L16 Peripheral Nervous System (PNS)

Learning objectives

Identify and explain the functions of various regions within the peripheral nervous system (PNS)

Compare and contrast the divisions of the autonomic nervous system (ANS)

Be able to apply knowledge of the ANS (on a basic level) to situations

Identify and describe the structure of a nerves

Describe cranial and spinal nerves (don’t need to remember all)

Describe the dorsal and ventral roots and explain their importance

Recognise and describe the plexuses

What is the peripheral nervous system (PNS)?

Everything outside of CNS

2 types of nerves:

• cranial nerves (12 pairs)

• Spinal nerves (31 pairs)

Ganglia

Enteric plexuses (GIT)

Other nerve plexuses

Sensory receptors

Why does the PNS?

Sciatica: a range of symptoms, including extreme back pain and radiating pain through the lower limbs

Compression or inflammation of sciatic nerve root = pain running down leg

Organisation of the PNS

In/output from the PNS

Afferent sensory nerves carry info from sensory receptors to the CNS

• somatic (skin, muscles, joints) - reflex action

• Visceral (visceral organs)

Efferent (motor) nerves carry information away from the CNS to the effector organs

• somatic motor nerves innervate skeletal muscle

• Automatic nerves innervates smooth muscle, cardiac muscle & glands (Parasympathetic and sympathetic)

Key anatomical differences between ANS divisions

1/ sites of origin

2/ fibre length

3/ location of ganglia

1/ site of origin

PS: brain stem, sacral region

S: lumbar region, thoracic region

2/ length of fibres

PS: long preganglionic, short postganglionic

S: short preganglionic, long postganglionic

3/ ganglia location

PS: within/near visceral effector organs

S: close to spinal cord

2 divisions of autonomic nervous system

Sympathetic (“fight or flight” → kicks in during stress)

Parasympathetic (“rest and digest”)

Key anatomical differences between ANS divisions

Parasympathetic

Fibers originate in brain stem/sacral spinal cord

Sympathetic

Fibers originate in the thoracic and lumbar spinal cord

The ANS divisions

3 types of nerves

Sensory

Motor

Inter

Sensory neuron

Activated by external physical or chemical stimuli

Picking up signals from the environment and send them affluently into our NS

Motor neuron

Found in the spinal cord that connect to organs, muscles, and different types of glands in the body

Efferent: sending signals out onto our organs, muscles, and glands

Interneuron

Connect motor neurons to sensory neurons, allowing signalling between the 2

High concentration of inter neurons in the spinal cord

3 layers in connective tissue

Endoneurium: surrounds individual axons within a fascicle

Perineurium: wraps around each fascicle

Epineurium: encases all the fascicles to form the whole nerve

Blood vessels

Provide oxygenated blood

Take away deoxygenated blood + other wastes

Nerve fiber types

Myelinated (white matter) axons: rapidly transmit impulses over long distances

Unmyelinated (grey matter) axons: slower or shorter-distance transmissions

Structure of a nerve

31 spinal nerves attach to the spinal cord

8 cervical

12 thoracic

5 lumbar

5 sacral

1 coccygeal

Mixed spinal nerves

Spinal nerves branch to form dorsal and ventral rami

Dorsal rami

Innervation to the skin + back muscles (smaller)

Ventral rami

Innervation to the rest of trunk and limbs (larger)

Nerve plexuses ???

Formed by ventral rami of spinal nerves

Cervical

Brachial

Lumbar

Sacral

Cervical

Located under the sternocleidomastoid, innervates the skin

Brachial

Located in neck and shoulder, innervates the upper limb

Lumbar

Located within psoas muscle (hip), innervates abdominal wall, anterior and medial thigh

Sacral

Located posterior to lumbar plexus, innervates buttocks, lower limb, pelvis

Brachial plexus injuries

Injuries to brachial plexus are common - can weaken or paralyse arm

Ulnar nerve is very vulnerable (“funny bone” - ulnar nerve is superficial, rests against medial epicondyle)

Eg.

• carpal tunnel syndrome - median nerve is compressed

• Brachial plexus birth palsy

  • Any injury to brachial plexus during difficult delivery (nerves are stretched or torn)

  • Most recover in 3-6m with physical therapy, otherwise surgery

Cranial nerves

12 pairs: sensory, motor, or both (mixed)

Arranged along the longitudinal axis of brain:

CNI attaches to the cerebrum

CNII attaches to diencephalon

CNIII-XII attach to the brain stem

Compare vs. contrast parasympathetic + sympathetic