Neuromuscularsynapse

The neuromuscular synapse, or neuromuscular junction (NMJ), is a highly specialized and efficient synapse that facilitates communication between motor neurons and muscle fibres, leading to muscle contraction (Figure 8.7 and Figure 8.8).

8.19.1 Synaptic Transmission at

Neuromuscular Synapse

Synaptic transmission at the neuromuscular junction involves a series of events that lead to the transmission of signals from a motor neuron to a skeletal muscle fibre.

Structurally, NMJ has small membrane-bound vesicles that contain the neurotransmitter acetylcholine. These vesi­cles are called Synaptic Vesicles. When an action potential reaches the presynaptic terminal, voltage-gated calcium channels open, allowing Ca²⁺ ions to enter the terminal. In active zones, which are the specialized areas of the pre- synaptic membrane, the synaptic vesicles dock and release their contents into the synaptic cleft through exocytosis. The entry of Ca2+ ions triggers the fusion of synaptic vesi­cles with the presynaptic membrane, releasing ACh into the synaptic cleft, a narrow gap (about 20-50 nm wide) between the presynaptic terminal and the muscle fibre membrane (sarcolemma). Acetylcholine released from the presynaptic terminal diffuses across this cleft to reach the postsynaptic membrane. The postsynaptic membrane is highly folded (junctional folds), increasing the surface area to accommo­date more acetylcholine receptors (nAChRs) on the motor end plate. These receptors bind to acetylcholine and open the channels, allowing Na+ ions to enter the muscle fibre and K+ ions to exit, resulting in depolarization of the muscle membrane (endplate potential).

Within the synaptic cleft, the basal lamina is a layer of extracellular matrix involved in providing alignment and stability to NMJ. It contains acetylcholinesterase (AChE), the enzyme responsible for breaking down acetylcholine in the synaptic cleft, terminating the signal and ensuring that muscle contraction does not persist indefinitely.

The neuromuscular synapse plays a critical role in controlling skeletal muscle contraction and movement. Dysfunctions at the neuromuscular junction can lead to muscle weakness, paralysis, and various neuromuscular disorders, such as myasthenia gravis.

8.19.2 Boutons en Passage

The autonomic nerve fibres consists of bouton (or synaptic bouton), which is a swelling at the end of an axon terminal that forms a synapse with another neuron or a target cell. These boutons contain synaptic vesicles filled with neu­rotransmitters (adrenergic NTs in sympathetic nerve fibre and cholinergic NTs in parasympathetic nerve fibres) and are responsible for releasing neurotransmitters into the syn­aptic cleft to transmit signals to the postsynaptic cell.

8.19.3 Sympathetic Signal Transduction

Sympathetic signal transduction refers to the process by which signals generated in the sympathetic division of the autonomic nervous system are transmitted from presynap- tic neurons to postsynaptic target cells, leading to physi­ological responses.

The sequence of sympathetic signal transduction includes origination of signals, synaptic transmission in the sympathetic ganglia, postganglionic transmission to target tissues and activation of adrenergic receptors. The first and foremost step is origination of signals wherein the sympathetic signals originate in the cell bodies of pregan­glionic neurons followed by synaptic transmission in the sympathetic ganglia. The axons of preganglionic neurons project to sympathetic ganglia located outside the spinal cord. Upon reaching the ganglia, the preganglionic neurons synapse with postganglionic neurons. This results in bind­ing of neurotransmitter released by preganglionic neurons (ACh) to nicotinic acetylcholine receptors on the postgan­glionic neurons. Postganglionic neurons of the sympathetic division project to various target tissues, including smooth muscle, cardiac muscle, glands, and adipose tissue. The neurotransmitter released by postganglionic sympathetic neurons is predominantly norepinephrine (noradrenaline), although some postganglionic neurons release acetylcho­line as well. Norepinephrine binds to adrenergic receptors on the target cells, initiating cellular responses. Adrenergic receptors are G protein-coupled receptors located on the plasma membrane of target cells. Activation of these recep­tors either alpha-adrenergic receptors (α1, α 2) or beta- adrenergic receptors (β1, β2, β3) leads to the activation or inhibition of intracellular signaling pathways, depending on the receptor subtype and the target tissue.

