Spinal Cord Anatomy PDF

Spinal cord -- - Each segment of spinal cord gives rise to pair of spinal nerves - 31 pairs of spinal nerves (one each side of the midline) - DRG -- dorsal root gangling -- contains the cell bodies of sensory neurones - The sensory neurones are pseudo unipolar (has 2 axons) 1. Peri...

Spinal cord -- - Each segment of spinal cord gives rise to pair of spinal nerves - 31 pairs of spinal nerves (one each side of the midline) - DRG -- dorsal root gangling -- contains the cell bodies of sensory neurones - The sensory neurones are pseudo unipolar (has 2 axons) 1. Peripherally projecting axon -- extends through a peripheral nerve, goes through dorsal root -\> spinal nerve, targets skin and muscles -- projects to PNS 2. Centrally projecting axon -- enters dorsal root and projects to spinal cord to relay sensory information to CNS - Ventral root -- allow motor neurons to exit spinal cord into spinal nerve and to skeletal muscle. Vertebrae and spinal nerves: - Humans have 32/33 bones in spinal column called vertebrae - 7 Cervical vertebrae: C1 to C7 -- located in neck - 12 Thoracic vertebrae: T1 to T12 -- located in upper back and attached to ribcage - 5 Lumbar vertebrae: L1 to L5 -- located in lower back - 5 Sacral vertebrae: S1 to S5 (fused) -- located in pelvis - Coccyx: 3-4 small fused vertebrae - Spine consists of cervical, thoracic, lumbar, sacral - Each vertebrae associated with pair of spinal nerves projecting from spinal cord e.g. c1 spinal nerves project out above c1 vertebrae, c2 nerves between c1 and c2 vertebrae etc... Spinal cord is protected within vertebral column (spinal column) - Front of body (ventral) -- vertebral body - Back of body (dorsal) -- vertebral arch  - Vertebral body and vertebral arch connected by pedicle (below the dorsal root ganglion) - Thick bony cylinder structure connecting front and back of each vertebrae (has 2 pedicles), serves as a bridge - Provides side protection for spinal cord + nerves - **Spinous process** -- muscles attach - **3 meninges** surrounding spinal cord 1. Dura -- tough, fibrous outer layer providing protection to spinal cord 2. Arachnoid -- web like structure filled with fluid, is continuous with that of the brain 3. Pia -- fine delicate layer which wraps tightly around spinal cord - Subarachnoid space (between arachnoid and pia) -- filled with CSF and small blood vessels - Epidural space (above meninges) -- contains small veins and fat - Dorsal root -- contains DRG which contains sensory axons - Ventral root -- contains motor neuron axons - T1 to L2 spinal nerves -- carry **preganglionic sympathetic neuron axons** - Spinal nerve -- contains mix of sensory and motor neuron axons - T1 to L2 spinal nerves -- carry **postganglionic sympathetic neuron axons** **[Rootlets, roots and nerves]** A diagram of the root system Description automatically generated - Dorsal root subdivides into dorsal rootlets - Carry sensory projection axons into spinal cord - Dorsal root and ventral root merge to form spinal nerve **[Organisation of the Vertebral Section]**  - Sympathetic trunk (chain ganglion) -- part of the sympathetic nervous system - Spinal nerve subdivides into 2 parts 1. Dorsal ramus -- supplies sensory, motor and sympathetic innervation to the back 2. Ventral ramus - supplies sensory, motor and sympathetic innervation to the trunk and limbs **[White and grey matter in the spinal cord]** - White matter (on outside of spinal cord) -- myelinated axon fibre tracts - Grey matter (on inside of spinal cord) -- neurons and glia - Split into different parts: 1. Dorsal horn 2. Ventral horn 3. Intermediate zone 4. Lateral horn (most pronounced in the thoracic region of spinal cord) Neurons associated with spinal cord 1. Dorsal root ganglion sensory neurons -- centrally projecting axons from the dorsal root, terminate I the dorsal horn of the spinal cord. - Make synaptic connections with other neurons 2. Motor neurons -- found in ventral horn and axons of ventral part of spinal cord, through ventral rootlets -\> ventral roots -\> spinal nerves 3. Preganglionic sympathetic neurons -- only found in thoracic region of spinal cord (intermedio-lateral horn T1-L2) 4. Interneurons -- found in all grey matter regions - Projection interneurons -- normally sensory, projections have very long axons which ascend up the spinal cord to high regions of the nervous system - Local circuit interneurons -- make connections within same action level of spinal cord, involved in computation, transmitting info to neurons and receive info from sensory neurons Spinal cord enlargements: - Spinal cord is not uniform in shape - Spinal cord ends at vertebral level L1/L2 1. Cervical enlargement - Spinal nerves that leave from this section make up brachial plexus to innervate upper limbs 2. Lumbosacral enlargement - Origin of lumbar and sacral plexi - Gives rise to 11 different nerves; L1 to L5, S1 to S5, coccygeal nerve - More compressed compared to cervical and thoracic segments - After spinal cord ends; L2 -- S5 spinal nerve roots + coccygeal nerve run caudally (downwards) in a structure called **cauda equina** until they emerge at appropriate vertebral level as spinal nerves **[Cauda equina and lumbae cistern]** - Spinal cord ends at L1/L2 vertebral level (**conus medullaris**) - After spinal cord ends, pia surrounding spinal cord fuses + forms **filum teminale,** anchors spinal cord to coccyx (stabilises spinal cord in vertebral column) - Dura and arachnoid continue to S2/S3 vertebral level, merges with film terminale - Causes subarachnoid space to enlarge -- called **lumbar cistern** A diagram of the spine Description automatically generated **[Clinical interventions associated with spinal cord]** 1. Epidural injection - Injection into epidural space (outside dura mater) - Used in pain relief for child birth 2. Lumbar punction - Relies on enlarged lumbar cistern - Needle is inserted between 3^rd^ and 4^th^ lumbar vertebrae - Can be used to analyse CSF to check for metabolic