Anatomy of Spinal Nerves

The Peripheral Nervous System Introduction: PNS – all neural structures outside the brain and spinal cord. Provides links to and from the external environment.

Peripheral Nervous System Structural division = cranial nerves & spinal nerves. Functional divisions = Afferent (sensory) & Efferent (motor).

The 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 Nerves No. & Name Type Function I. Olfactory sensory smell II.Optic vision III. Oculomotor motor Most eyeball movements IV. Trochlear Eyeball movements

The Cranial Nerves No. & Name Type Function V. Trigeminal Mixed Sensory: face Motor: chewing VI. Abducens motor Lateral eyeball movements VII. Facial Sensory: taste Motor: facial expression VIII. Vestibulocochlear Sensory: balance & hearing

The Cranial Nerves No. & Name Type Function IX. Glosso- pharyngeal mixed Sensory: taste Motor: swallowing X. Vagus Sensory: Viscera Motor: parasymp. to above muscles XI. Accessory motor Muscles of neck XII. Hypoglossal Muscles of tongue esp. speech

The Cranial Nerves

Spinal 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 Nerves

Mixed 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 nerves

Mixed Spinal Nerves Functionally: Ventral root – carries sensory input (messages in) Dorsal Root – carries motor output (messages out)

Distribution 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 Nerves

Spinal Nerve Plexuses All ventral rami except T2-T12 form interlacing nerve networks called plexuses. (There are 4 nerve Plexuses). This is achieved by small branches joining with those of adjacent nerves Each resulting nerve of a plexus contains fibers from several spinal nerves. Damage to one spinal segment cannot completely paralyze a muscle.

Nerve Plexuses Nerve Root Plexus Area supplied C1 – C4 (C3-C5 = phrenic) Cervical Scalp & neck skin, diaphragm C5 - T1 Brachial Arm T2 - T11 No plexus formed Intercostal nerves Serve chest wall T12 – L4 Lumbar Abdominal wall, anteromedial thigh L4 – S4 Sacral Posterior thigh, lower leg and foot

Eg: Lumbar Plexus

Reflexes 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 Arc Receptor – responds to stimulus Sensory Neuron – transmits message to CNS Integration Centre (within CNS) – link between sensory and motor neurons Motor Neuron – impulse transmitted from CNS to effector Effector – muscle or gland which responds to the nerve impulse

Characteristics 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 Arc Spinal cord (in cross-section) Stimulus Integration center 2 Sensory neuron 3 Integration center 1 Receptor 4 Motor neuron Skin 5 Effector Interneuron Figure 13.1