2022 Reflex arc Motor Functions of Spinal Cord (1).pptx

2022 Reflex arc Motor Functions of Spinal Cord

1. Motor Functions of Spinal Cord – Spinal Cord Reflexes DR. SHAZIA HASHMAT ASSOCIATE PROFESSOR PHYSIOLOGY

2. OVERVIEW--ORGANIZATION OF THE SPINAL CORD FOR MOTOR FUNCTIONS Sensory information is integrated at all levels of the nervous system and causes appropriate motor responses that begin in the spinal cord with relatively simple muscle reflexes, extend into the brain stem with more complicated responses, and finally extend to the cerebrum, where the most complicated muscle skills are controlled.

3. The spinal cord is responsible for the integration of many innate reflexes . The spinal cord is strategically located between the brain and the afferent and efferent fibers of the PNS; this location enables the spinal cord to fulfill : (1) serving as a link for transmission of information between the brain and rest of the body and (2) integrating reflex activity between afferent input and efferent output without involving the brain

4. Spinal Cord Gray Matter The centrally located gray matter is also functionally organized (❙Figure 5-27). The central canal, which is filled with CSF, lies in the center of the gray matter. Each half of the gray matter is arbitrarily divided into a dorsal (posterior) horn, a ventral (anterior) horn, and a lateral horn. The dorsal horn contains cell bodies of interneurons on which afferent neurons terminate. The ventral horn contains cell bodies of the efferent motor neurons supplying skeletal muscles.

5. The cord gray matter is the integrative area for the cord reflexes Sensory signals enter the cord almost entirely through the sensory roots, also known as the posterior or dorsal roots. After entering the cord, every sensory signal travels to two separate destinations: one branch of the sensory nerve terminates almost immediately in the gray matter of the cord and elicits local segmental cord reflexes and other local effects; another branch transmits signals to higher levels

6. Muscle Structure . Muscles function to produce force and  motion . maintaining and changing  posture ,  locomotion , movement of  internal organs , such as the contraction of the  heart  and the movement of food through the  digestive system  via  peristalsis .

7. Muscle receptors provide afferent information needed to control skeletal muscle activity. For effective control of motor output , the CNS needs continual information regarding ongoing changes in muscle length and tension . Two types of muscle proprioceptors — muscle spindles and Golgi tendon organs —provide this input. Muscle length is monitored by muscle spindles; changes in muscle tension are detected by Golgi tendon organs. Both these receptor types are activated by muscle stretch

8. Motor Unit    Each somatic neuron together with all the muscle fibers it innervates. Each muscle fiber receives a single axon terminal from a somatic neuron. Each axon can have collateral branches to innervate an equal # of fibers.

9. The specialized muscle fibers of the spindle are called INTRAFUSAL (distinguish from skeletal muscle fibers- extrafusal ) Intrafusal fibers do not contribute to muscle contraction The central parts of the intrafusal fibers are essentially no contractile ; ONLY THE POLAR REGIONS ARE ACTIVELY CONTRACT Serve as specialized sensory organs (proprioceptors) They detect the amount and rate of change in length of a muscle. They constitute the muscle spindle and are innervated by both sensory (afferent) and motor (efferent) fibers.

10. Short and slender Thicker in diameter There are two types dynamic and static 5:1 ratio

11. Nervous control of Muscle Contraction- Gamma motor neurons A gamma motor neuron is a type of lower motor neuron that takes part in the process of muscle contraction, and represents about 30% of ( A delta ) fibers going to the muscle. gamma motor neurons do not directly adjust the lengthening or shortening of muscles. their role is important in keeping muscle spindles taut, thereby allowing the continued firing of alpha neurons, leading to muscle contraction. play a role in adjusting the sensitivity of muscle spindles Like alpha motor neurons, their cell bodies are located in the anterior grey column of the spinal cord. They receive input from the reticular formation of the pons in the brainstem. gamma -Motor neurones innervate the striated portions of the intrafusal fibres . Therefore, they are also called fusimotor neurones

12. Brain Areas for Control of the Gamma Motor System The gamma efferent system is excited specifically by signals from the bulboreticular facilitatory region of the brain stem and, secondarily, by impulses transmitted into the bulboreticular area

13. The Golgi tendon organ is a proprioceptor, sense organ that receives information from the tendon, that senses TENSION. When you lift weights, the Golgi tendon organ is the sense organ that tells you how much tension the muscle is exerting. If there is too much muscle tension the Golgi tendon organ will inhibit the muscle from creating any force (via a reflex arc), thus protecting the you from injuring itself.

14. Renshaw Cells Transmit Inhibitory Signals to Surrounding Motor Neurons located in the anterior horns of the spinal cord, in close association with the motor neurons, are a large number of small neurons called Renshaw cells Renshaw cells are inhibitory cells that transmit inhibitory signals to the surrounding motor neurons. stimulation of each motor neuron tends to inhibit adjacent motor neurons, an effect called lateral inhibition to focus, or sharpen, its signals in the same way that the sensory system uses the same principle to allow unabated transmission of the primary signal in the desired direction

15. Multisegmental Connections From One Spinal Cord Level to Other Levels— Propriospinal Fibers. More than half of all the nerve fibers that ascend and descend in the spinal cord are propriospinal fibers. These fibers run from one segment of the cord to another. In addition, as the sensory fibers enter the cord from the posterior cord roots, they bifurcate and branch both up and down the spinal cord; some of the branches transmit signals to only a segment or two, whereas others transmit signals to many segments multisegmental reflexes

16. Reflex Arc Reflex Arc ,,,A reflex is any response that occurs automatically without conscious effort. The neural pathway involved in accomplishing reflex activity is known as a reflex arc, which typically includes five basic components: 1. Sensory receptor 2. Afferent pathway 3. Integrating center 4. Efferent pathway 5. Effector organ

