The somatic nervous system controls all voluntary systems within the body with the exception of reflex arcs (see below). This system is comprised of the afferent nerve network, which include all sensory nerves leading to the brain, and the efferent nerve network, which includes all motor nerves leading from the brain to the muscles. The somatic system is generally associated with all body movement. All of these nerves, with the exception of those involved in reflex arcs, are sorted and processed in the brain. Before discussing on reflex arc, let’s start with receptors and effectors sense organs.
Receptors and Effectors Sense organs make an animal aware of conditions or events in its own body and in the world around it. Information collected by sense organs is passed to the nervous system, which determines and initiates an appropriate response. The stimuli or the sensations are received by the nerves which are associated with special sensory cells called receptors (Figure 23.7).
Impulses from the receptors reach the brain (Central Nervous System), where they are interpreted or analyzed, following which a proper command is given to the effector organs. The effector organs are usually muscles or glands which contract or secrete chemicals producing suitable responses.
In higher animals , different types of cells (receptor cells) receive a variety of stimuli. These receptor cells are grouped together to form a sensitive structure known as sense organs. Generally, five senses are known in higher animals. These are sense of touch, smell, taste, hearing and sight. A particular sense receptor or a sense organ is specialized and responds to only one kind of stimulus. The main types of receptors, functions and locations are given in following table:
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S.No. Type Function
1 Mechanoreceptors Sensitive to tactile stimuli : vibrations or pressure change and sound
Location
Skin and ears
2 Thermoceptors Sensitive to thermal or pressure changes and sound.
Skin
3 Chemoreceptors Sensitive to concentrations of chemicals intensity and wave-length of light.
Tongue, olfactory chambers
4 Photoreceptors Sensitive to change in the internal and external environment
Eyes
Various types of receptor.
Many situations call for immediate action. If your finger touches a hot stone, you don’t want to waste precious tome sending sensory information to your brain to be filtered and analyzed. You want to be able to move your finger off the burner before additional injury can occur.
To help an organism avoid injury reflex arcs provide a means for immediate withdrawal from dangerous stimuli. While all sensory information does eventually get sent to the brain for analysis, the advantage of a reflex arc is that it can process the rapid, protective response directly in the spinal cord, without the need to wait for instructions from the brain. In this way, a response is elicited even before the pain is perceived at a conscious level.
The reflex arc is fundamental to physiology of posture and locomotion as well as to the clinical examination of the nervous system. The word reflex comes from the
Latin word reflecture, which means to bend backward. A reflex, in a sense, is
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reflected off the central nervous system. A reflex can be defined as an involuntary, qualitatively unvarying response of the nervous system to stimulus. The anatomy and function of are flex arc are programmed genetically and are fully developed at birth.
In animals, the passage of a nerve impulse from a receptor to a muscle is an automatic response to the stimulus. It is the simplest kind of animal behaviour.
Examples include the removal of the hand when it touches a hot object, blinking when an object comes close to the eye, and the narrowing of the pupil of the eye in bright light. A reflex arc differs from other responses by being automatic. This is achieved by not allowing the brain to ‘consider’
whether to take action in response to the stimulus or not. In a reflex arc, a particular stimulus virtually always results in the same response.
Reflex arcs involve actions that protect the body. By cutting out having to ‘consider’ whether to take action or not, they do not involve conscious thought. Impulses travel along fewer neurons and so produce a response in the shortest possible time.
The slowest steps in the pathway taken by nerve impulses are at the junctions between neurons – the synapses. In many reflex arcs the number of synapses to cross may be as low as three.
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Although reflex arcs send secondary signals to the brain during the reflex action, the primary response is "hard wired" through the spinal cord. Certain stimuli, such as touching a hot surface, cause a reflex arc to trigger where the nerve impulse travels up the afferent nerve, through an interneuron in the spine, and down the appropriate efferent nerves necessary to jerk the hand away from the hot surface.
This automatic response system allows quicker reaction times when confronted with sudden hazards.
The term reflex action refers to a rapid, automatic unlearned response to a stimulus. These actions are involuntary as opposed to voluntary actions, which are governed by willful or conscious control of the animal.
Diagram: Reflex arcs
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A familiar example of the reflex action in man is the "knee jerk" reflex or the
"hand withdrawal" after pin pricks. It is common understanding that if suddenly one is pricked by a sharp object such as a pin one immediately retracts from the object. In such actions, the impulses are carried by the sensory fibers from the receptor organ to the central nervous system and back to the effector organ. The path or the route through which the impulses travel from the receptor organ (via the Central Nervous System) is called a reflex arc.
Reflex hand withdrawal - Stimulus
(Pin
(Skeletal muscles)
If the controlling center of the reflex arc is located in the brain it is called cerebral reflex. If it is located in the spinal cord , it is called spinal reflex.
The reflex arcs are of two types.
1) Somatic reflex arc involving effector organs located in ( soma ) body structures, e.g. skeletal muscles.
2) Visceral reflex arc involves effectors located in the visceral organs; e.g. glands or smooth muscles .
