Spinal nerves up close

You have 31 pairs of spinal nerves (somatic nerves). They are located at each verebral level of the spinal cord:

• 8 cervical (C), in your neck or upper area of your spinal cord
• 12 thoracic (T), in your chest region
• 5 lumbar (L), in your middle to lower back
• 5 sacral (S), in your lower back
• 1 coccygeal (Cx), in your low back

Types of spinal nerves:

Spinal nerves consist of two types of nerves:

• sensory nerves
• motor nerves

Sensory nerves

Sensory nerves deliver information to your spinal cord from muscles and joints about your body position. They also transmit sensations such as touch, pressure, pain and temperature, which are experienced on the surface of your skin. This information enters your spinal cord via your posterior grey horn. It is then passed to the spinal tracts and transmitted up to the brain.

The following diagram is known as a dermatome chart. It is used to map the areas of the skin that relate to particular spinal nerves.

Motor nerves

Motor nerves (motor neurons) pass information received from your brain through your spinal tracts to your skeletal muscles to direct precise voluntary movements.

Spinal nerves are linked to specific muscles:

• Cervical spinal nerves supply the muscles of your neck, shoulders, arms and hands, and diaphragm.
• Thoracic spinal nerves supply your truck muscles and muscles involved with breathing.
• Lumbar and sacral spinal nerves supply your hip, leg and foot muscles. In addition the sacral nerves supply your anal and urethral sphincters.

 The following chart shows myotomes. Myotomes relate to the muscles or group of muscles (and their respective movements) that come from particular levels of the spinal cord.

Cervical spinal nerves

The spinal nerves in the cervical and 1st thoracic region of your spinal cord pass impulses or messages to your neck, arm and hand muscles. This allows movement such as bending or flexing, and straightening or extending your elbow, wrist and fingers.

• C 5 powers your biceps muscle and allows you to bend your elbow.
• C 6 powers your wrist muscles.
• C 7 powers your triceps muscle that allows you to straighten your arm.
• C 8 powers your fingers to grip an object.
• T1 powers your little finger and ring finger, so you can splay them apart.

These basics movements of the arms and fingers allow you to do many daily activities, including:

• writing
• working on a computer
• eating
• drinking
• dressing
• grooming
• driving

C3, C4 and C5 power your diaphragm. When your diaphragm contracts, it draws your lungs downward, expanding them and allowing them to fill with air.

Thoracic spinal nerves

T1-12 power the muscles that lie between the ribs (intercostal muscles). These also help you breathe by drawing the rib cage outwards and upwards, pulling the lungs in the same direction. The lungs expand, helping them fill with air.

The diaphragm and the intercostal muscles are your major breathing muscles.

Your lower thoracic spinal nerves T6-12 provide power to your abdominal muscles. These muscles help you cough and expel matter from your air passages. Abdominal muscles are also important in balance and posture.

Lumbar and sacral spinal nerves

Nerves in the lumbar (L) and sacral (S) sections of your spinal cord power leg muscles for walking, running and jumping.

L2 powers muscles that bend or flex your hip joint.

• L3 powers your quadriceps muscle so your leg straightens at the knee.
• L 4 powers muscles around your ankle, allowing your ankle to bend and draw the foot back towards your head (dorsi-flexion).
• S1 powers muscles around your ankle, allowing it to bend and your foot and toes to point downwards (plantar flexion)
• S2-S4 powers external sphincter muscles of your anal canal and urethra.

Autonomic nervous system

Your autonomic nervous system (ANS), which is part of the peripheral nervous system, consists of two divisions: parasympathetic and sympathetic, and they work in a complementary manner.

The ANS is responsible for bodily functions such as digestion, urination, changing the size of blood vessels in order to regulate blood pressure, regulating body temperature and keeping your heart beating.

Your parasympathetic and sympathetic nerves receive information from your brain down through your spinal cord. This information is then passed on to organs, glands and blood vessels.

Parasympathetic nervous system

Your parasympathetic division is subdivided into the cranial and sacral sections.

• The cranial section consists of the cranial nerves III, VII, IX, X which are located in the lower part of your brain, your brain stem.
  
• The sacral section is made up of autonomic nerves from the sacral section of your spinal cord, the S2, S3 and S4 levels.

• slowing your heart rate
• bronchial or air passage constriction
• increasing gastric secretions
• bladder function (e.g. bladder muscle contraction, release of urine)
• bowel function
• sexual function (e.g. erectile function and lubrication)

Sympathetic nervous system

The sympathetic system consists of nerves that are located in the thoracic and lumbar region of the spinal cord between T1-L2 levels.

The sympathetic system is responsible for:

• increasing your heart rate
• increasing blood pressure
• increasing respiratory or breathing rate
• regulating your temperature
• pupil dilation (enlargement)
• bronchial or air passage dilation
• decreasing gastric secretions
• bladder function (e.g. bladder muscle relaxation, storage of urine)
• sexual function

Spinal reflexes

Spinal nerves provide a pathway for reflex activity. Reflexes are fast and automatic responses (e.g. when an area of your skin contacts a hot surface, you quickly withdraw that part of your body away from the heat).

A variety of reflexes occur through your spinal cord.

• Reflex activity happens between your spinal cord and skeletal muscles via your spinal nerves (e.g. when the tendon of your quadriceps muscle is tapped below the knee joint, your knee jerks upwards).
• Reflex activity also happens between the spinal cord and organs and glands through autonomic nerves. These allow the many systems of your body to function without conscious control.

Reflexes assist:

• movement of food through the digestive tract
• in emptying your bladder
• in emptying your bowel
• in erection of the penis
• in stimulating the flow of secretions from sex glands that lubricate the vagina.