Anatomy of Anatomy of Spinal NervesSpinal Nerves
The Peripheral Nervous SystemThe Peripheral Nervous System
Introduction:
• 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 Nerves
No. & Name Type Function
I. Olfactory sensory smell
II.Optic sensory vision
III. Oculomotor
motor Most eyeball movements
IV. Trochlear motor Eyeball movements
The Cranial NervesThe Cranial Nerves
No. & Name Type Function
V. Trigeminal Mixed Sensory: faceMotor: chewing
VI. Abducens motor Lateral eyeball movements
VII. Facial Mixed Sensory: taste Motor: facial expression
VIII. Vestibulocochlear
motor Sensory: balance & hearing
The Cranial NervesThe Cranial Nerves
No. & Name Type Function
IX. 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 Nerves
Spinal 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 Arc
1. Receptor – responds to stimulus
2. Sensory Neuron – transmits message to CNS
3. Integration Centre (within CNS) – link between sensory and motor neurons
4. Motor Neuron – impulse transmitted from CNS to effector
5. 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 Effector
Motor neuron
Stimulus
Skin
Spinal cord (in cross-section)
Interneuron