit a trend towards the concentration of nerve cells in the cephalic region. Peripheral ganglia exit from this region, collaborating with cephalic ganglia in coordinating remote regions of the head.
Brain Structure and Function
Telencephalon
The cerebral hemispheres have folds called gyri and grooves called fissures that define the lobes.
Frontal Lobe
The frontal lobe houses the capacities for reasoning, emotions, thought, and the distinction between good and evil.
Parietal Lobe
The parietal lobe has a sensitive area that receives tactile sensations from all body parts and a motor area that governs the movement of all body parts.
Temporal Lobe
The temporal lobe is associated with hearing.
Occipital Lobe
The occipital lobe is associated with smell and the optic nerve.
Diencephalon
The diencephalon is an intermediate region formed by the epiphysis, which controls cycles of wakefulness and sleep, and the thalamus, which receives pain sensations and sends them to the hypothalamus.
Midbrain
The midbrain serves as a nerve passageway.
Hindbrain
The cerebellum, located below the occipital lobe, receives statoacoustic information, reporting the body’s position and restoring balance.
Myelencephalon
The medulla oblongata controls involuntary visceral actions such as cardiac and respiratory rates.
Spinal Cord
The spinal cord is a nerve cord that runs through the holes in the vertebrae from the brain, with a channel called the ependyma. Sensory nerves enter through the posterior horns, and motor nerves exit through the anterior horns.
Somatic System
Nerves are an aggregation of many nerve fibers, which in turn are collections of axons of neurons. There are two types:
- Cranial Nerves: These enter and exit the skull. There are 12 pairs, targeting tissues and organs of the head and neck. They can be sensory or motor.
- Spinal Nerves: These go in and out of the spinal cord. There are 31 pairs, heading to the chest and extremities. They are all mixed.
Autonomic Nervous System
The autonomic nervous system controls the activities of the body’s organs unintentionally. It consists of motor fibers that synapse before reaching the effectors. There are myelinated preganglionic fibers and postganglionic fibers. It consists of two antagonistic parts:
- Sympathetic Nervous System: Prepares the body for a state of emergency. The nerves leave the thoracic and lumbar spinal cord. The ganglia are located near the spinal cord and form a ganglionic chain.
- Parasympathetic Nervous System: Prepares the body for sleep states. They leave the skull, and the ganglia are located near the organs they innervate.
Reflex and Voluntary Actions
Involuntary Reflex Actions
These are performed with the intervention of the spinal cord, without the intervention of the cerebral cortex. They are not voluntary. They may be:
- Anatomical defects: When the basis on which they are based is formed during embryonic development.
- Conditioned: Acquired during the individual’s life.
Voluntary Actions
These involve stimulation of the cerebral cortex by neurons that start from the spinal cord. The response can be modulated.
Receptors
Receptors are modified cells that are excited by stimuli. They have high specificity, meaning that a receptor is sensitive to a particular type of energy and totally insensitive to other types of energy. According to their source, they are interoceptors or exteroceptors. According to their kind of energy, they are:
- Mechanoreceptors: Touch, pressure, and sound.
- Chemoreceptors: Taste and smell.
- Photoreceptors: Brightness (eyes).
- Thermoreceptors: Changes in temperature.
Hormones
Hormones are organic molecules of varied chemical composition that travel through the bloodstream to certain cells, called target cells, causing them to perform certain actions, such as coordinating and controlling different organs. Regulation is based on a mechanism called feedback, where a high concentration of the hormone in the blood inhibits secretion mechanisms.
Steroid Hormones
These are small in size and pass through cell membranes, binding to receptor molecules that modulate the gene involved.
Protein Hormones
These are large, so they do not cross the membrane. They bind to the receptor molecule, which induces the formation of a second messenger called cyclic AMP, which activates cellular enzymes that also modulate the expression of some genes.
Pituitary Gland
The pituitary is an endocrine gland, 1 cm in diameter, governed by the diencephalon’s nervous territory, the hypothalamus. It is located at the base of the sphenoid bone in a hole called the sella turcica. It has two parts:
1. Neurohypophysis: Stores hormones synthesized in the hypothalamus. It secretes fundamental hormones like oxytocin, which causes uterine contractions and milk ejection, and vasopressin, which causes water reabsorption from the nephron.
2. Adenohypophysis: Secretes hormones in response to the presence of hypothalamic releasing factors that induce the adenohypophysis to release hormones that activate certain glands. These include:
- TSH: Activates the thyroid gland with thyroxine.
- FSH: Stimulates the ovaries to produce estrogen and mature sperm.
- LH: Stimulates the production of progesterone and testosterone.
- STH: Causes the elongation of bones and cartilage.
- PRL: Causes milk secretion by the mammary glands.