Introduction to the Nervous System

Definitions

• Ganglion: collection of nerve cell bodies (in the PNS)

• Tract: bundle of axons in the CNS

• Nerve: bundle of axons in the PNS

• Bundles of axons can be all from the same modality but are normally ‘mixed’
• Modalities: somatic sensory, somatic motor, special sensory, visceral afferent, sympathetic, parasympathetic

• Spinal nerves and branches are mixed - somatic motor, somatic sensory, and sympathetic

• 5 cranial nerves are mixed

Motor or sensory

• Motor (efferent): action potential towards body wall, body cavity, or organ

• Sensory (afferent): action potential towards the brain

Central nervous system

• Brain and spinal cord - central controller

The brain

The spinal cord

• Connects with brain

• Passes through foramen magnum

• Protected by vertebral column

• 4 segments:
• Cervical (C1-C7)
• Thoracic (T1-T12)
• Lumbar (L1-L5)
• Sacral (S1-S5)/coccygeal (Co)

• 2 enlargements (cervical and lumbosacral) for innervation and control of limbs

• Ends at L1/L2 IV disc level - conus medullaris

• Lumbar and sacral spinal nerve roots descend in the vertebral canal and form the cauda equina

Peripheral nervous system

• All the other nerve tissues not within the CNS

• Peripheral nerves are bundles of axons wrapped in connective tissue, which are traveling to/from the same structure
• Bundles of nerves can leave as 'branches'

• Spinal nerves: connect with spinal cord

• Cranial nerves: connect with brain

• Autonomic nerves: innervate organs, smooth muscle, glands
• Visceral afferents, sympathetic nerves, parasympathetic nerves

Spinal nerves

• The spinal cord connects bilaterally with spinal nerves - 31 pairs

• A spinal nerve is named after the vertebrae above it, except for cervical region where they are named according to the vertebrae below (C8 exits between C7 and T1 vertebrae)

• The spinal nerves supply the soma (body wall)

• Located only within the IV foramina

• From the IV foramina, they connect with:
• Structures of the soma via rami
• The spinal cord via roots and rootlets

• Posterior rami: supply small posterior strip of the soma (back)

• Anterior rami: supply the remainder of the posterior part, the lateral and the anterior parts of the strips of the soma
• Supply all the limbs via plexus - networks of intertwined anterior rami

• Cervical plexus: C1-C4 anterior rami, supplies the posterior scalp, neck wall, diaphragm

• Brachial plexus: C5-T1 anterior rami, supplies the upper limb

• Lumbar plexus: L1-L4 anterior rami, supplies the lower limb

• Sacral plexus: L5-S4 anterior rami, supplies the lower limb, gluteal region and perineum

• Area of skin supplied by both the anterior and posterior rami of a spinal nerve
• Exception - limbs (no posterior rami)

• T4 dermatome - nipple

• T10 dermatome - umbilicus

• Dermatome maps used to illustrate dermatomes

Cranial nerves

Somatic nervous system

• Soma: body wall (external environment)
• Includes head and neck walls, chest walls, back, diaphragm, abdominal wall, pelvic wall, limbs
• Structures - skin, fascia, skeletal muscle, skeleton, internal lining of body cavities

Sensory receptors

• Mechanoreceptors: coarse and fine touch, vibration, proprioception

• Thermoreceptors: temperature

• Nociceptors: pain (sharp, stabbing, well localised)

Somatic sensation pathway to the brain

Motor innervation pathway to skeletal muscle

Spinal reflexes

• Rapid - misses out pathways to brain

• Involuntary - protective function

Autonomic nervous system

• Visceral motor system (internal environment): includes viscera, glands, smooth and cardiac muscle and external lining of organs
• Locations - internal organs in body cavities, body wall organs

• Sensory neurons: sense internal environment - heart rate, blood pressure, digestion, gland secretions
• Visceral afferent: organ sensory nerves
• Sense touch, temperature, pain (pain usually poorly localised and dull, may be sharp if colicky)

• Motor neurons: respond to changes in internal environment
• Dual motor control - sympathetic and parasympathetic nerve supply

Sympathetic division of ANS

• Supplies all internal organs, body wall organs and arterioles

• ‘Flight or fight’ response

1. Originates from autonomic centres in brain

2. Passes down spinal cord

3. Exits spinal cord with T1-L2 spinal nerves

4. Travel to sympathetic chains running the length of vertebral column

5. Pass into all spinal nerves
1. Anterior and posterior rami supply body wall structures

6. ‘Hitch a ride’ with arteries to all head and neck organs and skin

7. Travel via splanchnic nerves to reach organs

Parasympathetic division of ANS

• Often supplies same internal organs as sympathetic division, does not supply body wall organs or arterioles

• ‘Rest and digest’ response

• All parasympathetic axons leave the CNS via cranial nerves III, VII, IX and X and via sacral spinal nerves

• Parasympathetic ganglia in head travels to lacrimal gland and salivary glands

• Vagus nerve supplies organs of the neck, chest and abdomen as far as the mid-gut

• Sacral spinal nerves ‘carry’ parasympathetic axons to the hindgut, pelvis and perineum

Summary of neurone types