PTA2046 Advanced Brain - Neuroanatomy Flashcards

Nervous System: Introduction

• PTA2046 Advanced Brain Habilitation/Rehabilitation: Neuroanatomy Review
• Prepared by Sherman Mercado, PT, DPT
• Includes the brain and spinal cord.

Nervous System Function

• Functioning nervous system tasks:
  • Perceive sensory experiences.
  • Initiate movement.
  • Perform cognitive tasks
• Deals with various problems:
  * Reliably communicate with the rest of the body to accurately convey sensory and motor signals
  * Be flexible enough to change over time to learn and solve problems
• Divided into two parts composed of:
  • The flexible central nervous system (CNS).
  • The reliable peripheral nervous system (PNS).

Central Nervous System (CNS)

• Site of information integration.
• Composed of the brain and spinal cord.
• Goal: take in and process all information from outside and within the human body, put meaning to that data, and decide how to control the body’s response.

Peripheral Nervous System (PNS)

• Provides communication between the CNS and body.
• Comprises all components outside the cranium and spine.
• Consists of nerves leading to and from the CNS, including:
  • Cranial nerves exiting the brainstem.
  • Spinal roots exiting the spinal cord, many combine to form peripheral nerves.
• Divided into the somatic nervous system and the autonomic nervous system (ANS).
  • Somatic nervous system: composed of sensory nerves (cranial and peripheral nerves carrying information toward the CNS) and motor nerves (nerves carrying information from the CNS to the peripheral systems).
  • ANS: further subdivided into sympathetic and parasympathetic components.

Divisions of the Nervous System

• The nervous system is divided into the central and peripheral nervous systems.
• The peripheral nervous system is further divided into the autonomic and somatic nervous systems.
• The autonomic nervous system is divided into the sympathetic and parasympathetic nervous systems.
• The somatic nervous system involves sensory input and motor output.

Cranial Nerve Summary

• Olfactory (I):
  • Sensory: nose
• Optic (II):
  • Sensory: eye
• Oculomotor (III):
  • Motor: all eye muscles except those supplied by IV and VI
• Trochlear (IV):
  • Motor: superior oblique muscle
• Trigeminal (V):
  • Sensory: face, sinuses, teeth, etc.
  • Motor: muscles of mastication
• Abducent (VI):
  • Motor: external rectus muscle
• Facial (VII):
  • Motor: muscles of the face
  • Intermediate motor: submaxillary and sublingual gland
  • Sensory: anterior part of tongue and soft palate
• Vestibulocochlear (VIII):
  • Sensory: inner ear (vestibular and cochlear)
• Glossopharyngeal (IX):
  • Motor: pharyngeal musculature
  • Sensory: posterior part of tongue, tonsil, pharynx
• Vagus (X):
  • Motor: heart, lungs, bronchi, gastrointestinal tract
  • Sensory: heart, lungs, bronchi, trachea, larynx, pharynx, gastrointestinal tract, external ear
• Accessory (XI):
  • Motor: sterno-cleidomastoid and trapezius muscles
• Hypoglossal (XII):
  • Motor: muscles of the tongue

Parasympathetic vs. Sympathetic

• Parasympathetic Nervous System:
  • Associated Ganglia:
    • Ciliary ganglion
    • Pterygopalatine ganglion
    • Otic ganglion
    • Submaxillary ganglion
  • VII cranial nerve
  • IX cranial nerve
  • Affects the pupillary muscle, ciliary muscles, lacrimal gland, nasal gland, parotid gland, sublingual, submandibular glands.
  • Also affects the heart, larynx, trachea, bronchi, lungs, stomach, thin intestine, colon (ascending and transverse), and appendix, affects the rectum, kidney, and urinary bladder, gonads
• Sympathetic Nervous System:
  • Associated Ganglia: Celiac ganglion, Superior mesenteric ganglion, Inferior mesenteric ganglion
  • Affects the heart, larynx, trachea, bronchi, lungs, liver, gallbladder, pancreas, stomach, thin intestine, colon (ascending and transverse areas). appendix, rectum, the adrenal gland, the kidney and gonads

Cellular Components of the Nervous System

• Types of nerve cells: neurons and neuroglia.

