Biol 2401 A & P

Biol 2401 A & P Lecture Notes Nervous System Pathways Dr. Weis

NERVOUS SYSTEM PATHWAYS

The white matter of the spinal cord is divided into three white columns (funiculi) and named according to position :

1. Posterior (dorsal)

2. Lateral

3. Anterior (ventral)

Each column contains several fiber tracks with similar destinations

a. Ascending : up to higher centers for sensory inputs

b. Descending : down to cord from brain

c. Commissural : Crosses from one side of the cord to the other

All major spinal cord tracks are part of a multi neuron pathway.

1. Most cross (decussate)

2. Most consist of a chain of 2-3 neurons named first order, second order, third order, etc)

3. All are paired (right/left)

4. Reflect an orderly mapping of the body

Sensory and Motor Pathways

Sensory Pathways

found dorsally and laterally in the spinal cord white matter

ascending tracks

named for origin and destination

begins with spino- and ends with a structure in the brain region

conduct sensory impulses

Three major pathway locations :

1. Dorsal funiculus

fasciculus gracilis

fasciculus cuneatus

2. Lateral funiculus

dorsal spinocerebellar tract

ventral spinocerebellar tract

3. Lateroventral (anterior)

lateral spinothalamic track

ventral spinothalamic track

In the dorsal funiculus (column) -->

fasiculus gracilis

from the lower body receptors to medulla

ascends on the same side

for joint position, pressure, & fine (discriminative) touch

fasciculus cuneatus

from upper body to receptors (nuclei) to medulla

ascends on the same side

for joint position, pressure, & fine (discriminative) touch

In the lateral funiculus (column) --->

dorsal & ventral spinocerebellar track

proprioception from trunk and lower limbs to cerebellum

Stays on the same or can go across to opposite side

In the Lateroventral region —>

lateral spinothalamic

pain & temperature to thalamus

crosses in the spinal cord

ventral spinothalamic

crude touch, deep pressure to thalamus

crosses in the spinal cord

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Motor Pathways

medial and ventral portions of the spinal cord white matter

descending tracks

upper motor neurons.....control over Lower motor neurons (LMN or GSE)

named for origin in CNS and destination to spinal

Two major groups

Direct :: Pyramidal -----> Voluntary motor

Indirect :: Extra pyramidal -----> Involuntary motor

Pyramidal......(Direct Pathway)

motor neurons in brain stem and spinal cord called the UPPER MOTOR NEURONS

Clinically abbreviated : UMN.

controls skeletal muscle, direct pathway for axons

The lateral group will cross @ medulla & is called the "Decussation of the Pyramids"

Extra pyramidal (Indirect Pathway)

tracks include :

Rubrospinal

Reticulospinal

Vestibulospinal

Tectospinal

Rubrospinal....lateral, near mid gray region

red nucleus in Mesencephalon (midbrain)

muscle tone of flexors

crosses

Reticulospinal... anterior, medial, lateral

interconnects through a net work of nuclei in the brainstem (RAS)

autonomic function for visceral motor reflex

Vestibulospinal...anterior funiculus

vestibular nuclei are in medulla

associated with CN VIII and the cerebellum

position & movement of head for posture & balance

muscle tone for ipsilateral extensors

does NOT cross

Tectospinal....anterior funiculus

reflect visual and auditory stimulus to head and upper body reflexes

crosses

(superior colliculus nuclei in midbrain at the Tectum [roof])

Cerebellum ----->

integrates, regulates, modulates, balance

receives information from both direct and indirect motor pathways

Fxn = ~ rate, range, force

constant feed back between cerebrum and spinal tracks

With injury to the spinal cord, clinical signs are valuable clues to prognosis.

Spinal cord compression produces signs that vary with increasing compression.
Larger fibers will stop functioning earlier than smaller fibers.
Functional recovery is possible until pain sensation is lost.

Duration of the lesion also affects prognosis as nervous tissue tolerates injury for only a short time.

CEREBRAL CORTEX

can be divided into almost 100 different areas that have slightly different architectural characteristics. Neurohistologists have identified 6 major layers of the cortex.

All areas have different afferent and efferent connections with the thalamus, therefore the cortex operates in close association with the thalamus and can almost be considered both anatomically and functionally to be a large outgrowth of the thalamus and this is sometimes is referred to as the THALAMOCORTICAL system.

From electrical stimulation or post surgical patients, specific functions can be localized to certain areas of the cerebral cortex.

These regions of the cortex are not isolated, but work with associated areas. For instance, electrical stimulation of the visual cortex in the occipital lobe will causes the person to see flashes of light, lines, colors, but the visual cortex alone is not capable of complete analysis of complicated visual patterns and must operate in association with the adjacent regions of the occipital cortex, the VISUAL ASSOCIATION AREA.

Likewise, in the borders around the primary sensory areas (parietal lobe) are region called sensory association areas. The general function of this area is to provide a higher level of interpretation of sensory experience. Destruction greatly reduces the capability of the brain to analyze different characteristics of sensory experience.

The somatic, visual, and auditory association areas can be collectively called INTERPRETIVE AREAS and will meet each other where their corresponding lobes come together. This region has been called the GENERAL INTERPRETIVE AREA. It is important for most intellectual functions of the brain and a loss of this area usually leads to a demented existence. The General Interpretive area is also known as Wernicke’s area or the Intelligence area.

