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A. Introduction:
1.
In the previous sections/chapters, we discussed at length the major parts of the Central Nervous System (CNS).
2.
It is now time to start discussing the other part of the nervus system; the Peripheral Nervous System.
3.
What is the major difference between the Central and the Peripheral nervous system? The peripheral system is located outside the skull and the vertebrae; is therefore not protected by the bony structures.
3.
The Peripheral Nervous System consists of two major parts:
- the Somatic Nervous System
- the Autonomic Nervous System
4.
In this section (H.5.), we will discuss the Somatic Nervous System and in H.6. the Autonomic Nervous System (ANS).
B. Somatic Nerves:
1.
The somatic nervous system consists (again!) of two parts; afferent motor nerves that run to effectors (muscles, glands etc.) and efferent sensory nerves (such as taste and touch) to the CNS.
2.
All these nerves can be grouped into nerve bundles. In the upper part of the body, these nerves run though the 12 cranial nerves (I-XII).
See H.2.2. Cranial Nerves
3.
The first two cranial nerves (olfactory and optic nerve) are actually part of the CNS as they are located inside the skull but that is a minor issue.
4.
The other 10 cranial nerves (III-XII) originate from the brainstem and are mostly involved in the function of organs (muscles etc.) located in the head.
5.
For the rest of the body, the spinal nerves are responsible for the somatosensory network.
These spinal nerves come out of the spinal cord in the space between two adjoining vertebrae, towards and from the left and the right side of the body.
In humans, there are 31 pairs of spinal nerves:
- 8 cervical
- 12 thoracic
- 5 lumbar
- 5 sacral
- 1 coccygeal
7.
The last one, the coccygeal (= tailbone; remnant of ‘our’ tail) nerve is often forgotten! Even I forgot it (see H.3.2 Sensory pathways).
C. Remember the dermatomes?
1.
As we said before, the sensory nerves come for a large part from sensors located in the skin (touch, temperature and pain). These afferent nerves run to the nearest nerve bundle to enter the spinal cord and the brain.
2.
The origin of these nerves and their sensors can therefore be grouped together into ‘dermatomes’, as discussed in H.3.2. Sensory Pathways.
3.
More or less the same also applies to the efferent nerves; the nerves that run from the spinal cord to their effectors (muscles, glands etc.). However, these effectors are located inside the body, coupled to the skeleton and other organs and therefore not ‘visible’ on the surface of the body as are the sensors.
4.
In fact, the efferent nerves can be better described in anatomical then in physiological terms.
D. Nerves and Muscle groups in the Human Body:
Neck area:
The Phrenic nerve: from C3 and C4 and runs straight through the thorax to the diaphragm. It is a major nerve for our respiration!
Shoulder:
The muscles on and around the shoulder and the upper part of the thorax are innervated by:
- Thoracicus longus -> M. Serratus anterior
- Thoracodorsalis -> M. latissimus dorsi
- Pectoralis medialis and lateralis -> the major and minor pectoralis muscles
- Axillaris -> M. deltoideus
- Dorsalis scapulae etc. -> shoulder muscles
Arm:
All the nerves for the arm originate in the brachial plexus (C1 – C8):
- Musculocutaneus -> M. biceps, M. brachialis etc.)
- Medianus -> many muscles in the lower arm and hand
- Ulnaris -> many muscles in the hand
- Radialis -> many muscles in upper and lower arm
Thorax and Abdominal wall: these nerves originate from Th1 up to Th12.
- Intercostales -> innervate the muscles of the ribcage (Breathing!) and also the muscles in the abdominal wall (M. rectus abdominis etc.)
Hip and Pelvis: these nerves originate from the plexus lumbosacralis (from L1-L5 and S1 to S4)
- psoas major, minor
- ilioinguinalis and N. genitofemoralis innervate the skin (sensory) and muscles (motor) of the external genitalia of both males and females
- Gluteus superior and Inferior innervate the gluteus muscles in the hip (M. gluteus maximus etc.)
Leg: these nerves also originate from the plexus lumbosacralis
- Obturatorius innervate several adductor muscles in the legs
- Femoralis innervate the muscles that stretch our legs (M. quadriceps femoris and M. sartorius)
- Ischiadicus innervate the dorsal muscles and also divides int two other nerves:
- N. Peroneus and N. Tibialis who also innervate many muscles in different parts of the legs.
