Title: Reflexes and Reflex Arcs
Introduction: A reflex is a fast involuntary sequence of events that occur in response
to a particular stimulation. The actual reflex is a result of the reflex arc
1- Reflexes and Arcs
a- A reflex, is a fats, involuntary sequence of events that occurs in response
to a particular stimulus
1- Some reflexes are present at birth, i.e., pulling your hand away from a
painful situation, this type of reflex is “intrinsic” to the nervous
system.
2- Some reflexes are learned or acquired, i.e., the slamming on the car
brakes in response to injury
b- The brain and spinal cord serve as integration centers for reflex
information
1- a
2- a
c- Somatic Reflexes (most common and most well known) i.e., contraction of
the skeletal muscle
d- Autonomic (visceral) these reflexes are usually subconscious, i.e.,
contraction of the smooth muscle of the digestive tract
e- A pathway followed by nerve impulse that produces a reflex is called a
reflex arc (circuit, path) a reflex arc involves the following five steps
1- Sensory receptors
a-
2- Sensory neuron
a-
3- Integrating Center
a- In simple reflexes, the integration center is a single synapse
between a sensory and motor neuron – this is called a
monosynaptic reflex arc
b- Integrating centers with more than one interneuron, which
may relay signals to other interneurons and other motor
neurons are called polysynaptic reflex arcs
4- Motor neurons
a- impulses triggered by integrating centers propagate out of the
CNS along a motor neuron to a part of the body that will
respond
5- Effector Neurons
a- is a part of the body that responds to the motor nerve impulse,
such as a muscle or gland. This action is called a reflex
1- a
2-n
2-Because reflexed are normally so predictable they provide
useful information about our health. Any damage to the nerves along
the path will cause the reflex to be absent or abnormal. In general
though only somatic reflexes are easily observable. The only somatic
reflex that is diagnostic is the pupillary light reflex. Lack of this reflex
is indicative of brain damage
3-Stretch reflex – A stretch reflex is monosynaptic and ipsilateral
(The sensory receptors and the motor nerves are on the same side of
the body) a common example is the patella reflex
a- Stretch receptors in muscle are called muscle spindles
4-Flexor reflex also called the withdrawal reflex occurs when pain sensitive
receptors are activated, i.e., stepping on a tack. This is a polysynaptic ipsilateral
reflex arc and involves numerous segments of the brain it is therefore sometimes
called an intersegmental reflex
5-The somatic nervous system produces voluntary movements and the autonomic
nervous system produces in voluntary movements
a- The involuntary autonomic nervous system contains both sensory and
motor neurons
bcd-Autonomic responses cannot be consciously controlled, this is the basis
for lie detectors
6-Sympathetic Responses
a- During physical or emotional stress, sympathetic nerve cells support
physical activity and the rapid production of ATP. Body functions that
favor the support of energy are also decreased
b- The sympathetic nervous system controls “E” situations – Exercise,
Emotion, excitement and Embarrassment. All of these situations lead to
fight or flight situations, which include
1234-
56787- Parasympathetic responses – are the rest and relax functions. They reverse all of
the above situations and activate the SLUDD responses, these include and increase
in Salivation, Lacrimation, Urination, Digestion, Defecation