Neurons

• Specialized for communication through their ability to generate rapid electrochemical signals.
• Divided into three categories:
  • Afferent neurons (sensory neurons).
  • Interneurons.
  • Efferent neurons (motor neurons).
• Interneurons relay signals between two neurons.
• Motor neurons (efferent): convey output from the CNS to the muscles
• Sensory neurons (afferent): detect environmental or bodily stimuli and relay it to the CNS

Types of Neurons

• Sensory Neuron
• Motor Neuron
• Interneuron

Basic Neuron Types

• Unipolar (Sensory Neuron)
• Bipolar (Interneuron)
• Multipolar (Motoneuron)
• Pyrimidal Cell
• PURKINJE CELL

Parts of a Neuron with Function

• Dendrites:
  • Receive signals from other cells.
• Cell body (soma):
  • Organizes and keeps the cell functional.
• Cell membrane:
  • Protects the cell.
• Axon hillock:
  • Generates impulse in the neuron.
• Nucleus:
  • Controls the entire neuron.
• Node of Ranvier:
  • Allow diffusion of ions.
• Schwann cell:
  • Produces the myelin sheath.
• Axon:
  • Transfers signals to other cells and organs.
• Myelin sheath:
  • Increases the speed of the signal.
• Axon terminal:
  • Forms junctions with other cells.

Neuroglia (CNS)

• Diverse support cells that facilitate neuron function and survival.
• Nonneuronal supporting cells that provide critical services for neurons.
• Types: astrocytes, oligodendrocytes, microglia, and ependymal cells.

Astrocytes

• Maintain the capillary endothelium and provide a vascular link to neurons.
• Contribute to the metabolism of the CNS.
• Regulate extracellular concentrations of ions and neurotransmitters.
• Proliferate after an injury to create a glial scar.

Oligodendrocytes

• Wrap myelin sheaths around axons, forming the white matter of the CNS.
• Nonmyelinating oligodendrocytes (satellite oligodendrocytes): associate with the cell bodies of neurons and appear to regulate ion concentrations, similar to astrocytes.

Microglia

• The phagocytes of the CNS.
• Engulf and digest pathogens.
• Assist with nervous system repair after injury.

Ependymal Cells

• Line the ventricular system.
• Produce and circulate cerebrospinal fluid.

Neuroglia in the PNS

• Similar functions as in the CNS.
• Satellite cells buffer extracellular ion concentrations around neuronal cell bodies.
• Major neuroglial cell of the PNS is the Schwann cell; further divided into different functional classes.

Schwann Cells

• Myelinating Schwann cells ensheath axons in myelin, similar to oligodendrocytes.
• Nonmyelinating Schwann cells outnumber myelinating Schwann cells (4:1); similar functions to astrocytes, as they contact vasculature and participate in ion buffering.
• Terminal Schwann cells help maintain the neuromuscular junction.

Neuron Structures

• Consists of dendrites, a cell body, and an axon.

Dendrites

• Responsible for transducing extracellular physical or chemical input into an intracellular signal.
• Produces electrical currents, which are transferred to the cell body for processing.
• Neurons with no true dendrite: unipolar or pseudounipolar.
• Neurons with multiple dendrites: multipolar.
• Somatosensory neurons: pseudounipolar, allowing for linear communication of sensory stimuli.
• Multipolar Purkinje cells in the cerebellum: elaborate dendrites, allowing them to integrate a large number of inputs and allow for motor learning.

Cell Body or Soma

• Composed of a nucleus and a number of different cellular organelles.
• Responsible for synthesizing proteins and supporting functional activities of the neuron, such as transmitting electrochemical impulses and repairing cells.
• Cell bodies of neurons with similar functions: often grouped together to form nuclei in the CNS and ganglia in the PNS.

