Somatic Maps, Sensory Pathways, and Motor Pathways

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144 terms

Cerebral cortex functional areas

- Motor areas, that control voluntary motor
functions
- Sensory areas that provide for conscious
awareness of sensation
- Association areas that act mainly to integrate
diverse information for purposeful action

Primary somatosensory areas

The primary somatosensory cortex is in the
postcentral gyrus of the parietal lobe of the
cerebral cortex.

Somatotopy

Somatotopy is the orderly mapping of the
body such that the body is represented
spatially in proportion to the contribution of
particular body structures with regard to the
parameter being illustrated

Somatic sensory pathways relay info from somatic sensory receptors to:

- Primary somatosensory area in the cerebral
cortex
- Cerebellum

Pathways to the cerebral cortex comprise three sets of neurons:

- First-order neurons
- Second-order neurons
- Third-order neurons

What are first order neurons?

First-order neurons conduct impulses from
somatic receptors into the brain stem or spinal
cord

- Stimuli arising from the neck, body, and posterior aspect of the head propagate along spinal nerves
into the spinal cord

What stimuli propagates along cranial nerves into the brain stem?

- Stimuli arising from the face, mouth, teeth, and
eyes propagate along cranial nerves into the
brain stem

What stimuli propagates along spinal nerves into the spinal cord?

- Stimuli arising from the neck, body, and posterior aspect of the head propagate along spinal nerves into the spinal cord

What are second order neurons?

 Second-order neurons conduct impulses
from the brain stem and spinal cord to the
thalamus

Axons of second-order neurons decussate where?

 Axons of second-order neurons decussate
(cross over) in either the brain stem or the
spinal cord

Following decussation, the axons extend where?

 Following decussation, the axons ascend
to the ventral posterior nucleus of the
thalamus

What are third order neurons?

 Third-order neurons conduct impulses from
the thalamus to the primary somatosensory
area of the cortex
 Decussation occurred prior to the thalamus,
so both the thalamus and the primary
somatosensory area of the cortex receive
information from the opposite side of the
body

Somatic sensory impulses are either ...

- Consciously perceived (at least for the most part)
- Proprioceptive (generally not consciously perceived)

Those same somatic sensory impulses have one of two destinations associated with perception status ...

- Cerebral cortex (conscious perception)
- Cerebellum (non-conscious proprioception)

Somatic sensory impulses entering the spinal cord ascend to the cerebral cortex by two pathways ...

- The posterior column-medial lemniscus pathway
- The anterolateral (spinothalamic) pathways

Somatic sensory impulses entering the cerebellum do so by two tracts ...

- Posterior spinocerebellar tract
- Anterior spinocerebellar tract

Number of neurons in a pathway ...

- The pathways that give rise to conscious
perceptions comprise three neurons (first, second, and third order)
- Pathways that are not consciously perceived
comprise two neurons

Decussation of pathways ...

- Pathways that are consciously perceived
decussate
- Pathways that are not consciously perceived do
not decussate

Axons of first-order neurons form ascending tracts where?

Axons of first-order neurons form ascending tracts only in the fasciculi cuneatus and fasciculi
gracilis

Axons of second-order neurons form
ascending tracts in?

In all other sensory pathways with regard to the
spinothalamic pathway and trigeminothalamic pathway.

Example of posterior column-mediated lemniscus pathway

Example of Spinothalamic Pathway

Also called anterolateral pathway.

Example of Trigeminothalamic Pathway

What is the posterior-column medial lemnisucus pathway?

Two white matter tracts that convey the impulses:
- Posterior column of the spinal cord
- Medial lemniscus of the brain stem

 Carries nerve impulses for conscious
proprioception and most tactile sensations
 Destination is the cerebral cortex
 Composed of three-neuron sets

What is the medial lemnisucs?

A projection tract that extends from the medulla to the ventral posterior nucleus of the thalamus.

First-order neurons extend from the trunk and limbs and ascend to where?

Ascend to the medulla oblongata on the same side of the body.

Where are the cell bodies of first-order neurons?

In the posterior (dorsal) root ganglia of spinal nerves.

Axons of first-order neurons from the posterior (dorsal) columns comprise what?

