M06 | Peripheral Nervous System | Part Two

How many pairs of spinal nerves are there?

31 pairs of mixed nerves.

What are the regions of spinal nerves?

8 cervical (C1-C8), 12 thoracic (T1-T12), 5 lumbar (L1-L5), 5 sacral (S1-S5), and 1 coccygeal (C0).

Why are there 8 pairs of cervical spinal nerves despite having only 7 cervical vertebrae?

The first 7 exit superior to the vertebrae they are named for, while the 8th exits inferior to C7.

What are the two roots that each spinal nerve connects to the spinal cord?

Ventral roots (motor fibers) and dorsal roots (sensory fibers).

What do ventral roots contain?

Motor (efferent) fibers from ventral horn motor neurons that innervate skeletal muscles.

What do dorsal roots contain?

Sensory (afferent) fibers from sensory neurons in dorsal root ganglia.

What is the cauda equina?

A collection of spinal roots extending from the lumbar and sacral regions below the end of the spinal cord.

What are rami in relation to spinal nerves?

Branches of spinal nerves that include dorsal ramus (innervates back muscles) and ventral ramus (innervates anterior muscles).

What is a plexus?

An interlacing network of nerves formed by ventral rami, allowing multiple spinal nerves to innervate a single muscle.

What is the function of the phrenic nerve?

It innervates the diaphragm and is crucial for breathing.

What spinal nerves contribute to the brachial plexus?

Ventral rami of C5-C8 and T1.

Which nerve innervates the quadriceps muscle?

The femoral nerve from the lumbar plexus.

What is the longest nerve in the body?

The sciatic nerve from the sacral plexus.

What is a dermatome?

An area of skin innervated by cutaneous branches of a single spinal nerve.

What neurotransmitter is released at the neuromuscular junction?

Acetylcholine (ACh).

What is the role of the integration center in a reflex arc?

It processes the sensory input and coordinates the motor output.

What are the components of a reflex arc?

Receptor, sensory neuron, integration center, motor neuron, and effector organ.

What distinguishes inborn reflexes from learned reflexes?

Inborn reflexes are rapid and involuntary, while learned reflexes result from practice or repetition.

What is the function of the lumbar plexus?

It innervates the thigh, abdominal wall, and psoas muscle.

What is the function of the sacral plexus?

It serves the buttock, lower limb, pelvic structures, and perineum.

How do spinal roots change as they move inferiorly in the spinal cord?

They get longer as they need to reach the intervertebral foramina to exit.

What is the significance of the plexus arrangement?

It provides redundancy, ensuring that damage to one spinal nerve does not completely paralyze a muscle.

What is the role of rami communicantes?

They are branches that join ventral rami in the thoracic region and are associated with the autonomic nervous system.

What is the function of the musculocutaneous nerve?

It innervates the biceps brachii and skin of the lateral forearm.

What does the ulnar nerve supply?

It supplies the flexor carpi ulnaris, part of the flexor digitorum profundus, and most intrinsic hand muscles.

What happens at the neuromuscular junction during muscle contraction?

ACh binds to receptors, causing depolarization of the muscle cell and triggering an action potential.

What is an effector in the context of reflexes?

A muscle fiber or gland cell that responds to efferent impulses by contracting or secreting.

What are the five basic components of all reflex arcs?

Stimulus, receptor, sensory neuron, integration center, motor neuron, and effector.

What are the two functional classifications of reflexes?

Somatic reflexes (activate skeletal muscle) and autonomic (visceral) reflexes (activate visceral effectors).

Why do reflexes not require a conscious decision?

Reflexes involve sensory input that runs into the spinal cord, allowing for a motor response without brain involvement.

What is the integration center for spinal somatic reflexes?

The spinal cord.

What does the stretch reflex maintain?

Muscle tone in large postural muscles and adjusts it reflexively.

How does the stretch reflex work?

Stretch activates muscle spindle receptors, sensory neurons synapse directly with motor neurons, causing the stretched muscle to contract.

What is reciprocal inhibition in the context of the stretch reflex?

Specific fibers synapse with interneurons that inhibit motor neurons of antagonistic muscles.

What happens during the patellar reflex?

The quadriceps contracts while the hamstrings relax due to reciprocal inhibition.

What do positive reflex reactions indicate?

Intact sensory and motor connections between muscle and spinal cord, with response strength indicating spinal cord excitability.

What does a hypoactive or absent reflex indicate?

Peripheral nerve damage or ventral horn injury.

What does a hyperactive reflex response suggest?

Lesions of the corticospinal tract.

What is clonus?

A series of short rhythmic contractions and relaxations associated with hyperexcitability and upper motor neuron damage.

What is the flexor (withdrawal) reflex?

A reflex initiated by a painful stimulus that causes automatic withdrawal of the threatened body part.

What is the crossed extensor reflex?

A reflex that occurs with flexor reflexes in weight-bearing limbs to maintain balance, consisting of ipsilateral withdrawal and contralateral extensor reflex.

What are superficial reflexes?

Reflexes elicited by gentle cutaneous stimulation, depending on upper motor pathways and cord-level reflex arcs.

What does a positive plantar reflex indicate in adults?

Presence of nerve damage.

What is Babinski's sign?

An abnormal response in adults where the hallux extends and digits fan laterally, indicating damage to the motor cortex or corticospinal tracts.

What do abdominal reflexes test?

The integrity of the spinal cord from T8 to T12, resulting in contraction of abdominal muscles and movement of the umbilicus.

What does the cremasteric reflex involve?

Elevation of the testes in response to light touch on the medial thigh, indicating nerve function.

What is the significance of testing reflexes clinically?

It helps assess the condition of the nervous system by identifying exaggerated, distorted, or absent reflexes.

What type of reflex is the patellar reflex?

A monosynaptic reflex involving the quadriceps muscle.

What indicates a normal plantar reflex response in adults?

A negative response, meaning no pathology is present.

What is the role of interneurons in reflex arcs?

They facilitate communication between sensory and motor neurons, especially in polysynaptic reflexes.

What is the typical grading scale for deep tendon reflexes?

Ranges from 0 (absent) to 4 (brisk reflex with clonus).

What occurs during reciprocal inhibition in the stretch reflex?

The stretched muscle contracts while the opposing muscle relaxes.

What is the role of sensory neurons in the stretch reflex?

They transmit the stretch signal to the spinal cord where they synapse with motor neurons.