The Nervous System:
Nerve Plexuses, Reflexes, and Sensory and Motor Pathways.
By:
Avi Asraf
Roger Yee
Santiago Roybal
Sasha Buz
Valeria Muňoz
Vincent Cottrill
Nerve Plexuses
• Cervical plexus – innervates the muscles of the neck and extends into
the thoracic cavity to control the diaphragm.
• Brachial plexus – innervates the shoulder girdle and upper limb
• Lumbar plexus & Sacral plexus – supply the pelvic girdle and lower
limb.
▫ *all designate the lumbosacral plexus.
Reflexes
• A reflex is an automatic motor response to a specific stimulus.
• A reflex response usually removes or opposes the original stimulus.
• Reflexes help maintain homeostasis by making
rapid adjustments to the functions of
organs/ organ systems.
• Reflex arcs are an example of negative
• feedback.
Simple Reflex
• The “wiring” of a single reflex is called a reflex arc.
Simple Reflex
• In the simplest reflex arc, a sensory neuron synapses directly on a
motor neuron, which performs information processing function. This
is also known as a monosynaptic reflex.
• The sensory receptors in the stretch reflex are called muscle spindles.
Simple Reflex
• Stretch reflexes are important in maintaining normal posture and
balance and in making automatic adjustments in muscle tone.
• Doctors can use the sensitivity of the stretch reflex to test general
conditions of the spinal cord, peripheral nerves, and muscles.
• Example: knee jerk reflex
Complex Reflexes
• Polysyanptic reflexes include a longer delay between stimulus and
response.
• They can produce more involved responses because the interneurons
can control several muscle groups simultaneously.
• Withdrawal reflexes move stimulated parts of the body away from a
source of stimulation.
• The strongest withdrawal reflexes are caused by pain stimuli.
• A flexor reflex is a withdrawal reflex affecting the muscles of a limb.
Complex Reflexes
• When a specific muscle contracts, opposing muscles are stretched.
• Contraction of a flexor muscles should trigger in the extensors a
stretch reflex that would cause them to contract, opposing the
movement that is underway.
• Interneurons in the spinal cord prevent such competitions through
reciprocal inhibition.
Integration and Control of Spinal Reflexes
• Although reflexes are automatic, higher centers in the brain influence
these response by stimulating or inhibiting the interneurons and motor
neurons involved.
• Stroking the side of an infants sole produces a fanning of toes known
as the Babinski sign/positive Babinski reflex.
• In adults, the toes curl, which is called Plantar reflex/negative
Babinski reflex.
Sensory and Motor Pathways
Sensory Pathways
• A sensation, the information gathered by a sensory receptor, arrives
in the form of action potentials in an afferent (sensory) fiber.
▫ Posterior Column Pathway is an example of an ascending sensory
pathway.
The pathway of
messages sent from the
brain to the phalanges.
Sensory and Motor Pathways
Motor Pathways
• The corticospinal pathway/pyramidal system provides conscious,
voluntary control of skeletal muscles.
• The medial and lateral pathways provide subconscious, involuntary
control of muscle tone and movements of the neck, trunk, and limbs.
▫ These pathways were known as the extrapyramidal system because
it was thought that they operated independently of and parallel to
the pyramidal system.
▫ The corticospinal pathway begins at pyramidal cells of the cerebral
cortex.
Quick Review
• 1) What is another name for the simplest reflex arc?
• 2) Why are stretch reflexes important?
• 3) What is the difference between the positive Babinski reflex
and the negative Babinski reflex?
• 4) What is a sensation?
• 5) Where do corticospinal pathways begin?