Human Physiology: Reflex Arcs

Reflex Arcs

- reflex: preprogrammed responses to certain stimuli that are made without conscious thought or planning

- simple reflexes: built-in automatic responses

- conditioned (acquired) reflexes: automatic responses that are programmed in

the afferent and efferent nervous systems

- afferent nervous system: brings information in to the CNS (e.g. sensory transduction)

- efferent nervous system: sends instructions out to the appropriate end organs

- divisions

- somatic

- type: voluntary

- function: control of skeletal muscle

- structure: 1 neuron system

- mechanism: ACh released onto nAChR of motor neurons, causing an excitatory (ONLY) response

- cell bodies: located in ventral horn of the spinal cord

- stimulation: originates in motor cortex (except during spinal reflexes)

- activation: influenced by activity in the basal nuclei, cerebellum, brain stem, and spinal reflexes

- autonomic

- type: involuntary

- function: control of cardiac and smooth muscle, most exocrine and some endocrine glands

- structure: 2 neuron system

- transmitters

- acetylcholine (ACh)

- norepinephrine (NE)

spinal reflexes

- components of a reflex arc

- receptor

- afferent pathway

- integrator

- efferent pathway

- effector

- the stretch reflex

- reflex

- receptor: intrafusal fibers of the muscle spindle containing mechanosensitive receptors

- afferent: courses into spinal cord, synapses on the motor neuron

- integrator: response properties of the motor neuron

- efferent: axon of the motor neuron, coursing to the quadriceps muscle

- effector: quadriceps muscle

- properties

- type: monosynaptic circuit (not counting neuromuscular junction)

- speed: rapid

- division: tonic, phasic (phasic maintains constant muscle length during standing position

- the withdrawal reflex

- nociceptor: receptors for painful stimuli; often involved in withdrawal reflex

- reflex

- receptor: nociceptor (pain stimuli)

- afferent: courses into spinal cord, synapses on two interneurons

- integrator: interneurons that synapse on motor neurons

- excitatory: synapses on an excitatory motor neuron to the biceps

- inhibitory: synapses on an excitatory motor neuron to the triceps

- efferent: motor neurons

- effector: contract biceps, relax triceps (net effect: withdrawal)

- properties

- type: polysynaptic (reciprocal innervation)

- speed: slow