Spinal cord
Cylinder of nervous tissue; arises
from brainstem at foramen magnum of skull
� Passes through vertebral canal
- Averages 1.8 cm thick and 45 cm long
- Occupies the upper two-thirds of the vertebral canal
� Gives rise to 31 pairs of spinal nerves
- First p
Regions of the Spinal Cord
- Divided into the cervical, thoracic, lumbar, and
sacral regions
- Named for level of vertebral column from which
spinal nerves emerge (not for the vertebrae that
contain the cord itself)
� Two areas of the cord are thicker:
- Cervical enlargement: nerve
Cervical enlargement
Nerves to upper limb
Lumbar enlargement
Nerves to pelvic region and lower limbs
Medullary cone (conus medullaris)
Pointed end of cord; inferior to lumbar enlargement
Cauda equina
Bundle of nerve roots that occupy the vertebral canal from L2 to S5
Segment
Part of the spinal cord supplied by each pair of spinal nerves
Longitudinal grooves
On anterior and posterior sides
- Anterior median fissure
- Posterior median sulcus
Meninges
Three fibrous connective tissue membranes that enclose the brain and spinal cord
- Separate soft tissue of central nervous system
from bones of cranium and vertebral canal
- From superficial to deep: dura mater,
arachnoid mater, and pia mater
Dura Mater
Outermost layer; tough collagenous
sheath
Epidural Space
Between dura mater and vertebral bones� Contains blood vessels, adipose tissue, and loose connective tissue
Epidural anesthesia
Blocks pain signals during surgery or childbirth
Arachnoid membrane
simple squamous epithelium adhering to the inside of the dura
Subarachnoid space
Between the arachnoid membrane and the pia mater Filled with cerebrospinal fluid and fibrous connective tissue
Pia Mater
Deepest layer; delicate transparent membrane
Gray matter
Neuron cell bodies with little myelin
- Site of synaptic contact between neurons
- Neural integration/information processing
White matter
Abundantly myelinated axons
- Bundles of axons (tracts) that carry signals from one part of the CNS to another
Features of Gray Matter
� Central core that looks butterfly- or H-shaped
in a spinal cord cross section
� Pair of posterior (dorsal) horns
� Pair of thicker anterior (ventral) horns
Lateral horn
Only visible from T2 through L1
- Contains neurons of sympathetic nervous system
Gray commissure
Connects right and left sides
- Punctured by central canal lined with ependymal
cells and filled with CSF
Features of White Matter
White matter surrounds the gray matter
- Consists of bundles of axons that course up and down the cord
- Provide avenues of communication between different levels of the CNS
� Bundles are arranged into three pairs of columns
(funiculi)
- Posterior (dorsal
Spinal Tracts
� All nerve fibers in a given tract have a similar
origin, destination, and function
Ascending tracts
Carry sensory information up the spinal cord
Decussation
Some tracts of fibers cross over from the left to the right side and vice versa
Left side of brain
Can receive sensory information from and send motor commands to right side of body
Right side of brain
Can receive sensory information from and send motor commands to left side of body
Contralateral
When the origin and destination
of a tract are on opposite sides of the body
Ipsilateral
When the origin and destination of a
tract are on the same side of the body; does not
decussate
Ascending Tracts
� Carry sensory signals up the spinal cord
� Sensory signals travel across three neurons from
origin in receptors to the destination in the
sensory areas of the brain
First-order neurons
Detect stimulus and transmit signal to spinal cord or brainstem
Second-order neurons
Continues to the thalamus at the upper end of the brainstem
Third-order neurons
Carries the signal the rest of the way to the sensory region of the cerebral cortex in the brain
Cuneate fasciculus and gracile fasciculus
- Located in posterior column ; carry signals for deep touch, vibration, visceral pain, and proprioception (nonvisual sense of the position and movements of the body)
Spinothalamic
- Located in anterior and lateral columns; carries signals for light touch, pressure, temperature, and pain
Spinoreticular
- Located in anterior and lateral columns; carries pain signals resulting from tissue injury
Spinocerebellar (anterior and posterior)
- Located in lateral column; carry proprioceptive signals from limbs and trunk
Following the Cuneate Fasciculus Tract
- Originates from the level of T6 and up
- Carries signals from upper limb and chest
� Occupies lateral portion of the posterior column
� First order fibers end in the cuneate nucleus in
the medulla (part of brainstem)
� Second-order nerve fibers decussat
Descending tracts
Carry motor signals down the brainstem and spinal cord
- Carry motor information
down the spinal cord
Upper motor neuron
Originates in cerebral cortex or brainstem
� Axon terminates on a lower motor neuron
Lower motor neuron
Originates in brainstem or spinal cord
� Axon of lower motor neuron leads the rest of the way to the muscle or other target organ
Corticospinal (lateral and anterior)
Begins in cerebral cortex in brain; controls precise,
finely coordinated limb movements
Tectospinal
Begins in brainstem (tectum); reflex turning of head in response to sights and sounds
Reticulospinal (lateral and medial)
Begins in the brainstem (reticular formation); control muscles of upper and lower limbs (posture and balance); contain descending analgesic pathways (reduce pain signals to brain)
Vestibulospinal (lateral and medial)
Begins in brainstem (vestibular nuclei); receives
impulses for balance from inner ear (vestibule);
