Re: basic reflex terminology

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Chapter 13:  Basic Reflex Terminology

reflex - (1) general - A relatively rapid and predictable motor response to a stimulus which will be repeated in a similar fashion each time the stimulus is presented; it consists of five general components: the receptor(s) for the stimulus, the sensory neuron(s) transmitting afferent impulses to the CNS, the control/integration center(s) within the CNS, the motor neuron(s) transmitting efferent impulses away from the CNS, and the effector(s) which respond to the afferent impulses with the specific motor response(s); (2) inborn or intrinsic reflex - a reflex which is unlearned, unpremeditated, involuntary and in which the pathways are built into the neural anatomy of each individual during development; depending on the effectors involved, such reflexes may be termed somatic or autonomic; although unlearned in their original form, they are sometimes subject to modification by learning from experience; (3) learned or acquired reflex - a reflex which is learned through practice or repetition and may involve both a far more complicated set of triggering stimuli and a far more complicated pattern of motor response, e.g., the reflexive motor actions produced after one has learned to ride a bicycle or drive a car; most such reflexes are somatic because they involve complex response patterns from skeletal muscles.

 

spinal reflex - The inborn or intrinsic somatic (skeletal muscle response) reflexes  (relatively rapid and predictable motor responses to stimuli) mediated by the spinal cord; although the central pathway(s) involve only spinal cord segment control, some spinal reflexes require the participation of higher brain centers for completion and most spinal reflexes also report information about the reflex having occurred to higher centers in the brain.

cranial reflexes - The inborn or intrinsic somatic (skeletal muscle response) reflexes (relatively rapid and predictable motor responses to stimuli) mediated by control centers in the brain cord; the pathway(s) usually involve cranial and, perhaps, cervical spinal nerves.

autonomic reflex - The inborn or intrinsic visceral (cardiac or smooth muscle or glandular response) reflexes (relatively rapid and predictable motor responses to stimuli) mediated by control centers in the brain, particularly in the hypothalamus and brain stem, and in the spinal cord, particularly in the lateral horns of thoracic and lumbar spinal cord segments; these reflexes are involved in the automatic adjustment or negative feedback control of internal environment = internal homeostasis.  [For example:  The initial responses to cold-water immersion, evoked by stimulation of peripheral cold receptors, include tachycardia, a reflex inspiratory gasp and uncontrollable hyperventilation. When immersed naked, the maximum responses are initiated in water at 10 degrees C, with smaller responses being observed following immersion in water at 15 degrees C. Habituation of the initial responses can be achieved following repeated immersions, but the specificity of this response with regard to water temperature is not known. This gasp reflex seems to be a significant cause of death in kayakers.]

pupillary reflexes - The variety of autonomic reflexes (relatively rapid and predictable motor responses to stimuli) in which the effectors are the radial and circular muscles of the iris of the eye and the responses are constriction or dilation of the pupil; stimuli are quite variable; responses include components of the light adaptation and dark adaptation processes.

somatic reflex - Any reflex (relatively rapid and predictable motor response to a stimulus) in which the effectors are skeletal muscles, e.g., the patellar reflex, the flexor or withdrawal reflex, the crossed extensor reflex, the plantar reflex, the abdominal reflexes, etc.; most named somatic reflexes are inborn/intrinsic, but a variety of more complex learned/acquired reflexes may be included as somatic reflexes.

reflex neural pathway = reflex arc - The anatomical route(s) which connect the components: the receptor(s) for the stimulus, the sensory neuron(s) transmitting afferent impulses to the CNS, the control/integration center(s) within the CNS, the motor neuron(s) transmitting efferent impulses away from the CNS, and the effector(s) which respond to the afferent impulses with the specific motor response(s)) of a particular reflex; these pathways control automatic unconscious programmed (hard-wired) responses to particular sensory stimuli.

receptor -

1. Physiology.  A specialized cell or group of nerve endings or a specialized organ which responds to sensory stimuli of some modality.
   
