The document discusses reflexes and the nervous system. It defines reflexes as rapid automatic responses to specific stimuli. It describes the five steps in a neural reflex arc: 1) stimulus activates a receptor, 2) activation of a sensory neuron, 3) information processing in the central nervous system, 4) activation of a motor neuron, 5) response of a peripheral effector. Reflexes can be classified based on their development, processing site, response type, and complexity of the neural circuit involved. The stretch tendon reflex is provided as a detailed example of a polysynaptic reflex. The basal ganglia are also summarized as subcortical structures that receive input from and return output to the cerebral cortex to affect motor activity.

1. The Nervous System/Part V
Dr. Fawaz A. Mustafa
PhD in Medical Physiology and Pharmacology
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2. Reflexes
• Reflexes: they are rapid automatic responses to
specific stimuli
• Neural reflexes: in which sensory fibers delivers
information to the CNS & motor fibers commands to
peripheral effectors

3. Reflexes
• For any reflex in medicine neurology, we must
follow a rule in which a stimulus (which is
information converted to action potential) should fall
on a receptor connected to a nerve fibre which is
an axon that belongs to a sensory neuron (in the
spinal cord) whose dendrites are going to synapse
with neuron that belongs to the motor system. So,
there is a relation between motor and sensory
systems

4. Reflexes

5. Reflexes
• The Reflex Arc: a reflex arc begins at a receptor &
ends at a peripheral effecter such as a muscle fiber
or gland cell. There are five steps involved in a
neural reflex:

6. Reflexes

7. Reflexes
• Step I: arrival of stimulus & activation of a receptor:
receptors are specialized cells or dendrites of a
sensory neuron. The receptors are sensitive to
physical or chemical changes in the body or to the
external environment. If there is a pain stimulus
there will be activation to the pain receptor

8. Reflexes
• Step II: activation of a sensory neuron: stimulation
of pain receptor leads to the formation &
propagation of AP along the axon of sensory
neurons
• Step III: information processing: it begins when a
neurotransmitter is released by the synaptic knob &
affects the postsynaptic membrane of an
interneuron producing an EPSP. Sometimes the
sensory neuron innervates a motor nerve directly

9. Reflexes
• Step IV: activation of a motor neuron: the axons of
the stimulated motor neurons carry APs in to the
periphery over the ventral root of the spinal cord
• Step V: respond of a peripheral effecter: release of
neurotransmitter by the motor neurons at the
synaptic knobs → response by peripheral effecter,
skeletal muscle → contraction.

10. Reflexes

11. Reflexes

12. Reflexes
• Classifications of reflexes
a. According to their development.
b. Site where information processing occur.
c. Nature of the resulting motor response.
d. Complexity of neural circuit involved.

13. According to Development of
reflexes…
• Innate reflexes: result from the connections that
form between neurons during development. e.g.
removing your hand from a hot plate & blinking
when your eye lashes are touched
• Acquired reflex: learned motor patterns. These
motor responses are rapid & automatic but they
where learned rather than pre-established e.g. a
driver steps on the break when trouble appears on
the road

14. The Processing sites…
• Spinal reflex: the important interconnections &
processing occur inside the spinal cord
• Cranial reflex: the reflex is processed in the brain
e.g. movements in response to a loud noise, or
bright light

15. The Nature of the response…
• Somatic reflex: mechanism of involuntary control of
muscular system
• Visceral reflex: or autonomic reflex, control
activities of other systems

16. The Complexity of the circuit…
• Monosynaptic reflexes: it is the simplest reflex arc, the
sensory neuron synapse directly on a motor neuron which it
self serves as the processing center.
e.g. the starch reflex: Knee jerk reflex or patellar reflex,
because of the presence of muscle spindles (sensory
receptors)
• Polysynaptic reflexes: it have at least one interneuron
between the sensory & motor neurons.
e.g. stretch tendon reflex : presence of the sensory
receptors on the tendon called (Golgi tendon organs) →
these stimulate inhibition of interneuron that innervate motor
neuron of skeletal muscle

