Defination of Synapse
- The synapse is the anatomic site where the nerve cells communicate among themselves.
- There is no anatomical connection or continuity between different neurons. They are connected only functionally.
- So, synapse is the functional junction between two neurons.
TYPES OF SYNAPSE
A. Anatomical types
Depending upon the manner an axon terminates on the other neurons, the synapses can be of following types
- Axo-dendritic synapse. is the synapse between axon of a neuron with dendrite of another neuron.
- Most common type of synapse. Synapse on dendrites may be located on the spines or on the smooth areas between spines.
2. Axo-somatic synapse refers to the synapse between axon of a neuron with the soma (body) of another neuron.
3. Axo-axonic synapse. is the synapse between axon of a neuron with axon of another neuron. less common type of synapse.
B. Physiological types
Depending upon the process of transmission of impulse, the synapses can be classified as:
Chemical synapse are those in which transmission is carried out by neurotransmitter.
- Most comman type of synapse
- Conduct information only in one direction.
- More vulnerable to fatigue on repeated stimulation (synaptic fatigue) and to the effects of hypoxia and pH changes.
- Chemical synaptic transmission is definitely slower than the velocity of nerve conduction resulting in the synaptic delay.
- Although the plasma membranes of presynaptic and postsynaptic neurons in a chemical synapse are close, they do not touch.
- They are separated by the synaptic cleft, a space of 20–50 nm that is filled with interstitial fluid.
- In response to a nerve impulse, the presynaptic neuron releases a neurotransmitter that diffuses through the fluid in the synaptic cleft and binds to receptors in the plasma membrane of the postsynaptic neuron.
- The postsynaptic neuron receives the chemical signal and in turn produces a postsynaptic potential, a type of graded potential.
- Thus, the presynaptic neuron converts an electrical signal (nerve impulse) into a chemical signal (released neurotransmitter).
- The postsynaptic neuron receives the chemical signal and in turn generates an electrical signal (postsynaptic potential).
- The time required for these processes at a chemical synapse, a synaptic delay of about 0.5 msec, is the reason that chemical synapses relay signals more slowly than electrical synapses.
STRUCTURE OF A CHEMICAL SYNAPSE
- A typical chemical synapse between the axon of one neuron and dendrite of other neuron exhibits following characteristics
Synaptic knob or button. As axon of neuron approaches synapse, it loses the myelin sheath and divides into a number of fine branches which end in small swellings called the synaptic knobs or synaptic buttons, which make synapse with the soma or dendrite of the post-synaptic neuron.
- Each synaptic knob -Mitochondria and synaptic vesicles containing neurotransmitter. Mitochondria provide ATP required for the synthesis of neurotransmitter.
- Circular synaptic vesicles contain excitatory and elongated vesicles contain inhibitory neurotransmitters.
- The microtubules present in the synaptic knob transport the vesicles along the axons up to the pre-synaptic grid.
- Pre-synaptic membrane refers to the axonal membrane lining the synaptic knobs.
- On the inner aspect of pre-synaptic membrane are present zones of dense cytoplasm
- Synaptic cleft is a small gap (20–50 nm wide) b/w pre- and post-synaptic membranes and filled by ECF containing some glycoproteins.
- The extracellular matrix may be acting as an adherent between synaptic neurons.
- Post-synaptic process is the name given to the region of receiving neuron (e.g. dendritic spine) where the synaptic knob synapses.
- Post-synaptic membrane is the membrane lining the post-synaptic process.
- On the inner aspect of post-synaptic membrane is present a zone of dense cytoplasm
- Post-synaptic membrane contains large number of receptor proteins, which protrude outwards in the synaptic cleft.
- Neurotransmitter released in the synaptic cleft binds with these receptor proteins to cause the effect.
Receptor proteins are of two types:
1. Ion channel receptor proteins. These line the ion channels (Na+, K+, Cl−, etc.) and the neurotransmitter released in the cleft causes opening of the channels by reacting with these receptor proteins.