- Fasciculus gracilis, which carries impulses from the trunk and lower limbs

- Fasciculus cuneatus, which carries impulses
from the neck, upper limbs, and upper chest

First-order neurons terminate in a
correspondingly-named nucleus in the medulla:

- Gracile nucleus
- Cuneate nucleus

Where do first-order neurons synapse with second-order neurons?

 First-order neuron axon terminals synapse with
second-order neurons whose cell bodies are in that same (i.e. the correspondingly-named)
nucleus of the medulla

What happens to the axons of second-order neurons and wheer do they go?

 The axon of the second-order neuron decussates, and enters the medial
lemniscus

What happens in the thalamus with regards to second and third-order neurons?

 In the thalamus the axon terminals of the second-order neurons synapse with third-order neurons having their cell bodies in the
thalamus.

Third-order neurons project their axons where?

 The third-order neurons project their axons to the primary somatosensory area of the cerebral cortex.

What is fine touch?

the ability to recognize specific information about a touch sensation: the shape, size, and texture of the source, and what point on the body is touched

What is sterognosis?

the ability to recognize the size, shape, and texture of an object by feeling it

What is graphesthesia?

the ability to feel and identify a symbol drawn on the body

What is proprioception?

awareness of the precise position of body parts

What is kinesthesia?

the awareness of direction of movement of body parts

What is weight discrimination?

the ability to assess the weight of an object

What are vibratory sensations?

ability to perceive rapidly fluctuating touch stimuli

What is the spinothalmic pathway?

originates in the spinal cord and projects to the thalamus
 Also called the anterolateral pathway
 Carries nerve impulses for . . .
- Pain, thermal, tickle, and itch sensations
- Some impulses for crude, poorly localized touch
and pressure
- Vibration ( which is also carried by the posterior
column-medial lemniscus pathway)
 Destination is the cerebral cortex
 Composed of three-neuron sets

What do first-order neurons connect in the spinothalamic tract?

First-order neurons connect a receptor of the neck, trunk, or limbs with the spinal cord

Where are the cell bodies of first-order neurons in the spinothalamic pathway?

First-order neurons connect a receptor of the neck, trunk, or limbs with the spinal cord

Where are the cell bodies of second-order neurons in the spinothalamic pathway?

Cell bodies of second order neurons are in the posterior gray horn of the spinal cord

What do second-order neurons do before entering spinothalamic tract?

Axons of second-order neurons decussate,
then enter a spinothalamic tract.

What makes up the spinotahlamic tract?

- The lateral spinothalamic

- The anterior spinothalamic tract

What type of sensory impulses does the lateral spinothalamic tract carry?

- The lateral spinothalamic tract carries sensory
impulses for pain and temperature

What type of sensory impulses does the anterior spinothalamic tract carry?

- The anterior spinothalamic tract carries impulses for tickle, itch, crude touch, pressure, and vibration

Where do the axons of second-order neurons of the spinothalamic tract end?

Axons of second-order neurons end in the ventral posterior nucleus of the thalamus.

Where do axons of third-order neurons in the spinothalamic tract project to?

Axons of third-order neurons project to the primary somatosensory area of the cerebral cortex.

What is the trigeminothalamic pathway?

The trigeminal nerve (CN V) is the dermatome
for the face, and most somatic sensations
(tactile, thermal, pain) from the face, nasal
cavity, oral cavity, and teeth ascend to the
cerebral cortex along the trigeminothalamic
pathway.

Where do first-order neurons extend to in the trigemin pathway?

First order neurons extend from somatic
sensory receptors into the pons through the
trigeminal (V) nerves.

Where are the cell bodies of first-order neurons of the trigemin pathway?

Cell bodies for the neurons are in the trigeminal ganglion.

Where do first-order neurons synapse in the trigemin pathway?

First order neurons synapse with second-order neurons varies:

- Some first order neurons synapse with second
order neurons in the pons
- Other first order neurons descend to the medulla where they synapse with second order neurons

Do second-order neurons decussate in the trigemin pathway?

Regardless of where synapsis occurs, at the
level of synapsis second order neurons
decussate.

Where do second-order neurons of the trigemin pathway ascend to?