disturbances, signs of infection viruses, bacteria, antibodies Ratio of white to grey matter  - Alters along rostrocaudal axis of spinal cord - Cervical: skinny top butterfly - Thoracic -- underweight butterfly - Lumbar -- well proportioned butterfly - Sacral -- fat butterfly - In lower levels of sacral level, less ascending and descending fibre tracts so is predominantly made of grey matter **[Major ascending and descending axon tracts of spinal cord]** **Ascending tracts (sensory)** 1. **Spinothalmic tract:** pain, temperature, itch and simple touch A diagram of a human brain Description automatically generated - First order neuron -- small diameter nociceptive DRG neuron (pseudounipolar neurons within DRG) - Detects/responds to pain - Has an axon which goes into dorsal regions of grey matter of spinal cord, makes synaptic connection with long projection interneuron (second order neuron) - Second order neuron -- lies within posterior grey horn of spinal cord - Goes through process called decussation and ascend up the spinal cord anteriolaterally (now known as the spinaothalmic tract) via medulla -\> pons -\> midbrain - Makes synaptic connection with neuron in thalamus (thalmic nuclei/ ventral posterior thalmic nuclei) -- is somatotopically mapped, neurons within the nucleus are arranged in a map of the body - Third order neuron -- ventral posterior thalmic nuclei - Projects to primary sensory cortex (somatotopically mapped) 2. **Dorsal column medial lemniscus pathway:** discriminative touch and conscious proprioception  - First order neuron -- pseudounipolar neuron (low threshold mechanoreceptive DRG neuron - Projects an axon which passes through dorsal horn (grey matter) terminates in the more ventral part, to facilitate spinal reflexes, and branches to create a major branch which travels up dorsal column fibre tract -\> medulla - Second order neuron -- nucleus cuneatus & nucleus gracilis - At medulla, axon makes synaptic connection with 1 of 2 dorsal column nuclei; **nucleus cuneatus (NC)** & **nucleus gracilis (NG)** (cell bodies of projection interneurons). NC -- closer to midline, NG -- more lateral - Axons from dorsal ganglion sensory neurons project upwards - Originating from sacral and lumbar -- towards midline through **gracile fasciculus** (these axons make a synaptic connection in NG) - Originating from thoracic and cervical -- more laterally through **cuneate faciculous** (these axons make a synaptic connection in NC) - Dorsal column nuclei project axons which cross midline, ascend through medulla -\> pons -\> midbrain via medial lemniscus (contralateral). Makes connection with third order neuron in ventral posterior nucleus of thalamus - Third order neuron -- Vental posterior thalmic nuclei (ventral posterolateral neurons aka VPL -- are somatotopically mapped) - Axons project laterally out of thalamus through posterior limb into primary sensory cortex 3. **Spinocerebellar pathway:** non-conscious proprioception A diagram of a human body Description automatically generated Proprioceptive neurons innervate: 1. Muscle spindles -- detect muscle length 2. Golgi tendon organs -- detect muscle tension - First order neuron -- proprioceptive DRG neuron (very large, very heavily myelinated axons) - Projects through dorsal horn to deepest level of dorsal horn (Lamina VII) - Make connection with premotor neuron which connects with motor neuron - Basis of a reflex circuit - Proprioceptive DRG neuron also branches as it passes though grey matter of spinal cord, branch makes synaptic connection with neurons in nucleus of Lamina VII called **nucleus of Clarke** (found between C8-L3), these neurons are projection interneurons - Second order neuron - **Nucleus of Clarke** -- - The axons of these neurons extend and travel up the spines spinocerebellar tracts to make connection with third order neurone - Third order neuron -- Lateral cuneate nucleus - Axons project to ipsilateral cerebellum - This is why it is a non-conscious proprioception -- info never reaches primary sensory cortex **Descending tracts (motor)** 1. **Pyramidal motor pathway:** conscious movement -- lateral and anterior corticospinal tracts  - First order neuron -- upper motor neuron found in primary cortex - Have extremely long axons which project through the brain -\> midbrain -\> pons -\> medulla -\> spinal cord - Make a direct or indirect connection with lower motor neurons found in ventral horn of spinal cord (second order neuron) - Second order neuron - Axons project through brain to midbrain through internal capsule (IC) to medulla - At junction of medulla and spinal cord, 90% of motor neuron axons cross midline and go through decussation - 90% - travel down lateral corticospinal tract to make synaptic connections to lower moto neurons, innervate limb muscles - 10% - do not decussate, travel through anterior/lateral corticospinal tract (A.C.T.), make connections to lower motor neurons, innervate trunk/axial muscles (closer to vertebral column) 2. **Non-pyramidal motor pathways:** largely non-conscious muscle control for balance/posture/reflexes - Reticulospinal tract: muscle tone, posture, simple stereotyped movements - Vestibulospinal tract: coordinating balance, keeping head balanced, eyes horizontal when moving **Grey matter in spinal cord:** - Is divided into **10** distinct regions called **laminae** A close-up of a text Description automatically generated  A diagram of the muscles and ventricles Description automatically generated **1 --** contains central terminals of nociceptive DRG sensory neurons + soma of nociception specific projection interneurons, relays pain information to thalamus (spinothalamic tract) 2 -- central terminals of nociceptive DRG neurons + local inhibitory/excitatory interneurons that make connections to sensory projection interneurons in **I** and **V** **3-5 --** Central terminals of nociceptive **[Somatic motor functions associated with spinal nerves:]**  A diagram of the human brain Description automatically generated