MOTOR NEURON

Motor neur

In vertebrates, the term motor neuron (or motoneuron) classically applies to neurons located in the central nervous system (or CNS) that project their axons outside the CNS and directly or indirectly control muscles. Motor neuron is often associated with efferent neuron, primary neuron, or alpha motor neurons.
Function
The interface between a motor neuron and muscle fiber is a specialized synapse called the neuromuscular junction. Upon adequate stimulation, the motor neuron releases a flood of neurotransmitters that bind to postsynaptic receptors and triggers a response in the muscle fiber.
* In invertebrates, depending on the neurotransmitter released and the type of receptor it binds, the response in the muscle fiber could be either excitatory or inhibitory.
* For vertebrates, however, the response of a muscle fiber to a neurotransmitter can only be excitatory, in other words, contractile. Muscle relaxation and inhibition of muscle contraction in verterbrates is obtained only by inhibition of the motor neuron itself. Although muscle innervation may eventually play a role in the maturation of motor activity. This is why muscle relaxants work by acting on the motoneurons that innervate muscles (by decreasing their electrophysiological activity) or on cholinergic neuromuscular junctions, rather than on the muscles themselves.
Somatic motor neurons
Somatic motoneurons are further subdivided into two types: alpha efferent neurons and gamma efferent neurons. (Both types are called efferent to indicate the flow of information from the central nervous system (CNS) to the periphery.)
* Alpha motoneurons innervate extrafusal muscle fibers (typically referred to simply as muscle fibers) located throughout the muscle. Their cell bodies are in the ventral horn of the spinal cord and they are sometimes called ventral horn cells.
* Gamma motoneurons innervate intrafusal muscle fibers found within the muscle spindle.
In addition to voluntary skeletal muscle contraction, alpha motoneurons also contribute to muscle tone, the continuous force generated by noncontracting muscle to oppose stretching. When a muscle is stretched, sensory neurons within the muscle spindle detect the degree of stretch and send a signal to the CNS. The CNS activates alpha motoneurons in the spinal cord, which cause extrafusal muscle fibers to contract and thereby resist further stretching. This process is also called the stretch reflex.
Gamma motoneurons regulate the sensitivity of the spindle to muscle stretching. With activation of gamma neurons, intrafusal muscle fibers contract so that only a small stretch is required to activate spindle sensory neurons and the stretch reflex.
Symptoms of Motor neuron diseases (MND)

• * Clinical picture and progression of disease is variable.
  * It can be rapid and may result in death within a year of onset or may be slow progression.
  * Cramps are early symptom.
  * Usual history is clumsiness of one hand for skilled activity and later for gross activity.
  * Later on, the opposite hand is involved.
  * Overtime there is wasting of small muscles of hand, flaccidity, loss of tendon reflexes.
  * Fascinations are widespread.
  * The lower limbs manifest with difficult in walking with spastic gait.
  * Ankle and knee jerks are grossly exaggerated.
  * Bilateral extensor response is seen.
  * Characteristically abdominal reflexes are preserved till late.
  * Progressive palsy involves cranial nerve nuclei causing symptom like difficulty in swallowing, nasal regurgitation, speech is progressively affected.
  * There is a nasal twang to speech. Tongue shows atrophy and fasciculation (involuntary muscle contractions).
  * Upper motor neuron involvement results in pseudo-bulbar palsy with spasticity (muscles are stiff) of muscles of face, jaw, tongue and palate with exaggerated jaw jerks, pouting reflex and palatal reflexes.
  * Sensory loss is limited to subjective sensations like numbness, cramps and neuralgic pain. There is no loss of sphincter control.
  * Bedsores do not occur in spite of prolonged confinement to bed, a strange feature of the disease which has not been explained.