They ascend as the trigeminothalamic
tract to the ventral posterior nucleus of the
thalamus.

Where do third-order neurons of the trigemin pathway project to?

Third-order neurons which project to the
primary somatosensory are of the cerebral cortex

Summarize posterior (dorsal) column-medial lemniscus pathways.

 Dorsal column-medial lemniscus
- Encapsulated receptors
• Pacinian corpuscles
• Meissner corpuscles
- Touch and position (proprioception)
- All axons myelinated

Summarize spinothalamic (anterolateral) pathways.

 Anterolateral = Spinothalamic
- Free nerve endings
- Temperature and pain
- Many axons unmyelinated

Summarize the trigeminothalamic pathway.

 Trigeminothalamic
- Encapsulated receptors and free nerve endings
- Virtually all sensations for face

Explain somatic sensory pathways to the cerebellum.

 Sensory impulses conveyed to the
cerebellum are not consciously perceived

 The impulses are important for posture,
balance, and coordination of skilled
movements

What two major spinal cord tracts are the major routes that convey proprioceptive impulses to the cerebellum?

Two spinal cord tracts are the major routes
that convey proprioceptive impulses to the
cerebellum:
- Anterior spinocerebellar tract
- Posterior spinocerebellar tract

What is the anterior spinocerebellar tracts mostly concerned with?

The anterior spinocerebellar tract is mostly
concerned with proprioceptive information
from the lower limbs and with the control of
posture.

Impulses from the upper part of the body are mostly transmitted through what?

Impulses from the upper part of the body are
mostly transmitted through the posterior
spinocerebellar tract

First order (sensory neurons) of the cerebellum are involved in what?

- First order (sensory neurons)
• Have their cell bodies in the posterior root ganglia
• Synapse with second-order neurons in the posterior gray horn

Second order (sensory neurons) of the cerebellum are involved in what?

- Second order (association neurons)
• Have their cell bodies in the posterior gray horns of the spinal cord
• Terminate in the cerebellum

What is syphillis?

 Syphilis is a sexually transmitted disease caused by the bacterium Treponema pallidum
 Untreated syphilis infection leads to tabes dorsalis wherein somatic sensations are lost, and the gait becomes uncoordinated and jerky
because proprioceptive impulses fail to reach the cerebellum

What is tabes dorsalis?

Tabes dorsalis is a slowly progressive condition
caused by demyelination and deterioration of the
posterior portions of the spinal cord including
posterior white matter tracts (gracilis and
cuneatus), posterior spinocerebellar tracts, and
posterior roots

What are somatic motor pathways?

 Somatic motor pathways are descending
tracts that deliver efferent impulses from the
brain to the spinal cord

Somatic motor pathways are divided into what two groups?

Pyramidal tracts

Everything else

Where are the pyramids?

Axons of these neurons also form the pyramids of the medulla.

What are upper motor neurons?

Cells of the motor cortex of the brain, as well as
neurons other places in the brain that originate in
motor centers of the brain are called upper motor
neurons .

What are lower motor neurons?

 Neurons of the anterior horn of the spinal cord are truly motor neurons.
- They are called lower motor neurons
- Only lower motor neurons provide output from the CNS to skeletal muscle fibers
- They are also called the final common pathway

What are the pyramidal tracts?

 The pyramidal tracts are the major motor
pathways concerned with voluntary movement, especially precise or skilled movements.

What do the pyramidal tracts comprises?

 The pyramidal tracts comprise . . .
- The lateral corticospinal tract (left & right)
- The anterior corticospinal tract (left & right)

Also called direct pathways.

What are the 4 neural circuits?

Local circuit neurons

Upper motor neurons

Basal ganglia neurons

Cerebellar neurons

What are local circuit neurons?

- These are interneurons that provide both
excitatory and inhibitory inputs to lower motor
neurons
- They receive input from somatic sensory
receptors (nociceptors, muscle spindles) and
higher brain centers
- They help coordinate rhythmic activity in
specific muscle groups

Reiterate what upper motor neurons (UMN's).

- Most upper motor neurons synapse with
interneurons, which then synapse with lower
motor neurons
- UMNs from the cerebral cortex direct voluntary
movements
- UMNs of the brain stem regulate muscle tone,
postural muscles, balance

What are basal ganglia neurons?

