Chemical substances that transport nerve impulses from one neuron to another neuron or muscle or gland are called neurotransmitters. It is stored in vesicles of pre-synaptic neurons. Acetyl choline is the most commonly used neurotransmitter. Hormone-like chemicals released from neurons are called neurohormones. Neurosecretion is the chemical released from the end of a neuron.
1. Central Nervous System Neurotransmitters: Dopamine, glycine, serotonin, endorphin, dynophin, asinosin, neurotensin, somatostatin, prostaglandin, and nitric oxide.
2. Peripheral Nervous System Neurotransmitters: Acetyl Choline, Histamine, Epinephrine, Nor-epinephrine, Adrenaline and Nor-adrenaline.

The connective tissue in which neurons are protected is called neurogia. There are a billion or 100 million neurons in the brain. These cells do not divide, do not form synapses and do not conduct impulses. It is of four types. namely-
1. Astrocyte: It is stellate and has projections. They provide nutrients to neurons. Its abnormal division results in brain tumors.
2. Oligodendrocyte: It is small in size and has fewer projections. It forms the myelin sheath of the nerve cord.
3. Microglia: These are tiny cells. It has curved projections. They originate from the blood and cause phagocytosis.
4. Appendima: It is small cells. It has projections.

The small branches formed from around the cell body are called dendrites. Dendrites contain ribosomes, smooth endoplasmic reticulum, neurofilaments, etc. It transmits the impulse to the cell body.

The unbranched long-tube neurites arising from the cell body are called axons. The site from which the axon originates is called the hilocum. Each axon is covered by neurolemma. Between the axon and the neurolemma is the myelin sheath or medullary sheath. Myelin sheaths are formed from Swan cells. The area of the axon where there is no myelin sheath is called the segment of Ranvier. The branches that emerge from the side of the axon are called collateral branches. The end of the axon divides into tiny branches. These are called telodendria. The bulge at the end of the telodendria is called the synaptic knob or terminal button.
Inside the axon are microtubules, neurofilaments, mitochondria, lysosomes etc. Its length is more than 1 millimeter.

Branches arising from the cell body are called neurites. It is of two types. namely
(i) Axon: The unbranched long-tube neurites arising from the cell body are called axons. The site from which the axon originates is called the hilocum. Each axon is covered by neurolemma. Between the axon and the neurolemma is the myelin sheath or medullary sheath. Myelin sheaths are formed from Swan cells. The area of the axon where there is no myelin sheath is called the segment of Ranvier. The branches that emerge from the side of the axon are called collateral branches. The end of the axon divides into tiny branches. These are called telodendria. The bulge at the end of the telodendria is called the synaptic knob or terminal button.
Inside the axon are microtubules, neurofilaments, mitochondria, lysosomes etc. Its length is more than 1 millimeter.
(ii) Dendrites: The small branches formed from around the cell body are called dendrites. Dendrites contain ribosomes, smooth endoplasmic reticulum, neurofilaments, etc. It transmits the impulse to the cell body.

The cell body is the main part of a neuron. It may appear round, oval, star-shaped, triangular, pointed or triangular in shape. Its diameter is 5-120 micrometers. It consists of plasma membrane, cytoplasm and nucleus. Cytoplasm of neuron is called neuroplasm. Neuroplasm contains mitochondria, Golgibodies, neurofibrils, ribosomes, endoplasmic reticulum, lysosomes, glycogen and numerous Nisal granules. Polyribosomes join to form Nisal granules. Nisal seeds produce protein.

A typical neuron consists of two parts. Cell bodies and neurites.
1. Cell body: The cell body is the main part of a neuron. It may appear round, oval, star-shaped, triangular, pointed or triangular in shape. Its diameter is 5-120 micrometers. It consists of plasma membrane, cytoplasm and nucleus. Cytoplasm of neuron is called neuroplasm. Neuroplasm contains mitochondria, Golgibodies, neurofibrils, ribosomes, endoplasmic reticulum, lysosomes, glycogen and numerous Nisal granules. Polyribosomes join to form Nisal granules. Nisal seeds produce protein.
2. Neurite: Branches arising from the cell body are called neurites. It is of two types. namely
(i) Axon: The unbranched long-tube neurites arising from the cell body are called axons. The site from which the axon originates is called the hilocum. Each axon is covered by neurolemma. Between the axon and the neurolemma is the myelin sheath or medullary sheath. Myelin sheaths are formed from Swan cells. The area of the axon where there is no myelin sheath is called the segment of Ranvier. The branches that emerge from the side of the axon are called collateral branches. The end of the axon divides into tiny branches. These are called telodendria. The bulge at the end of the telodendria is called the synaptic knob or terminal button.
Inside the axon are microtubules, neurofilaments, mitochondria, lysosomes etc. Its length is more than 1 millimeter.
(ii) Dendrites: The small branches formed from around the cell body are called dendrites. Dendrites contain ribosomes, smooth endoplasmic reticulum, neurofilaments, etc. It transmits the impulse to the cell body.

When multiple genes carry the signal of a single amino acid, it is called redundant or redundant. For example, six codes UCU, UCC, UCA, UCG, AGU and AGC carry the signal of serine.

If a genetic code carries the signal of an amino acid, it is called ambiguous or Degeneracy. For example, a code carrying the signal for tryptophin is UGG.

Genetic codes contain genetic information from DNA in a specific sequence. The genetic code always carries signals in the 5→3 pathway. So the polarity of the genetic code is 5→3.