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uitgebreide Samenvatting hoofdstuk 13 silverthorn human physiology
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Fleur sam ch. 1313.1 Neural reflexes
The integration of sensory info into an involuntary response is the hallmark of a reflex.
All neural reflexes begin with a stimulus that activates a sensory receptor. The sensor sends info in
the form of an AP through sensory afferent neurons to the CNS. The CNS integrating center that
evaluates all incoming info and selects an appropriate response. It then initiates AP’s in efferent
neurons to direct the response of muscles and glands- the targets.
Key feature; negative feedback. Feedback signals from muscles and joint receptors keep the CNS
continuously informed of changing body position. Some reflexes have feedforward component that
allows the body to anticipate a stimulus and begin the response. Bracing yourself in anticipation of a
collision is an example of a feedforward response.
Reflex pathways in the nervous system consist of chains or networks of neurons that link sensory
receptors to muscles/glands.
Neural reflexes can be classified in several ways:
1. By the efferent division of the nervous system that controls the response : reflexes that
involve somatic motor neurons and skeletal muscles are known as somatic reflexes, if they
are controlled by autonomic neurons they are called autonomic reflexes.
2. By the CNS location where the reflex is integrated: spinal reflexes are integrated in the spinal
cord, these reflexes may be modulated by higher input from the brain, but they can occur
without that input. Reflexes integrated in the brain are called cranial reflexes.
3. By whether the reflex is innate or learned : many reflexes are innate (we are born with them),
they are genetically determined. Bv. Knee jerk, or patellar tendon reflex: when patellar
tendon at lower edge of kneecap is stretched with a tap from a reflex hammer, the lower leg
kicks out. Other reflexes are required through experience, such as Pavlov salivating upon
hearing a bell, these are called learned reflexes/ conditioned reflexes.
4. By the number of neurons in the pathway : the simplest reflex is a monosynaptic reflex, with
only a single synapse between the two neurons in the pathway: a sensory afferent neuron
and an efferent somatic motor neuron. These 2 neurons synapse in the spinal cord, allowing
a signal initiated at the receptor to go directly from the sensory neuron to the motor neuron.
(Synapse between somatic motor neuron and its muscle target is ignored.)
Most reflexes have 3 or more neurons in the pathway (and at least 2 synapses), leading to
their designation as polysynaptic reflexes. Polysynaptic reflexes may be quite complex, with
extensive branching in the CNS to form networks involving multiple interneurons.
Divergence of pathways allows a single stimulus to affect multiple targets.
Convergence integrates the input from multiple sources to modify the response. The
modification in polysynaptic pathways may involve excitation or inhibition.
13.2 Autonomic reflexes
Autonomic reflexes are also known as visceral reflexes, because they often involve the internal
organs of the body. Some visceral reflexes, such as urination and defecation, are spinal reflexes that
can take place without input from the brain.
, But spinal reflexes are often modulated by excitatory or inhibitory signals from the brain, carried by
descending tracts from higher brain centers. Bv. Urination may be voluntarily initiated by conscious
thought. Or it may be inhibited by emotion or a stressful situation, like presence of other people (this
syndrome is known as bashful(=verlegen) bladder). Often, the higher control of a spinal reflex is a
learned response. Bv. Toilet training as toddler is an example of a learned reflex that the CNS uses to
modulate the simple spinal reflex of urination.
Other autonomic reflexes are integrated in the brain, primarily in the hypothalamus, thalamus and
brain stem. These regions contain centers that coordinate body functions needed to maintain
homeostasis, like heart rate, blood pressure, breathing, eating, water balance, maintenance of body
temp.
Brain stem also contains integrating centers for autonomic reflexes like salivating, vomiting, sneezing,
coughing, swallowing, gagging.
Interesting type of autonomic reflex is conversion of emotional stimuli into visceral responses. Limbic
system – site of primitive drives like sex, fear, rage, aggression, hunger- has been called the visceral
brain bc of its role in emotionally driven reflexes. Gut-feeling & butterflies in stomach are all
transformations of emotion into somatic sensation and visceral function. Other emotion-linked
autonomic reflexes include urination, defecation, blushing, blanching, piloerection, in which tiny
muscles in the hair follicles pull the shaft of the hair erect.
Autonomic reflexes are all polysynaptic, with at least 1 synapse in the CNS between the sensory
neuron and the preganglionic autonomic neuron, and an additional synapse in the ganglion between
preganglionic and postganglionic neurons.
Many autonomic reflexes are characterized by tonic activity, continuous stream of AP’s that creates
ongoing activity in the effector. Bv. Tonic control of blood vessels is example of continuously active
autonomic reflex.
13.3 skeletal muscle reflexes
Skeletal muscle reflexes are involved in almost everything we do. Receptors that sense changes in
joint movement, muscle tension, muscle length feed this info to the CNS, which responds in 1 of 2
ways.
If muscle contraction is the appropriate response, the CNS activates somatic motor neurons to the
muscle fibers. If a muscle needs to be relaxed to achieve the response, sensory input activates
inhibitory interneurons in the CNS, and these interneurons inhibit activity in somatic motor neurons
controlling the muscle.
Excitation of somatic motor neurons always causes contraction in skeletal muscle. There is no
inhibitory neuron that synapses on skeletal muscles to cause them to relax. Instead, relaxation
results from the absence of excitatory input by the somatic motor neuron.
Inhibition and excitation of somatic motor neurons and their associated skeletal muscles must occur
at synapses within the CNS.
Skeletal muscle reflexes have the following components:
1. Sensory receptors, known as proprioceptors, located in skeletal muscles, joint capsules and
ligaments. They monitor the position of our limbs in space, our movements and the effort we
exert while lifting objects. The input signal from proprioceptors goes to the CNS through
sensory neurons. There are three types of proprioceptors: joint receptors, golgi tendon
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