- Neurons of the basal ganglia assist movement
by providing input to upper motor neurons
- Neural circuits interconnect the basal
ganglia with many other parts of the brain
- These circuits help initiate and terminate
movements, suppress unwanted movements,
and establish normal muscle tone

What are the cerebellar neurons?

- Neurons of the cerebellum aid movement
by controlling activities of upper motor neurons
- The thalamus interconnects the cerebellum, motor areas of the cerebral cortex, and the brain stem
- The cerebellum monitors Δ between intended and actual movements, then issues corrective
instructions to UMNs

What is flaccid paralysis?

 Damage or disease of lower motor neurons
produces flaccid paralysis of muscles on the
same side of the body
 Both voluntary movement and reflex actions
are absent
 Muscle tone is decreased or lost
 The muscle remains limp, or flaccid

What is spastic paralysis?

 Damage or disease of upper motor neurons
in the cerebral cortex causes spastic
paralysis of muscles on the opposite side of
the body since inhibitory influences of some
of these neurons on LMSs are lost
 Muscle tone is increased
 Reflexes are exaggerated
 Pathological reflexes such as the Babinski
sign appear

What are direct pathways?

Direct motor pathways, AKA pyramidal pathways, provide input to lower motor neurons via axons that extend directly from the cerebral cortex.

What are indirect pathways?

Indirect motor pathways, AKA multineuronal
pathways, provide input to lower motor
neurons from motor centers in the brain stem.
These brain stem centers receive signals from neurons in the basal ganglia, cerebellum, and cerebral cortex, hence the alternative term multineuronal pathways.

What are the 3 tracts of upper motor neurons belonging to the direct motor pathway?

- Lateral corticospinal tracts
- Anterior corticospinal tracts
- Corticobulbar tracts

What is the lateral corticospinal tract?

 Neurons originate in the primary motor area of the cerebral cortex
 Decussation occurs in the medulla
 Motor neurons control muscles located in distal
parts of the limbs responsible for precise,
skilled movements

What is the anterior corticospinal tract?

 Neurons originate in the primary motor area of the cerebral cortex
 Decussation occurs in the spinal cord
 Motor neurons innervate skeletal muscles of neck trunk and proximal parts of the limbs

What is the corticobulbar tract?

 Neurons originate in the primary motor
area of the cerebral cortex
 May or may not decussate
 Axons terminate in motor nuclei of nine
pairs of cranial nerves (all except I, II, and VIII)

What type of impulse do the lower motor neurons of the cranial nerves of the corticobulbar tract convey?

convey impulses that control precise,
voluntary movements of the eyes, tongue, neck, plus chewing, facial expression, and speech

What are the cranial nerves not involved in the corticobulbar tract? Why?

All except I, II, and VIII

What are indirect pathways?

 Also termed extrapyramidal pathways in
addition to the previously-mentioned term
multineuronal pathways
 Includes all somatic motor tracts other than
the corticospinal and corticobulbar tracts
 Pathways are very complex since they
descend from motor centers in the brain
stem, which in turn receive input from other
parts of the brain such as the cerebral cortex,
cerebellum, and various brain nuclei

What are the 5 indirect motor pathways?

- Rubrospinal
- Tectospinal
- Vestibulospinal
- Lateral reticulospinal
- Medial reticulospinal

What is the rubrospinal tract?

Conveys nerve impulses from the red nucleus (which receives input from the cerebral
cortex and cerebellum) to contralateral skeletal muscles that govern precise, voluntary movements of the distal parts of the upper limbs.

What is the tectospinal tract?

Conveys nerve impulses from the superior colliculus to contralateral skeletal muscles
that reflexively move the head, eyes, and trunk in response to visual or auditory stimuli.

What is the vestibulospinal tract?

Conveys nerve impulses from the vestibular nucleus (which receives input about
head movements from the inner ear) to ipsilateral skeletal muscles of the trunk and proximal parts of the limbs for maintaining posture and balance in response to head movements.

What is the lateral reticulspinal tract?