The key molecules involved in Sympathetic Signal Transduction are:

1. cAMP (Cyclic AMP): cAMP is a second messen­ger that mediates the effects of neurotransmitters and hormones. Adrenergic receptors (βι, β₂, and αι receptors) activate adenylate cyclase (AC), which converts ATP to cAMP. cAMP activates protein kinase A (PKA), which phosphorylates vari­ous target proteins, leading to cellular responses such as increased heart rate, glycogenolysis, and smooth muscle relaxation.

2. PKA (Protein Kinase A): PKA is a serine/threo- nine kinase that is activated by cAMP. Upon acti­vation by cAMP, PKA phosphorylates specific proteins, altering their activity and cellular func­tion. It is known to increase heart rate and force of contraction through phosphorylation of calcium channels and contractile proteins. Metabolically, it promotes glycogenolysis in liver and muscle cells. It induces relaxation of smooth muscle cells in bronchi and blood vessels through β₂ receptors.

3. Adenylate Cyclase: Adenylate cyclase (AC) is an enzyme that catalyzes the conversion of ATP to cAMP. It is stimulated by G-protein coupled receptors like β-adrenergic receptors. Found in the plasma membrane of cells, in sympathetic signal transduction, AC is activated by binding of norepi­nephrine to β-adrenergic receptors, leading to the production of cAMP and subsequent activation of PKA.

8.19.4 Parasympathetic Signal Transduction

Parasympathetic signal transduction refers to the process by which signals generated in the parasympathetic division of the autonomic nervous system (ANS) are transmitted from presynaptic neurons to postsynaptic target cells, lead­ing to physiological responses.

The sequence of parasympathetic signal transduction includes origination of signals, synaptic transmission in the parasympathetic ganglia, postganglionic transmission to target tissues and activation of muscarinic receptors. The parasympathetic signals originate in the cell bodies of pre­ganglionic neurons located in the brainstem and the sacral region of the spinal cord (craniosacral outflow). These preganglionic neurons project to parasympathetic ganglia located near or within the target organs. Upon reaching the parasympathetic ganglia, the axons of preganglionic neu­rons synapse with postganglionic neurons. The pregangli­onic neurons release acetylcholine which binds to nAChRs on the postganglionic neurons. Postganglionic neurons of the parasympathetic division project to various target tis­sues, including smooth muscle, cardiac muscle, glands, and visceral organs. The neurotransmitter released by post­ganglionic parasympathetic neurons is also acetylcholine. Unlike sympathetic neurons, the ACh binds to muscarinic acetylcholine receptors on the target cells in parasympa­thetic postganglionic neurons to initiate cellular responses. Activation of these muscarinic receptors are G protein- coupled receptors (M1, M2, M3, M4, and M5) leads to the activation or inhibition of intracellular signaling pathways, depending on the receptor subtype and the target tissue. The various physiological responses include decreased heart rate, increased gastrointestinal motility and secretion, contraction of smooth muscle in the bladder, constriction of the pupils (miosis), and stimulation of glandular secretion (e.g., saliva, tears, and digestive enzymes). These responses collectively promote relaxation, digestion, and conservation of energy.

8.20 REFLEXES OF NERVOUS SYSTEM

Living beings, from the simplest to the most complex, exhibit a remarkable ability to adapt to their environment.

Reflexes are involuntary and rapid actions of the system in response to a stimulus. They are essential for survival which help to protect from external harm and maintains internal homeostasis. Reflexes in addition to being invol­untary, are stereotypic motor responses to sensory stim­uli. These reflexes are mediated by neural circuits called reflex arcs which bypass the conscious processing centres of the brain, allowing rapid responses that are essential for survival.

There are four main categories of reflexes:

1. Somatic reflexes: These are quick, involuntary movements of the body’s skeletal muscles con­trolled by the somatic nervous system integrated at the level of spinal cord. Though living beings con­stantly use these muscles for activities like walk­ing, they don’t consciously trigger them. These autonomous actions happen seamlessly in the background of the conscious thought of the beings.

2. Autonomic reflexes: These reflexes involve auto­nomic nervous system and control involuntary functions like digestion, heart rate and metabolic responses to stress. Living beings are unaware of these reflexes happening and are mediated at the level of midbrain and medulla.

3. Somatic reflexes: As introduced earlier these are involuntary movements controlled by spinal cord and are unusually fast. The key to this instanta­neous response lies within a specialized neural pathway called the reflex arc. This pathway oper­ates in such a way that the body reacts swiftly without any conscious cognitive thought to ensure that the critical reflex happen in a split-second.

