CNS Lecture 7 PDF

Central nervous system By Dr : Basma Helal Lecturer of physiology  Types of the stretch reflex (1)Dynamic stretch reflex. (2)Static stretch reflex. (3)Inverse stretch reflex. (4)Negative stretch reflex: It is reflex relaxation of skeletal muscle in response to its s...

Central nervous system By Dr : Basma Helal Lecturer of physiology  Types of the stretch reflex (1)Dynamic stretch reflex. (2)Static stretch reflex. (3)Inverse stretch reflex. (4)Negative stretch reflex: It is reflex relaxation of skeletal muscle in response to its shortening secondary to reduction of discharge from muscle spindles. (5)Cerebellar stretch reflex: Such reflex is involved specially during carrying heavy loads. Impulses from muscle spindles are transmitted to cerebellum via spinocerebellar tracts.  Static stretch reflex (muscle tone) Definition: It is sustained mild (partial) contraction of skeletal muscles in response to maintained steady stretch of muscles during rest. It is a spinal stretch reflex. Pathway of static stretch reflex: The stimulus: Maintained stretch of skeletal muscles because muscles are in continuous degree of stretch between their origin and insertion because muscles are shorter than bones. Receptors: They are nuclear chain intrafusal muscle fibers of muscle spindle (very slowly adapting receptors). Afferent:  They are type Ia & II sensory fibers (primary & secondary endings). Center: AHCs ( motor neurons) of spinal cord. Efferent: A α efferent fibers (axons of α motor neurons) Effector organs: Extrafusal muscle fibers of all skeletal muscles (especially antigravity muscles). Result (response): It is a weak subtetanic (mild) contraction occurs during resting condition which is more marked in antigravity muscles. Reflex is potentiated by γ static efferent neurons which supplies contractile part of nuclear chain intrafusal fibers causing stretch to their central part. Functions of stretch reflex: 1. It maintains posture against withdrawal force of gravity. 2. It shares in maintaining body temperature  muscle tone increases heat production. 3. It helps venous return and lymph flow especially from the lower limbs. 4. It keeps and supports abdominal viscera inside abdomen and prevents visceral ptosis. 6.Increasing the power of muscle contraction (Load reflex) or (servoassistant function):  The stretch reflex assists the brain to produce and regulate the force of muscle contraction in the following way:  When muscle contracts against a load (e.g. during lifting a heavy load)  the load stretches the muscleso eliciting a stretch reflex.  However, this does not only activate alpha motor neurons but also, gamma motor neurons via alpha–gamma linkage which cause contraction of intrafusal fibers that leads to  an additional stretch reflex, via gamma spindle loop, that potentiates the initial one, resulting in powerful contraction of extrafusal fibers. 7.Damping (smoothing) function: If a muscle is activated by signals of irregular intensities, the stretch reflex acts to produce a smooth contraction as follows: A weak signal produces weak direct contraction so, less shortening of the muscle  so, the stretch reflex will be more active  leads to strong reflex contraction. The resultant total contraction will be the sum of the weak direct contraction and the strong reflex contraction. A strong signal produces strong direct contraction  so, more shortening of the muscle so, the stretch reflex will be less active  leads to weak reflex contraction. The resultant total contraction will be the sum of the strong direct contraction and the weak reflex contraction  In this way, different signal intensities produce equal average contractions so, this function of stretch reflex is called (signal averaging function) If muscle spindle afferent nerve is injured we get unsmooth jerky muscle contraction with oscillations  (II) Dynamic stretch reflex (tendon jerk) Definition: It is rapid short contraction of skeletal muscle followed by rapid relaxation in response to sudden stretch of that muscle by tapping its tendon sharply by a tendon hammer. It is a spinal stretch reflex. Pathway of dynamic stretch reflex: The stimulus: It is sudden stretch to muscle by trapping on its tendon.  Receptors: They are nuclear bag intrafusal muscle fibers of muscle spindle (rapidly adapting receptors)  Afferent: They are the primary endings (type Ia sensory nerve fibers). Center:  AHCs ( motor neurons) of the spinal cord. Efferent:  Type A α efferent fibers (axons of α motor neurons)  Effector organs: Extrafusal muscle fibers of stretched skeletal muscle.  