Motor Neuron Lesion
Most neurological symptoms include syncope (fainting) and seizure (both involve loss on consciousness) Lack of prodrome prior to attack favors syncope over seizure.
Focal transient neurological defect is most commonly due to transient ischemic attack, a cerebro-occusive event.
Low back pain after heavy lifting different from that after a car accident. In lifting concern is with muscle injury while in accident it is concerned with vertebral fracture.
Bladder dysfunction … severe nerve compression.
In evaluation, crucial to include symptoms of forgetfulness, speech and swallowing, headaches, syncope and incontinence.
Graded scale for muscular weakness: mild (4/5), moderate (3/5), severe (2/5, 1/5); Lack of insight into their deficit is also major clinical presenting sign. Ex. Anton’s syndrome, patient is blind but denies it due lesion in bilateral occipital lobe.
Upper motor lesion… cerebral hemisphere, cerebellum, brainstem
Lower motor lesion… anterior horn cell, spinal nerve root, peripheral nerve, NMJ
Lumbar puncture used in patients with subarachnoid hemorrhage. EEG assesses function of cerebral nervous systems used for altered consciousness. Electromyelogram asses conduction velocity used in patients with lower motor lesions.
Diagnostic Testing in Neurology:
Electroencephalogram- epilepsy
Nerve conduction studies- lower motor nerve lesions and EMG
Evoked potentials … speed of conduction from limb to cortex
Radiological Tests:
Cerebral angiogram, CAT scan, MRI, MRA
During patient history 2 major questions:
Type and Quality of Pain- sensory, motor, cognitive, change in consciousness
Severity, extent, radiation, exacerbating/mitigating factors
Onset and Progression over time
Acute pain/dysfunction symptoms:
-weakness, trauma, stroke, immune deficiency (Guillan Barre)
-Numbness, pain, vascular
-impaired motor control
-paroxysmal motor disorderà seizure
-altered consciousness (meningitis, encephalitis)
-amnesia (trauma, alcohol)
Upper Motor lesions have:
-INCREASED reflexes (3/5, 4/5) in extremities
-POSITIVE Babinski sign, (big toe up, digits flared)
-knee flexion weak
-minor muscular atrophy, NO fasciculation
Lower Motor Lesions have:
-REDUCED reflexes
-NEGATIVE Babinski sign
-Muscle weakness is proximal to injury
-Marked muscle atrophy
-Lots of fasciculation
Purpose of Neurological Evaluation: establishes causative disorder, differential diagnosis, confirms the need for more testing. History taking allows lesion localization.
Subacute pain/problems are least common, involve motor gain control, immune problems (multiple sclerosis), and tumor (brainglioma).
Chronic problems include;
Weakness- immune myopathy, multiple sclerosis
Pain numbness – metabolic, diabetic peripheral neuropathy
Impaired motor control – basal ganglia, Parkinsons, cerebellar, Friedrich’s Ataxia
Memory- degenerative, Alzheimer’s
Lhermittes Sign… electrical sensation down spine
Bilateral lesions cause quadriparesis and quadriplegia and indicates a high cervical lesion (MS, tumor, abcess) or a diffuse lower motor lesion.
Bilateral Weakness causes weak proximal muscles (indicated by climbing stairs and standing from chair) and distal muscle neuropathy (dropping objects from hands, tripping on feet).
Cranial nerve lesions indicate problem above foramen magnum, with ptosis, diplopia.
Ataxia leads to poor balance, worse at night, with eyes closed, and problems in limbs.
Six components to the neurological exam:
-Mental state/cognition
-Cranial Nerves
-Muscle strength
-Sensory exam
-Reflexes
-Gait and Coordination
Cognitive dysfunction indicates lesion above foramen magnum with any lesion of BILATERAL cerebral hemispheres impairing cognition.
Lesion of brain stem impairs alertness due to activation of reticular activating system.
Broca’s Lesion: NO fluent speech, Comprehension, NO repetition, NO naming
Wiernicke Lesion: Fluent speech, NO comprehension, NO repetition, NO naming
MRC Scale: 0-no movement, 1-trace movement, 2-movement in plane of gravity, 3- against gravity, 4- mild weakness, 5-normal
Motor Units and Muscle Receptors
Cell bodies of large motor neurons which supply skeletal muscle are contained within the ventral horn of the spinal cord.
Motor neurons are modulated by local circuits within the spinal cord and by descending pathways originating in the precentral gyrus.
Motor unit defined- comprised of a single motor neuron and all the muscle fibers associated with it; each muscle innervated by single motor neuron.
Innervation ratio is the number of muscle fibers innervated by a single motor neuron. Ratio is very large in muscles not requiring much fine control and low in muscles requiring fine motor control (hands).
Control of muscle force has two parameters:
1) Size principle- motor units recorded in fixed order from weakest to largest; small and weak ones activated first; larger inputs from cortex activate largest neurons and generate larger forces
2) Rate code- increase in firing rate produces increase in forceà summation
Two types of proprioceptive receptors: Muscle Spindle fibers and Golgi Tendon Organs; involved in length, velocity and load information from muscles.
Muscle Spindles
-Elongated structures within fleshy part of all striated muscles
-are fusiform in shape give rise to intrafusal fibers
-arranged in PARALLEL to skeletal muscle
-muscle spindle sense muscle LENGTH changes and RATE of change of length
-parallel structure causes them to STRETCH when skeletal muscle stretches
-spindle density high in muscles of FINE MOVEMENT
-innervated by GAMMA neurons but do NOT contribute to force of contraction
-Types of muscle spindles
1) nuclear chain fibers- short/thin, nuclear region filled with nuclei arranged in a single row along axis
2) nuclear bag fibers- filled with several long nuclei in clusters, longer and thicker
3) static bag fibers- sensitive to static position and steady state stretch
4) dynamic bag fibers- sensitive to changes in stretch
-sense functional muscle stretch in parallel attachment; stretch opens ion channels leading to AP
-Group 1a afferents supply intrafusal, dynamic bag and static bag fibers, very fast and spiral around cell body
-Group 2a- smaller and slower that innervated static bag
-Group 1a fire more during dynamic phase change of muscle
**muscle spindles function as STRETCH and LENGTH detectors
Extrafusal muscles are innervated by alpha motor neurons which ultimately causes contraction; therefore gamma neurons do not contribute to the contraction of the muscle.
Gamma motor neurons adjust the length of the muscle but do not contribute to the force of contraction.
Golgi Tendon Organs
-highly specialized to sense to FORCE or load placed on muscle fibers
-similar to Ruffini corpuscles
-informs muscle of force placed on muscles by tendonous structures
-Primary 1b afferents innervate and are UNMYELINATED
-arranged in SERIES with the muscle
-muscles pull directly on the collagen fibers increasing firing of 1b nerves
-active during CONTRACTION, quite during STRETCH
-are INDEP. Of length
Spinal Reflexes
Reflex defined- involuntary stereotyped motor response to specific sensory input. Function to maintain upright posture and balance.
Afferent component of reflexes from 1A muscle SPINDLES and efferent component is are alpha motor neurons which cause jerk-reflex.
Stretch reflexà dynamic + static component … maintains posture
Autogenic inhibition/reflex prevents the muscles and tendons from overload.
Tags: Anton's syndrome, Ataxia, Bladder dysfunction, Broca's Lesion, Cerebral angiogram, cerebro-occusive event, Electroencephalogram, epilepsy, ischemic attack, Muscle Spindles, prodrome, seizure, Subacute pain, syncope, Wiernicke Lesion
