Anatomy of Anatomy of Spinal NervesSpinal Nerves

The Peripheral Nervous SystemThe Peripheral Nervous SystemIntroduction:

• PNS – all neural structures outside the brain and spinal cord.

• Provides links to and from the external environment.

Peripheral Nervous SystemPeripheral Nervous System

Structural division = cranial nerves & spinal nerves.

Functional divisions = Afferent (sensory) & Efferent (motor).

The Cranial NervesThe Cranial Nerves• 12 pairs of nerves – first two arise from

forebrain and remaining 10 arise from the brain stem

• Numbered 1 to 12 from anterior to posterior• Names indicate primary functions or areas

served• Some are mixed nerves; some are purely

sensory; some are purely motor (although motor nerves may carry some afferent fibres from proprioceptors)

The Cranial NervesThe Cranial NervesNo. & Name Type FunctionI. Olfactory sensory smell

II.Optic sensory vision

III. Oculomotor

motor Most eyeball movements

IV. Trochlear motor Eyeball movements

The Cranial NervesThe Cranial NervesNo. & Name Type FunctionV. Trigeminal Mixed Sensory: face

Motor: chewingVI. Abducens motor Lateral eyeball

movementsVII. Facial Mixed Sensory: taste

Motor: facial expression

VIII. Vestibulocochlear

motor Sensory: balance & hearing

The Cranial NervesThe Cranial NervesNo. & Name Type FunctionIX. Glosso-pharyngeal

mixed Sensory: tasteMotor: swallowing

X. Vagus mixed Sensory: VisceraMotor: parasymp. to above muscles

XI. Accessory motor Muscles of neck

XII. Hypoglossal

motor Muscles of tongue esp. speech

The Cranial NervesThe Cranial Nerves

Spinal NervesSpinal Nerves

• 31 pairs of nerves coming off the spinal cord.

• They are all mixed nerves – i.e. contain both afferent (sensory) and efferent (motor) fibres.

• Supply all parts of the body except for the head and some areas of the neck.

• Named according to their point of emergence from the spinal cord.

Spinal NervesSpinal Nerves

Mixed Spinal NervesMixed Spinal Nerves

Structurally, a spinal nerve attaches to the spinal cord via:

• A Ventral Root – which attaches to the anterior horns of grey matter

&• A Dorsal Root – which attaches to the

posterior horns of grey matter.

Note: The dorsal root is characterised a swelling (the Dorsal Root Ganglion)

Mixed nervesMixed nerves

Mixed Spinal NervesMixed Spinal NervesFunctionally:

Ventral root – carries sensory input(messages in)

Dorsal Root – carries motor output(messages out)

Distribution of Spinal NervesDistribution of Spinal NervesSpinal nerves branch into several rami.

• Dorsal Ramus – supplies the skin and deep muscles of the back

• Ventral Ramus – supplies superficial back muscles, lateral & anterior trunk, limbs

• Rami Communicantes – serve visceral organs (part of Autonomic system)

Spinal NervesSpinal Nerves

Spinal Nerve PlexusesSpinal Nerve PlexusesAll ventral rami except T2-T12 form interlacingnerve networks called plexuses. (There are 4 nervePlexuses).

This is achieved by small branches joining with those of adjacent nerves

Each resulting nerve of a plexus contains fibers fromseveral spinal nerves.

Damage to one spinal segment cannot completelyparalyze a muscle.

Nerve PlexusesNerve PlexusesNerve Root Plexus Area supplied

C1 – C4(C3-C5 = phrenic)

Cervical Scalp & neck skin, diaphragm

C5 - T1 Brachial Arm

T2 - T11 No plexus formed Intercostal nervesServe chest wall

T12 – L4 Lumbar Abdominal wall, anteromedial thigh

L4 – S4 Sacral Posterior thigh, lower leg and foot

Eg: Lumbar PlexusEg: Lumbar Plexus

ReflexesReflexes• Reflex – a rapid, unconscious response to

a stimulus.examples - stretch reflex; flexor (withdrawal) reflex

• A Reflex Arc is a simple neural pathway by which sensory impulses from receptors cause a response in effectors without necessarily travelling to the brain

Components of a Reflex ArcComponents of a Reflex Arc1. Receptor – responds to stimulus2. Sensory Neuron – transmits message to

CNS3. Integration Centre (within CNS) – link

between sensory and motor neurons 4. Motor Neuron – impulse transmitted from

CNS to effector5. Effector – muscle or gland which responds

to the nerve impulse

Characteristics of reflexesCharacteristics of reflexes• Somatic reflex – activates skeletal

muscle• Autonomic reflex – activates visceral

muscle• Note: Although inborn; most reflexes

are subject to modification through learning & conscious effort

• Primary Function = protection

Reflex ArcReflex Arc

Figure 13.1

Receptor12 3

4

Sensory neuron Integration center

5 EffectorMotor neuron

Stimulus

Skin

Spinal cord (in cross-section)

Interneuron