Nervous System Anatomy

• Class notes dated January 6, 2025, 8:20 AM.

Central Nervous System Anatomy

• Includes the brain and spinal cord.
• The spinal cord connects to the brainstem and runs through the spinal cana
• Cranial nerves exit the brainstem
• Nerve roots exit the spinal cord.
• The spinal cord carries signals back and forth between the brain and peripheral nerves.
• Controls body functions like awareness, movements, thoughts, speech, and memory.

Peripheral Nervous System

• Consists of autonomic, enteric, and somatic systems.

Autonomic Nervous System

• Controls involuntary functions
• It receives sensory input from visceral organs
• Governs motor output to smooth muscles, cardiac muscles, and glands.
• The sympathetic component is responsible for "fight or flight" responses.
• The parasympathetic component handles "rest and digest" activities.

Enteric Nervous System

• Controls involuntary functions
• Involves sensory input from chemical changes and stretch in the GI tract
• Motor control of smooth muscles.

Somatic Nervous System

• Controls consciously controlled and voluntary functions.
• Sensory receptors and motor neurons connect to skeletal muscles.

Major Parts of The Brain

• Brain Stem: Continuous with the spinal cord, including the medulla oblongata, pons, and midbrain.
• Cerebellum: Located posterior to the brainstem.
• Diencephalon: Superior to the brainstem, containing the thalamus, hypothalamus, and epithalamus.
• Cerebrum: The largest part of the brain, positioned above the diencephalon.

Cranial Nerves

• Nerves emerge from or enter the skull.
• There are 12 cranial nerves that come directly from the brain through the skull.
• Olfactory (I): Smell.
• Optic (II): Vision.
• Oculomotor (III): Eye movement and pupil reflex.
• Trochlear (IV): Eye movement.
• Trigeminal (V): Face sensation and chewing.
• Abducens (VI): Lateral eye movement.
• Facial (VII): Face movement and taste.
• Vestibulocochlear (VIII): Hearing and balance.
• Glossopharyngeal (IX): Throat sensation, taste, and swallowing.
• Vagus (X): Movement, sensation, and abdominal organs.
• Accessory (XI): Neck movement (trapezius & SCM).
• Hypoglossal (XII): Tongue movement.

Motor Pathways

• Neural circuits transmit signals from the brain to muscles, leading to voluntary muscle movements.
• They coordinate and control muscle contractions.
• Two primary motor pathways work together for smooth, coordinated movements: the corticospinal and extrapyramidal pathways.
• Damage can result in motor impairments like weakness, tremors, or difficulties controlling movements.
• Understanding motor pathways aids in diagnosing and treating motor disorders.

Corticospinal Pathway

• Originates in the primary motor cortex of the brain.
• Upper motor neurons travel through the internal capsule, brainstem, and spinal cord.
• Upper motor neurons synapse with lower motor neurons in the spinal cord
• Lower motor neurons transmit signals to the muscles, causing voluntary muscle contractions.
• Disruptions can cause motor deficits such as weakness, paralysis, or impaired coordination.

Upper Motor Neurons (UMNs)

• Originate in the primary motor cortex of the cerebral cortex.
• They travel through the internal capsule, brainstem, and spinal cord.

Lower Motor Neurons (LMNs)

• Located in the spinal cord.
• Receive signals from the upper motor neurons.
• Extend axons out of the spinal cord to innervate skeletal muscles.

Muscle Contraction

• Occurs when LMNs receive signals from UMNs, transmitting electrical impulses to skeletal muscles.

Extrapyramidal Pathway

• Coordinates muscle tone, posture, and involuntary movements.
• Involves multiple subcortical nuclei and brain regions outside the primary motor cortex.
• Connections are more complex and indirect compared to the corticospinal pathway.

Spinal Segments

Cervical Spine

• There are 7 segments.
• Transmits signals to and from the head, neck, shoulders, arms, and hands.

Thoracic Spine

• There are 12 segments.
• Transmits signals to and from part of the arms, the anterior and posterior chest, and abdominal areas.

Lumbar Spine

• There are 5 segments.
• Transmits signals to and from the legs and feet
• Some pelvic organs.

Sacral Segments

• There are 5 fused vertebrae.
• Transmits signals to and from the lower back and glutes, pelvic organs, genital areas, and some areas in the legs and feet.

Coccygeal Segment

• 1 coccygeal remnant located at the bottom of the spinal cord.

