Central Neural IntegrationReflexes involve at least one sensory (afferent) neuron and one motor (efferent) neuron. Such a simple reflex is monosynaptic, but most reflexes involve many neurons and are polysynaptic. See Fig. 13-1 – next slide. Besides the number of neurons involved in the reflex, reflexes are classified by: - whether the reflex is inborn or learned (as conditioned reflexes).- division of the nervous system (as somatic reflexes or autonomic reflexes)-area of the CNS integrating the reflex, (as spinal reflexes or hypothalamic reflexes). Autonomic reflexes that are integrated in the hypothalamus, brainstem or spinal cord include cardiovascular and temperature reflexes, and the brainstem reflexes - salivating, vomiting, sneezing coughing, swallowing, and gagging

Although the motor cortex of the brain may sometimes voluntarily initiate the motor component of a reflex, the reflex is automatic (involuntary) once it has been initiated. For example, you can decide to swallow even if there is no stimulus in the back of your mouth to cause swallowing; however, once swallowing has begun, unconscious pathways control the process. Many reflexes are entirely involuntary and unconscious.

The spindle reflex and the Golgi tendon reflex are spinal reflexes that are important for controlling skeletal muscle length and tension. You already know that ventral horn motor neurons (sometimes called alpha () motor neurons) control muscle contraction.

The muscle spindle and the Golgi tendon organ are stretch receptors.

The muscle spindle and the Golgi tendon organ are stretch receptors. The muscle spindle is buried deep within the muscle and has intrafusal fibers that extend at each end – see Fig. 13-3 a/b. In discussions of the spindle, the normal muscle fibers are often referred to as extrafusal fibers to distinguish them from the intrafusal spindle fibers. Just as the muscle fibers are innervated by -motor neurons, the intrafusal spindle fibers are innervated by gamma () motor neurons. motor axon Stretch of the spindle sends a signal to

the spinal cord to stimulate the motor axon motor neuron to contract the muscle

Stretch of the spindle occurs:(1) tonically when the muscle is relaxed, and the tonic signals from the spindle stimulate the -motor neurons to contract the muscle slightly, which maintains muscle tone(2) during stretch of the muscle, which further stretches the spindle and sends higher frequency signals from the spindle, which stimulates the -motor neurons to contract the muscle to its resting length(3) Next slide

motor axon Stretch of the spindle sends a signal to the spinal cord to stimulate the

motor axon motor neuron to contract the muscle

(3) Stretch of the spindle also occurs during co-activation of and neurons. This maintains stretch of the spindle even when the muscle shortens.   Note how contraction of intrafusal fibers stretches the central core of the spindle.

The purpose of the spindle reflex is (1) to maintain the muscle’s resting length and tension (tone) (2) to prevent excessive stretch of the muscle

The Golgi tendon organ is a stretch receptor located at the junction of the muscle and the tendon

Contraction of the muscle indicated by the pink arrows below stimulates stretch of the tendon organ, which sends a signal to an inhibitory inter-neuron in the spinal cord that inhibits the motor neuron; which permits relaxation of the muscle.

muscle fiber

Golgitendonorgan

Golgi tendon reflex helps to protect the muscle from excessive contraction; for example from lifting excessive loads.

Drawthis reflex circuit.Note the importance of the inhibitory inter-neuron

Fig. 13-7 Patellar reflex – not need to know all details, but know how an inhibitory inter-neuron (green star) permits a signal from a sensory neuron to stimulate a neuron to one muscle and inhibit the neuron to the antagonistic muscle.

Fig. 13-8 Withdrawal reflex – Note when the flexor muscle in a limb is stimulated, the antagonistic extensor in that limb is inhibited to allow withdrawal, but the contralateral extensor (in the other leg) is stimulated and its antagonistic flexor is inhibited to extend the leg for support.

If an arm is withdrawn, the same reflexes occur in the two arms (as if a 4-legged animal). Draw a diagram showing withdrawal of one arm and extension of the other arm for support.

Rhythmic movements such as walking and running are controlled by both involuntary reflexes and voluntary input.

Voluntary signals initiate walking, but the reflex movement continues even if voluntary control is removed, as with the chicken that continues to run after its head is chopped off.

Central pattern generators are neurons in the CNS that control repetitive involuntary movements, reflexes or rhythms, which may be complicated.

Somatic Motor System Integration & Control of Skeletal musclesSpinal cord controls

Spindle reflex = stretch reflex (muscle length/toneGolgi tendon reflex, preventing excesssive stretchWithdrawal reflexWalking reflex (not the voluntary initiation of walking)

Brainstem including the vestibular nucleus, reticular nuclei, red nucleus, and tectum, controls posture & equilibrium. These reflexes require sensory input from visual and vestibular pathways, as well as motor output from the brainstem.Motor Cortex – Exerts both voluntary control and modulates involuntary reflexes – The motor cortex exerts a tonic inhibitory influence on the spindle reflex so the reflex still maintains muscle tone but is not so excessive as to cause muscle involuntary twitches. Damage to the motor cortex or its tracts causes a hyperactive spindle reflex tremors/ twitches

Corticospinal Tract = Pyramidal TractMost fibers cross to opposite side in the medulla (at level of medullary pyramids and exert voluntary control over ventral horn neurons that control muscles. These “upper motor neurons” also exert partial inhibition of the spindle reflex. A lesion in the top of the right motor cortex will cause loss of voluntary control where? Why will there be involuntary twitches in this same area?

Supplemental & Premotor CortexUnconscious planning & programming

of movementsPosterior parietal cortex

Hand manipulation, arm reachingCerebellum & Basal Ganglia (caudate, putamen, globus pallidus)

Coordination of movements

Other Motor Control areas:

Review the major anatomical and functional areas of the cerebral cortex

(Not to memorize), but notice the primary motor cortex & primary somatic sensory cortex.

Examples of motor system disordersLower motor neuron lesions (i.e., alpha motor neuron

lesion = ventral horn neuron lesions) ipsilateral hypoactive reflexes, flaccid weak muscles, paralysis. Because the motor neuron is the final neuron in any path controlling skeletal muscles, all involuntary and voluntary neural control of the muscle is lost – obviously on same side.

Upper motor neuron lesions (ex. Lesions of the motor cortex neurons or their fibers = corticospinal (pyramidal) tracts contralateral paralysis but hyperactive spinal reflexes such as the spindle reflex, resulting in involuntary muscle twitches and Babinski’s sign

Cerebellar lesions ataxia (loss of coordination), inaccurate movements, intention tremors

Parkinson’s resting tremors, slowed movements, progressive loss of control(loss of dopaminergic neurons in substantia nigra)

Multiple sclerosis – most common of the demyelinating diseases; an autoimmune disease (immune cells attack myelin basic protein)

Myasthenia gravis- an autoimmune disease in which immune cells attack the acetylcholine receptors in the neuromuscular junction.

Huntington’s Disease – loss of neurons in caudate and putamenspasmodic muscles and progressive loss of control; (a genetic disorder with many extra repeats of the CAG sequence on a particular gene)

Spinocerebellar ataxia (also many repeats of CAG within a different gene). Loss of control & co-ordination.

Myotonic muscular dystrophy (many repeats of CTG on another gene)

Autonomic neuropathy – often characterized by failure of blood pressure reflexes (example low pressure and fainting when standing), inability to control sphincters involved in urination and defecation, and sexual impotency.