Conveys nerve impulses from the reticular formation to ipsilateral skeletal muscles of the trunk and proximal parts of the limbs for maintaining posture and regulating muscle tone in response to ongoing body movements.

What is the medial reticulospinal tract?

Conveys nerve impulses from the reticular formation to ipsilateral
skeletal muscles of the trunk and proximal parts of the limbs for maintaining posture and
regulating muscle tone in response to ongoing body movements.

What is the amyotrophic lateral scelrosis?

a neuromuscular condition that involves
progressive destruction of anterior horn motor neurons and fibers of the pyramidal tract

What is the function of the basal ganglia?

 Input from sensory, association, and motor
areas of the cerebral cortex and substantia
nigra is received by the caudate nucleus
and putamen of the basal ganglia
 Output from the basal ganglia comes from
the globus pallidus and substantia nigra
(the latter being part of the midbrain, but
functionally linked with the basal ganglia) .
The output comprises feedback to upper
motor cortex neurons, and is routed to
them through the thalamus

What is the resulting cirucit in the initiation and termination of movements with regard to the basal ganglia? 1811

- From cerebral cortex to
- Basal ganglia (input received by caudate
nucleus and putamen of the basal ganglia) to
- Thalamus (output from the globus pallidus of the
basal ganglia and linked substantia nigra) to
- Cerebral cortex (from the thalamus)

The basal ganglion help initiate and terminate some cognitive proceeds such as:

Attention
Memory
Planning

What is Parkinson's disease?

 Dopamine-releasing neurons that extend
from the substantia nigra to the caudate
nucleus and putamen degenerate
 This apparently results in a situation in
which there is an imbalance of neurotransmitter activity: too little dopamine, too much acetylcholine

What is Huntington's disease?

Formerly called Huntington's chorea (KŌ-re-a) due to the rapid, jerky involuntary, nonpurposeful
movements produced
 Hereditary disorder in which the caudate nucleus and putamen degenerate, with loss of
neurons that normally release GABA or acetylcholine

What is Tourette syndrome?

 Tourette syndrome is a disorder characterized by involuntary body movements (motor tics) and use of inappropriate or unnecessary sounds or words (vocal tics)

 The cause is unknown, but research indicates it
may be due to a dysfunction of the cognitive
neural circuits between the basal ganglia and the
prefrontal cortex

What is PAP syndrome?

 AKA athymhormic syndrome, or psychic akinesia, PAP is the abbreviation for the French term meaning loss of psychic autoactivation
 PAP is characterized by extreme passivity, apathy, and a profound generalized loss of self-motivation and conscious thought. A patient might sustain severe burns on contact with a hot stove due to lacking the will to move away despite experiencing
severe pain. Drowning? Don't be concerned
 The syndrome is believed to be due to damage to areas of the basal ganglia or frontal cortex,
specifically the globus pallidus and striatum,
responsible for motivation and executive functions

What type of functions does the cerebellum participate in?

- Learning and performing rapid, coordinated, highly skilled movements
- Maintenance of proper posture and equilibrium

What type of activities does the cerebellar function in?

- Monitoring intentions for movement
- Monitoring actual movement
- Comparing intended movements with actual movements performed
- Sending out corrective feedback to upper motor neurons if there is a discrepancy

What are pontine nuclei and what do they do?

Nuclei in the pons and they provide information to the cerebellum concerning intended
movements.

What does the cerebellum have to do with proprioception? 1822

Proprioceptors in joints and muscles provide information to the cerebellum about actual movements via nerve impulses that
travel in the anterior and posterior
spinocerebellar tracts, as well as impulses
from vestibular apparatus and from the eyes

What does the cerebellum compare?1823

The cerebellum compares the command signals
(intended movements) with the sensory information (actual movement performed)

What does the cerebellum send out?

The cerebellum sends out corrective feedback
to upper motor neurons if there is a discrepancy
between intended and actual movements.
Corrective feedback to UMNs in the cerebral
cortex is via the thalamus, but corrective
feedback goes directly to UMNs in brain stem
motor centers

What is the consequence of cerebellar lesions?

 Disturbances due to cerebellar lesions lead to
errors in rate, range, or direction of voluntary
movements, but not to paralysis or paresis
(paresis = weakness, impaired movement)

What are some of the consequences of cerebellar lesions?

