Neuroanatomy Exam 2

Neurology

depends on diagnostic neurology “diagnostic criteria” - physician refers to clinico-anatomic correlations to infer what and where the deficit (lesion) is from physical examination, history and knowledge of nervous system anatomy and function 

• When physician makes a correlation of what happens to you with previous info., knowledge and history

Loss of function

this is due to neuronal damage, not glial damage

2 main types of CNS lesions

anatomic and physiologic, these two lesion types can cause each other to occur lesion = damage

Anatomic lesion

dysfunction resulting from structural damage to CNS (stroke, trauma, tumors)

• Damage to anatomy → results from damage to cells + pathway

Physiologic

 dysfunction in absence of obvious anatomic abnormalities (transient ischemia) 

• Dysfunction of not so obvious things like functions, like when cell is alive but function is impaired

Symptoms

subjective experiences of inflicted resulting from a neurologic disorder, listen for symptoms (told by patient) look for signs

• Subjective experience of something that is occurring in system → something that is observed

Signs

objective abnormalities detected by healthcare professional upon neurologic exam 

• Objective manifestation of what is going on 

• Can be reported/measured 

• Ex. you can measure the extent of blurriness

symptoms and signs reflect focal pathology within the nervous system

based on localized function - different parts of the NS serve different functions so, damage to one brain region will reciprocally affect (and predict) a loss of function of that area 

• Ex. lesion affecting hand area of brain or its pathways results in dysfunction of hand 

• Ex. hand dysfunction can predict hand area lesion 

manifestations of neurologic disease represent altered nervous system activity

negative and positive manifestations 

Negative manifestations

result from loss of nervous system function, ex. Weakness, hemiparalysis, sensation loss, memory loss, weakness, loss of function, memory loss, paralysis

Positive manifestations

 result from inappropriate NS excitation, ex. Seizure, spasticity, opposite to negative, result of excessive NS activity (like a seizure)

Gray matter lesions (neuronal loss)

• interfere with function of collections of neurons and their synapses (and their functions)

• Positive and negative manifestations can exist 

• Ex. ALS - degenerative motor disease - lower motor neuron dysfunction and death lead to motor spasticity (+) and loss of motor function (-) (may begin w/ + signs and then present w -) 

• With loss of cells you can see + and - manifestations

White matter lesions (axonal loss)

• Interfere with signal transmission between neurons (disconnection syndromes) 

• Negative manifestations exist 

• Ex. MS - degenerative white matter disease - loss of signaling between neurons leads to loss of motor function (-)

• Axonal damage → loss of function (-) manifestation

Neurologic disease can result in syndromes

syndromes are groups of signs and symptoms that are related → suggest a common origin, ex. Chronic fatigue syndrome, central pain syndrome 

• Can present themselves as cluster disorders = syndromes, things that have multiple causes w/ common overlap

The destruction dysfunction relationship

several conditions can lead to nervous system dysfunction

Destruction of neurons or axons in the nervous system

• Neuronal destruction (injury or death), ex. Cell death can result from stroke, parkinson's disease 

• Axonal destruction, ex. Multiple sclerosis - demyelinating disease that leads to loss of axons in spinal cord 

• Destruction can cause dysfunction 

• You don’t need to completely destroy something to cause dysfunction 

• ex) compression cause paralysis but it eventually heals saxon isn’t fully severed

Compression of the brain or spinal cord

can be reversible or irreversible depending upon whether cell death occurs, ex. Subdural hematoma - bleeding into subdural space

• Some impacts might not be long-term if impacts don’t cause cell death

Ventricular or vascular compromise

 tumor originating in regions surrounding ventricles can obstruct CSF flow, ex. Obstructive hydrocephalus, occlusion of cerebral arteries that feed multiple brain sites can cause cell death 

Where is the lesion/dysfunctional brain process causing the neurologic disease?

organization of the CNS makes it straightforward to diagnose based upon the clinical history (symptoms) and neurological exam (signs)

Focal pathology

causes signs and symptoms based on a single geographically isolated lesion, ex stroke - ischemia of a select territory leads to focal brain deficits

Multifocal pathology

 damage to CNS at numerous sites, ex. Multiple sclerosis - demyelinating (white matter) lesions in white matter across many areas of CNS

Diffuse process pathology

diffuse CNS damage due to exposure to toxins or metabolic abnormalities

Rostrocaudal and transverse localization

 signs and symptoms often make it possible to localize the lesion to a specific area within the 3D axis of the brain

What is the lesion

must consider the subject age, sex, and general medical context (other confounding variables)

• time course (just started or for a long time?)  of illness is significant

• Is the illness…recent onset (stroke), slowly progressing (degenerative disorder) relapsing and remitting (inflammatory)?