2. Enzymatic type of receptor proteins. The neurotransmitter released in the cleft reacts with enzymatic type of receptor proteins and causes activation of cellular gene for manufacture of additional receptor protein channels in the membrane.
PROCESS OF CHEMICAL SYNAPTIC TRANSMISSION
- Most synapses within the central nervous system (CNS) use chemical transmitters.
- The sequence of events
A .Release of neurotransmitter.
B. Development of the excitatory post-synaptic potential (EPSP) or inhibitory post-synaptic potential (IPSP).
C. Removal of neurotransmitter from the synaptic cleft.
D. Development of action potential.
RELEASE OF NEUROTRANSMITTER
- When the nerve impulse (action potential) travelling in a nerve fibre (axon) reaches the nerve terminal (synaptic knobs), there occurs depolarization of the pre-synaptic terminal.
- As a result of depolarization, the voltage-gated Ca2+ channels present on the pre-synaptic membrane open up increasing its permeability to Ca2+ ions. Consequently, the Ca2+ ions present in the ECF of synaptic cleft enter the axon terminal.
- Ca2+ triggers the release of acetylcholine (ACh) by exocytosis from a portion of the vesicles.
- Usually, only one type of neurotransmitter is released from all the terminals of a single neuron. This was first propounded by Dale and is called Dale’s phenomenon.
- After being released from the pre-synaptic terminal, the transmitter diffuses across the synaptic cleft and binds to the post-synaptic receptors.
- The time lapse (less than 1 ms) occurring between the arrival of nerve impulse at the pre-synaptic terminal and the effect of neurotransmitter on post-synaptic membrane is called synaptic delay.
DEVELOPMENT OF EXCITATORY POST-SYNAPTIC POTENTIAL AND INHIBITORY POST-SYNAPTIC POTENTIAL
- A neurotransmitter causes either an excitatory or an inhibitory graded potential.
- A neurotransmitter that depolarizes the postsynaptic membrane is excitatory because it brings the membrane closer to threshold .
- The most common excitatory neurotransmitter within the CNS is glutamate.
- The magnitude of the EPSP is 8 mV and depolarization starts with a latency of 0.5 ms, rises to its peak in 2.0 ms and then declines with a half-life of 4.0 ms
Ionic basis of EPSP.
- The excitatory neurotransmitter binds with a specific receptor protein and opens the ligand-gated Na+ or Ca2+ channels on the post-synaptic membrane.
- As a result, the Na+ diffuse inward and depolarize the membrane.
- However, since a very small area of post-synaptic membrane developes increased Na ion permeability,the amount of Na influx is able to produce only a brief depolarization.EPSP does not transmit over the cell
Summation of EPSP- It is graded response, does not follow all or none law like action potential
Spatial ,Temporal Summation of EPSP
- A typical neuron in the CNS receives input from 1000 to 10,000 synapses. Integration of these inputs involves summation of the postsynaptic potentials that form in the postsynaptic neuron.
- Spatial summation and temporal summation.
- Spatial summation is summation of postsynaptic potentials in response to stimuli that occur at different locations in the membrane of a postsynaptic cell at the same time.
- Temporal summation is summation of postsynaptic potentials in response to stimuli that occur at the same location in the membrane of the postsynaptic cell but at different times.
Spatial and temporal summation.
- (a) When presynaptic neurons 1 and 2 separately cause EPSPs in postsynaptic neuron 3, the threshold level is not reached in neuron 3.
- Spatial summation occurs only when neurons : 1 and 2 act simultaneously on neuron 3; their EPSPs sum to reach the threshold level and trigger a nerve impulse (action potential).
- (b) Temporal summation occurs when stimuli applied to the same axon in rapid succession (arrows) cause overlapping EPSPs that sum. When depolarization reaches the threshold level, a nerve impulse is triggered.