17. Classifying Reflexes Figure 13.17

18. Reflex Categories : 1. Spinal Reflexes Spinal reflexes are integrated by the spinal cord, an example of which is the withdrawal reflex, such as automatically withdrawing your hand from a hot object 2. Cranial Reflex : the reflex itself is subconsciously integrated by the brain at levels lower than the cortex, such as by the brain stem or hypothalamus. E. G Constriction of pupils due to bright light As innate or conditioned reflexes , depending on whether the reflex is inborn or learned. Innate, built-in, unlearned responses. The withdrawal reflex, the micturition reflex, and the pupillary constriction reflex are all examples of innate reflexes . Conditioned (or acquired) reflexes are a result of learning, such as increased secretion of saliva on smelling a favorite food being prepared. Increased salivation occurs via the innate salivary reflex (a cranial reflex) on tasting a favorite food, As somatic or autonomic . Output in a somatic reflex is transmitted by motor neurons to skeletal muscles, an example being the withdrawal reflex. Output in an autonomic (or visceral) reflex is to smooth muscle , cardiac ms and glands .Baroreceptor reflex ..

19. As monosynaptic or polysynaptic , depending on how many synapses are in the reflex arc The stretch reflex is a monosynaptic (“one synapse”) reflex because the only synapse in the reflex arc is the one between the afferent neuron and the efferent neuron Reflexes having more than one synapse in the reflex arc are called polysynaptic reflexes . Flexor reflexes ( withdrawal reflexes ) are the polysynaptic reflexes

20. Monosynaptic Reflex The knee jerk can be elicited by simply striking the patellar tendon lightly with a reflex hammer; this action instantaneously stretches the quadriceps muscle and excites a dynamic stretch reflex that causes the lower leg to “jerk” forward

21. Polysynaptic Reflex When a person receives any painful stimulus, a withdrawal reflex is initiated to withdraw from painful stimulus.

22. Withdrawal reflex Afferent neuron stimulates excitatory interneurons that stimulate the efferent motor neurons supplying the muscle, that flexes and pulls away from painful stimulus. The afferent neuron also stimulates inhibitory interneurons that inhibit the efferent neurons supplying the antagonistic muscle to prevent it from contracting.

23. MUSCLE STRETCH REFLEX

24. Muscle stretch reflex skeletal muscle fibers of stretched muscle and also of closely allied muscles. Is simplest manifestation of muscle spindle function Is monosynaptic. When muscle is stretched suddenly, excitation of spindles causes reflex contraction In which an afferent neuron originating at a stretch detecting receptor in a skeletal muscle terminates directly on the efferent neuron supplying the same skeletal muscle to cause it to contract and counteract the stretch. “

25. Dynamic Stretch Reflex and Static Stretch Reflexes The dynamic stretch reflex is elicited by potent dynamic signals transmitted from the primary sensory endings of the muscle spindles, caused y stretch /unstretch over within a fraction of a second after the muscle has been stretched (or unstretched) to its new length, Reflex opposes the sudden changes in length a weaker static stretch reflex continues for a prolonged period thereafter. This reflex is elicited by the continuous static receptor signals transmitted by both primary and secondary endings. The importance of the static stretch reflex is that it causes the degree of muscle contraction to remain reasonably constant “ Damping” Function of the Dynamic and Static Stretch Reflexes in Smoothing Muscle Contraction

26. Knee Jerk and Other Muscle Jerks Can Be Used to Assess Sensitivity of Stretch Reflexes. The knee jerk can be elicited by simply striking the patellar tendon lightly with a reflex hammer; this action instantaneously stretches the quadriceps muscle and excites a dynamic stretch reflex that causes the lower leg to “jerk” forward

27. FLEXOR REFLEX AND THE WITHDRAWAL REFLEXES withdrawing the limb from the stimulating object. This reflex is called the flexor reflex. the flexor reflex is elicited most powerfully by stimulation of pain endings, such as by a pinprick, heat, or a wound, for which reason it is also called a nociceptive reflex, or simply a pain reflex. Stimulation of touch receptors can also elicit a weaker and less prolonged flexor reflex

28. Neuronal Mechanism of the Flexor Reflex. The pathways for eliciting the flexor reflex do not pass directly to the anterior motor neurons but instead pass first into the spinal cord interneuron pool of neurons and only secondarily to the motor neurons

29. Crossed extensor reflex Reflex arc initiate to withdraw injured limb from painful stimulus, while opposite limb prepares to suddenly bear all the weight so that the person does not lose balance or fall. Bending of injured extremity’s knee is accomplished by concurrent reflex stimulation of muscles that flex the knee and inhibition of muscles that extend the knee. Extension of opposite limb’s knee is accomplished by activation of pathways that cross over to the opposite side of the spinal cord to reflexly stimulate extensors and inhibit flexors.

30. CROSSED EXTENSOR REFLEX About 0.2 to 0.5 second after a stimulus elicits a flexor reflex in one limb, the opposite limb begins to extend. This reflex is called the crossed extensor reflex. Extension of the opposite limb can push the entire body away from the object that is causing the painful stimulus in the withdrawn limb

31. Neuronal Mechanism of the Crossed Extensor Reflex signals from sensory nerves cross to the opposite side of the cord to excite extensor muscles. After 200 to 500 milliseconds after onset of the initial pain stimulus, it is certain that many interneurons are involved in the circuit between the incoming sensory neuron and the motor neurons of the opposite side of the cord responsible for the crossed extension Delayed after discharge -

32. Reciprocal inhibition The type of connection involving stimulation of nerve supply to one muscle and simultaneous inhibition of the nerves to its antagonistic muscle is known as reciprocal innervation .