Role of neuroimaging and lab investigations

synthesis of clinical data leads to a differential diagnosis (across medical fields) → list of possibilities based on what is presented to you

• List of diagnostic possibilities that fit the clinical picture leads the healthcare provider to differentiate between two or more clinical conditions 

• Can be further delineated by neuroimaging methods: 

  • XRays 

  • Angiography (to visualize blood vessels) 

  • Computerized Axial Tomography (CAT) scan 

  • Magnetic Resonance Imaging (MRI) Scan

Spinal column: (or vertebral column)

• 7 cervical vertebra (C1-C7)

• 12 thoracic spinal vertebra (T1-T12)

• 5 lumbar spinal vertebra (L1-L5) 

• 5 sacral spinal vertebrae (fused) (S1-S5) 

• Coccyx (tailbone) 

• Has 4 main regions → series of vertebrae stacked on eachother, # of vertebrae relate to amount of spinal nerves 

Spinal cord

• 8 cervical (neck)  spinal nerves (C1-C8) 

• 12 thoracic (thorax/body)  spinal nerves (T1-T12) 

• 5 lumbar (lower back)  spinal nerves (L1-L5) 

• 5 sacral spinal nerves (S1-S5) 

• 1 coccygeal spinal nerve

• * spinal cord ends at the lumbar region of the spinal column at lumbar vertebrae #1 region called the conus medullaris 

• 30 spinal cord segments give rise to 30 bilateral spine nerves projecting to/from the spinal cord 

• Segments of cord are similar to segments of column 

• Nerves come off bilaterally → 2 branches each (right and left nerve branches), we look at them in segments going down the cord 

• **nerve 1 is ABOVE C1

• ** nerve 8 goes UNDER C7 and then the rest start going under instead of over 

• *the nerve exits below the specific/corresponding vertebra, EXCEPT for in the cervical region as the nerves go above here and this is because there are more nerves in this region 

• **your brain cells don’t move muscles, your spinal neurons do

Conus medullaris

end/termination of spinal cord

Thecal sac (dura mater)

the best spot to enter is between L2 and L3, spinal taps are done here because if needle goes there, spinal cord won’t be damaged as these nerves live in a sac w/ CSF

Cauda equina

 beyond the spinal cords end (at L1) - spinal roots emanating form the spinal cord descend in the thecal sac (dura mater) and branch off to form the long nerves that travel to the buttocks and down the legs 

• Long spinal nerves emanating form the spinal cord that dangle down and branch off to the legs 

• Lowest spinal nerves of the cauda equina (S1-S5 and coccygeal) perforate the sacrum

Spinal cord enlargements

the spinal cord varies in width along its length, spinal cord is wider in cervical and lumbosacral regions 

• Cervical = largest, a lot of white matter 

• Lumbar and cervical are the 2 areas of which the spinal cord is larger → enlargements are because at these areas, you have a lot of larger nerves going to upper and lower extremities (arms and legs) 

• Lumbar = a lot of gray matter

Cervical and lumbosacral enlargements

• More large nerves to- and from- upper and lower extremities 

• More lower motor neurons in these areas of cord (feed the arms/legs)

Anterior (ventral) median fissure

shallow but wide fissure on the anterior surface of the spinal cord, shallow but very wide open but very separate groove/opening 

Posterior (dorsal) median fissure

deep but thin fissure on the posterior surface of the spinal cord, posterior = back, very thin and long but  tissue is right up against each other 

Anterolateral sulcus

shallow groove on the anterior-lateral surface of the spinal cord, anterior = front, where motor neurons/axons that feed muscles come out of spinal cord, going to muscles

Posterolateral sulcus

shallow groove on the posterior-lateral surface of the spinal cord, entrance of sensory nerves, sensory info always comes into the posterior aspect of the spinal cord 

Internal features of the spinal cord:

some CNS tissue is translucent and appears gray, other areas are opaque and appear white

White matter

bundles of myelinated axons running up and down the length of the spinal cord 

• Occupies the entire peripheral regions of the spinal cord 

• In the spinal cord, these bundles are called a tract, column, or pathway 

• In the spinal cord, they consist of the sensory (afferent) fibers and motor (efferent) fibers

Gray matter

unmyelinated areas in the spinal cord - collections of cell bodies (neuropil) 

