Chemical Coordination and Neural Control Notes By PW

Coordination allows different organs and organ systems to work together and maintain normal body functions. Neural coordination produces rapid responses through nerve impulses, while chemical coordination regulates physiological processes through hormones released by endocrine glands. Understanding these mechanisms is important for learning how the human body responds to internal and external changes.

PW supports NEET aspirants through courses for Class 11, Class 12 and droppers, along with live and recorded classes, daily practice problems (DPPs), mock tests and previous-year questions. Regular revision of neuron structure, nerve impulse transmission, brain regions, endocrine glands and hormone action can help you build conceptual clarity and practise chapter-based questions.

Meaning and Types of Coordination

Coordination occurs when two or more organs interact and complement each other's functions.

Neural coordination relies on electrical impulses along neurons and chemical communication at synapses. Chemical coordination involves hormones released into the blood, which act on target cells with the appropriate receptors.

Organisation of the Human Nervous System

The human nervous system has two major divisions:

• Central Nervous System (CNS): Consists of the brain and spinal cord.

• Peripheral Nervous System (PNS): Consists of nerves connecting the CNS with the rest of the body.

The human body has 12 pairs of cranial nerves arising from the brain and 31 pairs of spinal nerves arising from the spinal cord.

Afferent and Efferent Fibres

Memory tip: Afferent fibres carry information towards the CNS, while efferent fibres carry commands away from it.

Somatic, Autonomic and Visceral Nervous Systems

• Somatic nervous system: Carries motor information to skeletal muscles and is involved in voluntary movements.

• Autonomic nervous system: Regulates involuntary functions involving structures such as smooth muscles, cardiac muscle and glands.

• Visceral nervous system: Includes sensory and motor fibres associated with internal organs, along with related ganglia and nerve plexuses.

A ganglion is a collection of neuronal cell bodies outside the CNS. A nerve is a bundle of nerve fibres, while a plexus is a network of nerves.

The autonomic nervous system has two main divisions:

Sympathetic activity is associated with the fight-or-flight response. Parasympathetic activity generally supports rest-and-digest functions. Acetylcholine is a major neurotransmitter in parasympathetic pathways.

Structure and Classification of Neurons

A neuron is the structural and functional unit of the nervous system. It has three main parts:

1. Cell body: Also called the soma, cyton or perikaryon; it contains the nucleus and most cellular organelles.

2. Dendrites: Receive information and generally carry it towards the cell body.

3. Axon: Conducts nerve impulses away from the cell body towards other cells.

Nissl's granules are found in the cell body and dendrites but are absent from the axon. They consist of rough endoplasmic reticulum and are associated with protein synthesis.

The axon membrane is called the axolemma, while its cytoplasm is called the axoplasm. The axon terminal contains synaptic knobs and vesicles that store neurotransmitters.

Classification of Neurons by Polarity

Apolar neurons, described in some introductory classifications, lack distinct dendrites and axons; they are associated with simple organisms such as Hydra.

Myelinated and Unmyelinated Neurons

Myelin is an insulating layer around certain axons that helps nerve impulses travel faster.

• Oligodendrocytes form myelin in the CNS.

• Schwann cells form myelin in the PNS.

• Neuroglial cells support, protect and maintain neurons and contribute substantially to neural tissue.

Resting Membrane Potential and Action Potential

A resting neuron is in a polarised state. Its membrane potential is approximately −70 mV, meaning the inside is electrically negative relative to the outside.

The resting membrane potential is maintained by:

• Different sodium and potassium ion concentrations across the membrane.

• Selective permeability of the neuronal membrane.

• Ion leak channels.

• Negatively charged proteins and other anions inside the cell.

• The sodium-potassium pump, which uses ATP to transport three sodium ions out and two potassium ions into the cell.

Memory tip: Sodium concentration is higher outside the neuron, while potassium concentration is higher inside.

Stages of an Action Potential

An action potential is a rapid change in membrane potential that allows a nerve impulse to travel along an axon.

The peak of an action potential is often around +30 mV, while the temporary hyperpolarisation may approach −90 mV. These values are approximate and depend on the cell and conditions.

Memory tip: Resting → Depolarisation → Repolarisation → Hyperpolarisation → Resting.

Conduction of Nerve Impulses

Conduction is the movement of an impulse along a neuron. Transmission refers to the transfer of information from one neuron to another across a synapse.

• In unmyelinated axons, the action potential is regenerated continuously along the membrane.

• In myelinated axons, it is regenerated at the nodes of Ranvier, producing saltatory conduction.

Saltatory conduction is faster and more energy-efficient than continuous conduction along the entire membrane.

Synapses and Neuronal Communication

A synapse is a junction through which one neuron communicates with another neuron or a target cell.

Chemical Synapse

A chemical synapse consists of a presynaptic membrane, synaptic cleft and postsynaptic membrane.

The transmission process involves the following steps:

1. A nerve impulse reaches the axon terminal.

2. Voltage-gated calcium channels open.

3. Calcium ions enter the synaptic knob.

4. Synaptic vesicles release neurotransmitters by exocytosis.

5. Neurotransmitters bind to receptors on the postsynaptic membrane.

6. The resulting response may excite or inhibit the postsynaptic cell.

Chemical synapses are generally unidirectional because neurotransmitter release and receptor placement occur on opposite sides of the synapse.

Electrical Synapse

Electrical synapses contain gap junctions that allow ions to pass directly between adjacent cells. They transmit signals rapidly and can permit bidirectional communication, depending on the synapse.

