nervous system part 3 – section 2: reflexes and motor control (from week 5 – nervous system part iii, slides 53–60)

Reflex definition

Autonomic, involuntary, predictable response to a specific stimulus

reflex arc

pathway mediating a reflex response

components of a reflex arc

sensory receptor, afferent neuron, integration center, efferent neuron, effector organ

sensory receptor

detects stimulus and initiates nerve impulse

afferent neuron

transmits sensory information to the cns

integration center

processes information in spinal cord or brain

efferent neuron

carries command from cns to effector organ

effector organ

muscle or gland that performs the response

types of reflexes

spinal or cranial, somatic or autonomic, innate or learned, monosynaptic or polysynaptic

spinal reflex

reflex processed at spinal cord level without brain involvement

cranial reflex

reflex processed in the brainstem

somatic reflex

reflex involving skeletal muscles

autonomic reflex

reflex involving smooth muscle, cardiac muscle, or glands

innate reflex

reflexes present at birth

learned (conditioned) reflex

reflexes acquired through experience or training

monosynaptic reflex

single synapse between afferent and efferent neuron (example: stretch reflex)

polysynaptic reflex

multiple synapses involving interneurons; more complex

stretch reflex

example: patellar tendon (knee-jerk) reflex; helps maintain muscle tone and posture

withdrawal reflex

protective reflex causing withdrawal from painful stimulus

crossed-extensor reflex

extension of opposite limb to maintain balance when withdrawal reflex occurs

pupillary light reflex

type of cranial, autonomic, innate, and polysynaptic reflex

pupillary light reflex pathway

light activates photoreceptors → afferent neuron → midbrain → efferent neurons → pupil constriction

reflex importance

helps maintain posture, protect body, and coordinate movement

voluntary motor control

process of consciously generating and controlling movement

steps in voluntary motor control

idea → program → execution → feedback

idea phase

involves limbic system, association areas, and supplementary motor area

program phase

involves premotor cortex and supplementary motor area for planning movements

execution phase

involves primary motor cortex and brainstem motor nuclei activating motor neurons

feedback phase

involves sensory information and cerebellum adjusting movement accuracy

motor pathways

divided into lateral (pyramidal and rubrospinal) and ventromedial (reticulospinal, vestibulospinal, tectospinal) tracts

lateral pathways

control fine voluntary movements of distal muscles

pyramidal tracts

direct pathways from primary motor cortex for voluntary control

origin of pyramidal tracts

primary motor cortex

pyramidal decussation

point where pyramidal tracts cross over in medulla oblongata

rubrospinal tract

originates in red nucleus of midbrain; assists voluntary limb control

ventromedial pathways

control posture, balance, and gross body movements

reticulospinal tract

regulates involuntary posture and muscle tone via reticular formation

vestibulospinal tract

maintains balance and posture; receives input from vestibular apparatus

tectospinal tract

coordinates head and eye movements toward visual and auditory stimuli

integration of motor control

involves cortex, cerebellum, basal nuclei, and brainstem

cerebellum in motor control

coordinates voluntary movements, provides feedback, maintains tone, stores learned motor patterns

function of cerebellum in feedback

compares intended movement with actual movement and corrects errors

basal nuclei in motor control

select purposeful movements, inhibit unwanted ones, and support posture

feedback loops

basal nuclei and cerebellum send information to thalamus and motor cortex for coordination

motor learning

repetition of movement strengthens neural circuits in cerebellum and cortex

huntington’s chorea

disorder caused by genetic degeneration of basal nuclei pathways

symptoms of huntington’s chorea

involuntary jerking, loss of coordination, and cognitive decline

parkinson’s disease

disorder caused by loss of dopamine neurons in substantia nigra of basal nuclei

symptoms of parkinson’s disease

rigidity, tremors, slow movement (bradykinesia), shuffling gait, difficulty initiating and stopping movement

basal nuclei and parkinson’s disease

decreased dopaminergic input leads to excessive inhibition of movement

motor cortex role

initiates voluntary movement

brainstem role

integrates basic movement and posture

spinal cord role

executes motor commands through motor neurons and reflex pathways

upper motor neurons

location in brain and spinal cord; project to lower motor neurons

lower motor neurons

location in brainstem or spinal cord; project to skeletal muscles

descending motor pathways

carry motor commands from brain to spinal cord

ascending sensory pathways

carry feedback information from body to brain

importance of motor feedback

allows adjustment and fine-tuning of ongoing movements