• Occupies the central region of the spinal cord, so is called the central gray region 

• 4 lobed or “horned” structure to home spinal sensory and motor function 

• Dorsal horns of the central gray are home to spinal sensory function 

• Ventral horns of the central gray are home to spinal motor function - large, home of lower motor neurons, these are very large neurons so they need a lot of space

Peripheral white matter

outermost portion of spinal cord where there are bundles of axons/tracts/columns carrying info to and from brain

Spinal roots form spinal nerves

each segment of the spinal cord has 4 spinal roots (left and right ventral and dorsal roots) which join together as they exit the spinal cord to form a spinal nerve

• So, a spinal nerve is a mixed nerve, which carries both motor and sensory signals between the spinal cord and the body

Ventral spinal roots

• Motor output from the CNS to the body 

• Contains myelinated axons from motor neurons that lie in the ventral horn of the spinal cord 

• These axons innervate the body’s muscles, organs, and circulatory system 

• So, motor neurons send axons out of the CNS via the Ventral Spinal Root

Dorsal spinal roots

• Bring sensory input into the CNS from sensory receptors in the body 

• Contains myelinated axons from sensory neurons in the PNS 

• These axons carry information about pleasure, temperature, pain

• Cell bodies of these body’s sensory neurons lie outside the CNS in Dorsal Root Ganglia (DRG) 

• So, DRG neurons send axons into the CNS via the Dorsal Spinal Root

Somatic efferents (SE)

somatic motor fibers (axons) to skeletal muscle 

• Axons from lower motor neurons in ventral horn of the spinal cord to the muscles 

• Control voluntary motor movement 

• Output, voluntary (skeletal) motor movement 

• Efferent → can be involuntary 

• Afferents → cannot be involuntary (you can’t feel your stomach moving)

Visceral Efferents (VE)

 autonomic motor fibers to smooth muscle in viscera and glands 

• Control involuntary smooth muscle motor movement and secretion

• Autonomic NS, runs in your “background” such as digestion 

Somatic afferents (SA)

sensory fibers (axons) conveying info from skin, joints, muscles 

• Axons of unipolar neurons in the dorsal root ganglion (DRG) convey stretch and touch info

• Somatic = body, input 

• Specialized unipolar neurites form DRG that carry touch/pain signals

Visceral Afferents (VA)

sensory fibers conveying info from smooth muscle in viscera and glands 

Spinal Nerve distribution

• Each spinal nerve distributes to a defined body part

• Sp neurally, you are a segmented organism 

• Some spinal nerves overlap in their distribution pattern 

• Defined arrangement gives us dermatomes and myotomes 

• *** spinal nerves are formed by spinal roots which are either dorsal or ventral and posterior or anterior 

• Roots come together to form a nerve that goes out to the body, the spinal nerve wraps around a part of the body at ONE level 

• A spinal nerve is a MIXED nerve which carries sensory and motor info 

• ***some of these overlap but in a very limited way 

• Spinal nerves don't go to entire body, they go to ONE distinct point 

Cervical spinal nerves (C1-C8)

• Exit cervical spine 

• to/from neck, arm, shoulders, upper back

Thoracic Spinal nerves (T1-T12)

• Exit thoracic spine 

• to/from chest, abdomen, back

Lumbar Spinal Nerves (L1-L5)

• Exit lumbar spine 

• to/from hips, low back, legs, feet

Sacral Spinal Nerves (S1-S5)

• Exit sacrum 

• to/from buttock, crotch, back of leg, feet

Coccygeal Nerve Root (C0)

• Exit coccyx

• to/from tailbone area

Dermatomes and myotomes

sensory and motor component of each spinal nerve is distributed to a defined portion of the body termed a dermatome (sensory component) or myotome (motor component) 

• There is NO C1 dermatome, and some dermatomes overlap (ex. C5-8, T1 for arm) 

• Dermatome = sensory component of spinal nerve 

• *** No cervical nerve comes out of lumbar vertebrae, nerves that come out of lumbar vertebrae are lumbar nerves and cervical nerves come out of cervical vertebrae 

• * there is no C1 as these are sensory dermatomes and C1 is mostly MOTOR

Dermatome vs. Myotome

Dermatome: a dermatome refers to an area of skin innervated by the nerves from a single spinal root 

Myotome: a myotome refers to a group of refers to a group of muscles innervated by the nerves of a single spinal root 

Dermatome: a region of the skin innervated by a single spinal nerve 

Myotome: a group of muscles innervated by a single spinal nerve 

Dermatome: some consists of overlapping regions innervated by more than one spinal nerve 