Major Brain Divisions and Their Functions

The human brain is divided into the forebrain, midbrain and hindbrain.

Forebrain

The forebrain includes the cerebrum, thalamus, hypothalamus and associated structures such as the hippocampus and amygdala.

• Cerebrum: Contains sensory, motor and association areas. It is involved in voluntary actions, perception, memory, reasoning and language.

• Thalamus: Acts as a major relay centre for sensory information reaching the cerebral cortex, with olfaction being a notable exception.

• Hypothalamus: Regulates hunger, thirst, body temperature, water balance and several endocrine functions.

• Hippocampus: Plays an important role in learning and memory formation.

• Amygdala: Contributes to emotional processing, including fear and aggression.

• Pineal gland: Located in the epithalamus and secretes melatonin, which helps regulate the sleep-wake cycle.

The cerebral cortex consists mainly of grey matter, while deeper regions contain substantial white matter formed by nerve fibres. The two cerebral hemispheres are connected by the corpus callosum, a large bundle of nerve fibres.

Midbrain

The midbrain contains the corpora quadrigemina, which participate in visual and auditory reflexes. The cerebral aqueduct passes through the midbrain and connects the third and fourth ventricles.

Hindbrain

The hindbrain consists of the pons, cerebellum and medulla oblongata.

The spinal cord has a different arrangement from the cerebrum: grey matter lies internally, surrounded by white matter.

Important Endocrine Glands and Hormones

The endocrine system consists of glands that release hormones into the bloodstream. Hormones act on target cells and regulate growth, metabolism, reproduction, stress responses and internal balance.

Hypothalamus and Pituitary Gland

The hypothalamus links the nervous and endocrine systems. Its releasing and inhibiting hormones regulate the anterior pituitary through the hypophyseal portal system.

The pituitary gland, also called the hypophysis, has two major parts:

• Anterior pituitary (adenohypophysis): Produces hormones including growth hormone (GH), prolactin, thyroid-stimulating hormone (TSH), adrenocorticotropic hormone (ACTH), follicle-stimulating hormone (FSH) and luteinising hormone (LH). Melanocyte-stimulating hormone (MSH) is associated with the intermediate region of the pituitary.

• Posterior pituitary (neurohypophysis): Stores and releases oxytocin and vasopressin, also called antidiuretic hormone (ADH), which are synthesised in the hypothalamus.

NEET point: Prolactin supports milk production, whereas oxytocin causes milk ejection.

Thyroid and Parathyroid Glands

The thyroid gland produces thyroxine (T4) and triiodothyronine (T3). These hormones regulate basal metabolic rate, growth, development and several metabolic processes. Iodine is essential for their synthesis.

The thyroid's parafollicular cells produce calcitonin, while the parathyroid glands produce parathyroid hormone (PTH).

Important thyroid disorders include:

• Simple goitre: Enlargement of the thyroid gland that may result from iodine deficiency.

• Congenital hypothyroidism: Inadequate thyroid hormone during early development can impair growth and brain development.

• Graves' disease: An autoimmune condition that commonly causes thyroid hormone overproduction and may cause exophthalmos.

Adrenal Gland

The adrenal glands consist of the outer cortex and inner medulla.

The adrenal cortex has three zones:

Memory tip: GFR stands for Glomerulosa, Fasciculata and Reticularis, from outer to inner.

Aldosterone increases sodium reabsorption by the kidneys and contributes to blood pressure regulation. Cortisol influences glucose metabolism and the body's response to stress. The adrenal medulla secretes adrenaline and noradrenaline, which support the fight-or-flight response.

Pancreas

The endocrine part of the pancreas consists of clusters of cells called the islets of Langerhans.

Glucagon raises blood glucose through processes such as glycogenolysis and gluconeogenesis. Insulin promotes glucose uptake in responsive tissues and supports glycogen synthesis.

Diabetes mellitus is associated with impaired insulin secretion, insulin action or both. Type 1 diabetes primarily involves autoimmune destruction of pancreatic beta cells, while Type 2 diabetes commonly involves insulin resistance and progressive impairment of insulin secretion.

Reproductive Glands

The testes and ovaries produce sex hormones that regulate reproductive functions and secondary sexual characteristics.

• Testosterone: Secreted mainly by the testes; supports male reproductive development and secondary sexual characteristics.

• Oestrogens: Produced mainly by the ovaries; contribute to female reproductive development and endometrial growth.

• Progesterone: Helps prepare and maintain the endometrium for pregnancy.

Chemical Nature and Mechanism of Action of Hormones

Hormones differ in their chemical structure, solubility and mechanisms of action.

How Hormones Act on Target Cells

The mechanism depends partly on whether a hormone can cross the cell membrane.

• Lipid-soluble hormones: Steroid hormones and thyroid hormones can enter target cells and bind intracellular receptors. The hormone-receptor complex can regulate gene expression.

• Water-soluble hormones: Many peptide hormones and catecholamines bind to receptors on the cell membrane and activate intracellular signalling pathways.

Second messengers involved in hormone signalling include cyclic AMP (cAMP), calcium ions, inositol trisphosphate (IP3) and diacylglycerol (DAG).

A hormone affects only cells with suitable receptors, helping it regulate specific physiological processes.

Chemical Coordination and Neural Control is an important Biology chapter for understanding how the nervous and endocrine systems coordinate body functions. Physics Wallah (PW) supports NEET preparation with courses for Class 11, Class 12 and droppers, alongside live and recorded classes, DPPs, mock tests and previous-year questions to support revision and practice.

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