Spinal Reflexes What Is a Reflex? Neural reflexes are stereotyped, automatic, involuntary reactions of the Central Nervous System (CNS) to a specific sensory input. They are different to: - voluntary, goal directed movements, i.e. learned experiences (such as reaching for a glass). - rhythmic motor patterns (i.e. walking). What Are the Functions of Reflexes? Reflexes produce a rapid characteristic response to a stimulus. These responses can be: 1) protective – e.g. withdrawal of limb from hot surface or a cough reflex. 2) Important in postural control - so you don't fall over whenever you move a limb. Classification of reflexes Reflexes can be categorised into one of the following: Somatic reflexes: involve somatic motor neurones and skeletal muscles. Autonomic reflexes: the responses are controlled by autonomic neurones. Further classification of reflexes Reflexes can be further classified by considering: the CNS system where the reflex is integrated (spinal cord: spinal reflexes; brain: cranial reflexes) number of neurones in the pathway (N.B: All autonomic reflexes are polysynaptic, whereas somatic reflexes can be either monosynaptic or polysynaptic). Somatic Reflexes - Somatic motor neurones supply skeletal muscles. - Skeletal muscle reflexes are involved in almost everything we do. -The components of skeletal muscle reflexes form the Reflex arc. - The reflex arc is the most basic unit of integrated activity in the nervous system. There are 5 components to the reflex arc: (1) RECEPTOR (2) SENSORY AFFERENT (3) CENTRAL NEURONES (4) MOTOR (5) EFFERENT EFFECTOR 1) Receptors - detect stimuli which include light, sound, smell, pain muscle tension. - act as signal transducers, i.e. convert stimulus into electrical signals. Location: - on the surface of the skin e.g. nociceptors, which detect painful stimuli or thermoreceptors, which detect changes in temperature - within the muscle: receptors found in skeletal muscle include: a) Muscle spindles: detect changes in muscle length b) Golgi tendon organs: detect changes in muscle tension 2) Sensory Afferents Sensory afferents carry information from the receptors to the CNS. Various types of sensory afferents include: Group 1a Group 1b 3) Central Neurones Sensory afferent fibers enter the spinal cord and synapse onto either: - Interneurones: e.g. Ia and Ib interneurones; or - Cell body (soma) of efferent motor neurons : Sensory (afferent) input Motor (efferent) output ‘Lower Motor Neuron’ / Alpha Motor Neuron 4) Motor Efferent Efferent motor neurons are of various types: * Alpha – extrafusal (bulk of muscle) or * Gamma- intrafusal fibres of muscle spindles. 5) Effector Effectors convert electrical signals from efferent signals into an appropriate response. - i.e. smooth muscle, skeletal muscle. Examples of skeletal muscle reflexes Monosynaptic: stretch reflex Polysynaptic: Golgi tendon reflexes, crossed-extensor reflex a) The Muscle Spindle Detects muscle length changes. Specialised skeletal muscle fibres contained in a fibrous capsule. Group 1a sensory afferents wrap around the swollen middle of the capsule. Role of the Muscle spindle Any movement that increases muscle length also stretches the muscle spindle and causes its sensory fibers to fire rapidly, e.g placing a load on a hand stretches the muscle and the spindles. This creates a reflex contraction of the muscles, which prevents the muscle from over-stretching. (i.e. arm position is restored) This pathway is known as the stretch reflex. The Stretch Reflex Motor neuron Sensory afferent Alpha motor neuron Stretch sensory activity Motor activity Contraction b) The Golgi Tendon Organ Located at the junction of muscle and tendon. Acts like a strain gauge. Monitors tension. Innervated by 1b sensory axons. Golgi Tendon Reflexes The Golgi tendon reflex protects the arm from excessively heavy loads by causing the muscle to relax and drop the load. - prevents damage due to overwork. - Slows muscle contraction. Golgi Tendon Organ (Muscle Tension) muscle contraction muscle and tendon tension GTO activation sensory neuron activity inhibitory neuron activity motor activity Golgi tendon organ Sensory Inhibitory neuron interneuron Motor neuron Reciprocal Inhibition In addition to the excitation of the effector muscle during the stretch reflex, the antagonist muscle is inhibited at the same time. This is achieved by Reciprocal Inhibition caused by activation of the Ia inhibitory interneuron. Reciprocal Inhibition of Flexors and Extensors Complex Stretch Reflexes Polysynaptic reflexes: slower. Involves interneurons: inhibitory, excitatory, Renshaw cells. Receptor and effector may be in different parts of the body. For example, the Flexion (Withdrawal) reflexes are poly- synaptic reflexes that cause an arm or leg to move away from a painful stimulus e.g. pinprick or a hot stove. Flexion Reflex Sensory fibers carry information from the nociceptors to the spinal Cord, where they branch. These branches activate multiple excitatory interneurones at different levels: - Some of these interneurones excite alpha motor neurons leading to contraction of the flexor muscles. - Other interneurones simultaneously activate inhibitory interneurones that cause relaxation of the antagonistic muscle groups (extensors). So the limb is flexed, withdrawing it from the stimulus. Crossed-Extensor Reflex The quick withdrawal of the right foot from a painful stimulus is matched by extension of the left leg so that it can support the sudden shift in weight. This is brought about by the action of the Crossed- extensor Reflex: a postural reflex that helps maintain balance when one foot is lifted off the ground. The Crossed Extensor Reflex The activation of flexors and the inhibition of extensors on one side of the body is accompanied by the inhibition of flexors and activation of extensors on the other side. Involves sensory input to multiple excitatory and inhibitory interneurones. Central Pattern Generators The most complicated spinal reflex pathways are controlled by networks of neurones in the CNS called Central pattern generators (CPGs). Once activated, CPGs create spontaneous repetitive movement. In humans, rhythmic movements controlled by CPGs include locomotion and the unconscious rhythmicity of quiet breathing. Higher Centres and the Control of Movement Motor Cortex Planning and coordinating complex movements. Brain Stem Posture, hand and eye movements. Basal Ganglia Motor planning. The cerebellum Adjustment of fine movements.
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