4. The Reflex Arc - A speedy-bypass circuit: The cornerstone of the somatic reflex lies in the reflex arc. Reflex arc is a dedicated neural pathway that bypasses the brain for immediate response.

The reflex arc processes information at five different inter­connected levels.

1. Stimulus: A stimulus is anything in the environ­ment that can trigger a detectable change in an organism. Technically speaking, an event in the environment triggers a sensory receptor to send a signal. This change can be physical or chemi­cal and it may happen internally or externally. External stimuli come from outside the organ­isms’ environment for example light, temperature, sound, etc. Elevated blood pressure and altered sugar levels are examples of internal stimuli.

Modus operandi of stimulus includes sensory reception, signal transduction, transmission and processing and response.

Sensory reception: The stimulus comes into con­tact with sensory organs. Specialized cells called sensory receptors detect the stimuli form the envi­ronment and these cells are present not only in the sensory organs but are spread throughout the body.

Signal transduction: Sensory receptors that receive the stimulus convert it into electrical signal.

Transmission and processing: The electri­cal signals traverse through nerves to CNS for interpretation.

Response: The CNS generates response based on the interpretation of the processed informa­tion. The response can be either physiological or behavioural. Physiological responses include the changes in the bodily functions while the behav­ioural responses display the actions of the organ­isms that withdrew them from the environmental dangers.

Stimuli are indispensable for organisms’ survival. They allow interactions of the organisms with the environment, assess dangers, acquire resources and maintain homeostasis internally.

2. Sensory neuron: Sensory neurons are the por­ters of perception to the outer world. They are the afferent limbs of the somatosensory neural pathways. They are specialized nerve cells that detect and transmit information about changes in both the external and internal environments to the CNS. Based on the specialization to detect specific kind of stimulus, there are different types of sen­sory neurons.

Mechanoreceptors: these are sensory neurons to detect touch, pressure and vibration.

Thermoreceptors: These are sensory neurons to detect changes in temperature.

Photoreceptors: These are sensory neurons in the retina of the eye to detect light.

Chemoreceptors: These are sensory neurons to detect taste and smell.

Propioceptors: These are sensory neurons to detect the position and movement.

The primary function of the sensory neurons is to translate sensory stimuli into electrical signals that the CNS can comprehend. The process involves stimulus detection, sensory transduction, action potential generation, and signal transmission.

3. Integration centre: In a reflex arc, the integra­tion centre is the decision maker. It is located within the CNS, either in spinal cord or brainstem. Function of the integration centre can be sum­marized as receiving of sensory input, processing of information, generating response and sending motor output.

4. Motor neuron: also referred to as motoneuron or efferent neuron is a nerve cell that transmits signal from CNS to the peripheral effectors.

5. Effector muscle: Skeletal muscle is the heart of the reflex arc. The effector muscle receives the message and contracts or relaxes depending on the reflex type.

Uniqueness of the reflex arc: The reflex arc’s ele­gance lies in its lightning speed. Bypassing the brain, it elicits responses in milliseconds. This is critical for situations where a quick reaction is vital.

The role of Spinal Cord: The spinal cord plays main role in the reflex arc. It serves as an integration centre that receives sensory input, processing through interneurons and through the motor neurons finally triggering the motor response. This allows for a rapid, localized response without the involvement of extensive analysis of the brain.

The interplay of the somatic reflex and the brain: Though the reflex arc operates independent for quick responses, but the brain is still informed about the reflexes. The brain does get the sensory information along a sepa­rate pathway. This makes the subject aware of the stimulus after the initial reflex. In addition, the brain can modulate and refine the future reflexes based on the situations. This interplay can be categorised into 3 main events:

1. Information relay: Even the reflex arc stands alone, it still sends the information to the brain. This makes brain informed about the incidences which lead to informed decisions for later actions.

2. Learning and memory: The brain learns the reflex actions and makes conscious decisions to avoid similar situations in future.

3. Modulating reflexes: The brain can modulate the reflexes to some extent. For example, the Gag reflex triggers involun­tary cough when the back of the throat gets stimulated. It is a preventive measure to avoid choking. However, during eating or drinking, this Gag reflex is modulated momentarily for smooth movement of the ingesta down the oesophagus.

The following are the somatic reflexes each with a specific function.

Stretch reflex: Stretch reflex is a fundamental reflex that plays vital role in maintaining posture and balance. It is an involuntary response maintained by reflex arc which helps muscles react quickly to the changes in length. It supports weight bearing of the muscles and helps prevent falling under the influence of gravity.