Dermatomes and Myotomes

Dermatomes

• An area of skin supplied by sensory fibers from a single spinal nerve.
• Nerves transmit sensory information from specific areas of the skin to the spinal cord and brain.
• Organized in a segmented pattern corresponding to spinal nerves.
• C6–Thumb
• C7 – Index & Middle Finger
• C8 – Ring finger & Pinky
• L3 – Medial Knee
• L4 – Big toe
• L5 – Plantar and dorsal surface of foot/heel
• S1 – Pinky toe & lateral posterior leg
• S2 – Medial posterior leg

Myotomes

• A group of muscles primarily innervated by the motor fibers of a single spinal nerve.
• Motor neurons in the spinal cord send signals to specific muscles through these myotomes, enabling voluntary muscle movement.
• Organized in a pattern corresponding to specific spinal nerves.
• C1/C2 - Neck flexion/extension
• C3 - Neck lateral flexion
• C4 - Shoulder elevation
• C5 - Shoulder abduction
• C6 - Elbow flexion/wrist extension.
• C7 - Elbow extension/wrist flexion.
• T1 - Finger abduction.
• L2 - Hip flexion.
• L3 - Knee extension.
• L4 - Ankle dorsiflexion.
• L5 - Big toe extension.
• S1 - Ankle plantarflexion/ankle eversion/hip extension.
• S2 - Knee flexion.

Deep Tendon Reflexes

• Quickly confirm the integrity of the spinal cord
• Differentiate between upper motor nerve lesions (UMNL) and lower motor nerve lesions (LMNL).
• UMN = increase reflexes
• LMN = decreased reflexes

What to look for?

• Asymmetry
• Threshold for stimulus: how hard?
• Hyperactive = CNS lesion, test for weakness, and spasticity.
• Hypoactive = PNS, spinal roots, plexus, check for weakness, atrophy, and fasciculations.

Deep Tendon Reflexes (Spinal Level)

• Biceps brachii – C5
• Brachioradialis – C6
• Triceps brachii – C7
• Quadriceps – Patellar L4
• Gastrocnemius/soleus - Achilles S1

Grading of reflexes

• 0: Absent
• 1+: Trace
• 2+: Normal
• 3+: Brisk
• 4+: No sustained clonus
• 5+: Sustained clonus

Sensory Testing

• Touch perception – light vs deep
• Temperature perception – hot vs cold
• Pain perception - sharp vs dull
• Two-point discrimination
• Proprioception test
• Vibration sense with a tuning fork
• Make comparisons from one side to the other and from proximal to distal of each extremity

Manual Muscle Testing (MMT)

• MMT purposes include baseline for Rx, Dx and injury, set up a rehab program
• Break test with, gravity eliminated, and gravity assisted

Grading

• Grade 5: Normal-Able to hold against gravity with max resistance
• Grade 4: Good-As above except with moderate resistance
• Grade 3: Fair-Holding test position or moving in to test position against gravity
• Grade 2: Poor-Moving though the full range with gravity eliminated
• Grade 1: Trace-Mm contraction can be palpated
• Grade 0: Zero-No contraction can be elicited

UMN vs. LMN

• Upper motor neuron (UMN) lesions and lower motor neuron (LMN) lesions are two distinct types of neurological conditions
• They affect different parts of the nervous system and result in different patterns of symptoms
• Terms "upper motor neuron" and "lower motor neuron" refer to specific anatomical pathways

Upper Motor Neuron Lesion (UMN)

• Occur within the central nervous system (CNS), specifically within the brain or spinal cord.
• UMN lesions lead to increased muscle tone, a condition called hypertonia.
• Results in stiffness and resistance to passive movement
• Hyperactive reflexes are often present due to disrupted inhibitory signals from the brain
• Characterized by sudden muscle contractions or spasms
• weakness
• Stroke
• Traumatic brain injury (TBI)
• Multiple sclerosis (MS)
• Cerebral palsy
• Certain spinal cord injuries

Lower Motor Neuron Lesion (LMN)

• Occur outside the central nervous system, specifically within the peripheral nervous system (PNS), which includes the nerves that extend from the spinal cord to the muscles.
• LMN lesions lead to decreased muscle tone, a condition called hypotonia.
• Results in muscles feeling floppy and lacking resistance to passive movement
• Reflexes are typically reduced or absent due to the interruption of signals between the spinal cord and muscles
• Muscles may show signs of atrophy (wasting) due to the lack of neural input
• Muscle weakness or paralysis
• Amyotrophic lateral sclerosis (ALS)
• Spinal muscular atrophy
• Peripheral nerve injuries
• Certain types of neuropathy