Myotome: some are innervated by more than one spinal nerve 

Dermatome: responsible for the coordination of senses 

Myotome: responsible for the coordination of voluntary muscular movements

Central gray

• H-shaped columns of cell bodies in spinal cord 

• Bilaterally symmetric 

• Extend entire length of the cord 

• *shape of central gray changes at various levels - related to dermatomes and myotomes

Ventral (anterior) and lateral horns

where lower motor neurons exist - origin of ventral roots

Dorsal (posterior) horns

• Input zone of sensory pathways for nociception (pain) and mechanoreception (pressure)

• Where secondary sensory neurons involved in pain exist

white matter

Bundles of myelinated axons running up (ascending) and down (descending) the spinal cord 

• Occupy entire peripheral regions of spinal cord 

• Consist of sensory (ascending) fibers and motor (descending fibers) 

Ascending tracts

 UP spinal cord TO brain

Descending tracts

FROM brain DOWN to spinal cord

Columns (funiculi and fasciculi)

• Dorsal, lateral, ventral funiculi surround the central gray region 

• Columns of axons lading to and from brain 

• Acons go up and down like columns

Tracts

 fiber bundles (clusters of axons) with common functions, “columns” contain multiple “tracts”  (clusters of fiber bundles within a column that carry similar information)

Corticospinal tracts

cerebral cortex to spinal cord

• Axons from upper motor neurons (in brain) to lower motor neurons (in spinal cord) 

• Ex. lateral (decussates) and anterior (does not decussate) corticospinal tracts 

• Lateral corticospinal (pyramidal) tract 

  • Function: fine motor function (controls distal musculature) modulation of sensory functions 

  • Origin: motor and premotor cortex 

  • Ending: anterior horn cells (interneurons and lower motor neurons) 

  • Location in cord:lateral column (crosses in medulla at pyramidal decussation) 

  • Biggest descending tract laterally, cortex → neurons from cortical cortical neurons → spinal → neurons in spinal cord

  • This tract decussates from one side to the next 

• Anterior corticospinal tract

  • Function: gross and postural motor function (proximal and axial musculature) 

  • Origin: motor and premotor cortex 

  • Ending: anterior horn neurons (interneurons and lower motor neurons) 

  • Location in cord: anterior column (uncrossed until after descending)

Rubrospinal tract

red nucleus (in the midbrain) to spinal cord

• Regulates large muscle motor movement (particularly of the upper extremities) 

• Function: motor function 

• Origin: red nucleus 

• Ending: ventral horn interneurons 

• Location in cord: lateral column

Reticulospinal tracts

reticular formation (in brainstem) to spinal cord 

• Modify pain perception - important for pain control 

• Function:modulation of sensory transmission (especially pain) and modulation of spinal reflexes  

• Origin: brainstem reticular formation 

• Ending: dorsal and ventral horn 

• Location in cord:anterior column 

• Reticular formation → spinal tract, descending analgesia

Vestibulospinal tract

vestibular nuclei (in brainstem) to spinal cord 

• Activate quick movements to sudden changes in body position 

• Function: postural reflexes 

• Origin: lateral and medial vestibular nucleus 

• Ending: anterior horn interneurons and motor neurons (for extension) 

• Location in cord: ventral column

• Sensing and controlling position state

Tectospinal tract

superior colliculus (ex. In tectum in the midbrain) to spinal cord

• Coordinates head and eye movements 

• Function: reflex head turning 

• Origin: midbrain 

• Ending: ventral horn interneurons 

• Location in cord: ventral column 

• Tectum → superior (eye movements + responses)  + inferior colliculi 

Ascending Fiber Tracts

• Axons entering spinal cord via dorsal roots from cell bodies in Dorsal Root Ganglion (DRG) 

• Dorsal root → sensory info 

• Dorsal root ganglion = collection of cells (neurons), one group gathers pain info, the other group gathers pressure/touch info and stretch, pseudounipolar neurons  

• Ventral roots = myelinated axons that come from lower motor neurons that live in ventral horn of spinal cord, info goes out of CNS to move muscles 

• Dorsal + ventral roots = AXONS 

• Ventral = out of CNS, dorsal = in

Dorsal white columns tracts (called medial lemniscal system)

mechanoreception - carry info about touch, vibration, proprioception (position sense) 

• Function: fine touch, proprioception, two-point discrimination 

• Origin: skin, joints, tendons 

• Ending: dorsal column nuclei, second-order neurons project to contralateral thalamus (cross in medulla at lemniscal decussation) 

