Upper motor neuron units: A network of neurons whose cell bodies originate in the cerebral cortex and brainstem. Their axons descend through the spinal cord and synapse with lower motor neurons. Damage to upper motor neuron units produces weakness, hyperreflexia, and hypertonia, typically without muscle atrophy or fasciculations.
Lower motor neuron units: A network of neurons whose cell bodies lie in the spinal cord and brainstem motor nuclei, with axons that travel directly to the muscles they innervate. Damage to lower motor neuron units produces weakness, hyporeflexia or areflexia, and hypotonia, along with muscle atrophy and fasciculations.

Because the two lesions share weakness but differ on every other sign, it helps to line them up on the same rows rather than memorize two separate paragraphs.

The following table compares upper and lower motor neuron lesions on reflexes, muscle tone, atrophy, and fasciculations.

Sign	Upper motor neuron lesion	Lower motor neuron lesion
Reflexes	Hyperreflexia (increased)	Hyporeflexia or areflexia (decreased or absent)
Muscle tone	Hypertonia (increased)	Hypotonia (decreased)
Muscle atrophy	Absent	Present
Fasciculations	Absent	Present

The cerebral cortex and brainstem structures where upper motor neurons originate are described next.

Anatomy of the brain

The brain is divided into two hemispheres, separated externally by the longitudinal fissure and connected internally by the corpus callosum
- Right hemisphere
  - Spatial awareness
  - Emotional processing
  - Facial recognition
  - Creativity
  - Abstract thought
  - Controls the left side of the body
- Left hemisphere
  - Language
  - Logic and reasoning
  - Analytical thinking
  - Controls the right side of the body

The brain is divided into four lobes with distinct characteristics that work collectively to assist function:
- Frontal lobe
  - Executive function
  - Voluntary movement
  - Problem solving
  - Learning
  - Behavior
  - Impulse control
  - Personality
  - Social behavior
  - Expressive language (Broca’s area)
- Parietal lobe
  - Awareness of somatic sense - touch, pain, temperature, pressure, vibration
  - Processing somatic sensation - analyzing, recognizing, and developing memory of somatic sense
  - Spatial and body awareness
  - Coordination of visual, auditory, and somatosensory stimuli
- Temporal lobe
  - Hearing
  - Receptive language (Wernicke’s area)
  - Memory
    - Declarative: memory regarding names of people, places, or things
    - Procedural (not a temporal lobe function - relies mainly on the basal ganglia and cerebellum): memory of how to perform activities such as brushing teeth, putting on makeup, etc.
- Occipital lobe
  - Awareness of visual stimuli
  - Processing of visual stimuli
The cerebellum and brainstem are separate brain structures (not lobes), with their own distinct functions:
- Cerebellum
  - Motor learning
  - Coordinate movement
  - Balance and equilibrium
  - Proprioception sense
  - Maintain posture
- Brainstem
  - Ascending and descending tracts are located in the brainstem
  - Heart rate and respiration rate
  - Sleep and wake cycles
  - Digestion
  - Body temperature
  - Vomiting
  - Swallowing

Other important brain structures

Thalamus
- Receives sensory information from the body
- Sends these signals to the appropriate areas of the cerebral cortex for further processing
- Helps coordinate movements by sending signals to the motor cortex
- Assists with memory processing
Hypothalamus
- Maintains homeostasis within the body by regulation of hormones
Basal ganglia
- Initiation of movements
- Assists with maintaining posture and muscle tone
- Assists with controlling voluntary movements

Protective layers of brain

Diagram of the brain's protective layers, including the skull and meninges — Brain protective layers

External layers (scalp and skull)

Definitions

Scalp: The outermost skin, fat, and muscular tissue covering the head.
Skull bone: A hard, bony container that shields the brain from direct physical hits.
Periosteum: A thin, strong membrane covering the outside of the skull bone.

The meninges (protective membranes)

Definitions

Dura mater: The tough, thick, and leathery outermost membrane attached directly to the inner skull.
Arachnoid mater: The delicate, middle, web-like membrane. Below it lies the subarachnoid space filled with cerebrospinal fluid that cushions the brain from shocks.
Pia mater: The very tender and thin innermost membrane that hugs the soft outer folds of the brain.

Arterial circulation of the brain

Circle of Willis

A network of arteries that provides blood supply to the brain
Major arteries
- Anterior cerebral artery (ACA)
  - Supplies frontal, prefrontal, and supplementary motor cortex, as well as circulation to primary motor and sensory cortex
  - Injury to the ACA can cause hemiparesis with weakness of the lower extremity > upper extremity, with sparing of face, apraxia, abulia, akinetic mutism, and urinary incontinence
- Anterior communicating artery
  - The blood supply that connects the left and right ACA
  - Injury to the anterior communicating artery can cause visual disturbances, memory deficits, cognitive impairment, severe headache, altered mental status, and impaired executive function
- Middle cerebral artery
  - Blood supply to the frontal, temporal, parietal, and deeper structures
  - Injury to the middle cerebral artery can cause hemiparesis with weakness of the upper extremity > lower extremity, aphasia (the MCA supplies both Broca’s area and Wernicke’s area, so aphasia follows when the dominant hemisphere is involved), and neglect (when the non-dominant hemisphere is involved)
- Internal carotid artery
  - Provides oxygenation to the brain
  - Injury can lead to blurred vision, confusion, memory loss, hemiparesis, or sudden death
- Posterior cerebral artery
  - Supplies blood to the occipital and temporal lobes
  - Injury can cause visual field loss, visual impairment, headache, confusion, and memory impairment
- Posterior communicating artery
  - Connects the internal carotid artery to the posterior cerebral artery
  - Injury can cause visual field loss, ptosis, diplopia, headache, confusion, memory impairment, hemiparesis

PTA scope of practice: The PTA collects data and recognizes and communicates changes in a patient’s status to the supervising PT; evaluating the patient, interpreting findings, and changing the plan of care remain the PT’s responsibility. These arterial patterns help the PTA recognize a new or worsening sign, such as increasing weakness, new aphasia, or neglect, and report it to the supervising PT immediately (activating emergency services when stroke signs appear suddenly); localizing the lesion is not the PTA’s role. Supervision and delegation rules are covered in PTA responsibilities, research, and ADA.

