9. Name the three plexuses. Identify one spinal nerve from each plexus and describe its function
(i.e. what group of muscles does it innervate?). What would loss of function in that nerve
constitute? 10. What are the components of the polysynaptic reflex arc (ex. 16.6)? How do these
components work to improve reaction time?
Solution
9. There are five different spinal plexuses in the body. As per your recommendation, I am going
to mention the first 3 plexuses such as:
a) Cervical Plexus
b) Brachial Plexus
c) Lumbar Plexus
The remaining two plexuses are Sacral Plexus and Coccygeal Plexus.
a) Cervical Plexus: Cervical plexus is a network of nerve fibres that innervate some of the
structures in the head and neck. It is formed by the anterior rami (divisions) of cervical spinal
nerves C1-C4. Nervus Phrenicus i.e. c4 nerve innervates muscles of the diaphragm muscle which
controls breathing process. Hence the loss of function of c4 nerve could affect normal breathing
process.
b) Brachial Plexus: Brachial Plexus is formed by the ventral rami of the lower four cervical and
first thoracic nerve roots (C5-C8, T1). It innervates cutaneous and muscular muscles of the entire
upper limb with exceptions for trapezius muscle and an area of skin near the axilla. Loss of
function or injury of nerves rising from brachial plexus could result in paralysis.
c) Lumbar Plexus: Lumbar plexus interconnects with sacral plexus and generally referred to as
Lumbosacral plexus. The plexus is formed by the anterior rami of the lumbar spinal nerves L1,
L2, L3 and L4 and also from thoracic spinal nerve 12. It innervates the skin and the muscles of
the lower abdominal wall, the thigh and external genitals. The femoral nerve is the largest nerve
of the plexus and innervates the skin on the upper anterior thigh. Loss of function to the femoral
nerve results in weakness of hip flexion and knee extension and sensory loss in the anterior thigh
and medial leg particularly in cases of iliacus hematoma.
10. There are five essential components of the polysynaptic reflex arc:
a) The receptor that reacts to stimulus and initiates impulse via local depolarization
b) The sensory neuron that conducts nerve impulses and transmits afferent impulse towards the
CNS
(i.e. what group of muscles does it innervate?). What would loss of function in that nerve
constitute? 10. What are the components of the polysynaptic reflex arc (ex. 16.6)? How do these
components work to improve reaction time?
Solution
9. There are five different spinal plexuses in the body. As per your recommendation, I am going
to mention the first 3 plexuses such as:
a) Cervical Plexus
b) Brachial Plexus
c) Lumbar Plexus
The remaining two plexuses are Sacral Plexus and Coccygeal Plexus.
a) Cervical Plexus: Cervical plexus is a network of nerve fibres that innervate some of the
structures in the head and neck. It is formed by the anterior rami (divisions) of cervical spinal
nerves C1-C4. Nervus Phrenicus i.e. c4 nerve innervates muscles of the diaphragm muscle which
controls breathing process. Hence the loss of function of c4 nerve could affect normal breathing
process.
b) Brachial Plexus: Brachial Plexus is formed by the ventral rami of the lower four cervical and
first thoracic nerve roots (C5-C8, T1). It innervates cutaneous and muscular muscles of the entire
upper limb with exceptions for trapezius muscle and an area of skin near the axilla. Loss of
function or injury of nerves rising from brachial plexus could result in paralysis.
c) Lumbar Plexus: Lumbar plexus interconnects with sacral plexus and generally referred to as
Lumbosacral plexus. The plexus is formed by the anterior rami of the lumbar spinal nerves L1,
L2, L3 and L4 and also from thoracic spinal nerve 12. It innervates the skin and the muscles of
the lower abdominal wall, the thigh and external genitals. The femoral nerve is the largest nerve
of the plexus and innervates the skin on the upper anterior thigh. Loss of function to the femoral
nerve results in weakness of hip flexion and knee extension and sensory loss in the anterior thigh
and medial leg particularly in cases of iliacus hematoma.
10. There are five essential components of the polysynaptic reflex arc:
a) The receptor that reacts to stimulus and initiates impulse via local depolarization
b) The sensory neuron that conducts nerve impulses and transmits afferent impulse towards the
CNS
