The Nervous system Brain CNS Spinal cord NS Spinal n. PNS Cerebral n. Visceral n. Central Nervous System (CNS) Peripheral Nervous System (PNS)
Weihua Yu
Department of Anatomy Chongqing Medical University
Contents I. External features of the spinal cord II.Internal structures of the spinal cord III.Functions of the spinal cord
I. External features of the spinal cord
Location: lies in the vertebral canal and invested by meninges.
2 ends:
-The upper end: at the level of foramen magnum, where it continues with the medulla oblongata -The lower end: to the lower border of the 1st LV in adult. (to the 3rd LV in Child)
6 longitudinal grooves on its surface Ant. median fissure
●
Post. median sulcus
●
Anterolateral sulci (2)
● ●
Posterolateral sulci(2)
2 enlargements i. The cervical enlargement( C5-T1) supplies n. to the upper limbs. ii. The lumbosacral enlargement (L2-S3) supplies n. to the lower limbs.
The segments of spinal cord In terms of the nerve roots attached to the spinal cord, a portion of the cord that gives rise to a spinal nerve constitutes a segment. The spinal cord can be divided into 31 segments: Cervical 8; Thoracical 12; Lumbar 5; Sacral 5; Cocegeal 1
2 roots: Each segment has paired dorsal root and ventral root. The dorsal and ventral roots fuse to form spinal nerves and exit from the vertebral canal via the intervertebral foramina. (so there are 31 pairs of spinal n.) On the post. root, there is a oval swelling, called dorsal root ganglion
Inferior
to the lumbar enlargement, the spinal cord becomes tapered and conical- conus medullaris.
The
cord does not extend the entire length of the vertebral column – so a group of nerves leaves the inferior spinal cord and extends downward. It resembles a horses tail and is called the cauda equina
Filum
terminale -slender strand of fibrous tissue that extends from conus medullaris to the posterior surface of the coccyx to anchor the spinal cord.
The levels of spinal cord do not match the vertebral level In the fetus (within 3 months): Length of SC=length of vertebral canal In the fetus (after 3 months) and newborn: Length of SC
conus medullaris cauda equine Filum terminale
The relationship between spinal cord and vertebrae
Spinal segments
Vertebral bodies
C1-C4
The same number of vertebrae
C5-C8
The same number of vertebrae -1
T1-T4
The same number of vertebrae -1
T5-T8
The same number of vertebrae -2
T9-T12
The same number of vertebrae -3
L1-L5
At the level of T10 ~T12
S1-S5 、 C0
At the level of L1
※Clinical significance
The meninges of spinal cord Spinal cord is surrounded by a single layered dura mater, arachnoid and pia mater
epidural space
spinal dura mater subdural space
Arachnoid subarachnoid space
spinal pia mater denticulate ligament
Between the dura mater and periosteum of the vertebrae is the epidural space that contains many blood vessels and fat. Anesthetics can be injected here below the L3 vertebral level, from which it ascends to act upon sensory neurons to help dull pain. This procedure is called caudal block.(epidural block)
Space between dura mater and archnoid is the subdural space (no CSF).
Space between arachnoid and pia mater is the subarchnoid space: contains CSF, blood vessels, spinal roots.
¶ Cerebrospinal Fluid (CSF)
This is a clear watery ultra filtrate solution primarily derived from blood. The basic mechanism involves an active transport system and passive diffusion into the four ventricles. The CSF provides a cushion that protects the delicate tissues of the spinal cord. It is also involved in the exchange of nutrients between the blood and neurons of the brain and spinal cord.
II. Internal structure of the spinal cord
Cross Sectional Anatomy of the Spinal Cord
Anterior median fissure and posterior median sulcus partially divide it into left and right halves.
Gray matter is in the core of the cord and surrounded by white matter.
Gray matter resembles a butterfly. 2 lateral gray masses connected by the gray commissure. Posterior projections are the posterior or dorsal horns. Anterior projections are the anterior or ventral horns. In the thoracic and lumbar cord, there also exist
Gray Matter
The gray matter consists of nerve cell bodies, dendrites and axon terminals (unmyelinated) and neuroglia. It is pinkishgray color because of a rich network of blood vessels.
The posterior horns function in afferent input. The anterior horns function in efferent somatic output. The lateral horns function in efferent visceral output . Posterior horn （ sensory ）
Gray matter
Lateral horn: C8 － L3 ( sympathetic motor) S2-4 ( parasympathetic motor)
● posterior horn contains sensory interneurons and project neurons Nucleus posteromarginalis Substantia gelatinosa Nucleus proprius cornu posterior Nucleus thoracicus (Clarke’s column)
●anterior horn
cell bodies of motor neurons contains
Types of motor neurons ▪α-motor neuron: its axon passes through the white matter to contribute to the respective ant. root of the spinal n., which innervates the extrafusal fibers of the skeletal m. and produces contractile tension within a m. ▪ γ-motor neuron: give rises to fibers to supply the intrafusal muscle fibers of neuromuscular spindles, which is responsible for regulating of muscle tonus. ▪ Renshaw cell: to form a negative feedback pathway for alpha motor neurons
Ant. horn cell
Renshaw cell
The amount of ventral gray matter at a given level of the spinal cord is proportional to the amount of skeletal muscle innervated.
● Lateral horn (intermediate
zone)
Contains autonomic motor neurons serving visceral organs. Their axons also exit via the ventral root.
The lateral horn at the levels of C8(T1)-L3 contains the intermediolateral nucleus which is the preganglionic sympathetic neurons The lateral horn at the levels of S2-4 contains sacral parasympathetic nucleus which is the preganglionic parasympathetic neurons
Rexed layers (study by students) lamina
Nucleus
I
Posteromarginal Nu.
