The somatic nervous system provides output strictly to skeletal muscles. The lower motor neurons, which are responsible for the contraction of these muscles, are found in the ventral horn of the spinal cord. These large, multipolar neurons have a corona of dendrites surrounding the cell body and an axon that extends out of the ventral horn. This axon travels through the ventral nerve root to join the emerging spinal nerve.
The axon is relatively long because it needs to reach muscles in the periphery of the body. The diameters of cell bodies may be on the order of hundreds of micrometers to support the long axon; some axons are a meter in length, such as the lumbar motor neurons that innervate muscles in the first digits of the feet. The axons will also branch to innervate multiple muscle fibers.
Together, the motor neuron and all the muscle fibers that it controls make up a motor unit. Motor units vary in size.
Some may contain up to muscle fibers, such as in the quadriceps, or they may only have 10 fibers, such as in an extraocular muscle. The number of muscle fibers that are part of a motor unit corresponds to the precision of control of that muscle. Also, muscles that have finer motor control have more motor units connecting to them, and this requires a larger topographical field in the primary motor cortex.
Motor neuron axons connect to muscle fibers at a neuromuscular junction. This is a specialized synaptic structure at which multiple axon terminals synapse with the muscle fiber sarcolemma. The synaptic end bulbs of the motor neurons secrete acetylcholine, which binds to receptors on the sarcolemma. The binding of acetylcholine opens ligand-gated ion channels, increasing the movement of cations across the sarcolemma.
This depolarizes the sarcolemma, initiating muscle contraction. Whereas other synapses result in graded potentials that must reach a threshold in the postsynaptic target, activity at the neuromuscular junction reliably leads to muscle fiber contraction with every nerve impulse received from a motor neuron.
However, the strength of contraction and the number of fibers that contract can be affected by the frequency of the motor neuron impulses. This chapter began by introducing reflexes as an example of the basic elements of the somatic nervous system.
Simple somatic reflexes do not include the higher centers discussed for conscious or voluntary aspects of movement. Reflexes can be spinal or cranial, depending on the nerves and central components that are involved. The example described at the beginning of the chapter involved heat and pain sensations from a hot stove causing withdrawal of the arm through a connection in the spinal cord that leads to contraction of the biceps brachii.
The description of this withdrawal reflex was simplified, for the sake of the introduction, to emphasize the parts of the somatic nervous system. But to consider reflexes fully, more attention needs to be given to this example. As you withdraw your hand from the stove, you do not want to slow that reflex down.
As the biceps brachii contracts, the antagonistic triceps brachii needs to relax. Because the neuromuscular junction is strictly excitatory, the biceps will contract when the motor nerve is active. Skeletal muscles do not actively relax. In the hot-stove withdrawal reflex, this occurs through an interneuron in the spinal cord. The interneuron receives a synapse from the axon of the sensory neuron that detects that the hand is being burned.
In response to this stimulation from the sensory neuron, the interneuron then inhibits the motor neuron that controls the triceps brachii. This is done by releasing a neurotransmitter or other signal that hyperpolarizes the motor neuron connected to the triceps brachii, making it less likely to initiate an action potential.
With this motor neuron being inhibited, the triceps brachii relaxes. Without the antagonistic contraction, withdrawal from the hot stove is faster and keeps further tissue damage from occurring. Another example of a withdrawal reflex occurs when you step on a painful stimulus, like a tack or a sharp rock. The nociceptors that are activated by the painful stimulus activate the motor neurons responsible for contraction of the tibialis anterior muscle.
This causes dorsiflexion of the foot. An inhibitory interneuron, activated by a collateral branch of the nociceptor fiber, will inhibit the motor neurons of the gastrocnemius and soleus muscles to cancel plantar flexion. An important difference in this reflex is that plantar flexion is most likely in progress as the foot is pressing down onto the tack. Contraction of the tibialis anterior is not the most important aspect of the reflex, as continuation of plantar flexion will result in further damage from stepping onto the tack.
Another type of reflex is a stretch reflex. In this reflex, when a skeletal muscle is stretched, a muscle spindle receptor is activated. The axon from this receptor structure will cause direct contraction of the muscle.
A collateral of the muscle spindle fiber will also inhibit the motor neuron of the antagonist muscles. The reflex helps to maintain muscles at a constant length. A common example of this reflex is the knee jerk that is elicited by a rubber hammer struck against the patellar ligament in a physical exam.
