What is the Nervous and Endocrine System?

Any human body system is composed of a cluster of organs or cells that perform together to execute or maintain homeostasis. For example, the nervous system receives sensory input from the sensory receptors' environment, processes those signals, and directs the muscles and glands to respond to outside stimuli. On the other hand, the endocrine system produces hormones, which influence various functions in the human body like growth, metabolism, reproduction, sensation, etc. Together with the endocrine system, the nervous system maintains homeostasis and is responsible for perception, behavior, and memories, and controls all voluntary movements.

Types of the Nervous System

The nervous system can be divided into several types according to its functions. These are as follows:

Central Nervous System (CNS)

The main component of CNS is the brain and the spinal cord. It controls all other parts of the nervous system.

Peripheral Nervous System (PNS)

PNS consist of nerve fibers that carry information from the entire body to the periphery and retransmit the instruction back to the effector organs like muscles and glands. Components of PNS include cranial nerves and their branches, spinal nerve and their branches, originate from the spinal cord, ganglia, and sensory receptors.

PNS can further be divided into afferent and efferent divisions.

Afferent Division: The afferent division includes afferent nerve fibers that transmit stimuli or information (AP) from tissue and organ to the CNS.

Efferent division: It contains efferent nerve fibers that carry the instructions in action potential (AP) from the CNS to the effector organs like muscle and glands to execute the desired effect.

Somatic Nervous system (SNS)

The SNS consists of nerve fibers that transmit impulses from the CNS to the skeletal muscles and comprise the voluntary branch of peripheral efferent division.

Autonomic Nervous system (ANS)

It consists of the nerve fibers that transmit impulses from the CNS to the smooth muscles, cardiac muscles, and glands of the body and comprise the involuntary branch of the peripheral afferent division. Thus, the autonomic nervous system contains two components, one is sympathetic division, and another is parasympathetic division.

Enteric Nervous System (ENS)

In addition to the CNS and PNS, an extensive network of nerves in the digestive tract wall forms the ENS. It is sometimes considered the third component of the autonomic nervous system, but it can perform independently. The action of hormones also influences it.

Components of The Nervous System

Neurons

Neurons are the structural and functional units of the nervous system. They are unable to divide due to the absence of the centrosome. They have two extensions. A single and larger extension is called an axon, and many smaller extensions are called dendrons. Dendrons are further divided into smaller extensions called dendrites. The cell body is called soma. Axons may contain myelin sheath and terminate at terminal brush-border. The gap between the two neurons is called synaptic cleft, where they transmit action potential via neurotransmitters like acetylcholine, GABA, etc.

Nerves

Nerves are well-organized structures of the nervous system that contains bundles of nerve cells or neurons. They originate from either the spinal cord or from the brain. 

Spinal cord

It is a part of the CNS and extends from the base of the brain. It has 31 pairs of nerves that include eight cervical nerves, 12 thoracic nerves, five lumbar, five sacral, and one coccygeal nerve. It controls several nervous functions, including sympathetic and parasympathetic functions. The nerve signals pass through it are quicker than usual. They contain nerves made of sensory neurons and motor neurons.

Brain

It is the main part of the central nervous system and also controls the peripheral nervous system and others. It has three major parts, the forebrain (prosencephalon), midbrain (mesencephalon), and hindbrain (rhombencephalon). It contains nerve cells and different kinds of glial cells. It controls functions of different parts of the body like voluntary and involuntary functions, heart rate control, controlling blood pressure, social-cognitive behavior, etc.

Components of the Endocrine System

Endocrine Glands

In the endocrine system, there are several ductless glands present in the human body called endocrine glands. They secret hormones directly to the bloodstream for distribution throughout the body. 

Hormones

Hormones are chemical messenger or signal molecules of the endocrine system. It can be defined as the low molecular weight molecules, chemically heterogeneous non-nutrient substances produced in the trance amounts, and acts as intercellular signal molecules. Even though the blood distributes hormones throughout the body, only specific target cells can respond to each hormone because the target cells have receptors for binding with the particular hormone.

Hormone Signaling

In the endocrine system, hormone signaling involves the biosynthesis of a particular hormone, the chemical messenger in a particular tissue, storage, secretion of the hormone, transport of the hormone to their respective cell. Then the cell recognizes the hormone either by cell surface or intracellular receptor produce a signal transduction process and cellular response.

