UNIT 8 – Neurotransmitters Secretion (Q&A) | MZO-001 MSCZOO | IGNOU
SAQ 1
a) Write the name of two glands are the main secretary gland of brain.
The brain is not only the central organ of the nervous system but also contains two major secretory glands that are directly involved in endocrine functions. These are:
- Pituitary gland (Hypophysis)
- Pineal gland (Epiphysis cerebri)
These glands are part of the neuroendocrine system, which links the brain and hormonal regulation. They help in maintaining homeostasis, regulating growth, metabolism, stress response, sleep cycle, reproduction and many other vital processes.
1. Pituitary Gland: The Master Gland
The pituitary gland is called the "master gland" because it controls the activity of almost all other endocrine glands in the body. It is located at the base of the brain, in a bony cavity called the sella turcica of the sphenoid bone. It is connected to the hypothalamus by a stalk called the infundibulum, which carries signals from the brain to control hormone secretion.
The pituitary gland has two main lobes:
i. Anterior Pituitary (Adenohypophysis)
This part develops from Rathke's pouch (ectodermal origin) and is the true glandular part. It synthesizes and secretes the following hormones:
- GH (Growth Hormone) – Stimulates overall body growth, especially in bones and muscles.
- TSH (Thyroid Stimulating Hormone) – Controls thyroid gland and its release of thyroxine.
- ACTH (Adrenocorticotropic Hormone) – Stimulates adrenal cortex to release cortisol.
- FSH (Follicle Stimulating Hormone) – Stimulates growth of ovarian follicles and spermatogenesis.
- LH (Luteinizing Hormone) – Triggers ovulation in females and testosterone secretion in males.
- Prolactin – Helps in milk production after childbirth.
All these hormones are under control of releasing or inhibiting hormones secreted by the hypothalamus.
ii) Posterior Pituitary (Neurohypophysis)
This part is formed from the neural ectoderm and does not produce hormones itself. It stores and releases two hormones made by hypothalamic neurons:
- Oxytocin – Causes uterine contractions during labour and helps in milk ejection.
- ADH (Antidiuretic Hormone or Vasopressin) – Promotes water reabsorption in kidneys and helps in regulating blood pressure.
2. Pineal Gland: The Biological Clock Regulator
The pineal gland is a small, cone-shaped gland located deep in the brain, between the two hemispheres in a groove near the thalamus. It is part of the epithalamus and is most known for regulating the circadian rhythm or biological clock of the body.
Its main hormone is:
- Melatonin
- This hormone is secreted during darkness and inhibited by light. It helps regulate sleep patterns and biological timing such as seasonal reproduction in animals.
- Melatonin secretion is controlled by a pathway that involves the retina, suprachiasmatic nucleus (SCN) of the hypothalamus, and sympathetic nerves. Thus, light and dark cycles affect the pineal gland directly, allowing it to synchronise internal body functions with the environment.
b) Which hormone is called as sleep hormone?
The hormone that is called the sleep hormone is Melatonin. It is a naturally occurring hormone in the human body that controls the sleep-wake cycle, also known as the circadian rhythm. Melatonin is secreted by the pineal gland, which is a small, cone-shaped endocrine gland located deep in the brain, near the centre, just above the cerebellum and behind the third ventricle.
Melatonin secretion is directly controlled by the amount of light the eyes receive. When the environment becomes dark, the retina sends signals to a special region in the hypothalamus known as the suprachiasmatic nucleus (SCN). This SCN then sends nerve signals to the pineal gland, which begins to release melatonin into the bloodstream. The rise in melatonin levels at night makes a person feel sleepy and relaxed, preparing the body for sleep.
During the daytime, especially in the presence of sunlight or artificial light, melatonin secretion is stopped. This is why melatonin levels are high at night and very low during the day. The natural daily rhythm of melatonin makes it one of the most important hormones in controlling the body's internal clock.
Melatonin is chemically synthesized in the body from the amino acid tryptophan, which is first converted into serotonin and then into melatonin. This conversion mainly occurs during darkness. Because of its sleep-promoting effect, melatonin is also used as a medicinal supplement to treat problems such as insomnia, jet lag and sleep disorders caused by night shifts or irregular lifestyles.