A specialized reflex to protect the surface of the eye is the corneal reflex , or the eye blink reflex. When the cornea is stimulated by a tactile stimulus, or even by bright light in a related reflex, blinking is initiated. The sensory component travels through the trigeminal nerve, which carries somatosensory information from the face, or through the optic nerve, if the stimulus is bright light.
The motor response travels through the facial nerve and innervates the orbicularis oculi on the same side. This reflex is commonly tested during a physical exam using an air puff or a gentle touch of a cotton-tipped applicator.
Watch this video to learn more about the reflex arc of the corneal reflex. When the right cornea senses a tactile stimulus, what happens to the left eye? Explain your answer. Watch this video to learn more about newborn reflexes. Newborns have a set of reflexes that are expected to have been crucial to survival before the modern age. These reflexes disappear as the baby grows, as some of them may be unnecessary as they age.
The video demonstrates a reflex called the Babinski reflex, in which the foot flexes dorsally and the toes splay out when the sole of the foot is lightly scratched. This is normal for newborns, but it is a sign of reduced myelination of the spinal tract in adults. Why would this reflex be a problem for an adult? Connections between the body and the CNS occur through the spinal cord. The cranial nerves connect the head and neck directly to the brain, but the spinal cord receives sensory input and sends motor commands out to the body through the spinal nerves.
Whereas the brain develops into a complex series of nuclei and fiber tracts, the spinal cord remains relatively simple in its configuration Figure From the initial neural tube early in embryonic development, the spinal cord retains a tube-like structure with gray matter surrounding the small central canal and white matter on the surface in three columns. The dorsal, or posterior, horns of the gray matter are mainly devoted to sensory functions whereas the ventral, or anterior, and lateral horns are associated with motor functions.
In the white matter, the dorsal column relays sensory information to the brain, and the anterior column is almost exclusively relaying motor commands to the ventral horn motor neurons. The lateral column, however, conveys both sensory and motor information between the spinal cord and brain. The general senses are distributed throughout the body, relying on nervous tissue incorporated into various organs. Somatic senses are incorporated mostly into the skin, muscles, or tendons, whereas the visceral senses come from nervous tissue incorporated into the majority of organs such as the heart or stomach.
The somatic senses are those that usually make up the conscious perception of the how the body interacts with the environment. The visceral senses are most often below the limit of conscious perception because they are involved in homeostatic regulation through the autonomic nervous system.
The sensory exam tests the somatic senses, meaning those that are consciously perceived. Testing of the senses begins with examining the regions known as dermatomes that connect to the cortical region where somatosensation is perceived in the postcentral gyrus. To test the sensory fields, a simple stimulus of the light touch of the soft end of a cotton-tipped applicator is applied at various locations on the skin.
The spinal nerves, which contain sensory fibers with dendritic endings in the skin, connect with the skin in a topographically organized manner, illustrated as dermatomes Figure For example, the fibers of eighth cervical nerve innervate the medial surface of the forearm and extend out to the fingers.
In addition to testing perception at different positions on the skin, it is necessary to test sensory perception within the dermatome from distal to proximal locations in the appendages, or lateral to medial locations in the trunk. In testing the eighth cervical nerve, the patient would be asked if the touch of the cotton to the fingers or the medial forearm was perceptible, and whether there were any differences in the sensations.
Other modalities of somatosensation can be tested using a few simple tools. The perception of pain can be tested using the broken end of the cotton-tipped applicator. The perception of vibratory stimuli can be testing using an oscillating tuning fork placed against prominent bone features such as the distal head of the ulna on the medial aspect of the elbow. When the tuning fork is still, the metal against the skin can be perceived as a cold stimulus.
Using the cotton tip of the applicator, or even just a fingertip, the perception of tactile movement can be assessed as the stimulus is drawn across the skin for approximately 2—3 cm. The patient would be asked in what direction the stimulus is moving.
All of these tests are repeated in distal and proximal locations and for different dermatomes to assess the spatial specificity of perception. The sense of position and motion, proprioception, is tested by moving the fingers or toes and asking the patient if they sense the movement.
