THE NERVOUS SYSTEM: AN OVERVIEW
The nervous system is a complex network of nerves and cells known as neurons that transmit signals between different parts of the body.
It is essentially the body's electrical wiring.
It's comprised of the central and peripheral nervous systems, and it's fundamental to everything your body does, from sensing heat and light to controlling your muscles.
CENTRAL NERVOUS SYSTEM (CNS)
The central nervous system, comprising the brain and spinal cord, is the processing center for the nervous system.
It interprets incoming sensory information and issues instructions.
1.Brain:
The brain is the most complex part of the body.
It controls thought, memory, emotion, touch, motor skills, vision, breathing, temperature, hunger, and every process that regulates our body.
2.Spinal Cord:
The spinal cord is a long, thin, tubular bundle of nervous tissue and support cells that extends from the brain.
The brain and spinal cord together make up the central nervous system.
The spinal cord transmits signals between the brain and the rest of the body.
PERIPHERAL NERVOUS SYSTEM (PNS)
The peripheral nervous system consists of the nerves and ganglia outside of the brain and spinal cord.
It is divided into the somatic and autonomic nervous systems.
1.Somatic Nervous System:
This part of the PNS controls voluntary movements.
It consists of nerves that connect the brain and spinal cord with muscles and sensory receptors in the skin.
2.Autonomic Nervous System:
This system controls the involuntary functions of our body.
It has two divisions: sympathetic and parasympathetic.
The sympathetic division prepares the body for stressful or emergency situations—fight or flight—while the parasympathetic division controls body processes during ordinary situations.
NEURONS
Neurons, or nerve cells, are the basic working unit of the nervous system.
A neuron is a specialized cell designed to transmit information to other nerve cells, muscle, or gland cells.
NERVOUS SYSTEM FUNCTIONS
The nervous system has three primary functions:
1.Sensory Function:
The sensory nerves gather information from inside the body and the outside environment.
The nerves then carry the information to the central nervous system.
2.Integrative Function:
This is where the sensory information is processed by the brain, creating sensations, thought processes, and memories.
3.Motor Function:
The motor nerves convey information from the central nervous system to the muscles and glands of the body, allowing them to carry out their functions.
Understanding the nervous system and how it functions is a significant part of neuroscience and helps us understand more about human behavior and human health.
Any damage to the nervous system, through disease or injury, can lead to severe complications or even death.
4.Drugs Acting on the Autonomic Nervous System
The Autonomic Nervous System (ANS) is a component of the peripheral nervous system that controls involuntary physiological functions.
It is divided into two main branches: the Sympathetic Nervous System (SNS) and the Parasympathetic Nervous System (PNS).
Drugs that act on the ANS are used to treat a wide range of conditions, including hypertension, asthma, and heart failure.
Understanding these drugs requires knowledge of the neurotransmitters involved, primarily acetylcholine in the PNS and norepinephrine in the SNS.
Overview of the Autonomic Nervous System
1.Sympathetic Nervous System (SNS):
Often described as the "fight or flight" system, it prepares the body for stressful or energetic activity.
Norepinephrine is the primary neurotransmitter, acting on adrenergic receptors.
2.Parasympathetic Nervous System (PNS):
Known as the "rest and digest" system, it conserves energy and restores the body to a state of calm.
Acetylcholine is the primary neurotransmitter, acting on cholinergic receptors.
Drugs Acting on the Parasympathetic Nervous System
1.Cholinergic Agonists:
These mimic the action of acetylcholine.
They stimulate the PNS, leading to decreased heart rate, increased glandular secretions, and smooth muscle activity.
Examples include pilocarpine (used in glaucoma) and bethanechol (for urinary retention).
2.Cholinesterase Inhibitors:
Prevent the breakdown of acetylcholine, enhancing its action.
Used in conditions like myasthenia gravis (e.g., pyridostigmine) and Alzheimer's disease (e.g., donepezil).
3.Anticholinergics:
Block the action of acetylcholine at cholinergic receptors, thereby inhibiting PNS activity.
This can lead to increased heart rate, relaxation of bronchial muscles, and decreased secretions.
Examples include atropine (used in bradycardia) and ipratropium (for asthma and COPD).
Drugs Acting on the Sympathetic Nervous System
1) Adrenergic Agonists (Sympathomimetics):
These stimulate the SNS by acting on adrenergic receptors. They are subdivided based on their action on alpha and beta receptors.
Alpha Agonists:
Used in conditions like hypertension and nasal congestion (e.g., phenylephrine).
Beta Agonists:
Can be selective (acting on either β1 or β2 receptors) or non-selective. Examples include isoproterenol (non-selective), albuterol (β2 selective for asthma), and metoprolol (β1 selective for hypertension).
2) Adrenergic Antagonists (Sympatholytic):
Block adrenergic receptors, inhibiting SNS effects.
1.Alpha Blockers:
Used in hypertension and benign prostatic hyperplasia (e.g., prazosin).
2.Beta Blockers:
Commonly used in heart disease, hypertension, and anxiety (e.g., propranolol).
Clinical Applications
1)Cardiovascular Diseases:
Beta-blockers reduce heart rate and force of contraction, lowering blood pressure.
2)Respiratory Conditions:
Beta-2 agonists relax bronchial muscles, aiding in asthma management.
3)Ocular Conditions:
Cholinergic agonists like pilocarpine decrease intraocular pressure in glaucoma.
4)Neurological Conditions:
Cholinesterase inhibitors improve cognition in Alzheimer's disease.
Side Effects and Considerations
1)Specificity:
Selectivity for receptor subtypes is crucial to minimize side effects.
2)Systemic Effects:
ANS drugs can have widespread effects due to the pervasive nature of the nervous system.
3)Drug Interactions:
ANS drugs can interact with other medications, leading to enhanced or diminished effects.