Differences & Similarities

• The location of the lesion is the key distinguishing factor between UMN and LMN lesions
• UMN lesions often result in increased muscle tone, hyperactive reflexes, and spasticity
• LMN lesions typically result in decreased muscle tone and reduced or absent reflexes
• The conditions that cause UMN and LMN lesions vary, with different underlying causes and mechanisms

Neurological Signs & Symptoms

Spasticity

• Resistance of a limb to passive movement due to damage of UMN
• Examples in Spinal cord injury, MS, Cerebral palsy, stroke, TBI, ALS ect.
• Generally only during mm stretch (not at rest) usually accompanied with increase tendon reflexes
• Velocity depending
• More noticeable with fast movements
• Difference in resistance from one direction to another

Rigidity

• Resistance throughout ROM due to over firing of UMN - examples in Parkinson's & Huntington's
• Mm tone is increased even at rest, present during PROM in all directions across individual joints
• Absence of synergy
• Cogwheel rigidity: hypertonic state w/ ratchet-like jerkiness
• Lead pipe rigidity: hypertonic state throughout ROM, simultaneous co-contraction of agonist & antagonist

General Signs & Symptoms

• Flaccidity:Loss of normal mm tone due to deterioration of LMN
• Resting tremors, intention tremors (initiated with movement.)
• Circumduction gait in Hemiplegia or MS
• Bradykinesic or festinating gait seen in Parkinsons
• Decreased tissue health & edema leading to decubitus ulcers
• Seizures, speech dysfunction
• Paresthesia or dysesthesia, increase as sweated or secretions, general abnormalities in temperature regulation
• Autonomic dysreflexia – SCI
• Life threatening phenomenon that occurs in persons with spinal cord injuries ABOVE T6 level caused by reflex action of ANS in response to stimulus such as distended bladder, fecal mass, bladder irritation, rectal manipulation, thermal or pain stimuli, or visceral distention

Definitions:

• Dysarthria: Defective speech due to muscular dysfunction
• Dyskinesia: A defect in the ability to perform voluntary movement
• Dysmnesia: Any impairment in memory
• Dysphagia: Inability to swallow
• Dysphasia: Impairment of speech resulting from brain lesion
• Dyspnea: Laboured difficult breathing
• Dyspraxia: A disturbance in control and execution of voluntary movements
• Dystonia: Prolonged muscle contraction that causes twisting and repetitive movement or abnormal posture
• Dysesthesia: Abnormal sensation on the skin
• Ataxia: Defective muscular coordination
• Paresthesia: Sensation of numbness, prickling, tingling
• Dysreflexia: Individual with T6 or higher spinal cord injury experiences a life threatening uninhibited sympathetic response of the nervous system to a noxious stimulus
• Aphasia: Inability to speak may be due to lack of comprehension of words
• Paralysis: Temporary or permanent loss of function especially loss of sensation and voluntary control
• Can be spastic (upper motor neuron) or flaccid (lower motor neuron)

Developmental Reflexes

• Involuntary, or automatic, action that the body does in response to a stimulus
• Neonatal reflexes or primitive reflexes are the inborn behavioural patterns that develop during uterine life
• Should be fully present at birth and gradually inhibited by higher centres of the brain
• Related to UMN lesions

Types

• General body reflexes, facial reflexes, and oral reflexes

General Body Reflexes:

• Moro / Startle reflex integrated 2 – 4 months
• Palmar / plantar grasp reflex integrated 5 – 6 months
• Walking / Stepping reflex 2 – 4 months
• Asymmetric tonic neck reflex 6 months
• Symmetric tonic neck reflex 9 – 11 months
• Babinski's reflex 8 – 12 months

Facial Reflexes

• Blind reflex is permanent
• Auditory orienting reflex is permanent

Oral Reflexes

• Rooting integrates 3 – 4 months in Cerebral palsy
• Sucking is present in Cerebral palsy
• Gag is permanent
• Swallowing reflex is present in Cerebral palsy

Clinical Significance

• Automatic responses are measured in terms of timing, strength, and symmetry to determine signals sent to brain, spinal cord, face, and posture.
• They are related with leaning disorders, ADHD, autism spectrum
• Startle/Moro reflex causes issues with balance and coordination
• The Rooting reflex causes issues with feeding

Review of notes

• Review cranial nerves
• Review segments of the spine.
• Review deep tendon reflexes and grades
• Study the definitions page.
• Know spinal pathways.
• Know spasticity and rigidity.
• Know UMN and LMN characteristics.