Integrative Physiology I: Control of Body Movement 13

Neural Reflexes

Somatic Motor Reflexes Monosynaptic and polysynaptic somatic motor reflexes Figure 13-1a

Autonomic Reflexes Some visceral reflexes are spinal reflexes Stimulus Response Postganglionic autonomic neuron Preganglionic CNS integrating center Sensory Receptor Target cell Autonomic ganglion All autonomic reflexes are polysynaptic, with at least one synapse in the CNS and another in the autonomic ganglion. Figure 13-2

Skeletal Muscle Reflexes Proprioceptors are located in skeletal muscle, joint capsules, and ligaments Proprioceptors carry input sensory neurons to CNS CNS integrates input signal Somatic motor neurons carry output signal Alpha motor neurons Effectors are contractile skeletal muscle fibers Examples of proprioceptors Muscle spindle Golgi tendon organ Joint receptors Are found in capsules and ligaments around joints

Proprioceptors Muscle spindles are sensory receptors in muscle that control muscle tone and prevent injury from overstretching of the muscle. They are found in all muscles and are tonically active, firing increases as the muscle stretches. Figure 13-3a–b

Muscle Spindles Muscle spindles monitor muscle length and prevent overstretching of the same muscle. The tonic signaling produces muscle tone. Sensory neuron endings Intrafusal fibers of muscle spindle (a) Extrafusal muscle fibers at resting length Sensory neuron is tonically active. Spinal cord integrates function. Alpha motor neurons to extrafusal fibers receive tonic input from muscle spindles. Extrafusal fibers maintain a certain level of tension even at rest. Spinal cord Alpha motor 1 2 4 5 3 Figure 13-4a

Muscle Spindles During a stretch reflex increased firing by the sensory neuron increases signaling by the alpha motor neuron causing the muscle to contract. Figure 13-4b

Alpha-Gamma Coactivation*** Gamma motor neurons intervate the ends of intrafusal fibers in muscle spindles and keep the sensory neuron active even when the muscle contracts Alpha motor neuron fires. Stretch on center of intrafusal fibers is reduced. Firing rate of spindle sensory neuron decreases. Muscle contracts. (a) If gamma motor axons are cut, the spindle loses activity when muscle contracts. Muscle shortens Action potential Time Less stretch on intrafusal fibers Muscle length Action potentials of spindle sensory neuron 1 2 3 4 Figure 13-5a

Alpha-Gamma Coactivation When an alpha motor neuron fires the muscle contracts and shortens but the gamma motor neuron will also fire and thus there will be stretching at the spindle fiber to keep the tonic firing. Alpha motor neuron fires and gamma motor neuron fires. Muscle contracts. Stretch on centers of intrafusal fibers unchanged. Firing rate of afferent neuron remains constant. (b) Alpha-gamma coactivation maintains spindle function when muscle contracts. Muscle length Muscle shortens Intrafusal fibers do not slacken, so firing rate remains constant. Action potentials of spindle sensory neuron Time 1 2 3 Figure 13-5b, steps 1–3

Proprioceptors Golgi tendon organs are sensory receptors in muscle that respond to tension changes in the muscle and attempt to prevent injury from excessively strong contractions. When golgi sensory neuron fibers the efferent signal in inhibitory and thus there is a loss in contraction strength Figure 13-3a, c

Muscle Reflexes: response to load and overload

Patellar Tendon (Knee Jerk) Reflex Stimulus: Tap to tendon stretches muscle. Receptor: Muscle spindle stretches and fires. Afferent path: Action potential travels through sensory neuron. The patellar tendon (knee jerk) reflex illustrates a monosynaptic stretch reflex and reciprocal inhibition of the antagonistic muscle. Figure 13-7