In skeletal muscles, the muscle spindle acts as the sen­sory organ, detecting when the muscle is stretched. The voluntary striated muscle, innervated by alpha motor neu­rons from the ventral root of the spinal cord, serves as the effector organ. Together, these components form a negative feedback loop that regulates muscle length and maintains posture. Muscle spindles are encapsulated structures found within the fleshy part of the skeletal muscles. They are spindle shaped and are much thinner than the surrounding muscles. Muscle spindles house specialized muscle fibres called intrafusal muscle fibres; while the most common contractile fibres of the muscle are called extrafusal muscle fibres which make up the bulk of the muscle and are con­tractile in nature.

Intrafusal muscle fibres are further divided into two types: nuclear bag fibres and nuclear chain fibres. Nuclear bag fibres are larger and fewer in number compared to nuclear chain fibres. About two nuclear fibres per spindle exist. Numerous nuclei are clustered in a bag-like region in the central part of the fibre, hence the name. The ends of the nuclear bag fibres are striated, containing contractile machinery just like regular muscle fibres. These fibres are densely innervated by large-diameter primary sensory neu­rons, which wrap around the central region. When a muscle fibre is stretched, the nuclear bag fibres contract along with the entire muscle. This stretch is detected by the sensory neurons wrapped around them. The primary sensory neu­rons fire action potentials, sending signals to the spinal cord. The efferent signals trigger stretch reflex, causing the stretched muscles to contract and there might be indirect inhibition of the opposing muscles through interconnected neurons within the spinal cord. This in turn helps in coordi­nated movement to maintain posture and balance. Nuclear chain fibres, alongside the nuclear bag fibres, are another critical players within the muscle spindle, acting as sensory organs for detecting changes in muscle length. These are smaller and more in number compared to nuclear bag fibres (3-9 per spindle). As in the name, the nuclei are arranged in a single, chain like row within the central region of the fibre. Similar to the nuclear bag fibres, the ends of nuclear chain fibres are striated and contain contractile machin­ery. In contrast to nuclear bag fibres, nuclear chain fibres respond by contracting slowly and continuously (tonic con­traction), to the stretch. These are innervated by smaller diameter secondary sensory neurons compared to nuclear bag fibres. These secondary neurons fire action potentials that convey information about the static length of the mus­cle to the CNS. This continuous signal is critical for main­taining posture and body mindfulness.

In short, nuclear chain fibres update continuously, slowly and steadily on muscle length to the CNS. This is vital for maintaining posture and sense of body position. Nuclear bag fibres, on the other hand, function more like prompt responders, triggering the stretch reflex for immediate adjustments when a muscle is stretched too far. Together, these two types of intrafusal muscle fibres within the muscle spindle provide a comprehensive picture of muscle length, ensuring coordinated movement and proper posture.

Muscle spindles initiate two main types of large-diame­ter afferent (Sensory) fibres, Primary sensory fibres (Group Ia) and Secondary sensory fibres (Group II). Primary sen­sory fibres are associated with nuclear bag fibres. They are large in diameter for rapid transmission of the nerve impulses. Secondary sensory fibres are smaller in diameter than primary fibres and transmit signals slower.

The combined effect of large diameter afferent and efferent fibres coupled with monosynaptic pathway cre­ates a direct line of communication between muscle spin­dle and the motor neuron. This makes the effective quick reflex response possible. Example is knee-jerk response in humans. On the other hand, if the muscle shortens, the spindles calm down and send fewer signals allowing the muscle to relax and lengthen further. Though, the negative feedback mechanisms are in place, they are slow. Major of the problem with the feedback mechanisms is they take into consideration the static sensitivity alone. When an animal loses its balance, the initial error in length of the muscle is too small to be effective. This error in feedback system is overcome by dynamic sensitivity of the spindles. The sen­sitivity of the spindles relies on change in the length of the muscle or the posture rather than the state. This dynamism comes from the afferent group of fibres arising from nuclear bag fibres, which are sensitive to the rate of lengthening of the muscle. Those originating from nuclear chain fibres are static in responses.

The stretch reflex, triggered by sensory fibres and utiliz­ing efferent fibres, contracts a muscle via reciprocal inner­vation while the inverse stretch reflex relaxes it to prevent muscle damage.