Axon

• Message-sending component of neurons.
• Extend from the cell body and contact target cells that can include muscle cells, glands, or other neurons.
• Neurons communicate through action potentials down their axon (electrical signals down their axon).
• Action potentials stimulate the release of chemical signals called neurotransmitters that bind to receptors on target cells to elicit a response.

Electrical Conduction within Neurons

• Myelin increases the efficiency of action potential conduction through:
  • Increased conduction velocity.
  • Decreased metabolic expenditure.
• Occurs through the movement of ions across the membrane.
• Myelin facilitates electrical conduction in some axons.

Myelin Comparison (CNS vs PNS)

• CNS myelin (produced by oligodendrocytes): more compact due to space constraints within the skull.
• PNS myelin (produced by Schwann cells): less compact, providing additional protection for peripheral nerves.

Axon Regrowth

• PNS myelin: allows for axon regrowth.
• CNS myelin: appears to prevent axon regrowth following injury, potentially explaining limited recovery in spinal cord injuries compared to peripheral nerve injuries.

Myelin Function

• Insulates the axon.
• Myelinated regions of the axon: devoid of ion channels, prevents electrical charge from leaking out of the axon.
• Myelin sheath: prevents the charge within the axon from being stored at the membrane, allowing it to more rapidly flow within the axon.
• Myelin sheath: not continuous; contains interruptions or spaces within the myelin called the nodes of Ranvier (or simply nodes).
• Nodes: 2 μm in length and contain ion channels that restore the action potential as it travels along the axon.
• Lack of ion channels in internodes: action potentials “leap” from node to node in a process called saltatory conduction (saltare means “to leap” in Latin).

Multiple Sclerosis

• Neurodegenerative disorder caused by the autoimmune destruction of oligodendrocytes.
• Loss of myelin impairs action potential conduction.
• Demyelinated neurons must continue to spend higher amounts of ATP for each action potential.
• Production of additional ATP leads to the production of free radicals, which damage macromolecules and are generally toxic to cells.
• Myelination helps keep neurons alive.

Significance of Myelinated Axons

• Myelinated axons: fewer ions to pump back across the membrane, reduces the adenosine triphosphate (ATP) cost of each action potential.

Synapses

• Site of contact between the axon and its target cell.
• Site at which electrical signals within the axon (i.e., action potentials) are translated into a chemical signal that creates some effect on the target cell.
• Chemical messages are called neurotransmitters.

Neurotransmitters

• Chemicals that are released from neurons to communicate with target cells.
• Common neurotransmitters: acetylcholine, glutamate, γ-aminobutyric acid, dopamine, serotonin, and norepinephrine.

Acetylcholine

• Conveys information in the PNS; neurotransmitter used by lower motor neurons that synapse onto skeletal muscle fiber.
• Plays a role in regulating heart rate and other autonomic functions.

Glutamate

• Excitatory neurotransmitter used widely throughout the CNS.
• Excessive glutamate release is thought to contribute to neuron destruction after an injury to the CNS.

γ-Aminobutyric Acid (GABA)

• Major inhibitory neurotransmitter of the brain.
• Glycine is the major inhibitory neurotransmitter of the spinal cord.

Dopamine

• Influences motor activity, motivation, general arousal, and cognition.

Serotonin

• Plays a role in mood, behavior, and inhibits pain.

Norepinephrine

• Used by the sympathetic nervous systems; produces the “fight-or-flight response” to stress.

Synaptic Cleft

• Synapses are connections between neurons that allow different parts of the nervous system to communicate with and influence each other.
• Synaptic Cleft: Is a space that separates two neurons. It forms a junction between two or more neurons and helps nerve impulse pass from one neuron to the other

Anatomic Components of the CNS

• CNS components: brain and spinal cord.

Brain Composition

• Consists of the cerebrum, the thalamic complex, the brainstem, and the cerebellum.

