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

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

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

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

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