Comprehensive Reflexes and Neural Pathways in Human Nervous System

Reflex

Quick, involuntary, stereotyped reactions of glands or muscle to stimulation.

Somatic Reflexes

Reflexes involving the somatic nervous system, innervating skeletal muscle.

Visceral Reflexes

Reflexes involving organs such as heart and intestines.

Reflex Arc

A pathway that includes somatic receptors, afferent nerve fibers, integrating center, efferent nerve fibers, and effectors.

Innate Reflexes

Reflexes formed before birth, such as withdrawal from pain and suckling.

Acquired Reflexes

Learned motor patterns that are rapid and automatic, such as riding a bike.

Superficial Reflexes

Reflexes of skin and mucous membranes, such as corneal reflex and gag reflex.

Stretch Reflexes

Deep tendon reflexes, such as the patellar or 'knee-jerk' reflex.

Efferent Nerve Fibers

Nerve fibers that carry motor impulses to effectors.

Effectors

The muscles that carry out the response in a reflex action.

Afferent Nerve Fibers

Nerve fibers that carry information from receptors to the spinal cord or brainstem.

Integrating Center

A point of synaptic contact between neurons in gray matter of the spinal cord or brainstem.

Negative Feedback

A process where reflexes generally oppose the original stimulus.

Somatic Receptors

Receptors located in skin, muscles, or tendons that detect stimuli.

Motor Response

The nature of the resulting motor action in reflexes.

Complexity of Neural Circuit

A classification of reflexes based on the complexity of the neural pathways involved.

Site of Information Processing

A classification of reflexes based on where the information is processed in the nervous system.

Corneal Reflex

A superficial reflex that protects the eye by blinking when the cornea is stimulated.

Gag Reflex

A superficial reflex that causes a contraction of the throat muscles when the back of the throat is stimulated.

Patellar Reflex

A stretch reflex that causes the knee to jerk when the patellar tendon is tapped.

Spinal Cord

The part of the central nervous system that controls reflex actions without the need for the brain.

Monosynaptic reflex

Sensory neuron synapses directly onto motor neuron (only one synapse involved)

Polysynaptic reflex

At least one interneuron between sensory neuron and motor neuron (involves more than one synapse)

Spinal reflexes

Processing occur in spinal cord

Cranial reflexes

Processing occur in brain

Intersegmental reflex arcs

Many spinal segments interact producing highly variable motor response

Synaptic delay

The more synapses, the slower the reaction

Stretch reflex

When a muscle is stretched, it 'fights back' and contracts

Reciprocal inhibition

Reflex phenomenon that prevents muscles from working against each other by inhibiting antagonist when agonist is excited

Muscle spindle

Stretch receptors embedded in skeletal muscles that serve as proprioceptors

Proprioceptors

Specialized sense organs to monitor position and movement of body parts

Afferent neurons

Sensory nerve fibers that monitor fiber length and speed of length changes

Extrafusal fibers

The rest of the muscle's fibers that generate force for movement

Intrafusal fibers

Modified muscle fibers within the spindle

Gamma motor neuron

Innervates the ends of an intrafusal fiber and keeps it taut

Alpha motor neurons

Supply the extrafusal fibers

Postural reflex

A type of stretch reflex that helps maintain upright posture

Sensory nerve fibers

Monitor fiber length and speed of length changes

Primary afferent fibers

Monitor fiber length and speed of length changes

Secondary afferent fibers

Monitor length only

Polysynaptic reflex arc

Pathway in which signals travel over many synapses on their way to the muscle.

Interneurons

Neurons that control more than one muscle group and produce either EPSPs or IPSPs at CNS motor nuclei.

Five General Characteristics of Polysynaptic reflexes

1. Involve pools of interneurons. 2. Involve reciprocal inhibition. 3. Have reverberating circuits. 4. Are intersegmental in distribution. 5. Several reflexes cooperate.

