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Spinal Cord serves 4 principal functions:
1. Conduction. It contains bundles of nerve fibers that conduct information up and down the cord, connecting different levels of the trunk with each other and with the brain. This enables sensory information to reach the brain, motor commands to reach the effectors, and input received at one level of the cord to affect output from another level.
2. Neural integration. Pools of spinal neurons receive input from multiple sources, integrate the information, and execute an appropriate output. For example, the spinal cord can integrate the stretch sensation from a full bladder with cerebral input concerning the appropriate time and place to urinate and execute control of the bladder accordingly.
3. Locomotion. Walking involves repetitive, coordinated contractions of several muscle groups in the limbs. Motor neurons in the brain initiate walking and determine its speed, distance, and direction, but the simple repetitive muscle contractions that put one foot in front of another, over and over, are coordinated by groups of neurons called central pattern generators in the cord. These neural circuits produce the sequence of outputs to the extensor and flexor muscles that cause alternating movements of the lower limbs.
4. Reflexes. Spinal reflexes play vital roles in posture, motor coordination, and protective responses to pain or injury.
spinal cord
is a cylinder of nervous tissue that arises from the brainstem at the foramen magnum of the skull. It passes through the vertebral canal as far as the inferior margin of the first lumbar vertebra (L1) or slightly beyond. In adults, it averages about 45 cm long and 1.8 cm thick (about as thick as one’s little finger). Early in fetal development, the cord extends for the full length of the vertebral column. The cord gives rise to 31 pairs of spinal nerves.
Spinal Cord regions
The spinal cord is divided into cervical, thoracic, lumbar, and sacral regions. It may seem odd that it has a sacral region when the cord itself ends well above the sacrum. These regions, however, are named for the level of the vertebral column from which the spinal nerves emerge, not for the vertebrae that contain the cord itself.
Sinal Cord Enlargement Areas
In the inferior cervical region, a cervical enlargement gives rise to nerves of the upper limbs. In the lumbosacral region, there is a similar lumbar enlargement that issues nerves to the pelvic region and lower limbs. Inferior to the lumbar enlargement, the cord tapers to a point called the medullary cone (conus medullaris). Arising from the lumbar enlargement and medullary cone is a bundle of nerve roots that occupy the vertebral canal from L2 to S5. This bundle, named the cauda equina (CAW-duh ee-KWY-nah) for its resemblance to a horse’s tail, innervates the pelvic organs and lower limbs.
meninges (meh-NIN-jeez)
The spinal cord and brain are enclosed in three fibrous membranes called meninges (meh-NIN-jeez)—singular, meninx (MEN-inks). These membranes separate the soft tissue of the central nervous system from the bones of the vertebrae and skull. From superficial to deep, they are the dura mater, arachnoid mater, and pia mater.
dura mater (DOO-ruh MAH-tur) (superficial layer)
forms a loose-fitting sleeve called the dural sheath around the spinal cord. It is a tough collagenous membrane about as thick as a rubber kitchen glove. The space between the sheath and vertebral bones, called the epidural space, is occupied by blood vessels, adipose tissue, and loose connective tissue. Anesthetics are sometimes introduced to this space to block pain signals during childbirth or surgery; this procedure is called epidural anesthesia.
arachnoid (ah-RACK-noyd) mater (middle layer)
consists of a simple squamous epithelium, the arachnoid membrane, adhering to the inside of the dura, and a loose mesh of collagenous and elastic fibers spanning the gap between the arachnoid membrane and the pia mater. This gap, called the subarachnoid space, is filled with cerebrospinal fluid (CSF). Inferior to the medullary cone, the subarachnoid space is called the lumbar cistern and is occupied by the cauda equina and CSF.
pia (PEE-uh) mater (deep layer)
is a delicate, transparent membrane that closely follows the contours of the spinal cord. It continues beyond the medullary cone as a fibrous strand, the terminal filum, within the lumbar cistern. At the level of vertebra S2, it exits the lower end of the cistern and fuses with the dura mater, and the two form a coccygeal ligament that anchors the cord and meninges to vertebra Co1. At regular intervals along the cord, extensions of the pia called denticulate ligaments extend through the arachnoid to the dura, anchoring the cord and limiting side-to-side movements.
lumbar puncture (or colloquially, spinal tap)
When a sample of CSF is needed for clinical purposes, it is taken from the lumbar cistern by a procedure called lumbar puncture (or colloquially, spinal tap). A spinal needle is inserted between two vertebrae at level L3/L4 or L4/L5, where there is no risk of accidental injury to the spinal cord (which ends at L1 to L2). CSF drips from the spinal needle into a collection tube; usually 3 to 4 mL of CSF is collected.
Gray matter
has a relatively dull color because it contains little myelin. It contains the somas, dendrites, and proximal parts of the axons of neurons. It is the site of synaptic contact between neurons, and therefore the site of all neural integration in the spinal cord.
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