2. Biochemistry.  A molecular structure or site on the surface or interior of a cell that binds with substances such as hormones, antigens, drugs, or neurotransmitters.

sensory neuron = afferent neuron - A neuron, whose cell body generally is found in a peripheral ganglion such as a dorsal root ganglion, which conducts impulses representing information about an (external or internal) environmental change inwards to the brain or spinal cord.

integrating center = control center - Those cells, generally interneurons on the central nervous system, which receive sensory information from the (external or internal) environment, process that information, and, if appropriate, generate motor commands to effector organs in response to the stimulus from the (external or internal) environment.

association neuron = interneuron  = internuncial - A nerve cell found entirely within the central nervous system, often participating in a complex multicellular pathway, that acts as a link between sensory neurons and motor neurons or between other internal linking neurons and integrating or communicating or transmitting information between different parts of the CNS.

monosynaptic - Having one neural synapse and referring to direct neural connections between just two neurons, commonly between a primary sensory neuron and a motor neuron in a simple reflex arc where no interneurons are involved.

polysynaptic - Of or involving two or more synapses in the central nervous system and referring to direct neural connections between three or more neurons in any neural pathway, often used to describe reflex arcs in which one or more interneurons are involved.

motor neuron = efferent neuron - A neuron that conveys impulses outward from the central nervous system to a muscle, gland, or other effector tissue that regulates the activity of the effector.

effector - A muscle, gland, or other organ capable of responding to a stimulus which has been eval‎uated by a control center, mediated by a nerve impulse or a hormonal signal.

Sketch and Label:

1. An illustration of a simple reflex arc. What terminology would be used to describe the simplest possible reflex arc?

The simplest reflex arc would contain only four of the parts below, omitting the (3) interneuron functioning as the integration center.  Such a simplest reflex arc would be termed monosynaptic.

2. A homeostatic negative feedback pathway diagram of a reflex arc.

Using feedback control terminology, the sensor is the combination of the sensory receptor and the sensory neuron, the control center is the action of the CNS, either the cooperation of the sensory and motor neurons in a monosynaptic reflex or the action of the association neuron in a polysynaptic reflex, and the effector is the combination of the motor neuron and the skeletal muscle, cardiac muscle, smooth muscle, or endocrine or exocrine gland which responds to the nervous system command.  The feedback control is negative because the environmental change moves the body part or system in one direction while the response brings it back in the opposite direction; e.g., a somatic stimulus can be a stretching of the sensor and the response is the reverse, a muscle contraction which reduces the tension in the muscle and receptor.  [Note:  a similar diagram, with minor details, could be drawn of any somatic or autonomic reflex.]

Explain

3. the difference(s) between a somatic versus an autonomic reflex.

Somatic Reflexes

Autonomic Reflexes

Sensory receptor responds to stimuli at an exteroreceptor or proprioceptor. Motor neuron communicates directly (one cell path) with its effector. Motor neuron cell body resides in brain stem or ventral = anterior horn of spinal cord. Somatic effector is skeletal muscle (motor units). Acetyl choline is the excitatory transmitter at the neuro-muscular junction. Sometimes, they are perceived consciously, after the reflex has been initiated, especially if the reflex causes a dramatic movement of a body part, such as when the hand draws back reflexively after experiencing a painful stimulus, or when the reflex is checked by a test, such as a tap on the patellar tendon.  Many somatic reflexes are involved in minor postural adjustments and those would not generally be perceived consciously.

Sensory receptor responds to stimuli at an enteroreceptor or proprioceptor. Motor neuron communicates in a two cell path with effector; pre- and post-ganglionic autonomic neurons. Pre-ganglionic motor neuron cell body resides in brain stem or lateral horn of spinal cord; post-ganglionic motor neuron cell body resides in autonomic ganglion. Visceral effector is smooth muscle or gland cells. Acetyl choline or norepinephrine are the transmitters at the visceral effectors and their effects may be excitatory or inhibitory, depending on the receptor type at the target tissue. Generally, they are not perceived consciously.