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Reflex arc

 
Reflex Arc

 
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A reflex arc is the neural pathway
Neural pathway

A neural pathway is a neural tract connecting one part of the nervous system with another, usually consisting of bundles of elongated, myelin-insulated neurons, known collectively as white matter....
 that mediates a reflex action
Reflex action

A reflex action, also known as a reflex, is an involuntary and nearly instantaneous movement in response to a Stimulus . In most contexts, in particular those involving humans, reflex actions are mediated via the reflex arc; this is not always true in other animals, nor does it apply to casual uses of the term 'reflex'....
. In higher animals, most sensory neurons do not pass directly into the brain
Brain

The brain is the center of the nervous system in all vertebrate, and most invertebrate, animals. Some primitive animals such as cnidarian and echinoderm have a decentralized nervous system without a brain, while sponges lack any nervous system at all....
, but synapse in the spinal cord
Spinal cord

The spinal cord is a long, thin, tubular bundle of neuron and glia that extends from the brain. The brain and spinal cord together make up the central nervous system....
. This characteristic allows reflex actions to occur relatively quickly by activating spinal motor neurons without the delay of routing signals through the brain, although the brain will receive sensory input while the reflex action occurs. The main source of the reflex action is through the bottom muscles.

There are two types of reflex arc - autonomic reflex arc (affecting inner organs) and somatic reflex arc (affecting muscles).

a reflex arc consists of only two neurons in an animal (one sensory neuron
Sensory neuron

Sensory neurons or also known as afferent neurons are neurons that are activated by sensory input , and send projections into the central nervous system that convey sensory information to the brain or spinal cord....
 and one motor neuron
Motor neuron

In vertebrates, the term motor neuron classically applies to neurons located in the central nervous system that project their axons outside the CNS and directly or indirectly control muscles....
), it is defined as monosynaptic.






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Encyclopedia


A reflex arc is the neural pathway
Neural pathway

A neural pathway is a neural tract connecting one part of the nervous system with another, usually consisting of bundles of elongated, myelin-insulated neurons, known collectively as white matter....
 that mediates a reflex action
Reflex action

A reflex action, also known as a reflex, is an involuntary and nearly instantaneous movement in response to a Stimulus . In most contexts, in particular those involving humans, reflex actions are mediated via the reflex arc; this is not always true in other animals, nor does it apply to casual uses of the term 'reflex'....
. In higher animals, most sensory neurons do not pass directly into the brain
Brain

The brain is the center of the nervous system in all vertebrate, and most invertebrate, animals. Some primitive animals such as cnidarian and echinoderm have a decentralized nervous system without a brain, while sponges lack any nervous system at all....
, but synapse in the spinal cord
Spinal cord

The spinal cord is a long, thin, tubular bundle of neuron and glia that extends from the brain. The brain and spinal cord together make up the central nervous system....
. This characteristic allows reflex actions to occur relatively quickly by activating spinal motor neurons without the delay of routing signals through the brain, although the brain will receive sensory input while the reflex action occurs. The main source of the reflex action is through the bottom muscles.

There are two types of reflex arc - autonomic reflex arc (affecting inner organs) and somatic reflex arc (affecting muscles).

Monosynaptic vs. polysynaptic

When a reflex arc consists of only two neurons in an animal (one sensory neuron
Sensory neuron

Sensory neurons or also known as afferent neurons are neurons that are activated by sensory input , and send projections into the central nervous system that convey sensory information to the brain or spinal cord....
 and one motor neuron
Motor neuron

In vertebrates, the term motor neuron classically applies to neurons located in the central nervous system that project their axons outside the CNS and directly or indirectly control muscles....
), it is defined as monosynaptic. Monosynaptic refers to the presence of a single chemical synapse. In the case of peripheral muscle reflexes (patellar reflex
Patellar reflex

The patellar reflex or knee-jerk is a deep tendon reflex and is a myotatic reflex....
, achilles reflex), brief stimulation to the muscle spindle
Muscle spindle

Muscle spindles are sensory receptors within the belly of a muscle, which primarily detect changes in the length of this muscle. They convey length information to the central nervous system via sensory neurons....
 results in contraction of the agonist or effector muscle.

By contrast, in polysynaptic reflex pathways, one or more interneurons connect afferent
Afferent nerve

In the nervous system, afferent neurons , carry action potential from receptor s or sense organs toward the central nervous system. This term can also be used to describe relative connections between structures....
 (sensory
Sensory system

A sensory system is a part of the nervous system responsible for processing sense information. A sensory system consists of sensory receptors, neural pathways, and parts of the brain involved in sensory perception....
) and efferent
Efferent nerve

In the nervous system, efferent nerves ? otherwise known as motoneuron or effector neurons ? carry action potential away from the central nervous system to effectors such as muscles or glands ....
 (motor
Motor system

The motor system is the part of the peripheral nervous system that is involved to movement. It consists of the Corticospinal tract and extrapyramidal system....
) signals. All but the most simple reflexes are polysynaptic, allowing processing or inhibition of polysynaptic reflexes within the spinal cord.

The Patella Reflex (knee jerk)

Patellar reflex
Patellar reflex

The patellar reflex or knee-jerk is a deep tendon reflex and is a myotatic reflex....
: when the patellar tendon is tapped just below the knee, the patellar reflex is initiated and the lower leg kicks forward (via contraction of the quadriceps). The tap initiates an action potential
Action potential

An action potential is a self-regenerating wave of electrochemical activity that allows nerve cells to carry a signal over a distance. It is the primary electrical signal generated by nerve cells, and arises from changes in the permeability of the nerve cell's axonal Cell membranes to specific ions....
 in a specialised structure known as a muscle spindle
Muscle spindle

Muscle spindles are sensory receptors within the belly of a muscle, which primarily detect changes in the length of this muscle. They convey length information to the central nervous system via sensory neurons....
 located within the quadriceps. This action potential travels to the spinal cord, via a sensory axon which chemically communicates by releasing glutamate (see synapse) onto a motor nerve
Motor nerve

Motor nerves allow the brain to stimulate muscle contraction. A motor nerve is an efferent nerve that exclusively contains the axons of somatic and branchial motoneurons, which innervate skeletal muscles and branchial muscles ....
. The result of this motor nerve activity is contraction of the quadriceps muscle
Quadriceps muscle

The quadriceps femoris , also called simply the quadriceps, quadriceps extensor, or quads, is a large muscle group that includes the four prevailing muscles on the front of the thigh....
, leading to extension of the lower leg at the knee. The sensory input from the quadriceps also activates local interneurons that release the inhibitory neurotransmitter glycine onto motor neurons, blocking the innervation of the antagonistic (hamstring) muscle. The relaxation of the opposing muscle facilitates extension of the lower leg.

External links

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