Flexor reflex

The quick contraction of flexor muscles resulting in the withdrawal of a limb from an injurious stimulus; strength and extent of response depends on intensity and location of stimulus.

Tendon organs

Proprioceptors in a tendon near its junction with a muscle; involved in the tendon reflex.

Tendon reflex

Response to excessive tension on the tendon by inhibiting muscle from contracting strongly.

Tendon reflex mechanism

Moderates muscle contraction before it develops too much tension and tears a tendon or pulls it loose from the muscle or bone.

Crossed extension reflex

Contraction of extensor muscles in limb opposite of the one that is withdrawn.

Ipsilateral reflex arc

Reflex arc where stimulus and response are on the same side of the body.

Contralateral reflex arc

Reflex arc where input and output occur at opposite sides of the body.

Intersegmental reflex

One in which the input and output occur at different levels (segments) of the spinal cord.

Pain response in foot

Pain in foot causes contraction of abdominal muscles.

Reverberating circuits

Circuits that prolong reflexive motor response after initial stimulation is gone.

Reciprocal inhibition in flexor reflex

If inhibition does not occur, flexion will cause stretching of the antagonistic muscle, leading to reflex contraction of that muscle.

Tendon reflex inhibitory effect

The greater the tension generated, the greater the inhibitory effect on that muscle's contraction.

Antagonistic muscle contraction

Also causes antagonistic muscle to contract which reduces stress on the tendon.

Flexor and crossed extension reflexes

Reflexes maintained by reverberating circuits.

Tendon organ structure

0.5 mm long, consists of encapsulated bundle of collagen fibers and one or more nerve fibers.

Reinforcement of Spinal Reflexes

Occurs when higher centers stimulate excitatory neurons in the brain stem or spinal cord, creating EPSPs at reflex motor neurons.

Jendrassik maneuver

A technique where fingers are hooked together and pulled apart to reinforce the knee jerk reflex when the patellar ligament is tapped.

Inhibition of Spinal Reflexes

Occurs when higher centers stimulate inhibitory neurons, creating IPSPs at reflex motor neurons.

Babinski Reflex

A reflex where infants display fanning of toes when the medial side of the sole is stimulated; if displayed in adults, it may indicate CNS damage.

Negative Babinski Reflex

Observed in normal adults where curling of toes (plantar reflex) occurs when stimulated.

Neural Pools

Functional groupings of neurons, each consisting of thousands of interneurons concerned with a particular body function.

Diverging Circuit

A neural circuit where one nerve fiber branches and synapses with several postsynaptic cells, such as visual information going to the visual cortex and postural/balance areas.

Converging Circuit

A neural circuit where input from many different nerve fibers can be funneled to one neuron or neural pool, such as subconscious and conscious control of breathing.

Reverberating Circuit

A circuit where neurons stimulate each other in a linear sequence, with one or more later cells restimulating the first cell to restart the process, exemplified by the cross extension reflex.

Parallel After-Discharge Circuit

A circuit where an input neuron diverges to stimulate several chains of neurons, with different numbers of synapses converging on one or a few output neurons, leading to varying delays.

After-Discharge

Continued firing of neurons after the stimulus stops, such as seeing an image after staring at a bright light.

Serial Processing

A method where neurons and neural pools relay information along a pathway in a simple linear fashion, processing only one flow of information at a time.

Parallel Processing

A method where information is transmitted along diverging circuits through different pathways that act on it simultaneously for different purposes.

Complete Transection

A complete severance of the spinal cord, leading to immediate loss of motor control below the level of injury.

Spinal Shock

A condition that occurs upon spinal cord transection, characterized by flaccid paralysis and lack of reflexes below the injury.

Hyperreflexia

Exaggerated reflexes that occur as spinal shock subsides after a spinal cord injury.

Paraplegia

Paralysis of both lower limbs.

Quadriplegia

Paralysis of all four limbs.

Hemiplegia

Paralysis on one side of the body.

Paresis

Partial paralysis or weakness of the limbs.