Brachial plexus

Generality

The brachial plexus is the reticular complex of spinal nerves that provides the motor and sensory innervation of a part of the chest, shoulder, arm and hand.
To be precise, the anterior branches of the last 4 cervical spinal nerves (C5, C6, C7 and C8) and of the first thoracic spinal nerve (T1) make up the so-called brachial plexus.

Originating in the spinal cord and flowing into the axillary cavity, the brachial plexus can be divided into 5 sections: the section of the 5 roots (very first section), the section of the 3 trunks, the section of the 6 divisions, the section of the 3 cords and the section of the 5 terminal branches (last stretch).
The brachial plexus can be the victim of traumatic injuries and tumors.
Traumatic injuries - which represent the most common problems affecting the brachial plexus - are generally the consequence of motorcycle accidents, severe collisions during sporting activities or a birth in which the unborn child weighs a lot and is large (NB : in such circumstances, it should be pointed out, the child is the victim of the accident).

Brief review of what "is a nerve

To fully understand what a nerve is, it is necessary to start from the concept of a neuron.
Neurons represent the functional unit of the nervous system. Their task is to generate, exchange and transmit all those (nerve) signals that allow muscle movement, sensory perceptions, reflex responses, etc.
Typically, a neuron consists of three parts:

The so-called body, where the cell nucleus resides.
Dendrites, which are equivalent to antennas for receiving nerve signals from other neurons or from receptors located in the periphery.
The axons, which are cellular extensions having the function of spreading the nerve signal. The axon covered with myelin (myelin sheath) is also called the nerve fiber.

A bundle of axons makes up a nerve.
Nerves can carry information in three ways:

From the central nervous system (CNS) to the periphery. Nerves with this property are called efferents. The efferent nerves control the movement of muscles, so they are at the head of the motor sphere.
From the periphery to the SNC. Nerves with this ability are called afferents. The afferent nerves signal to the CNS what they have detected in the periphery, therefore they cover a sensory (or sensory) function.
From the SNC to the periphery and vice versa. Nerves with this double capacity are called mixed. Mixed nerves perform a double function: motor and sensory.

What is the brachial plexus?

The brachial plexus is an "important reticular complex of spinal nerves in the human body, in which the anterior branches of the last 4 cervical spinal nerves (C5, C6, C7 and C8) and the first thoracic spinal nerve participate. T1).
Spinal nerves are nerve structures belonging to the so-called peripheral nervous system (so they are peripheral nerves). The peripheral nervous system (or PNS) is the "arm" of the central nervous system (or CNS). In fact, his job consists in transmitting to the CNS all the information collected inside and outside the organism and in spreading all the processing originating in the CNS to the periphery. Without the SNP, the central nervous system could not function adequately.
N.B: to deepen the "spinal nerves" topic, readers are advised to read the article presented here.

Anatomy

The brachial plexus begins at the level of the spinal cord - where the anterior branches of the aforementioned 5 spinal nerves arise - crosses the cervico-axillary canal present in the neck, passes over the first rib and enters the armpit (or axillary cavity).

STRUCTURE OF THE BRACHIAL PLEXUS

According to anatomists, the brachial plexus can be divided into 5 sections. Starting from the spinal cord (considered the point of origin of the brachial plexus), these sections are: the section of the 5 roots, the section of the 3 trunks, the section of the 6 divisions, the section of the 3 cords and the section of the 5 branches terminals.

Roots. The 5 roots of the brachial plexus are the 5 anterior branches of the 5 spinal nerves mentioned at the beginning of the article, ie the spinal nerves C5, C6, C7, C8 and T1.
For the avoidance of doubt, the roots are indicated with the same alphanumeric characters as the spinal nerves.
Logs. Identified with the terms upper trunk, middle trunk and lower trunk, the 3 trunks of the brachial plexus derive from the aforementioned 5 roots. To be precise, the upper trunk arises from the roots of the spinal nerves C5 and C6; the middle trunk arises from the C7 spinal nerve root; finally, the lower trunk arises from the roots of the spinal nerves C8 and T1.
Divisions. The 6 divisions derive, in pairs, from the 3 trunks. In fact, at a certain point in its path, each trunk splits in two, giving rise to an anterior and a posterior division. Therefore, of the 6 divisions making up the brachial plexus, 3 are called anterior (anterior division of the upper trunk, anterior division of the middle trunk, and so on) and 3 are called posterior (posterior division of the upper trunk and so on).
Ropes. Identified with the terms posterior chord, lateral chord and medial chord, the 3 chords are a grouping of several divisions. To be more precise, the posterior chord groups all three posterior divisions, therefore it includes the nerve bundles belonging to the roots C5, C6, C7, C8 and T1; the lateral chord groups the anterior divisions of the upper and middle trunks, therefore it includes the nerve bundles belonging to the roots C5, C6 and C7; finally, the medial cord is a continuation of the anterior division of the lower trunk, therefore it includes the nerve bundles belonging to the roots C8 and T1.
Terminal branches. Originating from the 3 cords, the 5 branches of the brachial plexus are real nerves, called: musculocutaneous nerve, ulnar nerve, median nerve, radial nerve and axillary nerve.

Figure: the 5 terminal branches of the brachial plexus. In red, the median nerve is underlined, the protagonist of a well-known nerve compression syndrome, called carpal tunnel syndrome. The musculocutaneous nerve is in continuity with the lateral cord and groups the roots C5, C6 and C7.
The ulnar nerve arises from the medial chord and groups the C8 and T1 roots.
The median nerve arises, in part, from the medial chord (includes the C5, C6 and C7 roots) and, in part, from the lateral chord (includes the C8 and T1 roots).
The radial nerve continues the posterior chord and groups the roots C5, C6, C7, C8 and T1.
Finally, the axillary nerve is in continuity with a part of the posterior cord, to be precise the one comprising the roots C5 and C6.

