Chapters 13 & 16: Spinal Cord; Sensory and Motor Pathways
Chapter Objectives
SPINAL CORD ANATOMY
1. Describe the external features of the spinal cord and the Cauda equina. Discuss its
relative length within the vertebral column.
2. Describe the characteristics and purpose of the three layers and spaces of the meningeal
structures.
3. Match each horn of gray matter with the type of cell body it contains.
4. Match each column of white matter and the type of tracts it contains.
5. Describe where sensory information enters the spinal cord and where motor information
leaves the spinal cord.
SPINAL NERVES
6. Define a nerve and discuss the connective tissue layers that surround and protect nerves.
7. Discuss the naming and numbering of spinal nerves, the arrangement of spinal nerves
relative to the vertebrae, and the attachment of the spinal nerves to the spinal cord.
8. Discuss the branching of the spinal nerves once they emerge from the vertebral column.
9. Define a plexus; then list the names of major nerves of the cervical, brachial, lumbar, and
sacral plexuses.
10. Define a dermatome.
SOMATIC SENSATIONS
11. Identify the sensations and the receptors involved.
12. Describe the types of receptors in terms of microscopic features, location, and stimulus
type.
SOMATIC SENSORY PATHWAYS
13. Discuss the location of first order, second order and third order neuons.
14. Discuss the functions of the anterolateral pathway and where its ascending tracts are
located.
15. Discuss the functions of the posterior column-medial lemniscus pathway and where its
ascending tracts are located.
16. Discuss the functions of the cerebellar pathway and where its ascending tracts are located.
SOMATIC MOTOR PATHWAYS
17. List the neural circuits that are part of the somatic motor pathways.
18. Describe the origin and destination for the descending, motor tracts.
REFLEXES AND REFLEX ARC
19. Define a reflex.
20. Describe the components of a reflex arc and their specific functions.
Chapter Lecture Notes
Spinal Cord
Spinal cord - extends from foramen magnum to the level of the second lumbar vertebra (Fig
13.2)
Shorter than vertebral column because vertebral column grows faster than spinal cord
Cauda equina - nerves from lower cord don't leave vertebral column immediately, but instead
look like coarse hairs of a horse tail
Has grooves on surface – anterior median fissure and posterior median sulcus
Central canal – passageway for cerebrospinal fluid; flows from ventricles in brain (Fig 13.3)
Lined with ependymal cells that help circulate cerebrospinal fluid
The spinal cord is surrounded by meninges (Fig 13.1)
Dura mater – outermost layer which is single layered and not attached to the bony vertebrae
epidural space – space between dura mater and vertebrae
filled with adipose and blood vessels
Arachnoid mater – middle layer inside of dura mater
Pia mater – innermost layer bound very tightly to surface of spinal cord
Subarachnoid space – space between arachnoid mater and pia mater
contains cerebrospinal fluid
lumbar puncture – into subarachnoid space between L3 and L4 or L4 and L5 for
cerebrospinal fluid diagnosis or to introduce anesthetics
Gray Matter of the Spinal Cord
Gray matter (cell bodies and dendrites) - organized into horns and commissures
Posterior (dorsal) gray horn - contains cell bodies of interneurons which have synapsed with
sensory neurons (Fig 13.3 & 13.4)
Lateral gray horn - contains cell bodies of neurons from autonomic nervous system
Anterior (ventral) gray horn - contains cell bodies of motor neurons
Anterior gray commissure - gray communication between right and left section of cord
anterior to the central canal
Posterior gray commissure - gray communication between right and left section of cord
posterior to the central canal
White Matter of the Spinal Cord
White matter (myelinated axons) - organized into columns and commissures (tracts travel in
columns) (Fig 13.3 & 13.12)
Posterior white column - has ascending tracts only
Lateral white column - has both ascending and descending tracts
Anterior white column - has both ascending and descending tracts
Anterior white commissure
Posterior white commissure
Nerves
Nerves – bundles of axons in the PNS (Fig 13.5)
Surrounded by connective tissue
Epineurium – around whole nerve
Perineurium – around a fascicle of nerve fibers
Endoneurium – around a single axon
Mixed nerves – contain both afferent and efferent neurons
Ganglia – cell bodies of neurons in the PNS
Spinal Nerves
Spinal nerve - has bundles of axons of both motor and sensory neurons, therefore it is a mixed
nerve
The anterior root and the posterior root combine to form the spinal nerve
