Neuroscience Final

bilaterality

how is auditory inputted?

divergence

This is the spreading of activity within the CNS due to the extensive branching of nerve fibers. For example, sensory information carried by a single primary afferent fiber can diverge upon entering the cord or brainstem to influence many different reflex and ascending pathways. Divergence is important for increasing the "involvement" of different CNS regions, i.e., pain stimulus to activate reticular formation.

Rostral thalamic nucleus

This nucleus receives fibers from the mamillary body of hypothalamus via mammillothalamic tract and sends fibers to (and receives fibers from) the cingulate gyrus of the cerebrum via the internal capsule. Thus, this thalamic nucleus is part of the limbic system.

pyramidal and rubrospinal tracts

are generally considered to be the primary means of effecting voluntary fine muscle activity because: 1. These tracts are heavily influenced by the cerebral cortex. 2. Both of the tracts are somatotopically organized, permitting highly ordered interconnections between UMN's and LMN's. 3. Both of these tracts are facilitatory to flexor LMN's, meaning that the pyramidal and rubrospinal pathways are related to activity of non-antigravity muscles.

non-proprioceptive primary afferents

small thinly myelinated or unmyelinated fibers arising from mechanoreceptors, thermoreceptors and nociceptors in the skin. Strictly polysynaptic reflex connections with LMNs and ascending pathways are less specific and rapid due to high number of synaptic relays, slower neural responses

function of basal ganglia

modulate motor function

rostral peduncle middle peduncle caudal peduncle

cerebellar peduncles

plegia

complete paralysis

Hyperreflexia

enhanced reflex activity.

inhibit the LMN

Facilitation of an inhibitory interneuron will

function of ARAS

1. The afferent activity projected to the cerebral cortex is very nonspecific, so that instead of providing any specific afferent information the cortical projections of the ARAS function to bring about general activation of widespread areas of cortex. 2. The ARAS thus increases the "alertness" of the cerebral cortex; the ARAS is known to play a critical role in increasing and maintaining cortical arousal: Many CNS depressants and stimulants act on this system, and lesions of the ARAS can produce coma. Even though the ARAS is depressed during these states and sleep, the specific ascending systems are still active, i.e., experimental studies. 3. ARAS fibers synapsing in the hypothalamus may have the same effect on this diencephalic structure; this may also be a mechanism for inducing "physiological arousal" to accompany cortical arousal.

commissural fibers

Interconnect cortical areas across midline.

taste (gustation) area

A portion of the somatosensory area mentioned above functions as the perceptive area for taste, receiving afferents from the SVA nucleus of cranial nerves VII, IX, and X (solitary nucleus) via the VPM nucleus of the thalamus.

Ascending reticular activating system (ARAS)

A subdivision of the reticular formation, the "diffuse core" of gray matter found throughout the brainstem. The ARAS is an ascending multisynaptic pathway which extends from medulla to diencephalon and serves to enhance the state of arousal of the cerebral cortex. While in recent years specific pathways (i.e. chemically identified) have further eroded the "diffuseness" of this brainstem region, the ARAS still serves as an important concept in sleep-arousal states.

spinal portion of pathway

A. Approximately 85-100% of the pyramidal fibers decussate; these crossed fibers enter the lateral funiculus deep to the ascending spinocerebellar tracts and run caudad. These crossed fibers constitute the lateral corticospinal tract. Dogs - 100% cross, cats - >90%. B. The uncrossed part of pyramidal fibers enters the ventral funiculus and descend adjacent to the ventral median fissure as the ventral corticospinal tract. These fibers usually cross in the cord just before entering the gray matter to synapse. Clinically, it is probably very difficult to detect problems with this uncrossed part of the pyramidal tract and therefore not important.

caudal thalamic group

A. Medial geniculate nucleus - receives auditory fibers via brachium of caudal colliculus and projects to auditory sensory area of cerebral cortex. B. Lateral geniculate nucleus - Receives visual fibers from retina via optic tract and projects fibers to visual sensory cortex.

brain portion of pathway

A. UMN cell bodies lie in cerebral cortex, in both motor and sensory areas. B. UMN fibers project from the cortex through the cerebral hemisphere and diencephalon via the internal capsule. C. Descending pyramidal fibers consolidate on ventral surface of midbrain in the cerebral peduncles. Pyramidal fibers actually form the superficial layer of each peduncle called the crus cerebri. D. Beginning in the midbrain, fibers leave the pyramidal tract to supply motor nuclei of cranial nerves in the brainstem. This loosely organized group of UMN's is collectively called the corticobulbar tract. While it is difficult to localize a discrete bundle of corticobulbar fibers, these nerve fibers branch off from the pyramidal tract to innervate cranial nerve nuclei as they course through the brainstem. Hence, the pyramidal tract gets smaller and smaller as it courses caudally in the brainstem before reaching the spinal cord. E. Pyramidal fibers pass through the body of the pons, then appear on the ventral surface of the medulla as the paired pyramids. These corticobulbar fibers are very important in control of facial motor neurons, especially horses. F. Near the junction of medulla and spinal cord most of the pyramidal fibers cross the midline forming the pyramidal decussation.