Supportive and Protective Structures

• Bony skull and vertebral column: mechanical protection against injury.
• Meninges: three layers of membranes, below the skull and within the vertebrae.

Meninges

• Three layers of membranes:

Dura Mater

• Outermost layer; thick, fibrous connective tissue membrane that adheres to the skull.
• Two distinct projections of dura:
  • Falx cerebri: separates the cerebral hemispheres.
  • Tentorium cerebelli: provides a separation between the posterior cerebral hemispheres and the cerebellum.
• Epidural space: area between the dura mater and the skull, potential space (only exists in the case of injury).

Arachnoid Mater

• Middle layer
• Subdural space: potential space lies between the dura and the arachnoid.
• Subarachnoid space: Below the arachnoid mater, bona fide space contains the cerebral arteries; filled with cerebrospinal fluid, which allows it to act as a cushion for the CNS.

Pia Mater

• Third layer.
• Innermost layer and adheres to the brain and spinal cord.
• Delicate and fairly permeable compared with the other layers.
• Mostly, meninges are continuous with the connective tissues found in peripheral nerves.

Cerebral Cortex

• Surface of the cerebrum or cerebral cortex is composed of depressions (sulci) and ridges (gyri).
• Gray matter: outer surface of the cerebrum, approximately 2 to 4 mm thick.
• White matter: inner surface of the cerebrum.

Lobes of the Cerebrum

• Divided into four lobes—frontal, parietal, temporal, and occipital.

Frontal Lobe

• Contains the primary motor cortex (PMC).
• Responsible for voluntary control of complex motor activities.
• Exhibits a strong influence over cognitive functions, including judgment, attention, awareness, abstract thinking, mood, and aggression.
• Broca area (principal motor region responsible for speech) is located within the frontal lobe, near the primary motor regions that control the lips, tongue, and larynx.

Parietal Lobe

• Contains the primary somatosensory cortex.
• Incoming sensory information is processed within this lobe, meaning is provided to the stimuli.
• Perceptual learning requires a functioning parietal lobe (Perception: process of attaching meaning to sensory information and requires interaction between the brain, body, and the individual’s environment).
• Specific body regions are assigned locations within the parietal lobe for this interpretation; known as the sensory homunculus.

Temporal Lobe

• Contains the primary auditory cortex (decodes pitch and volume of sounds; meaning of sounds is distinguished in other cortical regions).
• Wernicke area: ascribes meaning to particular sounds (i.e., words).
• Involved in declarative memory function (i.e., factual memories).
• Important memory-relevant structures like the amygdala and hippocampus are located in the temporal lobe.

Occipital Lobe

• Contains the primary visual cortex (distinguishes fine details of an image (e.g., line angles); meaning of these fine details are determined in association regions).
• Visual association cortex: extensive, located throughout the cerebral hemispheres along two main streams:
  • Dorsal stream includes regions in the parietal lobes and determines object location.
  • Ventral stream includes regions in the temporal lobes and determines object identity.

Primary vs. Association Cortex

• Primary cortices: deal with granular details.
• Association cortices: create meaning from these details.
• Example: neurons in the primary visual cortex are responsive to very specific visual stimuli, such as the angle of a line, whereas visual association cortices will construct objects (e.g., an octagon) and ascribe meaning to these objects (e.g., a stop sign).
• Association areas: responsible for all higher-order functions of the CNS, including personality, intelligence, memory, and consciousness.

Motor Areas of the Cerebral Cortex

• PMC (primary motor cortex): located in the frontal lobe, primarily responsible for contralateral voluntary control of the upper and lower extremity and facial movements.
• PMC neurons are organized around movements, lower motor neurons in the spinal cord are organized around muscles.
• Premotor area and supplementary motor area (SMA): regions project directly to the spinal cord and to the PMC.
  • Premotor area: involved in well-patterned, bilateral movements and appears to direct our movements based more on external cues (i.e., sensory information).
  • SMA: involved with eye control and appears to create sequences of movements based more on internal cues (i.e., learned information).

