SOMATIC REFLEXES

Explain how the crossed extensor reflex functions

Crossed extensor reflex Flexor withdrawal reflexes are only useful if you can still maintain balance which is maintained by extending the other leg. Injured leg - flexors (hamstring) contract and the extensors (quads) relax to lift the leg from the ground. Supporting leg - flexors relax and extensors contract to stiffen the leg to it can support weight of the body. The crossed extensor reflex is a polysynaptic reflex arc which involves interneurons in the arc.

List and describe the general components of a typical reflex arc

A somatic reflex employs a reflex arc in which signals travel along the following pathways: 1.Sensory (somatic) receptor (skin, muscle and tendons) - distal end of a sensory neuron (dendrite). 2.Sensory neuron (afferent nerve fibres) - carries information from receptor to posterior horn of spinal cord or to the brainstem. 3.Integrating centre (interneuron) - Point of synaptic contact between neurons in grey matter of the spinal cord or brainstem. 4.Motor neuron (efferent nerve fibres) - carries motor impulses from spinal cord to the skeletal muscles. 5.Effector (skeletal muscle) - the muscles that carry out the reflexive response. In somatic reflexes the effector is always skeletal muscle.

Exemplify the combined functioning of the flexor withdrawal and crossed extensor reflex by using the example of stepping on glass with the right foot

Example of flexor withdrawal and crossed extensor reflex. •Stepping on glass with the right foot stimulates dendrites (pain receptors) of a pain-sensitive neuron. •The axon then travels to spinal cord through dorsal side (dorsal root then dorsal horn) and synapses on multiple interneurons. •On the ipsilateral side of the cord an excitatory interneuron stimulates a motoneuron which travels through ventral side (ventral horn then ventral root) to stimulate the flexor muscles (hamstrings) which flex the knee joint and lift the leg away from the painful stimulus. •Also on the ipsilateral side of the cord an inhibitory interneuron inhibits the alpha motoneuron to the ipsilateral extensor muscles (quadriceps) thus allowing knee joint to flex. •Interneurons synapsing on the incoming sensory input also cross-over to opposite side of grey matter. •Here on the contralateral side of the cord inhibitory interneurons synapse on an alpha motoneuron which inhibits activity in the flexor muscles (hamstrings) of the contralateral leg. •Also on the contralateral side of the cord an excitatory interneuron excites an alpha motoneuron which travels out ventral horn and ventral root to activate extensor muscles (quadriceps) on contralateral side which extends the knee joint of the left leg and allows weight to be born on that leg. •Note that reflex arcs often include reciprocal innervation to stimulate a flexor muscle and inhibit the corresponding extensor (or vice versa). Withdrawal reflexes involve regulation of both ipsilateral and contralateral muscles.

Explain how the flexor withdrawal reflex functions

Flexor (withdrawal) reflex This reflex is to protect from damage in response to a painful stimulus. The flexor reflex is a polysynaptic reflex arc (signals travel over many synapses on their way back to the muscle). Which involves interneurons in the arc. The flexor reflex is ipsilateral as the stimulus (stepping on glass) and response (muscle contraction) are on the same side of the body. Muscle spindle apparatus and golgi tendon organ are not involved.

Explain how the Golgi tendon reflex functions

Golgi tendon reflex: Monitors tension in the tendons produced by muscle contraction. Sensory neurons activated by these receptors input onto inhibitory interneurons that inhibit alpha motoneurons. Prevents excessive muscle contractions. Also functions when muscle contracts unevenly Has the opposite action on the agonist muscle (inhibition) when compared to a stretch reflex (excitation). Tendon organs are proprioceptors (tendon organs are sense organs specialised to monitor position and movement of body parts) in a tendon near its junction with a muscle. The golgi tendon organ is 1mm long, with nerve fibres entwined in collagen fibres of the tendon. Tendon reflex - is in response to excessive tension on the tendon. It inhibits agonist muscle from excessive tension. Moderates muscle contraction before it tears a tendon or pulls it loose from the muscle or bone. Summary of stretch reflex and golgi tendon reflex actions: •Skeletal muscles send sensory information to CNS about muscle length, from spindle apparatus and about muscle tension, from golgi tendon organ. •Stretch of muscle spindle activates stretch reflex arc to correct muscle tone and prevent over stretching. •Activation of golgi organ inhibits alpha motor neurons to relax muscle and prevent over contraction.

Describe the structure and explain the function of muscle spindles

Muscle fibres - skeletal muscles are composed of individual muscle fibres that contract when stimulated by a motor neuron. These may be referred to as extrafusal fibres. There are also other fibres found between extrafusal fibres. These extrafusal fibres form the muscle spindle apparatus. Muscle Spindle - Stress receptors embedded in skeletal muscles that monitors length of muscle and how fast muscles change in length (. They are composed of •Intrafusal muscle fibres (nuclear bag & nuclear chain) which are modified muscle cells that have sarcomeres and contractile ability but only at their two ends. •Innervated by sensory neurons - primary afferent fibres (large fast nerve fibre) that inform brain on muscle length and speed of muscle length change and secondary afferent fibres (slower speed) that inform brain on muscle length. •Gamma motor neurons (slower speed) - keep spindle fibres at good length for responding to stretch and motoneurons elsewhere in body called alpha motoneurons.

Explain and illustrate how the stretch reflex functions by using patellar tendon reflex as an example

The stretch reflex - When a muscle is stretched, it automatically contracts and maintains increased tone. It helps maintain equilibrium and posture. It stabilises joints by balancing tension in extensors and flexors smoothing muscle actions. For example - the head starts to tip forward as you fall asleep, muscles contract to raise the head. For example - knee jerk (patellar tendon reflex). The reflexive contraction of a muscle when its tendon is tapped. Activation of spindles - Stretching muscles causes spindles to stretch. The frequency of impulses along sensory neuron is proportional to length of muscle (the longer is gets the more stimulus it has) Activation of sensory endings in muscle spindles produces a reflex contraction. Stretch and activation of spindles elicits a reflex contraction which keeps normal muscle length and tension (muscle tone). Monosynaptic reflex - Sensory neuron synapses directly onto motor neuron in the ventral horn. Knee-jerk reflex (monosynaptic stretch reflex) - Only one synapse between afferent and efferent neuron there fore little synaptic delay and prompt response. Reciprocal inhibition, prevents muscles from working against each other. Agonist muscle contracted (excitation) and antagonist muscle relaxed. For example: Knee jerk - hamstring muscles remain relaxed due to motor neurons being inhibited to allow quadriceps to extend the knee.

Define a reflex and explain how reflexes differ from voluntary movement

•Fast, voluntary and predictable (stereotyped) sequence of actions by glands or muscles that occurs in response to a particular stimulus. •When integration takes place in the spinal cord grey matter, the reflex is a spinal reflex, such as the patellar tendon reflex. There is no involvement by the brain, only lower motor neurons. •When integration occurs in the brain stem, the reflex is a cranial reflex and involves the cranial nerves, such as eye tracking movements. •If the reflex involves the contraction of skeletal muscle as the effector it is called a somatic reflex. •If the reflex involves the contraction of smooth muscle, cardiac muscle or glands, it is called an autonomic (visceral) reflex. •Voluntary movement is under our control. It can be slow or fast. It uses higher and lower motor neurons, is variable and not stereotyped.