Inhibitory Postsynaptic Potentials
- A neurotransmitter that causes hyperpolarization of the postsynaptic membrane is inhibitory.
- A hyperpolarizing postsynaptic potential is termed an inhibitory postsynaptic potential (IPSP).
- Most common inhibitory neurotransmitters in CNS are Glycine and GABA
- Ionic basis of IPSP the inhibitory transmitter released at the synaptic cleft causes opening of K ion channel or Cl channels in post synaptic membranes leading to diffusion of large number of k ion from neuron to the ECF or Cl ions to diffuse to the interior of the neuron.
- This causes post-synaptic membrane potential to become more negative (hyperpolarization). This change in potential is called IPSP.
- Value of IPSP. The magnitude of IPSP is −2 mV. The hyperpolarization has a latency of 2.0 ms, attaining its maximum at 4 ms and then returning towards the resting membrane potential (RMP) with a half-life of 3 ms .
- Recording of IPSP can be made by a technique similar to that of the recording of EPSP .
- Summation of IPSP. Spatial and temporal summation also occurs, as seen with EPSP .
- This type of inhibition is called post-synaptic (or direct) inhibition
INACTIVATION OF NEUROTRANSMITTER FROM THE SYNAPTIC CLEFT
- The neurotransmitter in the synaptic cleft from the pre-synaptic terminal is soon inactivated in one of the three ways:
- Diffusion of the transmitter out of the cleft,
- Enzymatic degradation of the transmitter, e.g. dissociation of acetylcholine by acetylcholinesterase
- Active transport back into the pre-synaptic terminal (transmitter re-uptake), e.g. active re-uptake of norepinephrine at sympathetic post-ganglionic nerve endings.
DEVELOPMENT OF ACTION POTENTIAL
- The development of action potential (AP) from EPSP can be considered in three steps:
- Synaptic integration
- Generation of initial segment spike
- Generation of propagated signals, i.e. action potential.
Electrical synapses are those in which transmission occurs through gap junctions.
- Each gap junction contains a hundred or so tubular connexons, which act like tunnels to connect the cytosol of the two cells directly
- As ions flow from one cell to the next through connexons, the action potential spreads from cell to cell.
- Transmission of electrotonic conduction between two neurons and similar to process of nerve conduction.
- Can conduct in both directions.
- Electrical transmission is seen in the retina and in olfactory bulb in human nervous system.
- ❖Found mainly in invertebrates and lower vertebrates.
- ❖Electrical synapses have two main advantages:
- Faster communication. Because AP conduct directly through gap junctions, electrical synapses are faster than chemical synapses.
- Synchronization. Electrical synapses can synchronize (coordinate) the activity of a group of neurons or muscle fibers.
- The value of synchronized action potentials in the heart or in visceral smooth muscle is coordinated contraction of these fibers to produce a heartbeat or move food through the gastrointestinal tract.
3. Conjoint synapse refers to a synapse where both the chemical and electrical transmission co-exist.
PROPERTIES OF SYNAPTIC TRANSMISSION
- One-way conduction. The chemical synapse allows only one-way conduction of an impulse, i.e. from the pre-synaptic to the post-synaptic neuron and never in the opposite direction.
- This is called law of dynamic polarity or Bell– Magendie law.
Significance. The axons can conduct impulse in either direction with equal ease. However, the synapses act like a valve and are responsible for the orderly conduction of impulse in one direction only.
2. Synaptic delay.
- Synaptic delay refers to a time lapse, which occurs between arrival of nerve impulse at the presynaptic terminal and its passage to the post-synaptic membrane.
- Normally, synaptic delay occurs by approximately 0.5 ms (almost always less than 1 ms).
Causes of synaptic delay –time taken for: Release of neurotransmitter,
- Diffusion of transmitter through synaptic cleft to postsynaptic membrane,
- Action of neurotransmitter to bind with receptors on the post-synaptic membrane and to cause the opening of ion channels and Diffusion of ions causing changes in RMP (i.e. development of EPSP or IPSP).