Melatonin does not induce sleep forcefully like a sedative, but it signals the body that it is time to sleep. That is why it is scientifically and commonly known as the sleep hormone.
c) Write the name of neurotransmitter which act as neuromodulator as well as inhibitor of neurotransmitter.
One of the best-known neurotransmitters that shows both neuromodulatory and inhibitory functions is GABA (Gamma-Aminobutyric Acid). It is a major chemical messenger in the central nervous system (CNS) and plays a vital role in regulating brain activity. GABA is unique because it not only participates in fast synaptic transmission as an inhibitory neurotransmitter but also acts more broadly as a neuromodulator that controls the excitability of entire neural circuits.
1. GABA as a Neuromodulator
As a neuromodulator, GABA works at a slower and more widespread level than fast synaptic transmission. Instead of targeting a single postsynaptic neuron, it can influence large groups of neurons or entire regions of the brain. Neuromodulatory action of GABA usually happens through GABA-B receptors, which are metabotropic and linked with second messenger systems.
Through these pathways, GABA can regulate the activity of other neurotransmitter systems like:
- Glutamate (main excitatory neurotransmitter)
- Dopamine (involved in mood and reward)
- Serotonin (involved in emotion and sleep)
This kind of modulation helps in maintaining emotional stability, attention control, sleep-wake cycles and stress regulation. For example, in the limbic system and prefrontal cortex, GABA modulates circuits related to anxiety and decision-making. In this way, GABA does not always act by directly inhibiting action potentials but adjusts the general tone of neural activity.
2. GABA as an Inhibitory Neurotransmitter
In its classical role, GABA functions as the main inhibitory neurotransmitter in the CNS. It is released at synapses by inhibitory interneurons. It binds mainly to:
- GABA-A receptors, which are ionotropic and allow chloride ions (Cl⁻) to enter the neuron, making the inside more negative (hyperpolarization).
- GABA-B receptors, which are metabotropic and open potassium (K⁺) channels and close calcium (Ca²⁺) channels, also leading to inhibition.
This inhibition prevents the postsynaptic neuron from reaching threshold and firing an action potential. As a result, GABA helps maintain the balance between excitation and inhibition in the nervous system, and prevents problems like seizures, overstimulation and anxiety.
SAQ 2
a) Which neurological disorders are linked to increase dopamine secretion?
Dopamine is a crucial neurotransmitter in the brain that plays a significant role in controlling motor functions, emotional responses and reward pathways. It is involved in regulating mood, movement and several other physiological processes. However, when there is an imbalance in dopamine secretion or its activity, it can lead to several neurological and neuropsychiatric disorders. An increase in dopamine activity is linked to certain disorders, where excessive dopamine levels in specific brain regions can lead to abnormal behaviors and symptoms.
Here are two neurological disorders closely associated with increased dopamine secretion:
1. Tourette Syndrome
Tourette Syndrome (TS) is a neurological disorder characterized by repetitive, involuntary movements (motor tics) and sounds (vocal tics). It often begins in childhood and can persist into adulthood. In individuals with Tourette Syndrome, there is evidence of increased dopamine activity, particularly in the basal ganglia, a brain structure involved in motor control and coordination. Dopamine plays a crucial role in the functioning of the basal ganglia and its overactivity can disrupt the normal regulation of motor movements, leading to the appearance of tics.
While the exact cause of Tourette Syndrome is not entirely understood, it is believed that genetic factors, along with neurotransmitter imbalances (including dopamine), contribute to its development. The treatment of TS often involves the use of dopamine antagonists (medications that block dopamine receptors), which help reduce the excessive dopamine activity and alleviate the tics. Additionally, dopamine modulators are sometimes used to balance the neurotransmitter's effect in the brain.
