The peripheral nervous system

NEURONS

·         Neurons are the fundamental units of the brain and nervous system, the cells that receive and send messages from the body to the brain and back to the body.

·         Each neuron consists of a cell body and its processes: one axon and many dendrites.

·         Neurons need a continuous supply of O₂ and glucose for their survival.

Axon and dendrites

·         Axons and dendrites are extensions of cell bodies.

Axon:

·         Each nerve cell has only one axon, which begins at a tapered area of the cell body that is called the axon hillock.

·         It carries impulses away from the cell body and is usually longer than the dendrites; sometimes it is about 100cm long.

·         The membrane of the axon is called Axolemma.

·         Large axons and those of peripheral nerves are surrounded by a myelin sheath.

·         The axon in covered by number of layers of Schwann cell plasma membrane.

·         Between the layers of plasma membrane there is small amount fatty substance called myelin.

·         The outermost layer of Schwann cell plasma membrane is the Neurilemma.

Dendrites

·         It receives and carries impulses towards the cell body.

·         They have the same structure as axons but are usually shorter and branching.

·         In motor neurons, dendrites form part of the synapse, and in sensory neurons they form the sensory receptors that respond to specific stimuli.

The nerve impulse (Action potential)

·         An impulse is initiated by stimulation of a sensory nerve ending or by the passage of an impulse from another nerve.

·         Transmission of action potential is due to movement of ions across the nerve cell membrane.

·         When an impulse/AP occurs at one node, depolarization passes along the myelin sheath to the next node so that the flow of current appears to “leap” from one node to the next. This is called Saltatory condition.

The synapse and neurotransmitters

·         The neurone involved in the transmission of a nerve impulse from its origin to destination, i.e. sensory or motor transmission.

·         The neurotransmitters include NA, Adr, Dopamine, Serotonin, Gamma aminobutyric acid (GABA) and Acetylcholine.

·         Other substances such as Enkephalins, Endorphins, and Substance P have a specialized role in transmission of pain signals.

Nerves

·         It consists of numerous neurons collected into bundles. Each bundle has several coverings of protective connective tissue.

o   Endoneurium

o   Perineurium

o   Epineurium

Sensory or afferent nerves

·         Sensory nerves carry information from the body to the spinal cord.

·         The impulses may then pass to the brain or to connector neurons of reflex arcs in the spinal cord.

Sensory receptor

·         Specialised endings of sensory numerous responds to different stimuli inside and outside the body.

Somatic, cutaneous or common sense

·         This originate in the skin, these are: pain, touch, heat and cold. Sensory nerve ending in the skin are fine branching filaments without myelin.

Proprioceptor sense

·         These originate in muscles and joints and contribute to the maintenance of balance and posture.

Special sense

·         These are sight, hearing, balance, smell and taste.

Autonomic afferent nerves

·         These originate in internal organs, glands and tissue.

·         e.g.      Baroreceptor: Control BP

Chemoreceptor: Central respiration

Regulation of involuntary activity and visceral pain.

Motor or efferent nerve

·         Motor nerves originate in the brain, spinal cord and autonomic ganglia.

·         They transmit impulses to the effector organ, muscle and gland. There are two types;

1.       Somatic nerve: Voluntary and reflex smooth muscle contraction

2.       Autonomic nerve: (sympathetic and parasympathetic) involved in cardiac and smooth muscle contraction and glandular secretion.

Mixed nerves

·         These are the nerves that perform both the action of sensory nerves as well as motor nerves.

o   e.g.       Suprascapular nerve

Femoral nerve

Sciatic nerve

Vagus nerve

Autonomic nervous system (ANS)

·         Autonomic nervous system is the part of the peripheral nervous system responsible for regulating involuntary body functions, such as blood flow, heartbeat, digestion, breathing, etc.

·         Both the sympathetic and parasympathetic systems essentially consist of;

1.       Preganglionic nerve

2.       Ganglion

3.       Postganglionic nerve

4.       Effector organ

Sympathetic nervous system

·         The sympathetic nervous system is located in the sympathetic chain, which connects to the skin, blood vessels and organs in the body cavity.

·         The sympathetic chain is located on both sides of the spine and consists of ganglia.

Preganglionic fibre

·         The preganglionic fibres of the sympathetic nervous system arise from lateral horn cells of the spinal cord.

·         They pass through anterior nerve roots of spinal nerves and run for a short distance in the spinal nerve, and from the spinal nerves, they are communicated to the ganglia of the sympathetic chain through white rami communicants.

Postganglionic fiber

·         They are formed from grey ramus communicants, which arise from the ganglia of the sympathetic chain.

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Enter into the spinal nerve of the same level

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Reach the organ which they supply.

Chemical transmitters

·         The transmitter in the preganglionic sympathetic nerve is Ach, which is liberated at the ganglion. Ach is the transmitter in the preganglionic sympathetic nerve.

·         In postganglionic sympathetic nerves, NA is the transmitter. It is liberated at the postganglionic sympathetic nerve ending.

Parasympathetic nervous system

·         The parasympathetic nervous system is the branch of the autonomic nervous system (ANS) responsible for the body’s ability to recuperate and return to a balanced state after experiencing pain or stress.

Preganglionic fibre

·         The preganglionic fibres of parasympathetic nerves arise from cells present in the midbrain, medulla, and sacral position of the spinal cord.

o   From the midbrain, the fibers emerge through the oculomotor nerve.

o   From the medulla, they emerge through the facial, glossopharyngeal and vagus nerves.

o   From the sacral portion of the spinal cord, they arise from the anterior column of 2^nd, 3^rd and 4^th lumbar segments. Then pass through the anterior roots of the corresponding spinal nerves.

o   All these nerves end in a ganglion.

