CNS: Spinal Cord and Somatic Reflexes

spinal cord

“information highway” that connects brain with lower body

conduction

sensory and motor information moves up and down the spinal cord

neural integration

spinal neurons receive input from multiple sources, integrate it, and execute appropriate output

locomotion

spinal cord coordinates repetitive sequences of contractions for walking

reflexes

involuntary responses to stimuli that are vital to posture, coordination, and protection

funiculi

white matter in the spinal cord is arranged in this column form

tract

bundle of axons; all fibers in a given _____ have similar function

ascending tract

carries sensory information up to the brain

descending tract

carries motor information down to skeletal muscles

decussation

crossing of the midline that occurs in many tracts so that brain senses and controls contralateral side of body

ispsilateral

origin and destination on same side of body

3

ascending tracts usually involve _ neurons from origin (receptors) to destinations in sensory areas of the brain

thalamus

this brain structure filters sensory info except for smell and decides whether to send it to the cerebral cortex

spinothalamic tract

ascending tract, pain (nociceptors) and temperature signals, first, second, and third order neurons

first order neuron

spinothalamic tract

receptors → spinal cord

enter spinal cord via posterior (dorsal) horn

axon synapses with secondary neuron in posterior (dorsal) horn (gray matter)

second order neuron

spinothalamic tract

spinal cord → thalamus

axon decussates — ascends in contralateral white matter

axon synapses with tertiary neuron in thalamus

third order neuron

spinothalamic tract

thalamus → primary somatosensory cortex (postcentral gyrus)

2

descending tracts usually involve _ motor neurons

upper motor neuron in brain

lower motor neuron in spinal cord

corticospinal tract

descending tract, major pathway for voluntary motor control, upper and lower motor neurons

upper motor neuron

corticospinal tract

cortex → spinal cord

originates in primary motor cortex (precentral gyrus)

enters spinal cord via anterior (ventral) horn (gray matter)

axon synapses with lower motor neuron in spinal cord

lower motor neuron

corticospinal tract

spinal cord → skeletal muscle

somatic reflexes

quick involuntary reactions of skeletal muscle to stimulation; stimulation required, quick, involuntary, stereotyped

receptor

site of stimulus

sensory (afferent) neuron

transmits info from receptors to spinal cord

integration center

determines whether efferent neurons issue signal to effector, can be mono- or polysynaptic

monosynaptic

afferent and efferent synapses meet, only one synapse

polysynaptic

multiple synapses

motor (efferent) neuron

carries motor impulses to effector organ)

effector

muscle fiber that responds to efferent signal

stretch (myotatic) reflex

when a muscle is stretched, it “fights back” and contracts, helps maintain equilibrium and posture, stabilizes joints by balancing tension in extensors and flexors

knee-jerk reflex

monosynaptic reflex, afferent nerve excites efferent nerve, sends message to muscle to contract, foot kicks out, quadriceps stretch

reciprocal inhibition

prevents muscles from working against each other by inhibiting antagonist when antagonist is excited

e.g., quadriceps contract, hamstrings inhibited in order to kick foot out

flexor reflex

quick contraction of flexor muscles resulting in the withdrawal of a limb from an injurious stimulus (e.g., pulling foot up and away from sharp object on ground), contraction of flexors and relaxation of extensors in that limb, polysynaptic reflex, often stimulates crossed-extension reflex as well