Dominic Stone
The human body is a complex network of nerves and muscles, with nerves acting as the body's conduits for information, carrying electrical signals to and from the brain and spinal cord to control sensations, movement, and other functions. These nerves are clusters of cells called neurons, which carry messages to and from the brain through the spinal cord to muscles in the body. Motor neurons, a type of neuron, are responsible for muscle movement and control, both voluntary and involuntary. Each pair of nerves branching off the spinal cord is responsible for a specific area of the body, with 31 pairs of spinal nerves providing sensory function, motor function, or both. Cranial nerves, on the other hand, are a set of 12 nerves that emerge from the brain, sending electrical signals to different parts of the head, face, neck, and torso. These nerves play a crucial role in relaying sensory and movement information, with some controlling specific muscles, such as the facial nerve (CN VII) and the hypoglossal nerve (CN XII).
| Characteristics | Values |
|---|---|
| Number of cranial nerve pairs | 12 |
| Cranial nerve function | Relaying sensory and/or movement (motor) information |
| Nerve function | Sending electrical signals to control sensations, movement, and other functions |
| Nerve location | Throughout the body |
| Nerve structure | Long string-like branching structures |
| Nerve composition | Clusters of cells called neurons |
| Spinal nerves | 31 pairs branching out from the spinal cord |
| Brachial plexus nerves | C5, C6, C7, C8, T1 |
| Axillary nerve function | Provides function to the deltoid, teres minor, and long head of the triceps muscle |
| Somatic nervous system | Component of the peripheral nervous system associated with the voluntary control of body movements via skeletal muscles |
| Motor neurons | Responsible for movement or motor control |
| Sensory neurons | Responsible for sensation |
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What You'll Learn
The cranial nerve pairs
There are 12 cranial nerve pairs, which are responsible for relaying sensory and/or movement (motor) information. These nerves have paths within and outside the skull, with the paths inside the skull called "intracranial" and those outside called "extracranial". All cranial nerves are paired, meaning they occur on both the right and left sides of the body.
The 12 cranial nerve pairs are:
- Olfactory nerve (CN I): Provides the sense of smell.
- Optic nerve (CN II): Provides vision.
- Oculomotor nerve (CN III): Responsible for opening and moving the eyes and adjusting pupil width.
- Trochlear nerve (CN IV): Controls looking down and moving the eyes toward or away from the nose.
- Trigeminal nerve (CN V): Has both motor and sensory functions. It helps with chewing and clenching the teeth and gives sensation to muscles in the tympanic membrane of the ear.
- Abducens nerve (CN VI): This nerve is involved in eye movement.
- Facial nerve (CN VII): Controls several facial muscles, enabling facial expressions, and provides a sense of taste on part of the tongue.
- Vestibulocochlear nerve (CN VIII): Provides the senses of hearing and balance.
- Glossopharyngeal nerve (CN IX): Provides taste sensations to part of the tongue and controls muscles for swallowing. It also has parasympathetic nerve fibres that play a role in blood pressure regulation and saliva production.
- Vagus nerve (CN X): Regulates several automatic bodily processes, including digestion, blood pressure, heart rate, breathing, mood, and saliva production. It is the main nerve of the parasympathetic nervous system.
- Accessory nerve or spinal accessory nerve (CN XI): Controls shoulder and neck movement.
- Hypoglossal nerve (CN XII): Controls tongue movement, which is important for speaking, eating, and swallowing.
These cranial nerve pairs are essential for various functions, from sensory perception to controlling specific muscles and automatic bodily processes.
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The somatic nervous system
The SNS has 31 pairs of spinal nerves that branch out from the spinal cord and 12 pairs of cranial nerves that stem from the brain. The spinal nerves are mixed nerves, meaning they carry sensory information into and motor commands out of the spinal cord. They serve as a bridge between the environment and the central nervous system. These nerves are arranged into 31 pairs according to the regions of the spinal cord. To be precise, there are eight pairs of cervical nerves (C1–C8), twelve pairs of thoracic nerves (T1–T12), five pairs of lumbar nerves (L1–L5), five pairs of sacral nerves (S1–S5), and one pair of coccygeal nerves.