If the distal locations are not perceived, the test is repeated at increasingly proximal joints. The various stimuli used to test sensory input assess the function of the major ascending tracts of the spinal cord. The dorsal column pathway conveys fine touch, vibration, and proprioceptive information, whereas the spinothalamic pathway primarily conveys pain and temperature.
Testing these stimuli provides information about whether these two major ascending pathways are functioning properly. Within the spinal cord, the two systems are segregated. The dorsal column information ascends ipsilateral to the source of the stimulus and decussates in the medulla, whereas the spinothalamic pathway decussates at the level of entry and ascends contralaterally.
The differing sensory stimuli are segregated in the spinal cord so that the various subtests for these stimuli can distinguish which ascending pathway may be damaged in certain situations.
Whereas the basic sensory stimuli are assessed in the subtests directed at each submodality of somatosensation, testing the ability to discriminate sensations is important.
Pairing the light touch and pain subtests together makes it possible to compare the two submodalities at the same time, and therefore the two major ascending tracts at the same time. Mistaking painful stimuli for light touch, or vice versa, may point to errors in ascending projections, such as in a hemisection of the spinal cord that might come from a motor vehicle accident.
Another issue of sensory discrimination is not distinguishing between different submodalities, but rather location. The two-point discrimination subtest highlights the density of sensory endings, and therefore receptive fields in the skin. The sensitivity to fine touch, which can give indications of the texture and detailed shape of objects, is highest in the fingertips. To assess the limit of this sensitivity, two-point discrimination is measured by simultaneously touching the skin in two locations, such as could be accomplished with a pair of forceps.
Specialized calipers for precisely measuring the distance between points are also available. The patient is asked to indicate whether one or two stimuli are present while keeping their eyes closed. The examiner will switch between using the two points and a single point as the stimulus. Failure to recognize two points may be an indication of a dorsal column pathway deficit. Similar to two-point discrimination, but assessing laterality of perception, is double simultaneous stimulation.
Two stimuli, such as the cotton tips of two applicators, are touched to the same position on both sides of the body. If one side is not perceived, this may indicate damage to the contralateral posterior parietal lobe.
Because there is one of each pathway on either side of the spinal cord, they are not likely to interact. If none of the other subtests suggest particular deficits with the pathways, the deficit is likely to be in the cortex where conscious perception is based. The mental status exam contains subtests that assess other functions that are primarily localized to the parietal cortex, such as stereognosis and graphesthesia.
A final subtest of sensory perception that concentrates on the sense of proprioception is known as the Romberg test. The patient is asked to stand straight with feet together. Once the patient has achieved their balance in that position, they are asked to close their eyes. Without visual feedback that the body is in a vertical orientation relative to the surrounding environment, the patient must rely on the proprioceptive stimuli of joint and muscle position, as well as information from the inner ear, to maintain balance.
This test can indicate deficits in dorsal column pathway proprioception, as well as problems with proprioceptive projections to the cerebellum through the spinocerebellar tract.
Watch this video to see a quick demonstration of two-point discrimination. Touching a specialized caliper to the surface of the skin will measure the distance between two points that are perceived as distinct stimuli versus a single stimulus. The patient keeps their eyes closed while the examiner switches between using both points of the caliper or just one. The patient then must indicate whether one or two stimuli are in contact with the skin.
Why is the distance between the caliper points closer on the fingertips as opposed to the palm of the hand? And what do you think the distance would be on the arm, or the shoulder? The skeletomotor system is largely based on the simple, two-cell projection from the precentral gyrus of the frontal lobe to the skeletal muscles. The corticospinal tract represents the neurons that send output from the primary motor cortex.
These fibers travel through the deep white matter of the cerebrum, then through the midbrain and pons, into the medulla where most of them decussate, and finally through the spinal cord white matter in the lateral crossed fibers or anterior uncrossed fibers columns.
These fibers synapse on motor neurons in the ventral horn. The ventral horn motor neurons then project to skeletal muscle and cause contraction. Voluntary movements require these two cells to be active. The motor exam tests the function of these neurons and the muscles they control. First, the muscles are inspected and palpated for signs of structural irregularities.
Movement disorders may be the result of changes to the muscle tissue, such as scarring, and these possibilities need to be ruled out before testing function.
