section 2, chapter 11

Spinal Cord

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Spinal Cord The spinal cord is continuous with the brain and extends downward through the vertebral canal.

The spinal cord begins at the foramen magnum, and terminates between the first and second lumbar vertebra.

Two regions of the spinal cord are thickened1.Cervical Enlargement – nerves to upper limbs2.Lumbar enlargement – nerves to lower limbs

Structure of the spinal cord

31 pairs of spinal nerves arise from the spinal cord.

8 cervical nerves

12 thoracic nerves

5 lumbar nerves

5 sacral nerves

1 coccygeal nerve

Figure 11.29 The thirty one pairs of spinal nerves are grouped according to the level from which they arise.

Terminal end of the spinal cord

The spinal cord tapers at its terminal end into the conus medullaris

Filum terminale – thin cord of connective tissue arising from the conus medullaris

Cauda Equina “Horse’s tail” – spinal nerves at the conus medullaris fan outward, creating a structure that resembles a horse’s tail.

Cross Section of the spinal cord

Anterior Median Fissure & Posterior Median Sulcus• Grooves that divide the spinal cord into left and right halves

Central Canal – continuous with the ventricles in the brain

The spinal cord consists of white matter surrounding a core of grey matter.

structures of the spinal cord Grey Matter – unmyelinated tissue Posterior horns – located towards the dorsal surface Anterior horns – located towards the ventral surface Lateral horns – located in some regions of the spinal cord Grey commissure – connects the left and right grey matter

White Matter – myelinated axonsThe white matter can be separated into 3 columns

Posterior funiculus Anterior funiculus Lateral funiculus

The funiculi (sing. funiculus) are columns that provide pathways for axons, called nerve tracts.

Spinal Nerves

Ventral Root – Branch of spinal nerves that carry motor impulses away from the spinal cord

Dorsal Root – Branch of spinal nerves that carry sensory impulses towards spinal cord

Dorsal Root Ganglion – Mass of sensory neuron cell bodies in the dorsal root of the spinal cord

Functions of the spinal cord

The spinal cord is a conduit for nerve impulses to and from the brain and brainstem

The spinal cord is a center for spinal reflexes.

Most reflexes occur at the level of the spinal cord.

Reflex ArcsReflexes are automatic, subconscious responses to stimuli within or outside the body

Reflexes are the simplest response to a stimulus

Figure 10.7a. A schematic of a reflex arch.Figure 10.7a. A schematic of a reflex arch.

In a simple reflex arc a sensory neuron is directly connected to a motor neuron within the spinal cord.

Most reflex arcs also involve interneurons that connect the sensory neuron to a motor neuron.

Reflex ArcsFive components of a reflex arc.

1.Receptor - detects changes in environmentA receptor may be the dendritic end of a sensory neuron,

or it may be a specialized cell

2.Sensory neuron – conveys stimulus to spinal cord sensory neurons pass through the dorsal root

•Interneuron – connects the sensory neuron to the motor neuron

•Motor Neuron – transmits the impulse to the effectormotor neurons pass through the ventral root

5. Effector – muscle or gland that produces the reflex

Figure 10.7b the five components of a reflex arc.

Reflex Arcs

Reflex Arcs

Table 11.2 summarizes the components of a reflex arc.Table 11.2 summarizes the components of a reflex arc.

Tracts of the spinal cord

Ascending tracts conduct sensory impulses up the spinal cord to the brain

Descending tracts conduct motor impulses from the brain down the spinal cord to motor neurons reaching muscles and glands

Figure 11.11 Major ascending and descending tracts in a cross section of the spinal cord.

1. Fasciculus gracilis & fasiculus cuneatus are located within the posterior funiculus of the spinal cord.• They transmit sensory information from skin, muscles, and joints• Sense touch, pressure, and body movement• Fibers decussate (cross over) in medulla oblongata of brain

2. Lateral Spinothalamic tract• Conducts sensations of pain and

temperature to thalamus of the brain• Fibers decussate in spinal cord

3. Anterior Spinocerebellar tract• Conducts impulses from

muscles of lower limbs and trunk to cerebellum

• Coordinate muscle movements

Examples of ascending tracts

Figure 11.12

Sensory impulses originating touch receptors of the skin ascend in the fasciculus cuneatus tract and cross over in medulla oblongata of the brain.

Pain and temperature information ascends in the lateral spinothalamic tract, which crosses over in the spinal cord.

Ascending Tracts

Examples of descending tracts

1. Corticospinal tracts• Lateral and anterior tracts• Transmits motor impulses from the cerebral cortex to spinal

nerves and outward to various skeletal muscles• Fibers decussate in medulla oblongata• Corticospinal tracts are responsible for voluntary movement

2. Rubrospinal tract• Passes through the lateral funiculi• Transmits motor impusles that coordinate

muscles and controls posture.

3. Reticulospinal tractstransmits motor impulses responsible for sweat

glands and muscle tone.

Examples of descending tracts

Descending Tracts

Figure 11.13Most fibers of the corticospinal tract originate in the cerebral cortex.

They cross over in the medulla, and descend in the spinal cord

Corticospinal tracts synapse with neurons whose fibers lead to spinal nerves supplying skeletal muscles.

Some fibers cross over in the spinal cord.

End of Chapter 11, Section 2