2. Restless Legs Syndrome (RLS)
Restless Legs Syndrome (RLS) is a neurological disorder that causes a strong, uncontrollable urge to move the legs, usually due to uncomfortable sensations such as tingling, crawling, or burning. These sensations mostly occur during periods of rest or inactivity, especially in the evening or at night and often interfere with sleep. RLS is associated with increased dopamine activity in the brain, particularly in the regions involved in motor control such as the substantia nigra and basal ganglia.
Although the exact cause is not fully known, studies suggest that an overactive or imbalanced dopaminergic system plays a central role in RLS. The increased dopamine activity, especially in the evening hours, may contribute to the discomfort and restlessness. Treatment usually includes dopamine agonists like pramipexole or ropinirole, which mimic the action of dopamine and help normalize its effect in the brain, thus relieving symptoms and improving sleep quality.
b) What neurological condition is caused by inhibiting GABA?
When GABA is inhibited in the brain, it leads to a neurological condition called epilepsy. This happens because GABA normally controls brain activity by stopping too much nerve signalling. When GABA is not working properly, the brain loses its balance and nerve cells start becoming overactive. This overactivity can lead to seizures, which is the main feature of epilepsy.
Seizures are sudden, uncontrolled electrical disturbances in the brain that can cause changes in behavior, movements, feelings, or consciousness. They may last from a few seconds to minutes and can be caused by epilepsy, fever, head injury, or other neurological conditions. Seizures vary in type and severity.
Step-by-Step Explanation: How GABA Inhibition Leads to Epilepsy
To understand how epilepsy is caused by inhibiting GABA, we can break the process into simple steps. These steps explain what GABA normally does, what changes happen when it is blocked and how those changes result in epileptic seizures.
Step 1: Role of GABA in the Brain
GABA (Gamma-Aminobutyric Acid) is the major inhibitory neurotransmitter in the central nervous system. It works like a natural "brake" that prevents neurons from firing too much. It helps in maintaining a balance between excitation and inhibition in the brain.
Step 2: What Happens When GABA is Inhibited
When GABA is reduced or blocked, the inhibitory control over neurons is lost. This means neurons can now fire without control, even when they should be silent. This leads to excessive excitatory signals in the brain.
Step 3: Neuronal Overactivity and Seizures
Due to this lack of inhibition, large groups of neurons start firing together in an abnormal, uncontrolled way. This sudden electrical activity is what causes seizures, which are the main symptom of epilepsy.
Step 4: Support from Scientific Studies
Research has shown that chemicals which block GABA, like bicuculline or picrotoxin, can directly cause seizures. Also, people with epilepsy often have lower GABA activity in their brains. Medicines for epilepsy, like valproate, work by increasing GABA levels and restoring balance.
c) What is the full name for EPSP and IPSP?
The full name of EPSP is Excitatory Postsynaptic Potential, and the full name of IPSP is Inhibitory Postsynaptic Potential.
d) Which ions faciliate signal passage from presynaptic to postsynaptic knobs during chemical transmission?
The main ion that facilitates signal passage from the presynaptic to the postsynaptic knob during chemical transmission is Calcium ion (Ca²⁺). It triggers the release of neurotransmitters into the synaptic cleft. After that, Sodium (Na⁺) and Chloride (Cl⁻) ions in the postsynaptic neuron influence the response.
TERMINAL QUESTIONS
1. What a brief note about chemical and electrical neurotransmission.
Neurotransmission is the process by which neurons communicate with each other. It occurs mainly through two types: chemical neurotransmission and electrical neurotransmission. Both types serve the same basic function, that is, to transmit signals between neurons, but they differ in their mechanisms. These are two essential modes of synaptic transmission in the nervous system.
Chemical Neurotransmission
Chemical neurotransmission is the predominant form of signal transmission between neurons, involving the release and reception of neurotransmitters (chemical messengers). When an action potential reaches the presynaptic terminal of a neuron, it triggers the opening of voltage-gated calcium channels, allowing calcium ions (Ca²⁺) to enter the presynaptic neuron. This influx of calcium ions facilitates the fusion of synaptic vesicles with the presynaptic membrane, releasing neurotransmitters into the synaptic cleft. The neurotransmitters then diffuse across the synapse and bind to specific receptors on the postsynaptic membrane.
