Intro to Neuroscience

What type of neuron carries sensory information into the CNS?

• Afferent neurons

• Pseudounipolar neurons in DRG

What type of neuron carries motor information out of the CNS?

• Efferent neurons

• LMNs in ventral horn of SC

Where are dorsal root ganglia (DRG) located?

Just outside the SC on the dorsal root of each spinal nerve

Where do DRG central processes go?

Into SC through the dorsal horn

Where do DRG peripheral processes go?

Out to the body’s sensory receptors

What is located in the gray matter of the SC?

• Dorsal horn (sensory info)

• Ventral horn (motor output)

• Intermediate zone (interneurons & autonomic nuclei)

What is located in the white matter of the SC?

Ascending (sensory) & descending (motor) tracts

Which part of the white matter carries motor signals?

• Ventral & lateral funiculi

• Corticospinal tract

Which part of the white matter carries sensory signals?

• Dorsal columns

  • Fasciculus gracilis

  • Fasciculus cuneatus

• Lateral funiculi

What structures make up the monosynaptic reflex arc (MSR)?

• Muscle spindle

• Afferent neuron (from spindle —> DRG —> dorsal horn)

• Synapse in ventral horn

• Efferent (LMN) to same muscle

• Effector (skeletal muscle) contracts

What is the role of upper motor neurons (UMNs) in reflexes?

They excite & inhibit reflex arc responses

What happens to MSRs with an UMN lesion?

Increased reflexes (hyperreflexia) due to loss of inhibition

What happens to MSRs with a LMN lesion?

Decreased or absent reflexes (hyporeflexia or areflexia) due to direct loss of the arc

MSR responses in UMN damage?

• Hyperreflexia

• Possible clonus

MSR response in LMN damage?

• Hyporeflexia

• Areflexia

What are the Big Three ascending sensory pathways?

• Dorsal Column

• Spinothalamic

• Spinocerebellar tracts

What is the dorsal column pathway?

• Receptor —> peripheral nerve —> DRG

• Ascends IPSILATERALLY in dorsal column

• Synapse in medulla

• Crosses midline in medulla

• Ascends to thalamus —> somatosensory cortex

What are the neuron types for motor and sensory systems?

• Motor neurons = Multipolar (UMNs & LMNs)

• Sensory neurons = pseudounipolar (DRG)

Where are upper and lower motor neurons located?

• UMNs = pre-central gyrus (frontal lobe)

• LMNs = ventral horn of SC

Where do LMNs project?

• Out of the SC via ventral roots to skeletal muscles

• Release ACh to contract

Where are DRG cell bodies located?

• In the dorsal root ganglion

• Outside SC

What do the peripheral and central processes of DRG neurons do?

• Peripheral = from sensory receptors to cell body

• Central = from DRG into SC (via dorsal root)

What is the gray matter of the SC?

• Dorsal horn (sensory processing)

• Ventral horn (LMN cell bodies)

• Intermediate zone (interneurons, autonomic neurons)

What is the white matter of the SC?

• Myelinated axons

• Ascending and descending tracts

  • Dorsal columns (sensory)

  • Lateral funiculus (corticospinal tract)

  • Anterior funiculus (motor & sensory)

What is the MSR?

A spinal reflex involving a single synapse between afferent & efferent neurons

What structures are required for the MSR?

• Muscle spindle

• Afferent neuron —> DRG —> SC

• Synapse on LMN in ventral horn

• Efferent LMN exits ventral root

• Effector muscle contracts

What is the patellar tendon reflex testing?

• MSR arc integrity at L2-L4

• Especially L4

What happens to MSRs in UMN lesion?

• Hyperreflexia

• Loss of inhibition to LMNs

• Spasticity, possible clonus

What happens to MSRs in LMN lesion?

• Areflexia

• Hyporeflexia

• Reflex arc is broken

What is the Dorsal Column Pathway?

• Receptor —> DRG —> ascend ipsilaterally in dorsal columns

• Synapse in nucleus gracilis/cuneatus (medulla)

• Cross midline in medulla —> ascend to thalamus

• End at post-central gyrus (parietal lobe) of opposite side of signal

What is the Spinothalamic Tract pathway?

• Receptor —> DRG —> synapse in dorsal horn

• Cross midline immediately —> ascend in anterolateral tract

• Synapse in thalamus —> post-central gyrus of opposite side of signal

What is the Corticospinal Tract pathway?

• Starts in pre-central gyrus (UMNS) (frontal lobe)

• Descends through brainstem

• Crosses at medullary pyramids —> Lateral corticospinal tract

• Synapses on LMNs in ventral horn

• Carries voluntary motor control to body