PHYSIOLOGY: REFLEXES Page 1 REFLEXES Definition of Reflex – a stereotyped response to specific stimulus; any response that occur automatically without conscious effort Two types of Reflex a. Simple or Basic Reflex – built-in, unlearned responses, e.g., closing of eyes when an object moves towards them. Processed and integrated in spinal cord and brainstem b. Acquired or Conditional Reflex – Results from practice and learning, e.g., steering car wheel to follow a curve done automatically but after considerable training effort. Integrated, processed in higher brain centers. Relfex Arc - Basic unit of integrating neural activity; anatomical basis for the reflex - Stereotypical response to adequate stimuli *Stereotypical response: generally involuntary - May be innate or learned, but there are reflexes that you NEED to learn (i.e. in driving). - Simplest circuit involved for posture, walking, eating, laughing as well as control of autonomic functions. - *Bell-Magendie Law - Principle that states that the dorsal (posterior) roots of the spinal cord are sensory and that the ventral (anterior) roots are motor. Reflex Arc (parts) 1. Afferent limb - Would consist of a sensory receptor organ . - Afferent or sensory neuron and nerve - The sense organ may be part of the first order neuron (free or encapsulated nerve endings) or a specialized independent cell. 2. Center - The afferent limb will synapse with one or more neurons in CNS (either in the spinal cord, brainstem, thalamic nuclei, or the cerebral cortex). 3. Efferent limb - Efferent or motor neuron and nerve and an effector organ (glands- exocrine and endocrine and muscles – smooth, skeletal & cardiac) - Efferent fiber that innervates skeletal muscle - alpha (α) motor neurons (which is the FINAL COMMON PATHWAY towards skeletal muscles!). The soma of skeletal muscle neurons are found in the ventral horn. *Cardiac and smooth muscle have different fibers (Autonomic nerves – parasympathetic or sympathetic; pre- and post-ganglionic). - Intrafusal fibers – gamma (γ) motor efferents Reflex Arcs vary in complexity. Simple at level of the spinal cord More complex at the level of brainstem Most complex in the cerebral cortex Stimulus – a change in the external/internal environment. In the body, changes in temperate, pressure, electrolyte concentration, etc., can act as stimuli Adequate Stimulus – a stimulus to which a receptor is most sensitive or to which it has a low threshold, e.g., eye sensitive to lights; see „stars‟ when punched. Threshold Stimulus – the weakest stimulus that a sensory receptor can reliably detect and activate primary afferent fibers Motor Unit – a single motor neuron and the muscle cells it synapses on. Each muscle cell belongs to only one motor unit. Size of motor units varies and depends on muscle function: small muscle that generate very finely controlled movements (e. extra-ocular muscles of the eyes, motor units tend to be small and may contain just a few muscle fibers); large muscles that generate strong forces, e.g., gastrocnemius muscle of the leg, tend to have large motor units with more than several thousand muscle fibers. Two Types of motor neurons: a. Alpha Motor neurons – innervate the main force- generating muscle fibers (“extrafusal” fibers). b. Gamma Motor neurons – innervate only the fibers of the muscle spindle (“intrafusal” fibers) Motor Neuron Pool – the groups of all motor neurons innervating a single muscle **Not all reflex activity involves clear-cut reflex arcs: - response may be mediated through neurons or hormones or both - local response may not be due to hormones or nerves but by local metabolites SUBJECT: PHYSIOLOGY TOPIC: REFLEXES LECTURER: DANTE G. SIMBULAN JR. PhD DATE: FEBRUARY 2011
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PHYSIOLOGY: REFLEXES Page 1
REFLEXES
Definition of Reflex – a stereotyped response to specific
stimulus; any response that occur automatically without
conscious effort
Two types of Reflex
a. Simple or Basic Reflex – built-in, unlearned
responses, e.g., closing of eyes when an object
moves towards them. Processed and integrated in
spinal cord and brainstem
b. Acquired or Conditional Reflex – Results from
practice and learning, e.g., steering car wheel to
follow a curve done automatically but after
considerable training effort. Integrated, processed
in higher brain centers.
Relfex Arc
- Basic unit of integrating neural activity; anatomical
basis for the reflex
- Stereotypical response to adequate stimuli
*Stereotypical response: generally involuntary
- May be innate or learned, but there are reflexes
that you NEED to learn (i.e. in driving).
- Simplest circuit involved for posture, walking,
eating, laughing as well as control of autonomic
functions.
-
*Bell-Magendie Law - Principle that states that the dorsal
(posterior) roots of the spinal cord are sensory and that the
ventral (anterior) roots are motor.
Reflex Arc (parts)
1. Afferent limb
- Would consist of a sensory receptor organ.