Along with this inspection, muscle tone is assessed by moving the muscles through a passive range of motion. The arm is moved at the elbow and wrist, and the leg is moved at the knee and ankle. Skeletal muscle should have a resting tension representing a slight contraction of the fibers. The lack of muscle tone, known as hypotonicity or flaccidity , may indicate that the LMN is not conducting action potentials that will keep a basal level of acetylcholine in the neuromuscular junction.
If muscle tone is present, muscle strength is tested by having the patient contract muscles against resistance. The examiner will ask the patient to lift the arm, for example, while the examiner is pushing down on it. This is done for both limbs, including shrugging the shoulders.
Lateral differences in strength—being able to push against resistance with the right arm but not the left—would indicate a deficit in one corticospinal tract versus the other. An overall loss of strength, without laterality, could indicate a global problem with the motor system. Diseases that result in UMN lesions include cerebral palsy or MS, or it may be the result of a stroke.
A sign of UMN lesion is a negative result in the subtest for pronator drift. The patient is asked to extend both arms in front of the body with the palms facing up. While keeping the eyes closed, if the patient unconsciously allows one or the other arm to slowly relax, toward the pronated position, this could indicate a failure of the motor system to maintain the supinated position.
Reflexes combine the spinal sensory and motor components with a sensory input that directly generates a motor response. The reflexes that are tested in the neurological exam are classified into two groups.
A deep tendon reflex is commonly known as a stretch reflex, and is elicited by a strong tap to a tendon, such as in the knee-jerk reflex. A superficial reflex is elicited through gentle stimulation of the skin and causes contraction of the associated muscles.
For the arm, the common reflexes to test are of the biceps, brachioradialis, triceps, and flexors for the digits. For the leg, the knee-jerk reflex of the quadriceps is common, as is the ankle reflex for the gastrocnemius and soleus. The tendon at the insertion for each of these muscles is struck with a rubber mallet. The muscle is quickly stretched, resulting in activation of the muscle spindle that sends a signal into the spinal cord through the dorsal root.
The fiber synapses directly on the ventral horn motor neuron that activates the muscle, causing contraction. The reflexes are physiologically useful for stability. If a muscle is stretched, it reflexively contracts to return the muscle to compensate for the change in length.
In the context of the neurological exam, reflexes indicate that the LMN is functioning properly. The most common superficial reflex in the neurological exam is the plantar reflex that tests for the Babinski sign on the basis of the extension or flexion of the toes at the plantar surface of the foot.
The plantar reflex is commonly tested in newborn infants to establish the presence of neuromuscular function. An infant would present a positive Babinski sign, meaning the foot dorsiflexes and the toes extend and splay out. As a person learns to walk, the plantar reflex changes to cause curling of the toes and a moderate plantar flexion.
The descending input of the corticospinal tract modifies the response of the plantar reflex, meaning that a negative Babinski sign is the expected response in testing the reflex. Other superficial reflexes are not commonly tested, though a series of abdominal reflexes can target function in the lower thoracic spinal segments.
Watch this video to see how to test reflexes in the abdomen. Testing reflexes of the trunk is not commonly performed in the neurological exam, but if findings suggest a problem with the thoracic segments of the spinal cord, a series of superficial reflexes of the abdomen can localize function to those segments.
If contraction is not observed when the skin lateral to the umbilicus belly button is stimulated, what level of the spinal cord may be damaged? Many of the tests of motor function can indicate differences that will address whether damage to the motor system is in the upper or lower motor neurons. Signs that suggest a UMN lesion include muscle weakness, strong deep tendon reflexes, decreased control of movement or slowness, pronator drift, a positive Babinski sign, spasticity , and the clasp-knife response.
Spasticity is an excess contraction in resistance to stretch. It can result in hyperflexia , which is when joints are overly flexed. The clasp-knife response occurs when the patient initially resists movement, but then releases, and the joint will quickly flex like a pocket knife closing. A lesion on the LMN would result in paralysis, or at least partial loss of voluntary muscle control, which is known as paresis.
The paralysis observed in LMN diseases is referred to as flaccid paralysis , referring to a complete or partial loss of muscle tone, in contrast to the loss of control in UMN lesions in which tone is retained and spasticity is exhibited. Other signs of an LMN lesion are fibrillation , fasciculation , and compromised or lost reflexes resulting from the denervation of the muscle fibers. In certain situations, such as a motorcycle accident, only half of the spinal cord may be damaged in what is known as a hemisection.