Posterior (dorsal) root - connection to spinal cord from the peripheral nervous system that
carries only sensory information and ends at the posterior gray horn
Posterior (dorsal) root ganglia - cell bodies of sensory neurons (unipolar neurons)
Anterior (ventral) root - connection from the spinal cord to the peripheral nervous system
that carries only motor information
Motor neurons = multipolar neurons
Exits from vertebral column at the intervertebral foramen
31 pairs of spinal nerves (Fig 13.2)
8 cervical
12 thoracic
5 lumbar
5 sacral
1 coccygeal
each has 3 branches (rami) (Fig 13.6)
Visceral (Rami communicantes, part of autonomic nervous system)
Posterior (dorsal)
Anterior (ventral) branch (anterior rami) divides into plexus (braids) except thoracic (T1 and
possibly T2 exception)
Cervical plexus (C1-C5) - Phrenic nerve (phren = diaphragm) (Fig 13.7)
Brachial plexus (C5-C8, T1) (Fig 13.8)
Radial
Ulnar
Median nerve
Median nerve is compressed in carpal tunnel syndrome causing numbness,
tingling, and pain in the palm and fingers
Lumbar plexus - (L1-L4) femoral nerve (Fig 13.9)
Sacral plexus - (L4-S4) sciatic nerve (Fig 13.10)
Dermatomes
Dermatomes - letter and numbers indicating spinal nerve innervating a given region of skin (area
of the skin that supplies sensory input to the dorsal roots of one pair of spinal nerves) (Fig
13.11)
Sensations
Crude touch – can tell something has contacted skin but where and what can’t be determined
Fine touch – can tell where and what has contacted skin
Meissner’s corpuscles
Pressure – deformation of deeper tissues felt over larger area than touch
Pacinian corpuscles
Vibration
Itch
Tickle
Thermal sensations – warm and cold
Pain – nociceptors
Proprioceptive
Sensory Receptors Classified by Microscopic Features
Free (unencapsulated) nerve endings (Fig 16.1 & Table 16.1)
Bare dendrites associated with pain, thermal, tickle, itch, and some touch sensations
First-order neurons – conduct impulses from the PNS into the CNS
Encapsulated nerve endings
Dendrites enclosed in a connective tissue capsule
Meissner’s corpuscle or Pacinian corpuscle
First-order neuron
Receptor cell synapses with first-order neuron
Photoreceptors of the eye
Inner ear hair cells
Taste buds of the tongue
Sensory Receptors Classified by Location and Activating Stimuli
Exteroceptors – located near or at the external surface of the body
Hearing, vision, smell, taste, touch, pressure, vibration, temperature, and pain
Interoceptors – located in blood vessels, visceral organs and nervous system
Proprioceptors – located in muscles, tendons, joints, and inner ear
Body position, muscle length and tension, and the position and movement of joints
Sensory Receptors Classified by Stimulus Detected
Mechanoreceptors – mechanical pressure
Touch, pressure, vibration, proprioception, hearing and equilibrium, and stretch of blood
vessels and internal organs
Thermoreceptors – changes in temperature
Nociceptors – physical or chemical damage to tissues
Photoreceptors – light detection in retina
Chemoreceptors – taste and smell
Osmoreceptors – osmotic pressure of body fluids
Somatic Sensory Pathways
Ascending pathways
Neuronal composition
First-order neurons – body to spinal cord
Second-order neurons – spinal cord and brain stem to thalamus
Third-order neurons – thalamus to primary somatosensory of cerebral cortex
Pathways
Anteriolateral – pain, thermal, tickle, itch, crude touch, and pressure (Fig 16.6)
Uses spinothalamic tracts
Posterior column-medial lemniscus (lemniscus = ribbon) (Fig 16.5)
Uses spinothalamic and spinoreticular tracts
fine touch, stereognosis (recognize objects by touch) proprioception, and vibratory
sensations
Cerebellar – posture, balance, and coordination of skilled movements (Table 16.3 &
16.12)
Uses spinocerebellar tracts
Somatic Motor Pathways
Upper motor neurons – located in cerebral cortex (Fig 16.9)
Basal ganglia neurons
Cerebellar neurons (Fig 16.12)
Local circuit neurons – interneurons synapse on lower motor neurons in brain stem and spinal
cord – coordination of rhythmic activity
Descending tracts (Table 16.4 & Fig 16.10)
Corticospinal – cerebral cortex to spinal cord (crosses in decussation of pyramids)
Rubrospinal – red nucleus to spinal cord
Reticulospinal – reticular formation to spinal cord
Tectospinal – Corpora quadragemini to spinal cord
Reflexes
Reflex - quick involuntary response to an internal or external stimulus (results in either somatic
or visceral reflex)
Parts (Fig 13.13 – 13.17)
Receptor
Sensory neuron
CNS – integration center
Motor neuron
Effector - muscle or gland that responds to motor neuron impulse