Other important arteries of the brain

Basilar artery
- Supplies: Brainstem, occipital lobe, cerebellum, thalamus, medial temporal lobes
- Functions: Supports autonomic regulation
- Injury signs: Hemiparesis or quadriparesis, facial paralysis, dizziness, headache, dysarthria
- Involvement: Combines with vertebral artery → vertebrobasilar system → damage to both can cause locked-in syndrome
Vertebral artery
- Supplies: Brain and spinal cord
- Injury signs: Headache, neck pain, dysarthria, dysphagia, seizures, impaired coordination, sensory/motor deficits to the face and body
- Involvement: Joins basilar artery → vertebrobasilar system → dual injury may result in locked-in syndrome
Posterior inferior cerebellar artery (PICA)
- Supplies blood flow to the medulla, fourth ventricle, and cerebellum
- Occlusion causes Wallenberg syndrome (lateral medullary syndrome)
- Injury can cause diplopia, ptosis, facial pain, vertigo, slurred speech, hoarseness, balance deficits, and sensory deficits on the same-side face and the contralateral body

Definitions

Locked-in syndrome: Paralysis of all voluntary muscles, with only eye movement remaining intact. The individual remains conscious but is unable to speak or move.

Upper motor neuron units

Neurons originate in cerebral cortex, brainstem, spinal cord
Disease signs: weakness, hyperreflexia, hypertonia

Anatomy of the brain

Two hemispheres:
- Right: spatial awareness, emotion, creativity, controls left body
- Left: language, logic, analytical, controls right body
Six lobes:
- Frontal: executive function, voluntary movement, personality, Broca’s area
- Parietal: somatic sensation, spatial/body awareness
- Temporal: hearing, Wernicke’s area, memory (declarative & procedural)
- Occipital: visual awareness and processing
- Cerebellum: motor learning, coordination, balance, posture
- Brainstem: vital functions (HR, respiration, sleep, digestion, swallowing)

Other important brain structures

Thalamus: sensory relay, movement coordination, memory processing
Hypothalamus: hormone regulation, homeostasis
Basal ganglia: initiates movement, posture, muscle tone, voluntary movement control

Arterial circulation of the brain

Circle of Willis: main arterial network supplying brain
- Anterior cerebral artery (ACA): frontal/sensory cortex; injury—lower extremity weakness, apraxia, abulia
- Anterior communicating artery: connects ACAs; injury—visual/cognitive deficits, headache
- Middle cerebral artery: frontal/temporal/parietal; injury—upper extremity weakness, language deficits, neglect
- Internal carotid artery: brain oxygenation; injury—vision loss, confusion, hemiparesis, possible death
- Posterior cerebral artery: occipital/temporal lobes; injury—visual loss, headache, confusion
- Posterior communicating artery: connects ICA to PCA; injury—visual field loss, ptosis, memory impairment

Other Important arteries of the brain

Basilar artery: supplies brainstem, occipital, cerebellum; injury—hemiparesis, facial paralysis, locked-in syndrome (if vertebrobasilar system damaged)
Vertebral artery: supplies brain/spinal cord; injury—headache, dysphagia, coordination/motor deficits, locked-in syndrome (if both vertebral and basilar arteries involved)
Posterior inferior cerebellar artery (PICA): supplies medulla, cerebellum; injury—diplopia, vertigo, hoarseness, balance/sensory deficits (Wallenburg syndrome)

Locked-in syndrome

Complete paralysis except eye movement
Consciousness preserved, unable to speak or move

Next | 3.2 Anatomy and function of spinal cord

Central nervous system

Upper motor neuron units

Definitions

Upper motor neuron units: A network of neurons whose cell bodies originate in the cerebral cortex and brainstem. Their axons descend through the spinal cord and synapse with lower motor neurons. Damage to upper motor neuron units produces weakness, hyperreflexia, and hypertonia, typically without muscle atrophy or fasciculations.
Lower motor neuron units: A network of neurons whose cell bodies lie in the spinal cord and brainstem motor nuclei, with axons that travel directly to the muscles they innervate. Damage to lower motor neuron units produces weakness, hyporeflexia or areflexia, and hypotonia, along with muscle atrophy and fasciculations.

Because the two lesions share weakness but differ on every other sign, it helps to line them up on the same rows rather than memorize two separate paragraphs.

The following table compares upper and lower motor neuron lesions on reflexes, muscle tone, atrophy, and fasciculations.

Sign	Upper motor neuron lesion	Lower motor neuron lesion
Reflexes	Hyperreflexia (increased)	Hyporeflexia or areflexia (decreased or absent)
Muscle tone	Hypertonia (increased)	Hypotonia (decreased)
Muscle atrophy	Absent	Present
Fasciculations	Absent	Present

The cerebral cortex and brainstem structures where upper motor neurons originate are described next.