lateral thalamic group

A. Ventral layer - the most caudal two nuclei of this group (ventral posterior lateral and ventral posterior medial nuclei) are the thalamic relay sites for ascending afferent pathways: 1. VPL nucleus - Receives afferent input from body via spinothalamic, spinocervicothalamic, and dorsal column-medial lemniscal tracts. Projection fibers carry information via internal capsule to specific somatosensory area of cerebral cortex. 2. VPM nucleus - Receives afferent input from head via trigeminothalamic tract. Projection fibers carry information via internal capsule to specific somatosensory area of cerebral cortex. (The remaining ventral layer nuclei receive input related to motor activity from cerebellum and corpus striatum and project information to motor area of cerebral cortex) B. Dorsal layer - These nuclei receive input from diencephalic and telencephalic structures, not from ascending pathways, and project to diffuse widespread areas of cerebral cortex.

cerebral cortex organization

A. We have already seen that the surface of the cerebral hemisphere is covered with a thin layer of gray matter (cortex) which is highly convoluted. B. Specific gyri and lobes can be identified on the cerebral cortex; these landmarks are usually consistent within species but quite different between species C. Specific functional cortical areas (sensory):

extent of pathway

A. Whereas in primates the pyramidal tract is a critical descending UMN pathway (damage to the motor cortex in man results in loss of voluntary motor function), this pathway is greatly reduced in extent and function in the domestic animals (total removal of the motor cortex in a dog results in little or no discernible gait abnormality). B. In the carnivores the lateral corticospinal tract extends into the lumbar cord, although most corticospinal fibers terminate in the cervical spinal cord. The ventral corticospinal tract usually descends no further than the mid-thoracic region. (very little clinical significance.) C. In the large animals neither corticospinal pathway extends caudal to the cervical cord. Therefore, in large animals the extrapyramidal system serves a major role in controlling movement.

excitatory effects

As we have seen there are two basic groups of descending UMNs which can influence LMNs in cranial nerve motor nuclei and ventral horn of spinal cord.

function of the cerebellum

Balance and coordination

spinothalamic tract

Collaterals of small primary afferent fibers running in the dorsolateral fasciculus synapse in the substantia gelatinosa and/or nucleus proprius of dorsal horn. Most second order fibers cross the spinal cord to enter the ventral portion of the contralateral lateral funiculus to ascend toward the brainstem. A small number of second order nerve fibers do not cross over to contralateral side but remain ipsilateral. This is similar to dorsal column lemniscal tract but not precisely organized.

rubrospinal tract

Extrapyramidal motor tract responsible for motor input of gross postural tone, facilitating activity of flexor muscles, and inhibiting the activity of extensor muscles

polysynaptic

In this case the influence of the UMN is mediated by one or more interneurons. Since the interneurons utilized by descending pathways may be the same interneurons as those of a polysynaptic reflex pathway, UMN's may influence LMN's by altering their reflex activity (this will have considerable clinical significance). In other words, the UMN can make a LMN reflex more, or less, excitable. Much like a parent of a small child!!!

hypertonia

Increased muscle tone. Also called spasticity. Again, notice that while muscle tone is altered, there is some level of muscle tone detectable.

corticofugal

These projection fibers course from the cortex to some lower brain or spinal cord level. Corticothalamic fibers also exist, providing reciprocal interconnection of thalamus and cortex.

generally facilitate flexors and inhibit extensors

Medullary reticulospinal fibers

diffuse projections

Nuclei of dorsal layer and medial and rostral thalamic nuclei project to widespread areas of cerebral cortex.

pyramidal tract

This is a contralateral pathway composed of UMN's whose fibers descend through the brainstem and constitute the pyramids on the ventral surface of the medulla.

association fibers

These fibers are found within the cerebral hemisphere, interconnecting ipsilateral cortical areas with one another.

medial thalamic group

These nuclei, along with dorsal layer nuclei and rostral nucleus mentioned above receive input from a variety of structures in diencephalon and telencephalon and project diffusely to cerebral cortex Reticular nucleus and nuclei lying in medullary laminae and interthalamic adhesion - these nuclei function in ascending reticular activating system (see below).