Significance synaptic delay.
- When an impulse passes through a chain of neurons, it is delayed at every synapse. The synaptic delay is one of the causes for the latent period of the reflex activity.
- The number of neurons involved in the reflex can be estimated from the duration of reaction time of a reflex action.
3. Summation property of synapse. A synapse exhibits the property of both temporal and spatial summation of EPSP and IPSP
4. Convergence and divergence property is present in a chemical synapse.
Convergence refers to a phenomenon of termination of signals from many sources (i.e. many pre-synaptic neurons on a single post-synaptic neuron). For example, ventral horn cells of the spinal cord receive convergent signals from the corticospinal tract, reticulospinal tract, rubrospinal tract and sensory afferent from the dorsal root, etc.
Divergence. One pre-synaptic neuron may terminate on many post-synaptic neurons.
- causes magnification and amplification of an impulse. This is known as divergence.
5. Occlusion phenomenon.
- The term occlusion describes the situation in which response to stimulation of two presynaptic neurons is less than the sum total of the response obtained when they are stimulated separately.
- This happens because of the fact that some post-synaptic neurons are common to both the pre-synaptic neurons
6. Subliminal fringe effect.
- An afferent nerve fibre divides into many hundred branches. Of these, a large number may terminate on one efferent neuron, while a smaller number terminate on other efferent neuron lying nearby.
- When afferent neuron is stimulated, the efferent (post-synaptic) neuron which has many pre-synaptic terminals is excited to threshold level and AP is fired.
- Others in the peripheral zone (fringe area) are excited to subthreshold level only, i.e. their excitability is increased but an AP is not fired.
- This is known as subliminal fringe effect (subliminal means below threshold and fringe means border).
7. Facilitation. When pre-synaptic axon is stimulated with several consecutive individual stimuli, each stimulus may evoke a larger post-synaptic potential than that evoked by previous stimulus. This phenomenon is known as facilitation.
8. Synaptic plasticity and learning. Plasticity refers to the capability of being easily moulded or changed.
- Synaptic transmission can be increased or decreased on the basis of past experience.
- The changes in the synaptic transmission can occur due to alterations at pre-synaptic or post-synaptic location.
9. Synaptic fatigue. When the pre-synaptic neuron is stimulated separately, the rate of impulse discharge in the post-synaptic neuron is initially high but within a few seconds there occurs a gradual decrease and finally disappearance of the post-synaptic response.
- This phenomenon is called synaptic fatigue or habituation. Fatigue is a temporary phenomenon.
- Fatigue mainly occurs due to exhaustion of chemical neurotransmitter due to a gradual inactivation of Ca2+ channels, which decrease the intracellular Ca2+,
- Accumulation of waste products
10. Reverberation. Reverberation refers to the phenomenon of passage of impulse from pre-synaptic neuron and again back to pre-synaptic neuron to cause a continuous stimulation of pre-synaptic neuron.
13. Effect of acidosis and hypoxia. The CNS neurons cannot sustain oxygen lack.
- Synaptic transmission is particularly vulnerable to the effect of acidosis and hypoxia.
- This may explain why the first site of fatigue of the synaptic chain is located in the brain.
Reflexes and Reflex Arcs
- A reflex is a fast, automatic, unplanned sequence of actions that occurs in response to a particular stimulus.
- Some reflexes are inborn, such as pulling hand away from a hot surface before you even feel that it is hot. Other reflexes are learned or acquired. For instance, you learn many reflexes while acquiring driving expertise.
- Slamming on the brakes in an emergency is one example.
- When integration takes place in the spinal cord gray matter, the reflex is a spinal reflex. An example is the patellar reflex (knee jerk).
- If integration occurs in the brain stem rather than the spinal cord, the reflex is called a cranial reflex. An example is the tracking movements of eyes as we read this sentence.
- somatic reflexes,which involve contraction of skeletal muscles.