Forceful trauma to the trunk may cause ribs or vertebrae to fracture, and debris can crush or section through part of the spinal cord. The full section of a spinal cord would result in paraplegia, or loss of voluntary motor control of the lower body, as well as loss of sensations from that point down. A hemisection, however, will leave spinal cord tracts intact on one side. The resulting condition would be hemiplegia on the side of the trauma—one leg would be paralyzed.
The sensory results are more complicated. The ascending tracts in the spinal cord are segregated between the dorsal column and spinothalamic pathways. This means that the sensory deficits will be based on the particular sensory information each pathway conveys. Sensory discrimination between touch and painful stimuli will illustrate the difference in how these pathways divide these functions. On the paralyzed leg, a patient will acknowledge painful stimuli, but not fine touch or proprioceptive sensations.
On the functional leg, the opposite is true. The reason for this is that the dorsal column pathway ascends ipsilateral to the sensation, so it would be damaged the same way as the lateral corticospinal tract. The spinothalamic pathway decussates immediately upon entering the spinal cord and ascends contralateral to the source; it would therefore bypass the hemisection. The somatic nervous system. In: Reference Module in Biomedical Sciences. Elsevier; B Ganong's Review of Medical Physiology 25th Edition.
McGraw Hill Professional; Elsevier Health Sciences; Cleveland Clinic. Neuropathy peripheral neuropathy. Updated December 16, Akinrodoye MA, Lui F. Neuroanatomy, somatic nervous system. StatPearls [Internet]. Updated April 2, Neuropathy peripheral neuropathy : Prevention. Somatic nervous system. Dorland's Illustrated Medical Dictionary. Your Privacy Rights. To change or withdraw your consent choices for VerywellMind. At any time, you can update your settings through the "EU Privacy" link at the bottom of any page.
These choices will be signaled globally to our partners and will not affect browsing data. We and our partners process data to: Actively scan device characteristics for identification. I Accept Show Purposes. Table of Contents View All. Table of Contents. What Is the Somatic Nervous System? Somatic vs. Autonomic Nervous Systems. Parts of the Somatic Nervous System. Reflex Arcs. Impact of Damage. Preventing Peripheral Neuropathy.
ANS pathways are divided into sympathetic and parasympathetic around the sympathetic divisions and enteric plexuses. Preganglionic cell bodies for the sympathetic outflow are in the thoracic spinal cord. Preganglionic cell bodies for the parasympathetic outflow are in the brainstem cranial and in the sacral spinal cord sacral.
The idea that the divisions oppose each other is a misleading simplification. Neither division is ever activated in its entirety. Rather, each division consists of a series of discrete functional pathways that may be activated from the CNS either independently or in patterns, according to the particular requirement of the particular daily activity that is contributing to bodily homeostasis.
Different emergency states require different patterns of autonomic activity, and normal daily life apart from emergencies also requires patterned autonomic activity. The individual functions as a whole: there is just one nervous system. Sensory information visceral afferent information relevant to autonomic control eg degree of bladder distention or level of blood pressure travels in visceral afferent nerves and enters the CNS via spinal afferent pathways, or via vagal or glossopharyngeal afferents that project into the lower brainstem see white-filled black arrows in Figure 1.
All preganglionic autonomic neurons, both sympathetic and parasympathetic use acetylcholine ACh as their fast excitatory transmitter. In the ganglia, ACh acts on a subclass of nicotinic receptors , distinct from nicotinic receptors at the skeletal muscle neuromuscular junction. Many preganglionic autonomic neurons also contain neuropeptides, usually acting as co-transmitters that mediate slow excitatory post-synaptic potentials, facilitating cholinergic transmission. Most sympathetic final motor neurons utilise noradrenaline norepinephrine as their primary transmitter, together with co-transmitters such as adenosine triphosphate ATP and peptides, including neuropeptide Y NPY , galanin, somatostatin or opioid peptides.
Some sympathetic final motor neurons especially those innervating sweat glands use ACh as their main non-peptide transmitter. Parasympathetic final motor neurons pathways usually use ACh, nitric oxide, or both as non-peptide transmitters, as well as a wide range of co-transmitter peptides including vasoactive intestinal peptide VIP , calcitonin gene-related peptide CGRP , somatostatin and opioid peptides.