Components of the reflex arcs
Somatic reflex arc (Figure 23.8)
1. Receptor organ (e.g.) skin
2. Somatic sensory or afferent neuron
(unipolar)
3. Intermediate neuron (Multipolar)
4. Somatic motor or efferent neuron
(Multipolar)
5. Effector organ (e.g. Skeletal muscles)
Visceral reflex arc
1. Receptor organ (e.g. Epithelial lining of the gut)
2. Visceral sensory or afferent neuron
(unipolar)
3. Visceral motor of efferent (Preganglionic)
4. Visceral motor or efferent neuron
(Post-ganglionic)
5. Effector organ (e.g. gland or smooth muscles)
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A simple reflex arc consists of only one sensory neuron and one motor neuron
(monosynaptic reflex). But in a majority of reflexes, a sensory neuron influences several motor neurons through intermediate (association) neurons of the spinal cord. This forms a complex reflex arc, or polysynaptic reflexes.
Some common examples of reflex actions are knee jerk reflex, contraction of iris diaphragm (i.e. narrowing of pupil in response of intense light) blinking of eye lids when aimed at, withdrawal of hand on pin prick, sneezing, and secretion of saliva at the sight or smell of food. http://education.vetmed.vt.edu/Curr iculum/VM8054/Labs/Lab9/Examp les/exsomarc.htm
A somatic reflex arc is one in which there is the simplest possible arrangement of elements to permit a response to stimuli, and in which the final element in the chain is skeletal muscle. In the crude sketch given here, you see the basic elements of this system. 1 is some sensory transducer in the periphery, for example, a Pacinian corpuscle or other tactile sensor in the skin. Shown here in blue is 2, the pseudounipolar sensory neuron in the circuit. Its soma is physically located in a craniospinal ganglion (pictured here as a dorsal root ganglion, but it could also be on a cranial
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nerve). Drawn in black is 3, an interconnector neuron, whose soma is found in the
CNS. Drawn in red, 4 is a motor neuron whose soma is in the ventral horn of the gray H of the spinal cord. The last element involved is 5, the effector organ, which in the case of this type of arc, will always be skeletal muscle.
Here's how the system works: something impinges on the transducer, which causes the afferent fiber of the pseudo-unipolar sensory neuron to fire. That signal is transmitted via its efferent fiber into the CNS, specifically into a synapse with an interconnector neuron in the dorsal horn of the gray H. That neuron then sends a signal to a synapse with the motor neuron in the ventral horn. The afferent motor fiber (axon) of the motor neuron—which may actually be several meters in length— leaves the CNS and terminates at a motor end plate on some myofiber. When it fires it initiates contraction of 5.
Notice that this loop is completely independent; it's not necessary to have CNS involvement beyond the "relay" at the interconnector neuron. Let's say you inadvertently put your hand on a hot stove burner. You will of course immediately remove it, and in doing so you are making use of this type of arc, bypassing conscious thought. In fact, the sensation of uncomfortable heat makes it to the
CNS after the motor response to withdraw your hand is initiated. In other words, you move your hand away before you "know why" you're doing it. Of course, it's possible to override this loop with direct CNS input. As is always true, the Brain is
The Boss. If you really, really want to, it's possible to hold your hand on that hot burner, ignoring the somatic reflex. Some people have done things like this, but most of us haven't got that level of willpower.
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An autonomic reflex arc is similar to the somatic kind, but differs principally in the motor output side. The sensory side is similar in that a transducer, 1 , sends a signal via a nerve fiber ( 2 , drawn in blue) into the CNS. As with the somatic arc, this sensory fiber is associated with a pseudo-unipolar neuron in a craniospinal ganglion, usually one of the dorsal root ganglia. In the autonomic arc, the sensory transducer is often located in or associated with visceral organs.
Inside the CNS, there may or may not be an interconnector neuron involved. While in the somatic arc there is always a mediating interconnector neuron that receives the sensory signal and passes it to the motor neuron, the contact between the sensory fiber and the first motor neuron of the autonomic arc may be direct. (In this sketch, the interconnector neuron—labeled "3" in the sketch of the somatic arc—has been omitted.)
The motor output in an autonomic arc involves not one, but two motor neurons. Both are shown here in red. The first (4A), as in the somatic arc, is located in the spinal cord. Its efferent fiber (i.e., its axon) leaves the CNS to form a synapse with a second motor neuron (4B). The second neuron is located in the peripheral nervous system, not the CNS. More specifically, it's located in an autonomic ganglion of some kind. Autonomic ganglia are "switching stations" in the periphery; here the signal is passed from the first neuron of the motor chain to the second neuron.
More than one type of autonomic ganglion exists.
In a functional sense, autonomic reflex arcs are vital to what we think of as
"automatic" functions, such as gut peristalsis, sweating, etc. For the most part these activities are carried out without conscious thought or volition.
As mentioned earlier about fundamental components all reflex arcs contain five basic components. If any one of these five components malfunctions, the reflex response will be altered.
1) http://normandy.sandhills.cc.nc.us/psy150/somatic.html
2) http://www.tiscali.co.uk/reference/encyclopaedia/hutchinson/m0097305.html
3) http://www.pinkmonkey.com/studyguides/subjects/biology-edited/chap23/b2323401.asp
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