- Autonomic (visceral) reflexes, which generally are not consciously perceived. They involve responses of smooth muscle, cardiac muscle, and glands.
- Body functions such as heart rate, digestion, urination, and defecation are controlled by the autonomic nervous system through autonomic reflexes.
- Nerve impulses propagating into, through, and out of the CNS follow specific pathways, depending on the kind of information, its origin, and its destination.
- The pathway followed by nerve impulses that produce a reflex is a reflex arc (reflex circuit). A reflex arc includes five functional components.
- Sensory receptor. The distal end of a sensory neuron (dendrite). It responds to—a change in the internal or external environment—by producing a graded potential called a generator (or receptor) potential.
- Sensory neuron. The nerve impulses propagate from the sensory receptor along the axon of the sensory neuron to the axon terminals, which are located in the gray matter of the spinal cord or brain stem.
- Integrating center- Gray matter within the CNS act as an integrating center.
- Motor neuron. Impulses triggered by the integrating center propagate out of the CNS along a motor neuron to the part of the body that will respond.
- Effector. The part of the body that responds to the motor nerve impulse, such as a muscle or gland, is the effector.
- If the effector is skeletal muscle, the reflex is a somatic reflex.
- If the effector is smooth muscle,cardiac muscle, or a gland, the reflex is an autonomic (visceral) reflex.
- A reflex pathway having only one synapse in the CNS is termed a monosynaptic reflex arc . Integrating center consists one or more interneurons which may relay impulses to other interneurons as well as to a motor neuron.
- A polysynaptic reflex arc involves more than two types of neurons and more than one CNS synapse.
The Stretch Reflex
- A stretch reflex causes contraction of a skeletal muscle in response to stretching of the muscle. This type of reflex occurs via a monosynaptic reflex arc.
- The reflex can occur by activation of a single sensory neuron that forms one synapse in the CNS with a single motor neuron.
- Stretch reflexes can be elicited by tapping on tendons attached to muscles at the elbow, wrist, knee, and ankle joints. An example of a stretch reflex is the patellar reflex (knee jerk).
- A stretch reflex operates as follows :
- Slight stretching of a muscle stimulates sensory receptors in the muscle called muscle spindle. The spindles monitor changes in the length of the muscle.
- In response to being stretched, a muscle spindle generates one or more nerve impulses that propagate along a somatic sensory neuron through the posterior root of the spinal nerve and into the spinal cord.
- In the spinal cord (integrating center), the sensory neuron makes an excitatory synapse with and thereby activates a motor neuron in the anterior gray horn.
- If the excitation is strong enough, one or more nerve impulses arise in the motor neuron and propagate along its axon, which extends from the spinal cord into the anterior root and through peripheral nerves to the stimulated muscle. The axon terminals of the motor neuron form neuromuscular junctions (NMJs) with skeletal muscle fibers of the stretched muscle.
Acetylcholine released by nerve impulses at the NMJs triggers one or more muscle action potentials in the stretched muscle (effector), and the muscle contracts. Thus, muscle stretch is followed by muscle contraction, which relieves the stretching.
- In the reflex arc just described, sensory nerve impulses enter the spinal cord on the same side from which motor nerve impulses leave it. This arrangement is called an ipsilateral reflex . All monosynaptic reflexes are ipsilateral.
- Although the stretch reflex pathway is monosynaptic (just two neurons and one synapse), a polysynaptic reflex arc to the antagonistic muscles operates at the same time. This arc involves three neurons and two synapses.
- An axon collateral (branch) from the muscle spindle sensory neuron also synapses with an inhibitory interneuron in the integrating center.
- In turn, the interneuron synapses with and inhibits a motor neuron that normally excites the antagonistic muscles.
- Muscle spindles, which are distributed throughout the fleshy part of a skeletal muscle, consist of collections of specialized muscle fibers known as intrafusal fibers, which lie within spindle-shaped connective tissue capsules parallel to the “ordinary” extrafusal fibers .