Hemispheric Specialization

• Further divided into the right and left cerebral hemispheres.
• Hemisphere that is responsible for language is considered the dominant hemisphere.
• 95% of the population, including all right-handed individuals, are left- hemisphere dominant.
• Individuals who are left-hand dominant, the left hemisphere is the primary speech center in about 50% of these people.
• Language deficits (e.g., aphasia): suggestive of damage to the dominant hemisphere.
• Deficits in spatial awareness (e.g., hemineglect): suggestive of damage to the nondominant hemisphere.

Dominant Hemisphere Functions

• The verbal or analytic side of the brain.
• Processing of information in a sequential, organized, logical, and linear manner; processing of information in a step-by-step or detailed fashion allows for thorough analysis.
• Language is produced and processed in the frontal, temporal, and parietal lobes of the dominant hemisphere.
  • Frontal lobes: Broca area - plans movements of the mouth to produce speech.
  • Temporal and parietal lobes: Wernicke area - attributes meaning to words.
• Broca area and Wernicke area work together during speech production: damage to either one causes severe language deficits.
• Common impairments seen in patients with dominant hemispheric injury:
  • Inability to plan motor tasks (apraxia).
  • Difficulty in initiating, sequencing, and processing a task.
  • Difficulty in producing or comprehending speech.
  • Perseveration of speech or motor behaviors.

Nondominant Cerebral Hemisphere

• Responsible for an individual’s nonverbal and artistic abilities.
• Process information in a complete or holistic fashion without specifically reviewing all the details, able to grasp or comprehend general concepts.
• Principal function of the nondominant hemisphere: determine spatial relationships.
• Involved in hand-eye coordination, determining the location of objects, and perceiving one’s position in space.
• Plays an important role in nonverbal communication, gestures and adjustments of the individual’s tone of voice.
• Deficits with nondominant hemisphere damage:
  • Poor judgment and safety awareness.
  • Unrealistic expectations.
  • Denial of disability or deficits.
  • Disturbances in body image.
  • Irritability and lethargy.

Hemispheric Connections

• Communication between the two hemispheres is constant, analytic and yet still grasp broad general concepts.
• Possible for the right hand to know what the left hand is doing and vice versa.
• Corpus callosum: large group of axons that connect the right and left cerebral hemispheres and allow communication between the two cortices.

Subcortical Structures

• Deep within the brain, include the internal capsule, the basal ganglia, and the limbic system.

Internal Capsule

• Contains the major projection fibers that run to and from the cerebral cortex.
• Lesion within the internal capsule typically causes contralateral loss of voluntary movement and conscious somatosensation, visual and auditory deficits occur rarely.

Basal Ganglia

• Group of nuclei located at the base of the cerebrum.
• Form a subcortical structure made up of the caudate nucleus, putamen, globus pallidus, substantia nigra, and subthalamic nuclei.
• Project to motor regions of the thalamus to regulate posture and muscle tone, as well as volitional and automatic movements.
• Most common condition that results from dysfunction within the basal ganglia: Parkinson disease (symptoms of Parkinson disease, including bradykinesia (slowed movement), hypokinesia (reduced movement amplitude), akinesia (a lack of movement), rigidity, and postural instability).

Limbic System

• Group of deep brain structures that are involved in memory and emotion.
• Limbic system is a functional classification, rather than anatomic.
• Appears to control memory, pain, pleasure, rage, affection, sexual interest, fear, and sorrow.
• Limbic system plays a critical role in retention of new memory, retrieval of past memory, and communication with higher brain structures.
• Extremely sensitive to alcohol and drugs and can be damaged with recurrent use of any of those substances.

Thalamic Complex

• Composed of the thalamus, epithalamus, subthalamus, and hypothalamus.