The general interpretive area, as well as the areas for speech and motor control are usually more highly developed in one cerebral hemisphere than the other, and is usually the left hemisphere (9 out of 10 people).

Utilizing the fiber pathways in the corpus callosum, both hemispheres can receive sensory information and control motor activity to keep "in communication". The left hemisphere is primarily for analytical, logic, math, speech and language and reading, whereas the right hemisphere is primarily for sensory.

The speech center is primarily found in the left frontal lobe and lesions in this region (left frontal, left parietal) can affect speech and if extensive, the individual will be unable to speak, read, or understand. Examples of lesions can be stroke, edema, hemorrhage, or a tumor. In prosophenosia, the damage is to the medial undersides of both occipital lobes, and causes the inability to recognize faces.

Prefrontal Areas :

portions of the frontal lobes that lie anterior to the motor regions and functions to ::

MEMORY

is the storage and retrieval of previous experience for facts or skills

* occurs in stages

* involves the hippocampus and surrounding structures

* chemical or structural changes for encoding memory traces

mechanism is complex

recreate same pattern of stimulation in CNS @ some future date.

Types of information stored ::

degrees of classification of memory storage ::

Short term Memory (STM)

i.e. telephone #

Long Term Memory (LTM)

The transfer of information from STM can be affected by :

Mechanisms of memory

Long term memory will result in alterations of the synapses

Consolidation or permanently facilitated synapses must occur if memory is to last and be recalled.
Rehearsal of the same information again and again accelerates and potentiates the process of consolidation and
the transfer of short term memory into long term memory. One of the processes involves a process in which
| new memories are stored in direct association with other memories of the same type.
Rehearsal or repetition will develop a permanent memory trace and the memory will last a lifetime.

Brain structures involved with memory ::

hippocampus –> learning and consolidation of STM –> LTM

amygdala –> “gatekeeper” between all areas, ties sensory information together

diencephalon

Thalamus –> coding and storing

Hypothalamus –> ties to Limbic system

prefrontal cortex –> retrieving LTM to present tasks

basal nuclei –> retrieving LTM for skilled/learned tasks

Loss of the hippocampus and related limbic areas will prevent the change of short term memory to long term memory, and are unable to establish new memories.
The importance of the thalamus in coding, storing, and recalling memories can be seen with lesions causing retrograde amnesia.

RAS

In the brain stem, there is a region that produces the conscious alert state that makes perception possible. Known as the RETICULAR ACTIVATING SYSTEM (RAS), it extends from the medulla to the hypothalamus and is a complex polysynaptic pathway and can be activated by various different sensory stimuli, thereby making it nonspecific for particular sensory information.

Neurotransmitters such as norepinephrine or epinephrine can lower the reticular neuronal threshold thereby producing arousal and alertness. Drugs used in general anesthesia produce unconsciousness by depressing the conduction in the RAS and their effectiveness depends a great deal on their lipid solubility.

Patients with tumors or other lesions that interrupt the RAS are generally comatose.

EEG

Electroencephalography

records electrical activities in a particular part of the brain using needles or surface electrodes and records on paper or computer.

primarily measures cortical activity in the spontaneous waves from a large number of neurons.
There is a unique brain wave pattern that can change with age or physiochemistry.

One of four types of waves can be recorded :

1. Alpha waves ----> awake, low amplitude, slow

2. Beta waves ----> activity, frontal/parietal lobes

awake or mentally alert, higher frequency waves

3. Delta waves ---> Sleep, low frequency

4. Theta waves ---> stress or emotional disorders

normal in children and early stages of sleep (all ages)

EEG can be used in diagnosis of tumors, infections, lesions, infarcts, abscesses, epilepsy

Brain waves change with age, sensory stimuli, brain disease and chemical state.

Flat EEG --> clinical evidence of brain death

SLEEP

is the state of changed consciousness or partial unconsciousness from which one can be aroused by stimulation.

The awake state (alert cerebral cortex) is mediated by the RAS.

The Hypothalamus is responsible for the timing of the sleep cycle.

Two different kinds :

1. REM rapid eye movements

2. nonREM

NonREM is divided into 4 stages (1-4) and can be categorized based on EEG patterns that range from low amplitude fast frequency to slow large waves in stage 4.

REM, also called paradoxical sleep due to eye movement and the change in EEG activity from slow large waves to rapid low voltage, and large phasic potentials (spikes) that occur in groups. Humans awakened from REM sleep report that they were dreaming.

Most nightmares occur in stages 3 & 4 of NREM sleep.

Sleep cycles occur ---> 90 minutes cycles

with 4-6 REM periods per night

The cycle starts with stages 1 & 2 and progresses to

3 & 4 before going to REM. The time for stages

3 & 4 and REM will change, with more time spent in REM sleep toward the morning.

Levels of neurotransmitters change in certain brain regions

Decreased NE, Increased serotonin ==> sleep

In REM, increased NE and ACH occur

Sleep disorders .......

not usually associated with REM sleep

but with the slow wave sleep

examples :

sleepwalking

bed wetting

narcolepsy (sleep during the day)

AGING

anatomical changes can alter the physiological processes