- Unlike an ordinary extrafusal skeletal muscle fiber, which contains contractile elements (myofibrils) throughout its entire length, an intrafusal fiber has a noncontractile central portion, with the contractile elements being limited to both ends.
- Each muscle spindle has its own private efferent and afferent nerve supply. The efferent neuron that innervates a muscle spindle’s intrafusal fibers is known as a gamma motor neuron,
- The motor neurons that supply the extrafusal fibers are called alpha motor neurons.
- Two types of afferent sensory endings terminate on the intrafusal fibers and serve as muscle spindle receptors, both of which are activated by stretch.
- The primary (annulospiral) endings are wrapped around the central central portion of the intrafusal fibers; they detect changes in the length of the fibers during stretching as well as the speed with which it occurs.
- The secondary (flower-spray) endings, which are clustered at the end segments of many of the intrafusal fibers, are sensitive only to changes in length.
- Muscle spindles play a key role in the stretch reflex.
The Tendon Reflex
- Tendon reflex operates as a feedback mechanism to control muscle tension by causing muscle relaxation before muscle force becomes so great that tendons might be torn.
- Although the tendon reflex is less sensitive than the stretch reflex, it can override the stretch reflex when tension is great, making you drop a very heavy weight, for example. Like the stretch reflex, the tendon reflex is ipsilateral.
- The sensory receptors for this reflex are called tendon (Golgi tendon) organs which lie within a tendon near its junction with a muscle.
- A tendon reflex operates as follows :
- As the tension applied to a tendon increases, tendon organ (sensory receptor) is stimulated that is depolarized to threshold.
- Nerve impulses arise and propagate into the spinal cord along a sensory neuron.
- Within the spinal cord (integrating center), the sensory neuron activates an inhibitory interneuron that synapses with a motor neuron.
- The inhibitory neurotransmitter inhibits (hyperpolarizes) the motor neuron, which then generates fewer nerve impulses.
- The muscle relaxes and relieves excess tension.
- Thus, as tension on the tendon organ increases, the frequency of inhibitory impulses increases; inhibition of the motor neurons to the muscle developing excess tension (effector) causes relaxation of the muscle.
- In this way, the tendon reflex protects the tendon and muscle from damage due to excessive tension.
Golgi Tendon Organs
- Golgi tendon organs are in the tendons of the muscle, where they can respond to changes in the muscle’s tension rather than to changes in its length.
- Because a number of factors determine the tension developed in the whole muscle during contraction (for example, frequency of stimulation or length of the muscle at the onset of contraction), it is essential that motor control systems be apprised of :-
The Flexor and Crossed Extensor Reflexes :
- Another reflex involving a polysynaptic reflex arc results when, for instance, you step on a tack.
- In response to such a painful stimulus, you immediately withdraw your leg.
- This reflex, called the flexor or withdrawal reflex,
Patellar reflex (knee jerk).
- This stretch reflex involves extension of the leg at the knee joint by contraction of the quadriceps femoris muscle in response to tapping the patellar ligament .
- This reflex is blocked by damage to the sensory or motor nerves supplying the muscle or to the integrating centers in the second, third, or fourth lumbar segments of the spinal cord.
- It is often absent in people with chronic diabetes mellitus or neurosyphilis, both of which cause degeneration of nerves.
- It is exaggerated in disease or injury involving certain motor tracts descending from the higher centers of the brain to the spinal cord.
Achilles reflex (ankle jerk).
- This stretch reflex involves extension (plantar flexion) of the foot by contraction of the gastrocnemius and soleus muscles in response to tapping the calcaneal (Achilles) tendon.
- Absence of the Achilles reflex indicates damage to the nerves supplying the posterior leg muscles or to neurons in the lumbosacral region of the spinal cord.
- This reflex may also disappear in people with chronic diabetes, neurosyphilis, alcoholism, and subarachnoid hemorrhages.