No parasympathetic neurons use noradrenaline as a transmitter. ACh is also a major excitatory transmitter utilised by enteric neurons. Other enteric neurotransmitters include nitric oxide probably the main inhibitory transmitter to gut muscle , substance P, VIP, enkephalin, serotonin 5-hydroxytryptamine, 5-HT and ATP.
Axons of final motor neurons ramify throughout their target tissues, typically smooth muscle, secretory tissue or cardiac muscle. Axon terminals are specialized for neurotransmission, but they usually lack the structures characteristic of conventional synaptic contacts.
Many target tissues are innervated by both sympathetic and parasympathetic nerves eg the heart, the iris muscle, some salivary glands, the gastrointestinal tract and pelvic organs. The cranial parasympathetic pathways project to a wide variety of targets in the head, neck, thorax and abdomen Figure 1. Most final motor neurons in these cranial autonomic pathways are in four pairs of major ganglia: the ciliary ganglia III , sphenopalatine or pterygopalatine ganglia VII , submandibular ganglia VII , and otic ganglia IX.
The final motor neurons of the vagal autonomic pathways lie mostly in microganglia located near or within the target organs. The major target of cranial parasympathetic pathways are secretory glands associated with the eye tears , mouth saliva and nose mucus. They stimulate the secretion of watery fluid, often with a concomitant vasodilation.
Parasympathetic pathways also have a critical role in focusing the eye and regulating pupil diameter. Blood vessels in the brain also receive a parasympathetic vasodilator innervation, but the actual physiological function of these nerves is not well understood.
The vagus nerve innervates microganglia in the neck, thorax and abdomen, including the airways, heart, thyroid, pancreas, gall bladder and the upper gastrointestinal tract. Consequently, the vagus nerve has a vast array of actions.
It alters resistance to airflow and increases mucus secretion from the upper respiratory tract; it slows the heart; it stimulates secretion of digestive enzymes and bicarbonate from the pancreas; it either increases or decreases both secretory activity and smooth muscle contractility in the stomach. Some parasympathetic pathways tend to be tonically active eg vagal pathways that keep heart rate low when we are not exercising whereas others are activated only when required, eg salivary secretion during eating; relaxation of gastric smooth muscle; or near focus of the eyes when reading.
Neurons of the sympathetic division of the autonomic nervous system are aggregated into two main collections of ganglia: the paravertebral ganglia, which form the sympathetic chain each side of the vertebral column, and the prevertebral ganglia lying around the origins of the coeliac and mesenteric arteries Figure 1. Sympathetic neurons project to most tissues of the body, commonly reaching them by traveling with major nerves containing predominantly sensory and somatic motor nerve fibers.
Sympathetic pathways have a diverse range of activities. Many are active nearly all the time, eg, vasoconstrictor pathways to the muscles that maintain central blood pressure, vasoconstrictor pathways to the skin that help prevent excessive heat loss, or prevertebral pathways to the gastrointestinal tract that help prevent excessive water loss from the gut. Other sympathetic pathways are activated only on demand, eg those to that increase heart rate during exercise; sudomotor neurons stimulating sweating during high body temperature; or those stimulating ejaculation during sexual activity.
In some circumstances, sympathetic and parasympathetic pathways to a target tissue are co-activated eg sympathetic pathways to the salivary glands are co-activated with parasympathetic pathways when we eat something potentially noxious, such as hot chillies. The sympathetic co-activation results in the production of a thicker, more viscous saliva.
Sympathetic pathways normally are never activated all at once. Despite the widespread belief that they are only activated during stressful situations, on-going activity of specific sympathetic pathways are essential for our day-to-day health and well-being. Even when we are faced with extreme stress, only a subset of sympathetic pathways will be involved.
Regulation of the activity of many pelvic organs requires coordinated control via both sympathetic and sacral parasympathetic pathways, often in association with the relevant somatic motor pathways.
Indeed, many of the ganglia in pelvic pathways contain mixtures of neurons, some of which receive preganglionic inputs from lumbar spinal levels by definition, sympathetic and others of which receive preganglionic input from sacral spinal levels by definition, parasympathetic. Some individual neurons receive convergent inputs from both lumbar and sacral preganglionic neurons, and there may be considered to lie in both sympathetic and parasympathetic pathways.
Control of bladder function requires sympathetic activity to relax the bladder wall and combined sympathetic and somatic motor activity to keep sphincters closed during continence.
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