• Runs through the vertebral canalSpinal Cord • Extends from foramen magnum to second lumbar vertebra • Regions – Cervical – Thoracic – Lumbar – Sacral – Coccygeal • Gives rise to 31 pairs of spinal nerves – All are mixed nerves • Not uniform in diameter – Cervical enlargement: supplies upper limbs – Lumbar enlargement: supplies lower limbs • Conus medullaris- tapered inferior end – Ends between L1 and L2 • Cauda equina - origin of spinal nerves extending inferiorly from conus medullaris.
• Connective tissue membranesMeninges – Dura mater: outermost layer; continuous with epineurium of the spinal nerves – Arachnoid mater: thin and wispy – Pia mater: bound tightly to surface • Forms the filum terminale – anchors spinal cord to coccyx • Forms the denticulate ligaments that attach the spinal cord to the dura • Spaces – Epidural: external to the dura • Anesthestics injected here • Fat-fill – Subdural space: serous fluid – Subarachnoid: between pia and arachnoid • Filled with CSF
Cross • Anterior median fissure and posterior median sulcus Section of • – deep clefts partially separating left and right halves Gray matter: neuron cell bodies,Spinal Cord dendrites, axons – Divided into horns • Posterior (dorsal) horn • Anterior (ventral) horn • Lateral horn • White matter – Myelinated axons – Divided into three columns (funiculi) • Ventral • Dorsal • lateral – Each of these divided into sensory or motor tracts
Cross section of Spinal Cord• Commissures: connections between left and right halves – Gray with central canal in the center – White• Roots – Spinal nerves arise as rootlets then combine to form dorsal and ventral roots – Dorsal and ventral roots merge laterally and form the spinal nerve
Organization of Spinal Cord Gray Matter• Recall, it is divided into horns – Dorsal, lateral (only in thoracic region), and ventral• Dorsal half – sensory roots and ganglia• Ventral half – motor roots• Based on the type of neurons/cell bodies located in each horn, it is specialized further into 4 regions – Somatic sensory (SS) - axons of somatic sensory neurons – Visceral sensory (VS) - axons of visceral sensory neur. – Visceral motor (VM) - cell bodies of visceral motor neurons – Somatic motor (SM) - cell bodies of somatic motor neurons
White Matter in the Spinal Cord• Divided into three funiculi (columns) – posterior, lateral, and anterior – Columns contain 3 different types of fibers (Ascend., Descend., Trans.)• Fibers run in three directions – Ascending fibers - compose the sensory tracts – Descending fibers - compose the motor tracts – Commissural (transverse) fibers - connect opposite sides of cord
White Matter Fiber Tract Generalizations• Pathways decussate (most)• Most consist of a chain of two or three neurons• All pathways are paired – one on each side of the spinal cord
Descending (Motor) Pathways• Descending tracts deliver motor instructions from the brain to the spinal cord• Divided into two groups – Pyramidal, or corticospinal, tracts – Indirect pathways, essentially all others• Motor pathways involve two neurons – Upper motor neuron (UMN) – Lower motor neuron (LMN) • aka ‘anterior horn motor neuron” (also, final common pathway)
Pyramidal (Corticospinal) Tracts• Originate in the precentral gyrus of brain (aka, primary motor area) – I.e., cell body of the UMN located in precentral gyrus• Pyramidal neuron is the UMN – Its axon forms the corticospinal tract• UMN synapses in the anterior horn with LMN – Some UMN decussate in pyramids = Lateral corticospinal tracts – Others decussate at other levels of s.c. = Anterior corticospinal tracts• LMN (anterior horn motor neurons) – Exits spinal cord via anterior root – Activates skeletal muscles• Regulates fast and fine (skilled) movements
Corticospinal tracts1. Location of UMN cell body in cerebral cortex2. Decussation of UMN axon in pyramids or at level of exit of LMN3. Synapse of UMN and LMN occurs in anterior horn of s.c.4. LMN axon exits via anterior root
Extrapyramidal Motor Tracts• Includes all motor pathways not part of the pyramidal system• Upper motor neuron (UMN) originates in nuclei deep in cerebrum (not in cerebral cortex)• UMN does not pass through the pyramids!• LMN is an anterior horn motor neuron• This system includes – Rubrospinal – Vestibulospinal – Reticulospinal – Tectospinal tracts• Regulate: – Axial muscles that maintain balance and posture – Muscles controlling coarse movements of the proximal portions of limbs – Head, neck, and eye movement
Extrapyramidal TractNote:1. UMN cell body location2. UMN axon decussates in pons3. Synapse between UMN and LMN occurs in anterior horn of sc3. LMN exits via ventral root4. LMN axon stimulates skeletal muscle
Extrapyramidal (Multineuronal) Pathways• Reticulospinal tracts – originates at reticular formation of brain; maintain balance• Rubrospinal tracts – originate in ‘red nucleus’ of midbrain; control flexor muscles• Tectospinal tracts - originate in superior colliculi and mediate head and eye movements towards visual targets (flash of light)
Main Ascending Pathways• The central processes of first-order neurons branch diffusely as they enter the spinal cord and medulla• Some branches take part in spinal cord reflexes• Others synapse with second-order neurons in the cord and medullary nuclei
Three Ascending Pathways• The nonspecific and specific ascending pathways send impulses to the sensory cortex – These pathways are responsible for discriminative touch (2 pt. discrimination) and conscious proprioception (body position sense).• The spinocerebellar tracts send impulses to the cerebellum and do not contribute to sensory perception
Nonspecific Ascending Pathway• Include the lateral and anterior spinothalamic tracts• Lateral: transmits impulses concerned with pain and temp. to opposite side of brain• Anterior: transmits impulses concerned with crude touch and pressure to opposite side of brain• 1st order neuron: sensory neuron• 2nd order neuron: interneurons of dorsal horn; synapse with 3rd order neuron in thalamus• 3rd order neuron: carry impulse from thalamus to postcentral gyrus
Specific and Posterior Spinocerebellar Tracts• Dorsal Column Tract 1. AKA Medial lemniscal pathway 2. Fibers run only in dorsal column 3. Transmit impulses from receptors in skin and joints 4. Detect discriminative touch and body position sense =proprioception• 1st O.N.- a sensory neuron • synapses with 2nd O.N. in nucleus gracilis and nucleus cuneatus of medulla• 2nd O.N.- an interneuron • decussate and ascend to thalamus where it synapses with 3rd O.N.• 3rd-order (thalamic neurons) •transmits impulse to somato- sensory cortex (postcentral gyrus)Spinocerebellar Tract• Transmit info. about trunk and lowerlimb muscles and tendons to cerebellum• No conscious sensation
A particular slide catching your eye?
Clipping is a handy way to collect important slides you want to go back to later.