1. Reciprocal Innervation: it ensures coordinated muscle movement. When a muscle (agonist) con­tracts due to a stretch reflex, the opposing mus­cle (antagonist) relaxes. This is achieved through inhibitory signals sent from sensory neuron to the motor neurons innervating the antagonist muscle. The stretch reflex on the agonist muscle and the reciprocal inhibition on the antagonist muscle hap­pen simultaneously.

2. Efferent Fibres: Efferent fibres are motor neurons that carry signals from the CNS to muscles, caus­ing them to contract. In stretch reflex, the motor neurons carrying the signal for muscle contraction are efferent fibres.

3. Inverse Stretch Reflex (Golgi Tendon Reflex): The inverse stretch reflex, also known as Golgi tendon reflex, acts as a safety mechanism to pre­vent excessive muscle force generation. Golgi tendon organs, located near muscle-tendon junc­tions, detect excessive tension and send inhibitory signals to the motor neurons of the contracting muscle, causing it to relax.

4. Knee-jerk Reflex - A Classic Example of Stretch Reflex: The knee-jerk reflex also called as patellar reflex, is a stretch reflex present in many animals including humans. It’s a type of stretch reflex that tests the integrity of the reflex arc. When the ten­don below the kneecap (patellar tendon) is given a blow, it stretches the quadriceps muscle, a mus­cle in the front of thigh. Stretch receptors in the quadriceps muscle, the muscle spindles, detect this stretch. Sensory neurons send a signal from the muscle spindles to the spinal cord. In the spinal cord, the sensory neurons synapse with motor neu­rons that control the quadriceps muscle. The motor neurons send a signal back to the quadriceps mus­cle, causing it to contract. The contraction of the quadriceps muscle straightens the leg at the knee joint. Doctors often use the knee jerk reflex as a way to assess the health of the subject’s nervous system. A normal response to a tap on the patel­lar tendon is a quick extension of the lower leg. An abnormal response, such as a weak or absent reflex, could indicate a problem with the spinal cord or the nerves in the leg. Knee-jerk reflexes are

shown by dogs, cats, rabbits, horses, sheep, cows, pigs along with humans.

5. The Withdrawal and Crossed Extensor Reflex:

The withdrawal reflex is considered pre-potent to the stretch reflex and inverse stretch reflex. This means that when there is a conflict between the reflexes, the withdrawal reflex takes priority. The main function of the withdrawal reflex is to pro­tect the body from harmful stimuli like pain or extreme heat. It’s an instantaneous response that allows the subject to pull away quickly without needing to analyse cognitively. This reflex uses polysynaptic connections (uses multiple connec­tions in the spinal cord) to activate certain flexor muscles and deactivate extensor muscles. The withdrawal reflex is elicited by the detection of nociceptive stimuli at the cutaneous innervation level. There will be a flexion of the associated limb when the subject stands on a sharp or hot object. The magnitude of the response increases with the increase in stimulus so much so that if the stimu­lus is stronger enough, the whole weight bearing of the subject will be shifted to another/opposite (contralateral) limb for body support. This coordi­nated response involves activation of the ipsilateral extensor muscles (extensor reflex) in the sup­porting limb. Additionally, the upper limbs may exhibit a crossed extensor reflex, with extension of the ipsilateral arm (on the same side as the sup­porting leg) potentially serving a pushing motion away from the stimulus, while the contralateral arm flexes. Stretch reflex maintains posture with a momentary response. In contrast, the flexor reflex is a more prolonged action, protecting the body by keeping it withdrawn from danger for longer dura­tion. Flexor reflex exhibits prolonged activation due to polysynaptic pathways. This multi-synaptic architecture allows for repeated stimulation of alpha-motor neurons through pathways of vary­ing lengths. Additionally, positive feedback loops within the circuit (reverberating circuits) further extend alpha-motor neuron activation, resulting in after-discharge.

6. Autonomic Reflexes: As already discussed, the somatic nervous system controls voluntary stri­ated skeletal muscles for cognitive interaction with the environment and situations where as, the autonomic nervous system (ANS) acts as an internal conductor, managing involuntary func­tions involving smooth muscles, like blood flow, digestion, etc., to maintain homeostasis. The ANS comprises three anatomically distinct divisions: Sympathetic nervous system (SNS) which orches­trates the fight-or-flight response via widespread activation; Parasympathetic nervous system (PNS) that promotes a rest-and-digest state through tar­geted activation of organs; Enteric nervous system (ENS) that functions as a semi-autonomous net­work governing gastrointestinal (GI) tract pro­cesses. ENS is strongly influenced by both the other divisions of the ANS.