II
Substantia gelatinosa
III 、 I V
Proper sensory Nu.
V VI VII VIII IX X
Intermediate gray
Dorsal Nu.(of Clarke) Ventral horn Anterior horn cells Commissural neurons
Substantia gelatinosa is the distinctive region which caps the posterior horn, it is related to the transmission of pain and temperature
White Matter
The white matter gets its name because it is mainly composed of myelinated nerve fibers, and myelin has a whitish color. The white matter is divided into three pairs of columns or funiculi of myelinated fibers: anterior, posterior and lateral funiculi. The bundles of fibers within each funiculus are divided into tracts called fasciculi. Ascending tracts: carry sensory impulse up the spinal cord to the brain. Descending tracts: transmit motor impulse from the brain down the Post. funiculi spinal cord.
lateral funiculi
Ascending tract
Ant. funiculi
decending tract
Tracts in the white matter Faciculus gracilis(*) Long ascending tract
Faciculus cuneatus (*) Spinothalamus tract Lateral corticospinal tract (*)
White matter
Long descending tract
Anterior corticospinal tract (*) Rubrospinal tract Reticulospinal tract
Short proprius
Many tracts are named after their nuclei of origin, their termination as well as their location in the spinal cord
● Ascending tract (Sensory conducting tract)
i. Fasciculus gracilis (FG)& Fasciculus cuneatus(FC) Function: convey the sensations of body posture, movement, vibration, pressure and fine touch.
Cell bodies (pseudounipolar sensory neurons ) lies in the dorsal root ganglia (spinal ganglia)
FG: the fibers derived from spinal segments below T4 form the fasciculus gracilis
FC: the fibers derived from spinal segments above T4 form the fasciculus cuneatus
muscles,tendons, joints and periosteum Receptor for fine touch in skin
Spinal ganglion
Peripheral processes
Central processes enter into ipsilateral posterior funiculus of spinal cord and ascends Fasciculus gracilis (below T5) Fasciculus cuneatus (above T4)
Spinal ganglion T4 T5 T6
Fasciculus cuneatus Fasciculus gracilis
ii. spinothalamic tract (STT) convey the sensations of pain, temperature, and rough touch.
Cell bodies lies in the posterior horn of gray matter
Send out fibers cross to the opposite side through the anterior white commissures and form the spinothalamic tract lateral spinothalamic tract (pain and temperature) anterior spinothalamic tract (rough touch)
Spinal ganglion Skin exteroceptor
Peripheral
Central processes to form posterior root
processes
posterior horn
Ascending 1 - 2 segments and cross to the contralateral side of spinal cord to form STT
posterior horn spinothalamic tract spinal ganglia
lateral spinothalamic tract (pain and temperature) anterior spinothalamic tract (rough touch)
fasciculus gracilis fasciculus cuneatus posterior spinocerebell ar tract lateral spinothalamic tract anterior spinothalamic tract
● Descending tract (Motor conducting tract)
Pyramidal tract Lat. corticospinal tract Ant. corticospinal tract
Ant. corticospinal tract Tectospinal tract Reticulospinal tract
Extrapyramidal tract Rubrospinal tract Tectospinal tract Vestibulospinal tract Reticulospinal tract
Corticospinal tract （ CST) CST arises from the cerebral cortex, descends though the internal capsule and brainstem, and divides into 2 tracts at the pyramidal decussation
Lateral corticospinal tract (lateral CST): decussates in the medulla oblongata and descends in the lateral funiculus of spinal cord on the opposite side , and terminates in the anterior horn. To innervate the muscle fibers of the limbs on the opposite side.
Anterior corticospinal tract (anterior. CST): descends in the ant. funiculus of spinal cord on the same side. This tract contains another 2 tracts, one tract never decussate, another tract decussates before terminating in the ant. horn. To innervate the muscle fibers of trunk on both side, the muscle fibers of limbs on the opposite side.
Lateral CST
Anterior CST
s m. of limobf trunk m.
spinal cord
Questions
Above the pyramidal decussation, what symptons and signs will occur if the right corticospinal tract is damaged? Below the pyramidal decussation, what symptons and signs will occur if the right corticospinal tract is damaged?
Injuries of the CST at the level of above the pyramidal decussation will cause paralysis of the contralateral limbs. Injures of the CST at the level of below pyramidal decussation will cause the paralysis of the ipsilateral limbs
III. Functions of the spinal cord The spinal cord with its 31 pairs of spinal nerves serves two important functions. i. It is the connecting link between the brain and most of the body. ii. It is involved in spinal reflex actions, both somatic and visceral.
Questions
What signs and symptoms will appear if the complete hemisection of T6 on the right side is occured?
Signs and symptoms: 1. Paralysis of ipsilateral muscles of trunk in the segment T6, due to damage to the ant. horn of segment T6 on the right side. 2. Paralysis of ipsilateral lower limbs, due to the damage to the lateral corticospinal tract on the right side. 3. Ipsilateral zone of cutaneous anesthesia in the segment T6 on the right side, due to the damage to the afferent fibers that have entered the cord and have not yet crossed. 4. Ipsilateral loss of deep sensation and fine touch sense below the T6, due to the damage to the fasciculus of gracilis in the segment T6 on the right side. 5. Contralateral loss of pain and temperature sense and rough tough sense below 1 or 2 segments (namely, below segment T7 or T8) at the juried level, due to the damage to the spinothalamic tract on the right side.