- An exaggerated Achilles reflex indicates cervical cord compression or a lesion of the motor tracts of the first or second sacral segments of the cord.
- This reflex results from gentle stroking of the lateral outer margin of the sole. The great toe dorsiflexes, with or without a lateral fanning of the other toes.
- This phenomenon normally occurs in children under one and half years of age and is due to incomplete myelination of fibers in the corticospinal tract.
- A positive Babinski sign after age 1&1/2 is abnormal and indicates an interruption of the corticospinal tract as the result of a lesion of the tract, usually in the upper portion.
- The normal response after age 1&1/2 is the plantar flexion reflex, or negative Babinski—a curling under of all the toes.
- This reflex involves contraction of the muscles that compress the abdominal wall in response to stroking the side of the abdomen.
- The response is an abdominal muscle contraction that causes the umbilicus to move in the direction of the stimulus.
- Absence of this reflex is associated with lesions of the corticospinal tracts.
- It may also be absent because of lesions of the peripheral nerves, lesions of integrating centers in the thoracic part of the cord, or multiple sclerosis.
- Most autonomic reflexes are not practical diagnostic tools because it is difficult to stimulate visceral effectors, which are deep inside the body.
- An exception is the pupillary light reflex, in which the pupils of both eyes decrease in diameter when either eye is exposed to light.
- Because the reflex arc includes synapses in lower parts of the brain, the absence of a normal pupillary light reflex may indicate brain damage or injury.
PROPERTIES OF REFLEXES
- Adequate stimulus. Reflex response is obtained only when a precise stimulus for a given reflex activity is applied. The precise stimulus which involves a reflex response is called adequate stimulus for that particular reflex
- Delay. Delay refers to the time interval between the application of stimulus and starting of the response. It is attributed to a synaptic delay and to time required for passage of impulse along the nerves. Therefore, delay is minimum in a monosynaptic reflex.
- One-way conduction. During reflex activity, impulses are transmitted in only one direction through the reflex arc as per the Bell–Magendie law. Impulses pass -receptors to the centre and then from centre to the effector organ
4. Summation of stimuli
5. Irradiation. When a sensory stimulus is too strong, impulse spreads to many neighbouring neurons in the centre and produces wider response.
6. Final common pathway. Efferent pathway of the reflex arc is formed by α-motor neurons that supply the extrafusal muscle fibres. All neuronal influences (excitatory and inhibitory) affecting muscular contraction ultimately funnel through the motor neurons called common final pathway.
7. Facilitation. When a reflex is elicited repeatedly at proper intervals the response becomes progressively higher for first few occasions, i.e. each subsequent stimulus exerts a better effect than the previous one. This is due to the facilitation occurring at the synapse.
8. Inhibition. During a reflex activity, impulses through sensory fibres from protagonist muscles inhibit the action of antagonist muscles.
9. After discharge. When a reflex action is elicited continuously for some time, and then the stimulation is stopped, the reflex response (contraction) may continue for some time even after cessation of the stimulus. This is called after discharge.
10. Fatigue or habituation. When a particular reflex is elicited repeatedly at frequent intervals, the response is reduced progressively and then disappears all together. This is called fatigue or habituation. The first site of fatigue is synapse, then the motor endings and lastly the muscle.
11. Rebound phenomenon. The reflex activity can be inhibited for some time . However, once the inhibitory effect is over, the reflex activity reappears and becomes more powerful. This is called rebound phenomenon.
12. Fractionation. The force of a muscle contraction is much higher when it is stimulated directly through motor nerve as compared to when it is stimulated reflexly through a sensory nerve. This is due to phenomenon of occlusion of the motor neurons when sensory nerve is stimulated.
13. Sensitization. When an injurious stimulus is repeatedly applied, there occurs intensification of response. This is known as sensitization. Sensitization, in fact, is the presynaptic facilitation of an impulse.
Other Physiology Notes
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