- Afferent or sensory neuron and nerve
- The sense organ may be part of the first order
neuron (free or encapsulated nerve endings) or a
specialized independent cell.
2. Center
- The afferent limb will synapse with one or more
neurons in CNS (either in the spinal cord,
brainstem, thalamic nuclei, or the cerebral cortex).
3. Efferent limb
- Efferent or motor neuron and nerve and an
effector organ (glands- exocrine and endocrine and
muscles – smooth, skeletal & cardiac)
- Efferent fiber that innervates skeletal muscle -
alpha (α) motor neurons (which is the FINAL
COMMON PATHWAY towards skeletal muscles!).
The soma of skeletal muscle neurons are found in
the ventral horn.
*Cardiac and smooth muscle have different fibers
(Autonomic nerves – parasympathetic or
sympathetic; pre- and post-ganglionic).
- Intrafusal fibers – gamma (γ) motor efferents
Reflex Arcs vary in complexity.
Simple at level of the spinal cord
More complex at the level of brainstem
Most complex in the cerebral cortex
Stimulus – a change in the external/internal environment.
In the body, changes in temperate, pressure, electrolyte
concentration, etc., can act as stimuli
Adequate Stimulus – a stimulus to which a receptor is most
sensitive or to which it has a low threshold, e.g., eye
sensitive to lights; see „stars‟ when punched.
Threshold Stimulus – the weakest stimulus that a sensory
receptor can reliably detect and activate primary afferent
fibers
Motor Unit – a single motor neuron and the muscle cells it
synapses on. Each muscle cell belongs to only one motor
unit. Size of motor units varies and depends on muscle
function: small muscle that generate very finely controlled
movements (e. extra-ocular muscles of the eyes, motor
units tend to be small and may contain just a few muscle
fibers); large muscles that generate strong forces, e.g.,
gastrocnemius muscle of the leg, tend to have large motor
units with more than several thousand muscle fibers.
Two Types of motor neurons:
a. Alpha Motor neurons – innervate the main force-
generating muscle fibers (“extrafusal” fibers).
b. Gamma Motor neurons – innervate only the fibers
of the muscle spindle (“intrafusal” fibers)
Motor Neuron Pool – the groups of all motor neurons
innervating a single muscle
**Not all reflex activity involves clear-cut reflex arcs:
- response may be mediated through neurons
or hormones or both
- local response may not be due to hormones
or nerves but by local metabolites
SUBJECT: PHYSIOLOGY
TOPIC: REFLEXES
LECTURER: DANTE G. SIMBULAN JR. PhD
DATE: FEBRUARY 2011
PHYSIOLOGY: REFLEXES Page 2
Classification Systems of Reflexes
A. According to NUMBER of SYNAPSES at level of the
SPINAL CORD:
1. Monosynaptic – only one synapse
2. Polysynaptic – two or more synapses in between your 1st
order neuron and your final common
pathway.
B. According to LOCATION of SENSORY RECEPTOR
1. Superficial – skin (cutaneous) and subcutaneous;; result
from stimulation of the receptors present in the skin and
mucous membrain – they are all polysynaptic, e.g.,
withdrawal reflex or scratch reflex.
2. Deep – deep tissues (i.e. muscle, bone, joints); reflexes
that result from stimulation of receptors present in muscles
or tendons, e.g., stretch reflex
3. Visceral – i.e. carotid sinus, carotid arch, baroreceptors,
chemoreceptors; clinically important reflexes
– micturation, defacation and erection.
C. According to LOCATION of EFFECTOR RESPONSE
1. Somatic – i.e. skeletal muscle contraction
2. Autonomic – i.e. tachycardia, bradycardia, increase in
stroke volume
*overlaps such as somato-autonomic (i.e. You get pinched
and then there is an increase in blood pressure aside from
the withdrawal response.)
D. According to SITE of INTEGRATION (in the CNS)
1. Spinal Cord
2. Brainstem
3. Thalamus
4. Hypothalamus
5. Cerebral Cortex (where most learned reflexes are
mediated)
TWO MAIN TYPES OF REFLEX ARCS:
1. Monosynaptic Reflex (e.g. stretch reflex of muscle
spindles) – made up of a two-neuron pathway from
receptor to effector (the afferent or sensory neuron has a
direct synaptic connection with the efferent / motor neuron,
without the intervention of interneurons), e.g., stretch or
myotactic reflex – which is the most rapid of all reflexes,
utilizing 1A afferent which are the largest diameter, fastest/
conducting of any afferent nerves
2. Polysynaptic Reflex / Multi-synaptic
- More than one synapse
- Made up of a few or several interneurons
intersposed between the afferent and efferent
neurons, e.g., withdrawal reflex
- i.e. pain reflex, inverse stretch (of golgi tendon),