• Location in cord: dorsal column

 Spinothalamic tracts

spinal cord to thalamus (ventrolateral anterior system) 

• Nociception- carry info about sharp (noxious) pain, temperature

• Function: sharp pain, temperature, crude touch 

• Origin: skin 

• Ending: dorsal horn, second order neurons project to contralateral thalamus (cross in spinal cord close to level of entry)

• Location in cord: ventrolateral column, ventral column 

• Pain (sharp pain and temperature pain)

Spinocerebellar Tracts

spinal cord to cerebellum

• Dorsal (posterior) and ventral (anterior) tracts - motor control, state and position sense 

• Dorsal 

  • Function: movement and position mechanisms 

  • Origin: muscle spindles, golgi tendon organs, touch and pressure receptors (via nucleus dorsalis/clarke's column) 

  • Ending: cerebellar paleocortex (via ipsilateral inferior cerebellar peduncle)

  • Location in cord: lateral column

  • Fasciculus gracilis: will ultimately meet gracilis nucleus in brainstem 

  • Fasciculus cuneatus: will ultimately meet cuneatus nucleus in brainstem, on lateral side of dorsal columns

• Ventral 

  • Function: movement and position mechanisms 

  • Origin: muscle spindles golgi tendon organs, touch and pressure receptors

  • Ending: cerebellar paleocortex (via contralateral and ipsilateral superior cerebellar peduncle) 

  • Location in cord: lateral column

Spatial orientation within ascending and descending fiber systems

ascending and descending pathways have a specific medial to lateral orientation, particularly apparent in dorsal columns and the spinothalamic and corticospinal tracts

Spatial orientation in ascending and descending fiber systems changes with location

ascending and descending pathways change size and shape but not relative position in the spinal cord as pathways from different dermatomes and myotomes enter and exit

Functional segregation in the spinal cord

sensory and motor function are separated in the spinal cord

Ascending sensory pathways in the spinal cord

in the first stages of sensory processing, synaptic connections are systematic and hierarchical 

• The first cell in sensory system is the first-order neuron

  • The cell body of all first order neurons lie within the PNS in the DRG

• The first order neuron synapses onto a second-order neuron in the CNS 

• The second-order neuron synapses onto a third order neuron in the CNS 

Nerve fibers often cross the midline (decuss) and contact neurons on the other side of the CNS

• Decussation 

  • The left brain therefore controls the right side of the body and vice versa 

• A tract is contralateral when nerve fibers cross the midline 

• A tract is ipsilateral when nerve fibers stay on the same side

Somatic sensation - two principal organs of touch

Sensation from the body surface (Somatic Sensation) in the form of pressure from the body surface, vibration of the skin, and deflection of hair on the body surface contribute to bodily awareness in space

Somatic Sensation comes from different neural systems and information regarding somatic sensation is carried to the CNS by different afferent fiber systems

• 1. Large, quick fibers (specialized neurites of Mechanoreceptor neurons in the DRG) • activated by all somatic sensation (ie, pressure, vibration, tissue damage)

• 2. Small, slow fibers (specialized neurites from Nociceptor neurons in the DRG)• activated by noxious stimuli (ie, tissue damage)

Mechanoreceptors for Pressure, Stretch, Vibration

First order (1o) mechanoreceptor neurons in the DRG project specialized neurites to the body that produce and conduct electrical signals in response to pressure, vibration, and stretch

Nociceptors for Pain

 First order (1o) nociceptive cells in the DRG project specialized neurites to the body that produce and conduct electrical signals in response to noxious stimuli (tissue damage)

Mechanoreceptors for Pressure, Stretch, Vibration

•  First order (1o) mechanoreceptor neurons in the DRG project specialized neurites to the body

• The axons of 1o mechanoreceptor cells enter the Dorsal Column (white matter) of the spinal cord directly and ascend ipsilaterally (same side of spinal cord) without synapsing in the spinal cord.

• The axons of 1o mechanoreceptor cells synapse with neurons in the Medulla (in the brainstem).

• The 2o mechanoreceptor cells in the Medulla then project axons contralaterally (decussate) and ascend in the Medial lemniscus to the thalamus.

•  Thalamic neurons then project axons to the appropriate area of the Somatosensory cortex

• Note The Level Of DECUSSATION

Nociceptors for Pain

•  First order (1o) nociceptive neurons in the DRG project specialized neurites to the body.