The ANS, often referred to as the involuntary nervous system, plays a critical role in maintaining physiological equilibrium within the body. Autonomic reflexes, a funda­mental component of the ANS, represent a complex inter­play between sensory organs, the CNS, and effector tissues, ensuring the body’s constant adaptation to internal and external stimuli.

The autonomic reflex arc shares similarities with its somatic counterpart but exhibits key distinctions. Sensory information from visceral organs and glands, relayed by visceral afferent neurons, reaches the brainstem or spinal cord. Within dedicated autonomic centres, complex inte­gration of the sensory information occurs. These centres, located in the brainstem and thalamus, coordinate the appropriate autonomic response. For instance, the nucleus tractus solitarius (NTS) in the medulla oblongata integrates cardiovascular and respiratory information, while the pon­tine micturition centre in the pons controls bladder func­tion. The response is then relayed through efferent neurons to target tissues, which include smooth muscle, cardiac muscle and glands.

Unlike, the single-synapse connection in the somatic system, the autonomic reflex arc involves two - a pregan­glionic neuron that synapses within an autonomic ganglion, and postganglionic neuron that innervates the effector tis­sue. This two-neuron arrangement allows for more intricate modulation and integration within the CNS.

Numerous autonomic reflexes ensure optimal physiolog­ical function. Here are a few key autonomic reflexes:

1. Baroreceptor Reflex: This reflex maintains blood pressure stability. When blood pressure rises, baroreceptors in the carotid sinus and aortic arch detect the change. Signals travel to the brainstem, leading to parasympathetic activation and sym­pathetic inhibition, resulting in vasodilation and decreased heart rate, ultimately lowering blood pressure. Conversely, a fall in blood pressure trig­gers the opposite response.

2. Pupillary Light Reflex: Light intensity is detected by retinal photoreceptors. In bright light, a reflex arc mediated by the parasympathetic nervous sys­tem constricts the pupil, reducing light intake. Conversely, in dim light, the sympathetic system dilates the pupil to allow for more light entry.

3. Gastrointestinal Reflexes: Autonomic reflexes govern digestion from food intake to waste elimi­nation. Salivation, gastric acid secretion, and intestinal motility are all under autonomic con­trol, ensuring proper food breakdown and nutrient absorption.

4. The Cough Reflex: This protective reflex safe­guards the respiratory system. When irritants stimulate cough receptors in the airways, the brainstem initiates a forceful expulsion of air through the glottis, aiming to dislodge the irritant. Both the sympathetic and parasympathetic sys­tems contribute to this reflex, with the sympathetic system causing bronchoconstriction to increase air pressure and the parasympathetic system coordi­nating the diaphragm and abdominal muscle con­tractions for forceful expulsion.

5. The Micturition Reflex: This complex reflex governs bladder emptying. As the bladder fills, stretch receptors in the bladder wall send signals to the pons in the brainstem. When filling reaches a certain threshold, the parasympathetic system triggers bladder muscle contraction, while simul­taneously relaxing the urethral sphincter, allow­ing for urine flow. Conversely, the sympathetic system can inhibit bladder contraction and main­tain urethral closure during socially inappropriate moments.

6. The Thermoregulatory Reflex: The body strives to maintain a constant core temperature (around 37^oC). When skin thermoreceptors detect a decrease in ambient temperature, the sympathetic system activates. This response includes vasocon­striction to decrease blood flow to the skin, conserv­ing body heat. Additionally, shivering, mediated by the sympathetic system, generates heat through involuntary muscle contractions. Conversely, in hot environments, the parasympathetic system pro­motes vasodilation, sweating, and increased blood flow to the skin to facilitate heat dissipation.

7. The Oculocardiac Reflex: This reflex helps maintain cardiovascular stability during certain manoeuvres. When pressure is applied to the eye­balls, such as during valsalva manoeuvre (straining with closed glottis), the vagus nerve (parasympa­thetic) is stimulated, leading to a transient decrease in heart rate. This reflex helps prevent excessive blood pressure spikes during straining activities.

8. The Hering-Breuer Reflex: This pulmonary reflex plays a role in regulating breathing depth and pre­venting overinflation of the lungs. Stretch recep­tors in the alveoli (air sacs) signal the brainstem when lung inflammation reaches a certain point. This triggers the parasympathetic system to inhibit the diaphragm and intercostal muscles, leading to exhalation and preventing lung overinflation.

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