The binding of neurotransmitters to receptors induces either excitatory or inhibitory responses in the postsynaptic neuron. For example, glutamate (excitatory neurotransmitter) leads to depolarization and an action potential, while GABA (inhibitory neurotransmitter) causes hyperpolarization and reduces neuronal signaling. The effects of neurotransmitters are short-lived because they are quickly removed from the synaptic cleft either by reuptake into the presynaptic neuron, enzymatic degradation, or diffusion away from the synapse. This process allows the nervous system to modulate various functions, such as mood, cognition, motor control and perception.
Common neurotransmitters involved in chemical transmission include dopamine, serotonin, norepinephrine, acetylcholine, glutamate and GABA. Chemical neurotransmission is slower compared to electrical transmission but is essential for complex neural processes such as learning, memory, emotional regulation and synaptic plasticity.
Electrical Neurotransmission
Electrical neurotransmission, also known as electrical synaptic transmission, involves the direct flow of electrical signals from one neuron to another through specialized channels called gap junctions. These junctions are formed by connexins, proteins that create pores allowing ions and small molecules to pass directly between adjacent cells. Unlike chemical transmission, electrical transmission does not require neurotransmitter release and receptor binding. As a result, it is much faster and allows for synchronized activity across connected neurons.
Electrical synapses are commonly found in regions of the nervous system where rapid and coordinated response is required, such as in the heart muscle (for synchronized contraction) and certain brain regions involved in rhythm generation and reflexes. In these systems, electrical transmission helps maintain synchronous activity across groups of neurons or cells, ensuring quick and efficient responses.
Unlike chemical transmission, electrical transmission does not offer the same level of modulation or complexity. However, it is highly efficient and suited for rapid communication. Electrical synapses are often used for reflex circuits and functions that require high-speed transmission with minimal delay.
Key Differences Between Chemical and Electrical Neurotransmission:
1. On the Basis of Mechanism: Chemical transmission involves neurotransmitter release and binding to receptors, while electrical transmission relies on the direct flow of ions between neurons through gap junctions.
2. On the Basis of Speed: Electrical transmission is faster because it does not involve the complex steps of neurotransmitter release and receptor activation.
3. On the Basis of Modulation: Chemical transmission allows for complex modulation of signals, enabling diverse effects such as inhibition, excitation and plasticity. Electrical transmission is faster but more simplistic, typically enabling rapid, synchronized responses.
4. On the Basis of Complexity: Chemical synapses are involved in most brain functions and allow for intricate signaling patterns, whereas electrical synapses are limited to specific tissues like the heart or certain brain regions requiring synchronized activity.
2. Write a brief note about the secretary glands of brain.
Secretory glands of the brain, also known as neuroendocrine glands, are specialized structures that secrete hormones directly into the bloodstream. These hormones affect various organs and tissues throughout the body and play a vital role in maintaining homeostasis. They help regulate important physiological functions such as growth, metabolism, sleep, reproduction and response to stress. The primary secretory glands of the brain are the pituitary gland and the pineal gland. These glands are located within the central nervous system but also act as endocrine organs. They are directly or indirectly controlled by the hypothalamus and are involved in the production and secretion of hormones that regulate body functions like stress response, reproductive cycles, circadian rhythms and metabolic activity.
1. Pituitary Gland (Hypophysis)
The pituitary gland is a small pea-shaped endocrine gland located at the base of the brain in a bony cavity called the sella turcica of the sphenoid bone. It is connected to the hypothalamus by a stalk called the infundibulum. It is often referred to as the "master gland" because it controls the function of many other endocrine glands by releasing stimulating hormones.
There are two major parts of the pituitary gland:
A. Anterior Pituitary (Adenohypophysis)
The anterior pituitary is made up of glandular epithelial tissue and develops from Rathke's pouch (oral ectoderm origin). It is the largest part of the pituitary and is richly supplied with blood via the hypophyseal portal system, which carries hormones from the hypothalamus. These hypothalamic hormones stimulate or inhibit the secretion of anterior pituitary hormones.