Result (response): Rapid contraction followed by rapid relaxation of stretched muscle. Reflex is potentiated by γ dynamic efferent neurons which supplies contractile part of nuclear bag intrafusal muscle fibers causing stretch to their central part. Significance of dynamic stretch reflex (tendon jerk): Testing tendon jerks are fundamental part of neurological examination as following: 1. It helps to localize site of the lesion in nervous system. 2. It helps in diagnosis of nervous diseases: a) Complete absence of tendon jerk indicates a lesion in their reflex arc e.g. Tabes dorsalis & lower motor neuron lesion. a) Decrease tendon jerk occurs in many diseases as chorea & neocerebellar syndrome. b) Exaggerated tendon jerks occur in many diseases as upper motor neuron lesion and tetany due to increase neuromuscular excitability.  Differences between static and dynamic stretch reflexes: Items of differences Static stretch reflex Dynamic stretch reflex 1. Stimulus Maintained stretch (by Sudden stretch (by tapping gravity) on tendons) 1. Receptors Nuclear chain intrafusal Nuclear bag intrafusal fibers fibers 1. Adaptation of Very slowly adapting Rapidly adapting receptors receptors 1. Afferent Ia& II sensory fibers Type Ia fibers 1. Response Weak subtetanic maintained Brisk contraction followed contraction by rapid relaxation 1. Existence Exists normally in all Only elicited clinically by a muscles hammer  Examples of tendon jerks Knee jerk: Sudden tapping on patellar tendon while knee is semiflexed result incontraction of quadriceps femoris muscle & forward movement of leg. Its center is α motor neurons of AHCs of L 2, 3, 4 segments. Pendular knee jerk: This is a peculiar pattern of the knee jerk that occurs especially in the chorea & neocerebellar syndrome. These diseases are characterized by hyporeflexia and hypotonia so, the knee jerk is weaker than normal and during relaxation of the quadriceps muscle, the leg falls like a dead weight & swings for sometimes like a pendulum before resting  thus, it is called pendular knee jerk. Ankle jerk: Sudden tapping on tendo-achilles while foot is slightly dorsiflexed result in contraction of gastrocnemius and soleus muscle and planter flexion of the foot. Its center is α motor neurons of AHCs of S 1& 2 segments. Biceps jerk: Sudden tapping on examiner left thumb placed on biceps tendon while, forearm is midway between flexion and extension result in contraction of biceps muscle and flexion of elbow. Its center is α motor neurons of AHCs of C 5&6 segments. Triceps jerk: Tapping with sudden gentle strike on triceps muscle while, elbow is semiflexed leads to  contraction of triceps muscles & extension of elbow. Its center is α motor neurons of AHCs of C6 &7 segments. Reinforcement of the tendon jerk: The response of the tendon jerks can be reinforced (i.e. increased) by facilitating the spinal centers. In the lower limb jerks facilitation can be produced by asking the patient to grasp his hands together and exert maximal effort to pull them apart  This is called Jendrassik's maneuver. In the upper limb jerks  facilitation can be produced by asking the patient to clench his teeth or to squeeze his thigh by the opposite hand. These maneuvers act through: a) Sending signals from the contracted muscles which stimulate the gamma efferent nerves. b) Distracting the attention of the patient which prevents any voluntary inhibition of the reflex.  (III) Inverse stretch reflex Definition: It is reflex relaxation of skeletal muscle in response to its overstretch through activity of Golgi tendon organs. The stimulus: Stimulus is excessive stretch to muscle high tension in its tendon. Receptors: They are Golgi tendon organs found in tendons of muscles. Result (response): Relaxation of excessively stretched muscle. So, this is protective reflex prevent tearing of overstretched muscles.  Supraspinal centers affecting stretch reflex (1) Supraspinal facilitatory centers: (a)Motor area 4 of the cerebral cortex: It sends collateral to facilitatory reticular formation. (b)Vestibular nucleus: It sends collateral to facilitatory reticular formation. (c) Basal ganglia (caudate nucleus):  It sends impulses to facilitatory reticular formation & to vestibular nucleus. (d) Neocerebellum: It sends impulses to facilitatory reticular formation. (e) Facilitatory reticular formation: It is present in pons. (2) Supraspinal inhibitory centers: (a)Certain cortical areas especially (premotor area 6): It activates inhibitory reticular formation & also inhibits vestibular nucleus. (b)Red nucleus: It sends collateral to inhibitory reticular formation. (c) Basal ganglia (lentiform nucleus): It sends impulses to inhibitory reticular formation. (d)Paleocerebellum: It sends impulses to inhibitory reticular formation. (e) Inhibitory reticular formation: It is present in medulla oblongata