Motor and sensory functions of the terminal branches

Terminal branch of the brachial plexus

Motor functions

Sensitive functions

Musculocutaneous nerve

Innervates the coracobrachialis muscle, the brachialis muscle and the brachial biceps muscle (N.B: these are some of the muscles of the arm)

It innervates a cutaneous portion of the forearm

Ulnar nerve

It innervates the ulnar flexor carpus muscle, part of the deep flexor muscles of the fingers of the hand, the extrinsic muscles of the hand (except the thenar muscles and the two lateral lumbricals).

It innervates the skin of the medial side of the hand, the palmar skin of the first 3 and a half fingers (thumb, index, middle and ring half) and the dorsal skin of the first two and a half fingers (thumb, index and middle half).

Median nerve

At the level of the forearm, it directly innervates the muscles: pronator teres, radial flexor of the carpus, long palmar and superficial flexor of the fingers of the hand; it innervates, on the other hand, indirectly (ie by means of branches) the muscles: long flexor of the thumb , pronator quadrate and deep flexor of the fingers of the hand (only the radial half).
At the hand level, it indirectly innervates the thenar muscles of the hand and the lateral lumbrical muscles of the hand.

The sensory functions of the median nerve belong only to its branches.
Through the latter, therefore, it innervates the skin of the lateral portion of the palm of the hand, the palmar surface of the thumb, index, middle finger and part of the ring finger and the dorsal surface of the tip of the thumb, index and middle fingers.

Radial nerve

Innervates the brachial triceps muscle, the supinator muscle, the anconeus muscle, the extensor forearm muscle and the brachioradialis muscle

Through its branches, it innervates: the skin of the posterior portion of the arm, the skin of the lateral portion of the arm, the skin of the posterior and central region of the forearm and, finally, the skin of the dorsal surface of the thumb, index, middle and middle annular.

Axillary nerve

It innervates part of the deltoid muscle and the teres minor muscle.

It transmits sensitive information from the shoulder joint.

PRETERMINAL OR COLLATERAL BRANCHES

In reality, the brachial plexus also possesses other nerve branches, which however anatomists tend to exclude from the description of the main structure, as “less important”. These other nerve branches are called preterminal and collateral branches, since they originate in points of the brachial plexus preceding the cords or from one of the terminal branches.
The "list of collateral branches of the brachial plexus includes:

The dorsal scapular nerve. It comes from the root C5. It provides the innervation of the great rhomboid and small rhomboid muscles.
The long thoracic nerve. It comes from the roots C5, C6 and C7. Innervates the anterior serratus muscle.
The phrenic nerve. It originates from the roots C3, C4 and C5 (N.B: C3 and C4 are not part of the brachial plexus). Provides for the innervation of the diaphragm.
The subclavian nerve. It originates in a point of the upper trunk of the brachial plexus. Check the subclavian nerve.
The suprascapular nerve. It originates in a point of the upper trunk of the brachial plexus, therefore it includes the nerve bundles belonging to the cervical spinal nerves C5 and C6. It provides for the innervation of the supraspinatus and infraspinatus (or infraspinatus) muscles.
The lateral pectoral nerve. It originates in a point of the lateral cord of the brachial plexus, therefore it includes the nerve bundles belonging to the cervical spinal nerves C5, C6 and C7. It innervates a portion of the pectoralis major muscle and the pectoralis minor muscle.
The medial pectoral nerve. It originates in a point of the medial cord of the brachial plexus, therefore it includes the nerve bundles belonging to the cervical spinal nerve C8 and the thoracic spinal nerve T1. Provides for the innervation of a part of the pectoralis major muscle and a part of the pectoralis minor muscle.
The medial cutaneous nerve of the arm. It originates from the medial cord of the brachial plexus. It transmits sensitive information from the skin of the medial portion of the arm to the CNS.
The medial cutaneous nerve of the forearm. It originates in a point of the medial cord of the brachial plexus. It transmits sensitive information to the CNS from the skin of the medial portion of the forearm.

Function

With its innumerable nerve branches, the brachial plexus provides the motor and sensory innervation of a part of the chest, shoulder, arm and hand.

WHAT DOES NOT INVERTER?

The brachial plexus does not innervate the trapezius muscle or even the skin area near the armpit. The innervation of the trapezius muscle belongs to the accessory nerve (a cranial nerve), while the innervation of the skin area near the armpit belongs to the intercostobrachial nerve.

Associated pathologies

The brachial plexus can be the victim of traumatic injuries or - albeit much more rarely - of tumors.

TRAUMATIC ACCIDENTS

In most cases, traumatic injuries to the brachial plexus are the consequence of motorcycle accidents, of hard crashes during sports activities, in which physical contact is expected, and, finally, the consequence of births, in which the unborn child weighs a lot and is large (NB: in such circumstances, it should be noted, the child is the victim of the accident).
An injury to the brachial plexus can have repercussions both from a motor and a sensitive point of view.
The symptom picture varies from case to case and depends on which portion of the brachial plexus has been injured. For example, a brachial plexus injury, involving injury to the medial cord, has different effects than a brachial plexus injury. which involves the injury of the lateral cord.
From a diagnostic point of view, to understand the exact injured portion of the brachial plexus, doctors compare the motor and sensory abilities of the injured upper limb with those of the healthy upper limb.