The anterior pituitary synthesizes and secretes six major peptide hormones, which regulate various physiological processes in the body.
- Growth Hormone (GH): Promotes growth of bones and muscles and regulates metabolism.
- Thyroid Stimulating Hormone (TSH): Stimulates the thyroid gland to produce T3 and T4 hormones.
- Adrenocorticotropic Hormone (ACTH): Stimulates the adrenal cortex to secrete cortisol and other glucocorticoids.
- Follicle Stimulating Hormone (FSH): Controls the development of follicles in ovaries and spermatogenesis in testes.
- Luteinizing Hormone (LH): Triggers ovulation in females and testosterone production in males.
- Prolactin (PRL): Promotes milk production in lactating females.
B. Posterior Pituitary (Neurohypophysis)
The posterior pituitary is made up of neural tissue and does not produce hormones itself. It is formed from an outgrowth of the hypothalamus and consists mainly of axon terminals of hypothalamic neurons. It stores and releases hormones that are synthesized in the hypothalamus.
It acts as a storage and release site for two important hormones which are produced in the hypothalamus and transported through axons.
- Oxytocin: Causes uterine contractions during childbirth and helps in milk ejection from mammary glands.
- Antidiuretic Hormone (ADH) or Vasopressin: Regulates water balance by increasing water reabsorption in kidneys.
2. Pineal Gland
The pineal gland is a small, pine cone-shaped structure located near the center of the brain between the two cerebral hemispheres in a groove where the two halves of the thalamus join. It is part of the epithalamus and has both neural and endocrine features.
The pineal gland is responsible for the secretion of the hormone melatonin, which regulates sleep-wake cycles (also called circadian rhythms). Its activity is influenced by the light-dark cycle sensed through the retina.
- Melatonin
- This hormone is secreted during darkness and inhibited by light. It helps regulate sleep patterns and biological timing such as seasonal reproduction in animals.
- Melatonin secretion is controlled by a pathway that involves the retina, suprachiasmatic nucleus (SCN) of the hypothalamus, and sympathetic nerves. Thus, light and dark cycles affect the pineal gland directly, allowing it to synchronise internal body functions with the environment.
3. What is the difference between excitatory and inhibitory postsynaptic potentials.
When a nerve impulse reaches the axon terminal of a presynaptic neuron, neurotransmitters are released into the synaptic cleft. These neurotransmitters bind to specific receptors present on the membrane of the postsynaptic neuron. This binding leads to the opening of ion channels and causes changes in the membrane potential of the postsynaptic neuron. These changes can either increase or decrease the likelihood of generating an action potential. If the change leads to depolarization, it is called Excitatory Postsynaptic Potential (EPSP) and if it leads to hyperpolarization, it is known as Inhibitory Postsynaptic Potential (IPSP). Both are types of graded potentials and are crucial for the integration of synaptic inputs in the central nervous system.
Here is the detailed comparison of EPSP and IPSP based on different criteria:
1. Based on Definition and Nature of Response
EPSP (Excitatory Postsynaptic Potential) is a type of postsynaptic potential that increases the probability of the neuron initiating an action potential. It results in depolarization of the membrane.
IPSP (Inhibitory Postsynaptic Potential) is a type of postsynaptic potential that decreases the probability of the neuron initiating an action potential. It results in hyperpolarization of the membrane.
2. Based on Direction of Membrane Potential Change
EPSP makes the inside of the neuron less negative, bringing the membrane potential closer to the threshold.
IPSP makes the inside of the neuron more negative, taking the membrane potential farther from the threshold.
3. Based on Type of Neurotransmitters Involved
EPSP is usually caused by excitatory neurotransmitters like glutamate (main excitatory neurotransmitter in the brain) and acetylcholine (especially at neuromuscular junctions).
IPSP is caused by inhibitory neurotransmitters like GABA (gamma-aminobutyric acid, the main inhibitory neurotransmitter in the brain) and glycine (important in the spinal cord).
4. Based on Location of Occurrence
EPSP is common in excitatory synapses, especially in brain regions like hippocampus, cortex and thalamus.