controls extensor muscles of limbs for balance control
Following the Corticospinal Tracts
� Carry motor signals from cerebral cortex in brain
for precise, finely coordinated limb movements
� Nerve fibers form ridges on anterior surface of
the medulla (called pyramids)
� Upper motor neuron fibers come from cerebral
cortex and lead down, decussa
how many nerves
- 8 cervical (C1-C8)
- 12 thoracic (T1-T12)
- 5 lumbar (L1-L5)
- 5 sacral (S1-S5)
- 1 coccygeal (Co)
Posterior (dorsal) root
Sensory input to spinal cord
- ganglion�contains somas of sensory neurons carrying signals to spinal cord
- Rootlets contain the axons of sensory neurons that enter posterior horn of cord
Anterior (ventral) root
Motor output out of spinal cord
- Rootlets containing axons of motor neurons leave spinal cord and converge to form anterior root
Anterior ramus
Innervates the anterior and lateral skin and muscles of the trunk; gives rise to nerve plexuses
Posterior ramus
Innervates the muscles and joints in the region of the spine and the skin of the back
Meningeal branch
Reenters the vertebral canal and innervates the meninges, vertebrae, and spinal ligaments
Cervical plexus
In the neck, C1 to C5 - supplies neck and phrenic nerve to the diaphragm
Brachial plexus
Near the shoulder, C5 to T1 - supplies upper limb and some of shoulder and neck
Lumbar plexus
In the lower back, L1 to L4 - supplies abdominal wall, anterior thigh, and pelvic region
Sacral and Coccygeal plexuses
In the pelvis, L4 to Co1
- supply remainder of pelvic region, lower trunk,
and lower limbs
Sensory nerve fibers
Carry afferent signals from bones, joints, muscles, and skin to CNS ("somatosensory")
- For sensations such as touch, heat, cold, pain
- Also for proprioception (brain receives information about body position and movements from nerve endings in muscles, t
Motor nerve fibers
Carry efferent signals from CNS to skeletal muscles
- For stimulation of contraction
Cervical Plexus
� Formed from anterior rami of nerves C1 to C5
� Gives rise to several major nerves:
- Lesser occipital, greater auricular, transverse
cervical, ansa cervical, supraclavicular, phrenic
� Carries sensory information from ear, neck,
chin, shoulder, chest, d
Brachial Plexus
� Formed from anterior rami of nerves C5 to T1
� Gives rise to several major nerves:
- Musculocutaneous, axillary, radial, median, ulnar
� Carries sensory information from skin and joints
of upper limbs
� Motor fibers innervate upper limb and neck/shoulde
Lumbar Plexus
� Formed from anterior rami of nerves L1 to L4
� Gives rise to several major nerves:
- Iliohypogastric, ilioinguinal, genitofemoral, lateral femoral, femoral, obturator
� Carries sensory information from skin and joints
of lower limbs
� Motor fibers inner
Sacral and Coccygeal Plexuses
- Formed from anterior rami of nerves L4 to Co1
� Give rise to several major nerves:
- Superior gluteal, inferior gluteal, posterior cutaneous, sciatic (tibial and fibular), pudendal
� Carry sensory information from pelvic region and
lower limbs
� Motor f
Reflexes
Quick, involuntary, stereotyped reactions of glands or muscle to stimulation
Somatic reflexes
Involve the somatic nervous system
- Unlearned skeletal muscle reflexes mediated by
the brainstem and spinal cord
- Testing somatic reflexes helps diagnose many
diseases
Stimulation (reflexes)
- Not spontaneous actions, but responses to sensory input
Quick reflexes
- Involve few if any interneurons and minimum synaptic delay
Involuntary reflexes
- Occur without intent and difficult to suppress
- Automatic response
Stereotyped Reflexes
- Occur essentially the same way every time
- Very predictable responses
Afferent nerve fibers
Carry sensory information from receptors to spinal cord or the brainstem
Integrating center
Point of synaptic contact between
neurons in gray matter of spinal cord (spinal reflex) or brainstem (cranial reflex)
Monosynaptic reflex
The sensory neuron synapses directly with the motor neuron
Polysynaptic reflex
Interneurons are involved in complex
reflexes involving multiple synapses in the CNS
Efferent nerve fibers
Carry motor impulses to skeletal muscle
Muscle spindle
- Stretch receptor embedded in skeletal muscle
- Located in perimysium; concentrated near tendons
- Contains modified muscle fibers and nerve fibers
wrapped in a fibrous capsule
� Muscle spindles inform the brain of muscle length
and body movement (propri
Stretch (myotatic) reflex
When a muscle is stretched, it "fights back" and contracts
- Helps maintain equilibrium and posture
- Stabilize joints by balancing tension in extensors
and flexors, smoothing muscle actions
� Example: Falling asleep in class
- Head starts to tip forward
Tendon reflex
Reflexive contraction of a muscle when its tendon is tapped (stretches the muscle suddenly)
- Example: Knee-jerk (patellar) reflex
Reciprocal inhibition
Reflex phenomenon that prevents muscles from working against each other by inhibiting the antagonist
- Knee-jerk reflex: quadriceps femoris muscle
contracts and hamstring muscles stay relaxed
Flexor reflex
Quick contraction of flexor muscles resulting in the withdrawal of a limb from an injurious stimulus
- Requires contraction of the flexors and relaxation of the extensors in that limb - reciprocal inhibition
� Example: stepping on a sharp object
- Even be
Crossed extension reflex
The contraction of extensor muscles in limb opposite of the one that is withdrawn
- Maintains balance by extending other leg
Contralateral reflex arc
Input and output are on opposite sides
- Crossed extension reflex
Ipsilateral reflex arc
Input and motor output are on the same sides of the spinal cord
- Flexor reflex