- Nystagmus (involuntary eye movement)
- Ataxia
- Hypotonia (abnormally low muscle tone)
- Intention tremor

What is ataxia?

 Ataxia is an abnormal condition in which
movements are jerky and uncoordinated
 It is the typical observation in persons who
have cerebellar trauma or disease

What is an intention tremor?

 Intention tremor is a shaking that occurs
during deliberate voluntary movement,
especially during visually guided movement
toward a target

What are the integrative functions of the cerebrum?

Sleep and wakefulness. Learning and memory. Emotional responses.

What is circadian rhythm?

Alternating cycles of sleep and wakefulness that involve the whole brain.

What structure establishes the circadian rhythm?

The suprachiasmatic nucleus of the hypothalamus.
Perhaps based on light/dark cycles perceived by the eyes.

What is sleep?

Sleep is a state of changed consciousness, or partial unconsciousness, from which a person can be aroused by stimulation.

What is the reticular activating system (RAS) involved in?

It is of the reticular formation of the brainstem and is involved in arousal from sleep and in maintaining consciousness.

What stimuli can activate the RAS?

Pain.
Touch and pressure.
Noise.
Light.
But not smell.

What is consciousness?

State of wakefulness.

Normal sleep consists of two components:

Non-rapid eye movement (NREM) sleep. Rapid eye movement (REM)

What is learning?

The ability to acquire new info or skills through instruction or experience.

What is memory?

The process by which info acquired thru learning is stored and retrieved.

What is immediate memory?

The ability to recall ongoing experiences for a few seconds, thereby providing perspective to the present time that allows us to know where we are and what we a doing.

What is short term memory?

The temporary ability to recall a few pieces of info for seconds to minutes.

What is long term memory?

Is a more permanent type of memory that for days or years.

What structure functions in memory processing?

The limbic system in general, and the hippocampus of the limbic system in particular, functions in
processing STM into LTM

What are some of the factors of the transfer of info from STM to LTM?

- Emotional state (witness to a shocking event;
association with food as an evolutionary
adaptation)
- Rehearsal (repetition, rehearsal)
- Association of "new" information with "old"
information already stored in LTM
- Automatic memory (non-conscious formation of impressions: tie or behavioral aspects of a
speaker may be committed to LTM in addition to
the content of the presentation)

What is fact memory? 1842

- Entails learning explicit information: names, faces, words, dates
- Often forgotten quickly
- In LTM they are filed with the context in which they were learned

What is skill memory?

- Usually involves motor skills
- Acquired only through practice
- Circumstances of learning are not usually preserved
- Once learned, skill memories are hard to unlearn

Schizophrenia

excess dopamine activity in the brain causes a person to experience delusions, distortions of reality, paranoia, and hallucinations.

OCD

experience repetitive thoughts (obsessions) that cause repetitive behaviors (compulsions) that they feel obligated to perform

Amnesia

Refers to the lack or loss of memory.
It is a total or partial inability to remember past experiences

Anterograde amnesia

there is memory loss for events that occur after the trauma or disease that caused the condition. In other words, it is an inability
to form new memories.

Retrograde amnesia

there is a memory loss for events that occurred before the trauma or disease that caused
the condition. In other words, it is an inability to recall past events.

Bradykinesia

slowness of movements

Hypokinesia

decreasing range of motion

Cerebral palsy (CP)

motor disorder that results in the loss of muscle
control and coordination; caused by damage of the motor areas of the brain during fetal life, birth, or infancy. Radiation during fetal life, temporary lack of oxygen during birth, and hydrocephalus during infancy may also cause cerebral palsy.

Coma

A state of unconsciousness in which a person's responses to stimuli are reduced or absent

Insomnia

Difficulty in falling asleep and staying asleep.

Narcolepsy

A condition in which REM sleep cannot be inhibited during waking periods. As a result, involuntary periods of sleep that last about 15
minutes occur throughout the day.

Sleep apnea

A disorder in which a person repeatedly stops breathing for 10 or more seconds while sleeping. Most often, it occurs because loss of muscle tone
in pharyngeal muscles allows the airway to collapse.

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