Types of Hormones

They can be classified into two main groups in the endocrine system.

Lipid-Soluble Hormones

They are soluble in lipids, and most of them diffuse through the plasma membrane and binds to the intracellular receptors. These are two types, Steroid hormones like estrogen, progesterone and thyroid hormones like T₃ and T₄.

Water-Soluble Hormones

They are water-soluble and binds to the cell surface receptors to carry out their effect through signal transduction. These are also two types. Amine hormones or modified amino acids. Example: serotonin and melatonin are derived from tryptophan, histamine from histidine, etc., and peptide or protein hormones ADH, oxytocin, glucagon, insulin, etc.

Types of Glands Present in the Endocrine System

Hypothalamus

It is located at the forebrain, near the base of the diencephalon. The hormones originating from hypothalamic neurons pass through the axon and reach the anterior pituitary and control the releasing of hormones from the anterior pituitary.

Hormones Released from the Hypothalamus

• Thyroid-releasing hormone (TRH) regulates the pituitary gland to release TSH.
• Gonadotropin-releasing hormone (GnRH) controls the release of FSH and LH from the anterior pituitary.
• GH-releasing hormone (GHRH) stimulates the release of growth hormone (GH) from the anterior pituitary.
• GH-inhibitory hormone (GHIH) or somatostatin inhibits the secretion of GH from the anterior pituitary.
• Corticotropin-releasing hormone (CRH) is released from the hypothalamus and stimulates the anterior pituitary gland to secrete ACTH.
• Prolactin-inhibitory hormone (PIH) inhibits the release of prolactin from the anterior pituitary.
• Antidiuretic hormone (ADH) and oxytocin are two hormones stored in the posterior pituitary and produced by the hypothalamus.

Pituitary Gland

It is also called the master gland of the endocrine system and located in the sella turcica of the sphenoid bone and attached to the hypothalamus by infundibulum. It has two parts, named anterior and posterior pituitary.

Hormones Released from the Anterior Pituitary

• GH stimulates body growth.
• Adrenocorticotropic hormone (ACTH) acts on the adrenal gland and stimulates it to secrete hormones.
• Follicle-stimulating hormone (FSH) stimulates gamete production in both sexes.
• Luteinizing hormone (LH) stimulates ovulation and synthesis of progesterone in females. In males, it stimulates the testis to synthesize testosterone.
• Thyroid-stimulating hormone (TSH) controls thyroxine secretion.
• Prolactin acts on the mammary glands for the production of milk.
• Melanocyte-releasing hormone acts on melanocytes and promotes pigmentation of the skin.

Hormones of the Posterior Pituitary

• Oxytocin stimulates uterine contraction during labor pain.
• ADH reabsorb water from DTC and reduces water from urine.

Pineal Gland

It is located in the brain and secretes melatonin. It controls the sleep-wake and wake-sleep cycle in the human body and is called the hormone of darkness.

Thyroid Gland

It secretes thyroxine (T₃ and T₄) and has several functions in the body. e.g., growth, regulate BMR, stimulate transport protein synthesis, etc.

Adrenal Gland

The human body has one pair of adrenal glands that are present superior to the kidney. It has two parts: the adrenal cortex, which secretes glucocorticoids, and the adrenal medulla, which releases emergency hormones or quick-acting hormones like epinephrine, norepinephrine which controls senses like fear, controls blood pressure, heart rate, etc.

Gonads

Although they are reproductive organs but produce several hormones, male gonad or testis produce male sex hormone testosterone involved in the development of sperms and secondary sexual characters. In females, the ovary and corpus lutetium produce estrogen and progesterone by stimulating FSH and LH, respectively.

Other Glands

There are several glands present apart of the glands mentioned above, e.g., the parathyroid gland. It produces the parathyroid hormone. The pancreas is another gland that produces insulin, glucagon, and somatotropin hormone.

Context and Applications

This topic is significant in the professional exams for both undergraduate and postgraduate courses, especially for

• Bachelor and Master of Science (Biological sciences)
• Bachelor and Master of Science(Physiology)
• Master of Science in Biochemistry
• Master of Science in Clinical Biochemistry

Related Concepts

1. Endocrine system
2. Cell signaling
3. Action potential and action of the neurotransmitter
4. Sympathetic and parasympathetic actions
5. Hormonal disorders