IPSP is common in inhibitory synapses, such as those in cerebellum, spinal cord and basal ganglia.
5. Based on Type of Receptors Involved
EPSP usually involves receptors like AMPA, NMDA (for glutamate) or nicotinic receptors (for acetylcholine).
IPSP usually involves GABA-A receptors (ligand-gated chloride channels) or glycine receptors.
6. Based on Ion Channel Activity
EPSP results from the influx of sodium ions (Na⁺) or simultaneous influx of Na⁺ and efflux of potassium (K⁺), leading to depolarization.
IPSP results from the influx of chloride ions (Cl⁻) or efflux of potassium ions (K⁺), both of which cause hyperpolarization.
7. Effect on Action Potential Generation
EPSP increases the probability of the neuron reaching threshold potential and initiating an action potential.
IPSP reduces the probability of reaching threshold, thus preventing action potential generation.
8. Based on Summation
EPSPs can add up with other EPSPs through spatial summation (multiple inputs from different neurons) or temporal summation (repeated signals from one neuron) to reach the threshold.
IPSPs can add up or also cancel out the effect of EPSPs. A strong IPSP can nullify the depolarization caused by EPSP.
9. Based on Functional Importance
EPSP helps in activating postsynaptic neurons and is involved in processes like learning, memory formation and sensory signal transmission.
IPSP provides a controlling mechanism, helping to suppress unnecessary excitation. It is vital in preventing overstimulation, seizures, or uncontrolled firing.
4. Make a chart of neurological disorder and associated neurosecretion.
Neurological disorders are often caused by imbalances in specific neurotransmitters or neurosecretions in the brain. These chemicals help neurons communicate and their deficiency or excess can disturb normal brain function. Below is a detailed explanation of major neurological disorders and their associated neurosecretions:
- Parkinson's Disease: Parkinson's Disease is mainly caused by a reduction in dopamine secretion. The dopaminergic neurons of the substantia nigra in the midbrain slowly degenerate, leading to low dopamine in the striatum. This causes tremors, rigidity, slow movements and posture imbalance.
- Alzheimer's Disease: Alzheimer's Disease is linked with reduced secretion of acetylcholine. In this disorder, cholinergic neurons in the hippocampus and cortex degenerate. Acetylcholine is needed for learning and memory, so its deficiency results in memory loss and confusion.
- Myasthenia Gravis: Myasthenia Gravis is an autoimmune disorder where the body produces antibodies against acetylcholine receptors at the neuromuscular junction. Although acetylcholine is released normally, it cannot bind properly to the receptors. As a result, the communication between nerve and muscle is weakened, causing muscle fatigue, especially in facial muscles, eyes and limbs.
- Depression: Depression is associated with low levels of serotonin and norepinephrine. These neurotransmitters regulate mood, emotions and motivation. Their deficiency leads to sadness, fatigue, lack of interest and hopelessness.
- Schizophrenia: Schizophrenia is often linked to overactivity of dopamine in specific brain areas like the mesolimbic pathway. This causes hallucinations, delusions and disorganized thoughts.
- Anxiety Disorders: Anxiety Disorders are generally caused by low levels of GABA (gamma-aminobutyric acid). GABA is an inhibitory neurotransmitter. Its deficiency leads to over-excitation in the brain, resulting in worry, tension and fear.
- Epilepsy: Epilepsy results from an imbalance between glutamate (excitatory) and GABA (inhibitory). When GABA is low and glutamate is high, abnormal electrical activity is triggered, leading to seizures.
- Huntington's Disease: Huntington's Disease is associated with degeneration of neurons that release GABA and acetylcholine in the basal ganglia. This leads to involuntary muscle movements, behavioral changes and cognitive decline.
- Insomnia: Insomnia is often caused by reduced secretion of melatonin from the pineal gland. Melatonin controls the sleep-wake cycle. If its production is low, the person experiences difficulty falling asleep.
- Addiction Disorders: Addiction Disorders, such as drug or alcohol dependence, involve overactivation of the dopamine reward pathway. The repeated release of dopamine reinforces drug-seeking behavior and causes strong cravings.
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