Thalamus

• Area where the major sensory tracts (dorsal columns and lateral spinothalamic) and the visual and auditory pathways synapse.
• Serves as a central relay station for sensory impulses, channels them to appropriate primary and association areas of the cortex for interpretation.

Hypothalamus

• Regulates homeostasis, which is the maintenance of a balanced internal environment.
• Involved in automatic functions, including the regulation of hunger, thirst, digestion, body temperature, blood pressure, sexual activity, and sleep-wake cycles.
• Responsible for integrating the functions of both the endocrine system and the ANS through its regulation of the pituitary gland and its release of hormones.

Brainstem

• Located between the thalamus and the spinal cord and is divided into three sections (midbrain, pons, and medulla).

Midbrain

• Connects the diencephalon to the pons, acts as a relay station for tracts passing between the cerebrum and the spinal cord or cerebellum.
• Houses reflex centers for visual, auditory, and tactile responses.

Pons

• Contains bundles of axons that travel between the cerebellum and the rest of the CNS.
• Functions with the medulla to regulate breathing rate, chewing, and swallowing.
• Contains reflex centers that assist with orientation of the head in response to visual and auditory stimulation.
• Cranial nerve nuclei can also be found within the pons, specifically cranial nerves V through VIII, which carry motor and sensory information to and from the face.

Medulla

• Extension of the spinal cord and contains the fiber tracts that run through the spinal cord.
• Motor and sensory nuclei for the neck and mouth region are located within the medulla, as well as the control centers for heart rate and respiration.
• Reflex centers for vomiting, sneezing, and swallowing are also located within the medulla.

Reticular Formation

• Collection of relay nuclei within the brainstem that extends vertically throughout its length.
• Adjust an individual’s level of arousal, including sleep-wake cycles.
• Facilitates the voluntary and autonomic motor responses necessary for certain self-regulating, homeostatic functions, involved in the modulation of muscle tone throughout the body.

Cerebellum

• Controls balance and complex muscular movements.
• Uses primarily proprioceptive information from the head and body and sends that information back to modify muscle and joint activity.
• Structure compares the blueprint for a movement to the final product and makes the necessary adjustments.
• Consists of two symmetric hemispheres and a midline vermis.

Cerebellum Components

• Vermis: controls the trunk and central regions of the body and assists in balance and posture.
• Hemispheres: control the limbs and allows us to smoothly execute complex, multijoint movements.

Damage to the Cerebellum

• Characterized by gait instability, intention tremor, uncoordinated movements, slurred speech, and difficulty with smooth eye movements, problems with trajectory of movement (the patient will often overshoot or undershoot).

Spinal Cord

• Direct continuation of the brainstem, specifically the medulla.
• Two primary functions: coordination of movement patterns and communication of sensory information.
• Extends approximately to the level of the intervertebral disc between the first two lumbar vertebrae.
• Approximately the vertebral L1 level, the spinal cord becomes a cone-shaped structure called the conus medullaris.
• Below the conus medullaris, the spinal cord becomes a mass of spinal nerve roots called the cauda equina (consists of the nerve roots for spinal nerves L2 through S5).
• Filum terminale: extends from the caudal end of the spinal cord and attaches to the coccyx.

Spinal Cord Enlargements

• Two enlargements:
  • Third cervical segment to the second thoracic segment.
  • Extends from the first lumbar to the third sacral segment.
  • Enlargements accommodate the great number of neurons needed to innervate the upper and lower extremities located in these regions, give rise to the cervical, brachial, and lumbosacral nerve plexuses.

Gray Matter and White Matter

• Gray matter:
  • Areas that contain large numbers of nerve cell bodies and dendrites.
  • Cell bodies give the region its grayish coloration.
  • Gray matter covers the entire surface of the cerebrum and is called the cerebral cortex.
• White matter:
  • Composed of axons and associated glia.
  • High concentration of myelin appear white because of the fat content within the myelin.