The cranial nerves play a role in relaying sensory and/or movement (motor) information. For example, the facial nerve (CN VII) controls several facial muscles to make facial expressions and provides a sense of taste on the tongue. The optic nerve (CN II), on the other hand, is technically part of the brain, not the SNS, as it connects to the eyes.
The SNS controls all voluntary muscular systems within the body and a specific type of involuntary muscle response known as reflexes, which are controlled by a neural pathway called the reflex arc. The SNS is also responsible for regulating the body's voluntary movements.
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The spinal nerves
Spinal nerves are bundles of nerve fibres connected to the spinal cord. They carry information to and from the spinal cord, supplying all areas of the body except for the head and neck regions (these are supplied by cranial nerves). Spinal nerves are essential for the control of body parts by the higher centres of the nervous system. They can provide sensory function, motor function, or both. For example, spinal nerves may carry sensations from joints and muscles to the spinal cord, and they also control some of our reflexes or involuntary responses, such as pulling your hand away from a hot stove.
There are 31 pairs of spinal nerves branching out from the spinal cord, with each nerve formed by the combination of nerve fibres from the dorsal and ventral roots of the spinal cord. The nerves emerge from the spinal column through an opening (intervertebral foramen) between adjacent vertebrae. The nerves are numbered from superior to inferior positions, with each nerve named for the level of the spinal cord at which it emerges. There are eight cervical spinal nerve pairs (C1-C8), 12 thoracic pairs (T1-T12), five lumbar pairs (L1-L5), five sacral pairs (S1-S5), and one coccygeal pair.
The brachial plexus is a group of nerves that control the muscles of the shoulder, arm, forearm, and hand. The roots are labelled C5, C6, C7, C8 for the cervical vertebrae and T1 for the first thoracic vertebra. The lumbar plexus arises from axons of the ventral rami of spinal nerves T12 through L4 and gives rise to nerves enervating the pelvic region and the anterior leg. The femoral nerve is one of the major nerves from this plexus, which gives rise to the saphenous nerve, extending through the anterior lower leg. The sacral plexus comes from the lower lumbar nerves L4 and L5 and the sacral nerves S1 to S4. The most significant systemic nerve from this plexus is the sciatic nerve, which is a combination of the tibial nerve and the fibular nerve.
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The brachial plexus
The lateral cord is formed by the anterior divisions of the upper and middle trunks and gives rise to the lateral pectoral nerve (C5, C6, and C7). This nerve supplies the pectoralis major muscle. The medial cord is formed by the anterior division of the lower trunk and gives off the medial pectoral nerve (C8 and T1), the medial brachial cutaneous nerve (T1), and the medial antebrachial cutaneous nerve (C8 and T1), which provide sensory innervation to the skin of the arm and forearm. The posterior cord is formed by the posterior divisions of each of the three trunks and gives rise to the upper, middle, and lower subscapular nerves.
The final subdivision of the brachial plexus consists of five terminal branches, including the musculocutaneous nerve (C5 and C6) and the axillary nerve (C6-T1). The musculocutaneous nerve innervates the teres minor and deltoid muscles, while the axillary nerve innervates the flexor muscles in the forearm, the thenar muscles, and the lateral lumbricals associated with the index and middle fingers. The brachial plexus is susceptible to injuries, ranging from mild stretch injuries to complete root tears, which can result in a loss of function and sensation in the affected arm.
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The axillary nerve
The posterior terminal branch provides motor innervation to the posterior aspect of the deltoid muscle and the teres minor, a muscle that is part of the rotator cuff and acts to stabilise the glenohumeral joint. It also innervates the skin over the inferior part of the deltoid, known as the regimental badge area. The anterior terminal branch provides motor innervation to the anterior aspect of the deltoid muscle.
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Frequently asked questions
The hypoglossal nerve (CN XII) controls the tongue and is responsible for tongue movement, which plays a role in speaking, eating and swallowing.
The brachial plexus is a group of nerves that control the muscles of the shoulder, arm, forearm and hand.
T1 is the lowest nerve "root" that becomes part of the brachial plexus. It eventually becomes the nerve that feeds the muscles in the hand.
The glossopharyngeal nerve (CN IX) provides taste sensations to part of the tongue and controls the muscles used for swallowing.
The facial nerve (CN VII) controls several facial muscles, enabling facial expressions and providing a sense of taste to part of the tongue.