cytoarchitecture

The arrangement of neuronal cell bodies in various parts of the brain. 1. Large Purkinje cells send very extensive dendritic "trees" into the molecular layer. These dendritic "trees" are oriented perpendicular to the long folds of cerebellar cortex. 2. Granule cells (which, like Purkinje cells, are varieties of neurons) of the granular layer send long processes (axons) into the molecular layer. These granule cell axons run parallel to the folds of cerebellar cortex. 3. As the granule cell axons run through the molecular layer, they can form synapses (not terminal synapses but "synapses-in-passing") with the extensive dendrites of Purkinje cells. Because of the large number of granule cell axons which are present in the molecular layer and the very extensive dendrites of the Purkinje cells, literally thousands of granule cells can form synaptic connections (excitatory) with a single Purkinje cell. Therefore, a tremendous amount of convergence occurs in the cerebellar cortex.

auditory area

The gyrus surrounding the pseudosylvian sulcus (sylvian gyrus) receives projection fibers from the medial geniculate nucleus of the thalamus. Therefore, the auditory area lies largely in the temporal lobe.

corticopetal

These projection fibers course from some lower brain level to the cortex; thalamocortical fibers are corticopetal projections.

organization of descending motor pathways

The motor system is composed of lower motor neurons (LMN's) lying in the ventral horn of the spinal cord and motor nuclei of certain cranial nerves, and neurons found at various brain levels which send descending fibers to establish synaptic connections with LMN's; these higher-level neurons are referred to collectively as upper motor neurons (UMN's). The LMN's are themselves capable of nothing more than reflex or spontaneous activity - it is the influence of UMN's therefore which allows voluntary control of muscle contraction. Also, any movement, such as a reflex, does not mean nor imply that the animal perceives this response. Movement and perception are two separate phenomena during an examination.

convergence

The opposite of divergence, this is the gathering together of many different sources of information onto a single location. For example, one neuron receives synapses from many other neurons lying elsewhere in the CNS or in cerebrospinal ganglia. Thus, many individual bits of information (in the form of nerve impulses) are said to converge on a single neuron.

somatosensory area

The postcruciate gyrus is often described as the "somatosensory area" (based upon data from the human cerebrum), but in the domestic animals, the cortical area just caudal and ventral to the postcruciate gyrus (parts of marginal and coronal gyri) is more accurate. The somatosensory area thus occupies the rostral part of the parietal lobe. This area is somatotopically arranged, with the caudal end of the body represented near the dorsal midline, and the head represented more laterally; the area with the most representation corresponds to the body region with the greatest number of receptors (e.g. hand of primates, muzzle of domestic animals). The somatosensory area receives projection fibers from the VPL and VPM nuclei of the thalamus.

olfactory area

The pyriform lobe (part of rhinencephalon - "smell brain") receives olfactory input from olfactory bulbs. It is noteworthy that olfactory sensory information is the only afferent input which is not relayed to the cerebral cortex via the thalamus.

pupillary light reflex

The visual system provides the afferent limb for the pupillary light reflex. The light shown in the eye is carried to the CNS via the ganglion cell axons comprising the optic nerve/tract. Fibers mediating this reflex synapse in the midbrain. Neurons in the midbrain project to the preganglionic parasympathetic nucleus which gives rise to preganglionic parasympathetic fibers that run with the oculomotor nerve and synapse in the ciliary ganglion. Postganglionic parasympathetic fibers originating from neurons in this ganglion then project to and synapse on ciliary muscles which when stimulated, contract and cause constriction of the pupils.

vestibulospinal tract

There are actually two vestibulospinal tracts - lateral and medial - but the lateral vestibulospinal tract is by far the more important. Lateral vestibulospinal fibers arise from the lateral vestibular nucleus, one of the complex of brainstem nuclei related to CN VIII. Vestibulospinal fibers descend ipsilaterally throughout the entire spinal cord in the ventral funiculus, synapsing via interneurons with LMN's in the cord. This descending pathway is primarily facilitatory to extensor LMN's and is the major pathway for maintaining balance and coordinating extensor muscle tone (i.e. tone of antigravity muscles) with changes in body position. As we will see later, the lateral vestibulospinal pathway is influenced almost exclusively by the cerebellum and by input from the vestibular apparatus of the inner ear.

reticulospinal tract

There are two distinct pathways in this group, medullary and pontine reticulospinal tracts, which arise from medullary and pontine reticular formation, respectively. These fibers have a bilateral influence although there may be a bias towards an ipsilateral or contralateral influence and pass caudally from their origin to enter the spinal cord as follows:

inhibitory effects

These descending pathways also have a negative influence over LMNs which innervate skeletal muscles of opposite function (i.e. antagonistic) to those which are facilitated. This is an example of an important and ubiquitous phenomenon called reciprocal inhibition.

