The Reflex

Describe what is happening where…..

Learning Outcomes To describe reflex actions include knee jerk reflex and blinking reflex, Explain why and how reflex actions are associated with survival M0.1, M0.2, M1.1, M1.2, M1.3, M1.6 PAG11 Use an appropriate number of significant figures Find arithmetic means Recognise and make use of appropriate units in calculations Recognise and use expressions in decimal and standard form Construct and interpret frequency tables and diagrams, bar charts and histograms Select and use a statistical test -the chi squared test (χ2) to test the significance of the difference between observed and expected results -the Student’s t-test -the Spearman’s rank correlation coefficient. Understand the terms mean, median and mode

Proprioceptors and the Control of Movement Proprioceptors – specialized receptors found in tendons, muscles, and joints Provide sensory information about the state of muscle contraction, the position of limbs, and body posture and balance This feedback is provided primarily by afferent (sensory) input from two sensory receptors: tendon organs and muscle spindles

Golgi Tendon Organs (Tension Reflex)‏ Sensory receptors that terminate where tendons joint to muscle fibres Since they are aligned with muscle, any stretching of the muscle also stretches the GTO The job of GTO is to detect increased tension exerted on the tendon When a change in tension is detected an impulse is sent along afferent (sensory) neurons to the CNS The efferent (motor) neurons transmit an impulse causing the muscle to relax This prevents injury

Muscle Spindles Lie parallel to the muscle fibre Send constant signals to the spinal cord Help maintain muscle tension They are sensitive to changes in muscle length. Responds to changes in length by sending a message to the spinal cord

The Stretch Reflex Simplest spinal reflex Depends only on the single connection between primary afferent fibres and motor neurons of the same muscle i.e. Knee-jerk test

1. The receptor muscle senses the action of the hammer against the patella ligament through the muscle spindle's sensory neuron 2. The message is transmitted along the afferent (sensory) nerve axon to the spinal cord 3. The afferent neuron synapses with the efferent pathway (motor neuron) of the same muscle 4. An impulse is transmitted along the efferent pathway (motor neuron) to the muscle 5. The motor units contract (knee-jerk to accomodate additional stretch)‏

Reciprocal Inhibition During a reflex, the opposing muscle group is simultaneously stimulated In a knee-jerk reflex the quadriceps contract to extend the knee while the hamstrings are inhibited in a slightly delayed response (they do not flex)‏

Categorising reflexes Knee jerk reflex is a spinal reflex. The pathway consists of only two neurones: sensory and motor neurone. There is no relay or interneurone. This means that there is only ONE synapse – therefore it is a very fast response. This also means that because there is no relay neurone – the brain cannot inhibit the reflex - (the inhibition would have to be provided by inhibitory synapses before the motor neurone is depolarised). The pain withdrawal reflex is a multisynaptic spinal reflex

Categorising reflexes The blink reflex is a cranial reflex. It is a reflex arc because the receptor and effector are in the same place. The corneal reflex – stimulated by touching the cornea – has two synapses and so can be overridden by inhibitory signals from the cerebral cortex.

Questions Explain why higher reasoning or conscious thought are not necessary for reflex behaviours. Distinguish between a spinal reflex and a cranial reflex and give an example of each. Describe the survival value of the following reflexes: Knee-jerk Corneal blink reflex Grasp reflex Pupillary light reflex

Homework Compare Coordination by nerves and hormones Use the following headings Communication Speed Duration Target pathway Action