Patellar Tendon (Knee Jerk) Reflex Stimulus: Tap to tendon stretches muscle. Receptor: Muscle spindle stretches and fires. Afferent path: Action potential travels through sensory neuron. Integrating center: Sensory neuron synapses in spinal cord. Efferent path 2: Interneuron inhibiting somatic motor neuron Efferent path 1: Somatic motor neuron The patellar tendon (knee jerk) reflex illustrates a monosynaptic stretch reflex and reciprocal inhibition of the antagonistic muscle. onto Figure 13-7

Patellar Tendon (Knee Jerk) Reflex Stimulus: Tap to tendon stretches muscle. Receptor: Muscle spindle stretches and fires. Afferent path: Action potential travels through sensory neuron. Integrating center: Sensory neuron synapses in spinal cord. Efferent path 2: Interneuron inhibiting somatic motor neuron Effector 2: Hamstring muscle Response: Hamstring stays relaxed, allowing extension of leg (reciprocal inhibition). Efferent path 1: Somatic motor neuron Effector 1: Quadriceps Response: Quadriceps contracts, swinging lower leg forward. The patellar tendon (knee jerk) reflex illustrates a monosynaptic stretch reflex and reciprocal inhibition of the antagonistic muscle. onto Figure 13-7

Flexion Reflex and the Crossed Extensor Reflex White matter Gray Spinal cord Ascending pathways to brain Painful stimulus activates nociceptor. Primary sensory neuron enters spinal cord and diverges. One collateral activates ascending pathways for sensation (pain) and postural adjustment (shift in center of gravity). Sensory neuron Nociceptor Painful stimulus Spinal cord 1 2 3a Figure 13-8, steps 1–3a

Flexion Reflex and the Crossed Extensor Reflex White matter Gray Spinal cord Ascending pathways to brain Alpha motor neurons Painful stimulus activates nociceptor. Primary sensory neuron enters spinal cord and diverges. One collateral activates ascending pathways for sensation (pain) and postural adjustment (shift in center of gravity). Withdrawal reflex pulls foot away from painful stimulus. Sensory neuron Nociceptor Painful stimulus Spinal cord 1 2 3a 3b Extensors inhibited Flexors contract, moving foot away from painful stimulus. Figure 13-8, steps 1–3b

Flexion Reflex and the Crossed Extensor Reflex White matter Gray Spinal cord Ascending pathways to brain Alpha motor neurons Extensors inhibited Flexors inhibited Flexors contract, moving foot away from painful stimulus. Extensors contract as weight shifts to left leg. Painful stimulus activates nociceptor. Primary sensory neuron enters spinal cord and diverges. One collateral activates ascending pathways for sensation (pain) and postural adjustment (shift in center of gravity). Withdrawal reflex pulls foot away from painful stimulus. Crossed extensor reflex supports body as weight shifts away from painful stimulus. Sensory neuron Nociceptor Painful stimulus Spinal cord 1 2 3a 3b 3c Figure 13-8, steps 1–3c

Movement Classification Three types of movement: Reflex (simplest) voluntary (most complex) Rhythmic (a combination of relex and voluntary).

CNS Integrates Movement Spinal cord integrates spinal reflexes and contains central pattern generators Brain stem and cerebellum control postural reflexes and hand and eye movements Cerebral cortex and basal ganglia Voluntary movement- can become reflexive once well learned Rhythmic movement is initiated at the cerebrum but maintained by interneurons in the spinal cord

Integration of Muscle Reflexes Reflexes are managed by the spinal cord, cerebellum, and brain stem. They do not require input from the cerebrum. However, sensory input is send to the cerebrum so we aware of what happens. Figure 13-9

CNS Control of Voluntary Movement Voluntary movement can be planned based on postural reflex information. There are 3 phases, sensory feed back is used in the first two. Figure 13-10

Voluntary Movement Feedforward reflexes and feedback of information during movement Figure 13-13

Visceral Movement Contraction of cardiac and smooth muscle Moves material in hollow organs by changing the shape of the organ Controlled by ANS as a reflex Some create own action potentials Muscle can respond to hormones or signaling from neighboring cells through gap junction