17. The Complexity of the circuit…

18. The Complexity of the circuit…

19. Stretch tendon reflex
• There are two types of receptors in the skeletal
muscle: muscle spindle and the Golgi tendon organ
• The muscle spindle (detecting the length and the
rate of change in the length of muscle) is connected
to a sensory axon through which conducts an
action potential, ending in a neuron cell body which
sends dendrites to synapse with the motor neuron,
for example, the spinal cord
• When the length is increased (during stretching),
the muscle spindles will send more action potential
to the sensory side of the spinal cord

20. Stretch tendon reflex
• Golgi tendon organ is present in the tendon, and it
detects the force or tension of the muscle. It is
connected to a sensory nerve fibre. The Golgi
tendon organ is activated when muscle is
contracted, so action potential is produced in this
case. When the muscle is relaxed, the frequency of
action potential becomes less. The previous
discussion was about the sensory side of this
reflex, the following is the motor side

21. Stretch tendon reflex
• Motor neurons give axons to the skeletal muscle.
These neurons are of two types, the first is big and
with myelinated axon known as “a-motor neuron”,
and the other is small with less myelinated axon
known as “g-motor neuron”

22. Stretch tendon reflex
• The skeletal muscle fibres are of two types.
• One is the muscle spindles lying in the centre and
known as “intrafusal muscle fibres”, and the other
is the ordinary fibres including the rest fibres, and
they are known as “extrafusal muscle fibres”
• The a-motor neurons supply the extrafusal fibres,
while g-motor neurons supply either ends of the
muscle spindle. So, the centre of the muscle
spindle is supplied by sensory nerve, and the ends
are supplied by the g-motor neurons

23. Stretch tendon reflex

24. Stretch tendon reflex

25. Stretch tendon reflex
• In human, we can stretch a muscle by tapping the tendon
with a hammer (such as tapping a hammer on the knee).
The tapping here is considered as a mechanical stimulus
causing stretching of muscle, thus, the muscle spindles
(receptors) are sensitive to this increase in length, and the
frequency of action potential conducted is increased. The
action potential is conducted by sensory nerve fibre of a
neuron in the posterior horn of the spinal cord, which will
synapse of the a- or g-motor neurons
• Once a-motor neurons are stimulated, this leads to
conducting action potential to the muscle fibres. Thus, the
muscle contracts back to the same length or less (i.e. to the
normal length). If the g-motor neurons are activated, this will
lead to contraction of both ends of muscle spindle leading to
stretching of the whole muscle

26. Stretch tendon reflex

27. Stretch tendon reflex
• Stretch tendon reflex is acting against gravity,
gives our muscles certain texture or consistency
(muscle tone: defined as partial contraction of
muscle or resistance of muscle to stretch), ensures
the normal response, and maintains normal limit
of contraction

28. Stretch tendon reflex
• The tone can be normally decrease (hypotonia) so
that we can move any part of the body easily, as in
sleep
• Abnormally, the muscle tone can be increased
(hypertonia) which is of two types: spasticity and
rigidity
• Spasticity is when the tone of a contracted muscle
increases up to a certain level then it is decreased,
while rigidity is when there is continuous increase
in muscle tone without reaching the critical level
where the stretch tendon reflex is activated to
cause muscle relaxation

29. Stretch tendon reflex

30. The basal ganglia
• The subcortical structures are called “the basal
ganglia” including caudate nucleus, putamen (both
which are called corpus striatum), globus pallidus,
substantia nigra and subthalamic nucleus
• There is internal connection by chemical ways (i.e.
releasing neurotransmitters among these nuclei cell
bodies and axons)
• There are many neurotransmitters in the internal
connections, such as ACh, g-aminobutyric
(inhibitory), glutamate (inhibitory), dopamine
(excitatory and inhibitory) and noradrenaline

31. The basal ganglia
• The basal ganglia are also connected to the
cerebral cortex (mainly precentral areas)
• The basal ganglia receive virtually all their input
signals from the cerebral cortex and, in turn, return
almost all their output signals back to the cortex
• This connection is the only way by which the basal
ganglia can affect the motor activity, since they
have no direct effect on the lower motor neurons

32. The basal ganglia