Postganglionic fiber

·         They arise from the ganglia and then reach the structures which these nerves supply.

 Difference between sympathetic and parasympathetic nervous system

 Spinal nerves

Spinal nerves or nerve roots branch off the spinal cord and pass out through a hole in each vertebra called the foramen. 31 pairs of spinal nerves leave the vertebral canal by passing through the intervertebral foramina formed by adjacent vertebrae. They are named and grouped according to the vertebrae with which they are associated.

·         8 cervical

·         12 thoracic

·         5 lumbar

·         5 sacral

·         1 coccygeal.

Nerve roots

·         The spinal nerves arise from both sides of the spinal cord and emerge through the intervertebral foramina.

·         Each nerve is formed by the union of a motor (anterior) and a sensory (posterior) nerve root and is, therefore, a mixed nerve.

·         Thoracic and upper lumbar (L1 and L2) spinal nerves have a contribution from the sympathetic part of the autonomic nervous system in the form of a preganglionic fibre.

Anterior nerve root

The anterior nerve root consists of motor nerve fibres, which are the axons of the lower motor neurones from the anterior column of grey matter in the spinal cord and, in the thoracic and lumbar regions, sympathetic nerve fibres, which are the axons of cells in the lateral columns of grey matter.

Posterior nerve root

·         The posterior nerve root consists of sensory nerve fibres. Just outside the spinal cord there is a spinal ganglion (posterior, or dorsal, root ganglion), consisting of a little cluster of cell bodies.

·         Sensory nerve fibres pass through these ganglia before entering the spinal cord. The area of skin whose sensory receptors contribute to each nerve is called a dermatome.

Nerve plexus

·         At certain region of the spinal cord some individual nerve trunks unit to form plexuses.

·         There are five large plexuses of mixed nerves formed on each side of vertebral column.

1.       cervical plexuses

2.       brachial plexuses

3.       lumbar plexuses

4.       sacral plexuses

5.       coccygeal plexuses.

CERVICAL PLEXUS

·         It is formed by the anterior rami of the first four cervical nerves.

BRACHIAL PLEXUS

·         The anterior rami of the lower four cervical nerves and a large part of the 1st thoracic nerve form the brachial plexus.

·         The branches of the brachial plexus supply the skin and muscles of the upper limbs and some of the chest muscles.

·         Branches of brachial plexus

1.       axillary (circumflex) nerve: C5, 6

2.       radial nerve: C5, 6, 7, 8, T1

3.       musculocutaneous nerve: C5, 6, 7

4.       median nerve: C5, 6, 7, 8, T1

5.       ulnar nerve: C7, 8, T1

6.       medial cutaneous nerve: C8, T1.

Axillary (circumflex) nerve

·         At the surgical neck level, the axillary (circumflex) nerve winds around the humerus.

·         The deltoid muscle, shoulder joint, and surrounding skin are then supplied by tiny branches that split off from the main branch.

Radial nerve

·         The radial nerve is the largest branch of the brachial plexus.

·         It supplies the triceps muscle behind the humerus, crosses in front of the elbow joint then winds round to the back of the forearm to supply extensors of the wrist and finger joints.

Musculocutaneous nerve

·         The lateral side of the forearm is reached by the musculocutaneous nerve moving downwards.

·         It supplies the muscles of the upper arm and the skin of the forearm.

Median nerve

·         The median nerve and brachial artery travel together through the midline of the upper arm.

·         It travels in front of the elbow and descends to supply the forearm muscles.

Ulnar nerve

·         In the upper arm, the ulnar nerve travels down medially to the brachial artery.

·         The muscles of the ulnar side of the forearm are supplied by this artery as it travels behind the medial epicondyle of the humerus.

·         It continues downwards to supply the muscles in the palm of the hand and the skin of the whole of the little finger and the medial half of the third finger.

LUMBAR PLEXUS

·         The lumbar plexus is generated by the anterior rami of the first three lumbar nerves and a portion of the fourth lumbar nerve.

·         The plexus is situated in front of the transverse processes of the lumbar vertebrae and behind the psoas muscle.

·         The main branches are;

o   Iliohypogastric nerve: L1

o   ilioinguinal nerve: L1

o   genitofemoral: L1, 2

o   lateral cutaneous nerve of thigh: L2, 3

o   femoral nerve: L2, 3, 4

o   obturator nerve: L2, 3, 4

o   lumbosacral trunk: L4, (5).

Sacral plexus

·         The sacral plexus is formed by the anterior rami of the lumbosacral trunk and the 1^st, 2^nd and 3^rd sacral nerves.

·         The lumbosacral trunk is formed by the 5th and part of the 4th lumbar nerves.

·         It lies in the posterior wall of the pelvic cavity.

Coccygeal plexus

·         The coccygeal plexus is a very small plexus formed by part of the 4th and 5th sacral and the coccygeal nerves.

·         The nerves from this plexus supply the skin around the coccyx and anal area.

Thoracic nerves

·         There are 12 pairs and the first 11 are the intercostal nerves. They pass between the ribs supplying them, the intercostal muscles and overlying skin.

·         The 12th pair comprise the subcostal nerves.

·         The 7th to the 12th thoracic nerves also supply the muscles and the skin of the posterior and anterior abdominal walls

Cranial nerve