Reflexes and Reflex Arcs

The spinal cord also promotes homeostasis by serving as an integratingcenter for some reflexes. A reflex is a fast, involuntary, unplannedsequence of actions that occurs in response to a particularstimulus. Some reflexes are inborn, such as pulling your handaway from a hot surface before you even feel the heat. Otherreflexes are learned or acquired. For instance, you learn manyreflexes while acquiring driving expertise. Slamming on the brakesin an emergency is one example.

Spinal reflex:When integration takes place in the spinal cord gray matter, the reflex is a. An example the familiar patellar reflex (knee jerk)

Cranial reflex:If integration occurs in the brain stem rather than the spinal cord, the reflex is an example is the tracking movements of your eyes as you read this sentence.

Somaticreflexes, which involve contraction of skeletal muscles.

Autonomic (visceral) reflexes: These reflexes involve responses of smooth muscle, cardiac muscle, and glands. body functions such as heart rate, digestion, urination, and defecation are controlled by the autonomic nervous system through autonomic reflexes.

Nerve impulses propagating into, through, and out of theCNS follow specific pathways, depending on the type of information,its origin, and its destination. The pathway followed bynerve impulses that produce a reflex is a reflex arc (reflex circuit).

Using the patellar reflex (knee jerk) as an example, the basiccomponents of a reflex arc are as follows

1 Sensory receptor. The distal end of a sensory neuron (dendrite)or an associated sensory structure serves as a sensory receptor.Sensory receptors respond to a specific type of stimulus(a change in the internal or external environment) by generatingone or more nerve impulses. In the patellar reflex, sensoryreceptors known as muscle spindles detect slight stretching ofthe quadriceps femoris muscle (anterior thigh) when thepatellar (knee cap) ligament is tapped with a reflex hammer.

2 Sensory neuron. The nerve impulses conduct from thesensory receptor along the axon of a sensory neuron to itsaxon terminals, which are located in the CNS gray matter.From here, relay neurons send nerve impulses to the area ofthe brain that allows conscious awareness that the reflex hasoccurred.The CNS act as an integrating center. In the simplest type ofreflex, such as the patellar reflex in our example, the integratingcenter is a single synapse between a sensory neuron and amotor neuron in the spinal cord. A reflex pathway in theCNS that involves one synapse is called a monosynaptic reflexarc. In other types of reflexes,the integrating center includes one or more interneuronsand thus more than one synapse. These reflex pathwaysare referred to as polysynaptic reflex arcs

4 Motor neuron. Impulses triggered by the integrating centerpass out of the spinal cord (or brain stem, in the case of acranial reflex) along a motor neuron to the part of the bodythat will respond. In the patellar reflex, the axon of the motorneuron extends to the quadriceps femoris muscle. While thequadriceps femoris muscle is contracting, the antagonisthamstring muscles are relaxed.

5. Effector. The part of the body that responds to the motornerve impulse, such as a muscle or gland, is the effector. Thepatellar reflex is a somatic reflex because its effector is a skeletalmuscle, the quadriceps femoris muscle, which contractsand thereby relieves the stretching that initiated the reflex.In sum, the patellar reflex causes extension of the knee bycontraction of the quadriceps femoris muscle in responseto tapping the patellar ligament. If the effector is smoothmuscle, cardiac muscle, or a gland, the reflex is an autonomic(visceral) reflex.

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