Internal Anatomy of the Spinal Cord

• Gray matter:
  • Center of the spinal cord; distinguished by its H-shaped or butterfly-shaped pattern, contains cell bodies of motor and sensory neurons, as well as interneurons that link motor and sensory neurons to create spinal circuits.
  • Ventral or anterior horns: house the lower motor neurons.
  • Dorsal or posterior horns: house sensory neurons
• White matter:
  • Composed of sensory (ascending) and motor (descending) fiber tracts.
  • Carry impulses between the spinal cord and brain.
  • Fiber tracts cross over from one side of the body to the other at various points within the spinal cord and brain.
  • Point at which fiber tracts cross the midline, or decussate, varies between tracts, but this crossing occurs once for all of the major tracts involving the cerebral cortex.
  • Left side of our brain controls the right side of our bodies (this is called contralateral innervation).

Major Afferent (Sensory)/Ascending Tracts

• Two primary ascending sensory tracts: Posterior Column-Medial Lemniscus, Spinothalamic Tract.

Dorsal or Posterior Columns

• Carry information about position sense (proprioception), vibration, two-point discrimination, and deep touch.
• Medullary neurons form the medial lemniscus, which crosses the midline and projects to the thalamus.
• Thalamic neurons project to the cortex to create conscious perceptions of somatosensory information.

Spinothalamic Tract

• Transmits pain and temperature sensations.
• Part of a larger collection of fibers called the anterolateral tract; conveys crude touch and nociceptive input to brainstem nuclei to carry out reflexive responses to painful stimuli.
• Lateral spinothalamic tracts ascend two to four segments on the same side before crossing.

Location of Neurons in the Major Sensory Tracts of the Spinal Cord

• Posterior Column-Medial Lemniscus
  • Primary neuron: Ipsilateral dorsal root ganglion
  • Secondary neuron: Ipsilateral medulla, crosses midline within medulla
  • Tertiary neuron: Contralateral thalamus: ventral posterior lateral nucleus
• Spinothalamic Tract
  • Primary neuron: Ipsilateral dorsal root ganglion
  • Secondary neuron: Ipsilateral dorsal horn of spinal cord, crosses midline within three spinal segments
  • Tertiary neuron: Contralateral thalamus: ventral posterior lateral nucleus, dorsomedial nucleus, and interstitial nuclei

Major Efferent (Motor)/Descending Tract

• Corticospinal tract: the primary motor pathway; controls skilled movements of the extremities; originates from upper motor neurons in the frontal lobes.
• Fairly straightforward two neuron circuit in which upper motor neurons stimulate lower motor neurons (stimulate individual muscles to cause contraction and carry out movements).
• To carry out movements, the corticospinal tract has to stimulate lower motor neurons as well as inhibit spinal reflexes.
• Common indicators of corticospinal tract damage: muscle weakness and/or paralysis, spasticity, clonus, and the reemergence of primitive reflexes such as the Babinski sign.

Corticospinal Tract Damage

• Spasticity: caused by the loss of inhibition from the corticospinal tract and leads to hyperreflexia of tendon reflexes.
• Babinski reflex: completely normal in infants up to 2 years old. A positive Babinski sign in adults suggests damage to the corticospinal tract.
• Damage to both upper and lower motor neurons leads to muscle weakness.
• Lower motor damage leads to severe muscle atrophy, muscle fasciculations, and hyporeflexia; only lower motor neuron damage leads to hyporeflexia.

Other Descending Tracts

• Rubrospinal tract:
  • Originates in the red nucleus of the midbrain and terminates in the anterior horn.
  • Synapses with lower motor neurons that primarily innervate the upper extremities.
• Lateral vestibulospinal tract:
  • Assists in postural adjustments through facilitation of proximal extensor muscles.
• Medial vestibulospinal tract: Regulation of muscle tone in the neck and upper back is a function of the.
• Medial reticulospinal tract: facilitates limb extensors,.
• Lateral reticulospinal tract: facilitates flexors and inhibits extensor muscle activity.
• Tectospinal tract.