•  The axons of 1o nociceptive neurons enter the Dorsal Horn (gray matter) of the spinal cord synapse with 2o nociceptive neurons in the dorsal horn.

• The 2o nociceptive cells in the spinal cord neurons then project axons contralaterally (decussate) and ascend in the Spinothalamic Tract(s) to the thalamus.

• Thalamic neurons then project axons to the appropriate

area of the Somatosensory cortex.

•  Note The Level Of DECUSSATION

Projections of the Nociceptor System

The Spinothalamic Tracts: The 2o nociceptive cells in the dorsal horn of spinal cord that receive pain signals are of two types: nociceptive specific cells and wide dynamic range cells 

• The 2o nociceptor cells in the DRG project axons that cross the midline (decussate) at the spinal cord level and then ascend to the brain in the Spinothalamic and Neospinothalamic tracts

• • Each tract carries different information about the painful stimulus

Nociceptive specific cells

2o cells located in superficial parts of the dorsal horn

•  receive synaptic contacts from 1o nociceptive cells in the DRG

•  only respond to noxious stimuli (tissue damage) so pain only

 Wide dynamic range cells

2o cells located in deep intermediate parts of the dorsal horn

•  receive contacts from 1o DRG nociceptive cells and DRG mechanoreceptor neurons

• respond to noxious stimuli AND pressure so pain and somatic sensation

Upper Motor Neurons Project Axons through the Spinal Cord

Upper Motor Neurons send two fiber tracts (axon bundles) as part of Corticospinal Tracts down either side of the cord (these also called the Pyramidal tracts) activate lower motor neurons

The Pyramidal System

 Upper motor neurons lie in the motor cortex of brain

Pyramidal Cells

 largest neurons in mammalian brain

•  Axons leave upper motor neurons and form the Corticospinal Tracts (Pyramidal Tracts)

Corticospinal Tracts

axons of upper motor neurons

• The majority of axons cross the midline of the brainstem at the Pyramidal Decussation (in the lower medulla),

and descend contralaterally in the spinal cord

• Corticospinal tracts then descend in the lateral white matter of the spinal cord and insert into the ventral horn of spinal cord and synapse with lower motor neurons

Descending Motor Pathways in the Spinal Cord

Motor pathways are divided into Upper- and Lower- Motor Neuron Regions

• The origin of motor commands is the Upper Motor Neurons in the cerebral cortex (motor cortex)

• The output for motor commands is the Lower Motor Neurons in the ventral horn of the spinal cord

Upper Motor Neuron

Lower Motor Neurons

 in the ventral horn of the spinal

cord send axons that exit the CNS via the ventral

roots and contact and activate skeletal muscles in

the body (in the drg)

The Monosynaptic Reflex

• Each muscle has: (1) sensory structures and is (2) innervated by lower motor neurons

• Eg, Muscle Spindle Organ

• Intrafusal muscle fibers – sensory component of muscle - contain a sensory stretch receptor

•  Extrafusal muscle fibers (bulk of muscle) – motor component of muscle – moves the muscle

• When intrafusal fibers of a muscle are stretched, its sensory afferents activate synapses on a lower motor neuron which activates extrafusal fibers of the same muscle, causing muscular contraction – so stretch of a muscle is thus countered with a contraction of that same muscle

Inhibition of Antagonist Muscles

• Every muscle in the body has a monosynaptic reflex. But what has to happen for a reflex to occur?

• Muscles exist in antagonistic pairs of flexors (muscle flexion) and extensors (muscle extension)

•  When intrafusal fibers of the agonist muscle are stretched, its sensory afferents activate synapses on a lower motor neuron which activates extrafusal fibers of the same muscle

• • At the same time, these sensory afferents activate inhibitory interneurons that interact with a lower motor neuron that contacts extrafusal fibers of the antagonist muscle - suppressing extrafusal fibers of the opposing muscle.

LESIONS IN THE MOTOR PATHWAYS

Two main types of motor neuron lesions

1. Lower motor neuron lesions 

2. Upper motor neuron lesions

 Lower Motor Neuron Lesions

Damage to cells of the ventral horn of the central gray (or brain stem) which constitute the ventral roots of spinal and cranial nerves

•  Result of toxins, trauma, infections

• Signs included flaccid paralysis

Upper Motor Neuron Lesions

Damage to cells of cerebral cortex or Lateral Corticospinal Tract in the spinal cord (axons from upper motor neurons in the motor cortex)

•  Results from strokes, infections, tumors

•  Signs include spastic paralysis, little/no muscle atrophy, hyperactive reflexes (hyper-reflexia)