tectospinal tract

This tract arises from the roof of the midbrain (tectum), mostly from the rostral colliculus. Fibers decussate in the midbrain tegmentum. Tectospinal fibers run in the ventral funiculus as far caudally as perhaps the midthoracic region, although most fibers terminate in the upper cervical cord. The rostral colliculus is an important brainstem reflex center, receiving afferent input from visual and somatosensory pathways, as well as some auditory input. The tectospinal tract therefore interconnects the rostral colliculus with LMN's of neck and thoracic limb mm which enable the animal to turn it head and reposition itself in response to sudden visual, auditory or somatosensory stimuli.

direct projections

VPL, VPM and other ventral layer nuclei of lateral group, and medial and lateral geniculate nuclei project to specific cortical areas.

domestic animal cerebral cortex

have reduced emphasis upon the cerebral cortex and pyramidal tract: extensive cortical lesions seldom produce motor deficits, such as contralateral paresis (weakness) with little discernible abnormality in such "automatic" activities as walking. Thus, while the cerebral cortex is still an important center for voluntary initiation of motor activity, much more emphasis has shifted to the subcortical nuclear groups.

projection fibers

We have already been discussing this fiber classification in reference to thalamocortical projections. may run in two general directions.

Lateral thalamic group, rostral thalamic nucleus, Caudal thalamic group, medial thalamic group

What are the major nuclear groups of thalamus?

spiral ganglion

a collection of neurons in the modiolus of the cochlea that receives input from hair cells and sends output to the cochlear nuclei in the medulla via the auditory nerve

reticulospinal and vestibulospinal tracts

are less heavily influenced by cerebral cortex, but more by other "subcortical centers." Also, because these tracts are not somatotopically arranged and generally (except for medullary reticulospinal tract) facilitatory to extensor LMN's, these pathways are more concerned with regulations and maintenance of posture and balance. For example, an animal which has had its brainstem completely transected just rostral to the rostral colliculi exhibits a high level of extensor muscle tone and can stand and maintain balance and posture in a fairly normal manner; such an animal (called decerebrate preparation) can even walk and right itself (reorient its body from an abnormal position). The decerebrate animal retains intact the nuclei of origin of the rubrospinal, vestibulospinal and reticulospinal fibers and afferent connections from ascending pathways and the cerebellum but has lost the connection with cerebral cortex and other critical subcortical centers. Decerebrate animals will only react (although in a very complex fashion) to external stimuli and evidence no voluntary activity.

cochlear nuclei

brainstem nuclei that receive input from auditory hair cells and send output to the superior olivary complex

cerebellar peduncles

bundles of fibers which attach and functionally connect cerebellum to brainstem.

extrapyramidal tracts

constitute a diverse group of brainstem nuclei and descending pathways which share only the common trait that none of the descending fibers pass through the pyramids. This system consists of multisynaptic pathways which can be subdivided into:

hypotonia

decreased muscle tone

pontine reticulospinal fibers

descend in the ventral funiculus (ventral reticulospinal tract).

medullary reticulospinal fibers

descend in the ventral portion of the lateral funiculus (this tract is sometimes called the lateral reticulospinal tract).

Hyporeflexia

diminished reflex activity. In "humanoids" there may be an absence of reflex activity after an insult to brain and spinal cord. Notice that while reflex activity is altered, it is not absent.

Thalamic nuclei of ascending reticular activating system

do not project to cortical levels at all, but rather influence the diffuse projection nuclei which then relay activity to widespread cortical areas.

middle peduncle

extends dorsally from pons to cerebellum; composed almost exclusively of afferent fibers to cerebellum

caudal peduncle

extends from medulla to cerebellum; composed mainly of afferents to cerebellum, but many efferents from cerebellum also pass through this bundle.

rostral peduncle

extends from midbrain to cerebellum; composed primarily of efferent fibers from cerebellum.

facilitate the LMN

facilitation of an excitatory interneuron will

spinocervicothalamic tract

first order neuron synapses in dorsal horn of spinal cord which synapse with second order neurons they decussate to go up spinothalamic track up the spinal cord third order neurones arise from contralateral thalamus which then ascends to cortex

monosynaptic

in this case the descending UMN fiber synapses directly with the dendrites and/or soma of the LMN. connections between UMN's and LMN's are rare in the domestic animals, but very common and important in humanoids! connections result in more direct control over movement.

inhibit the LMN

inhibition of an excitatory interneuron will

facilitate the LMN

inhibition of an inhibitory interneuron will

basal ganglia

is a loosely organized description of many of the clusters of nuclei located in the base of the telencephalon and often other parts of the brainstem as well.

proprioceptive primary afferents

large heavily myelinated fibers arising from mechanoreceptors in the skin, joints and skeletal mm. can participate in monosynaptic reflexes with LMNs ascending pathways are specific involving few synaptic relays, mediate rapid, well localized neural activities

primary fissure caudo lateral fissure

lobes of the cerebelum

vermis and hemispheres

major subdivision of cerebellum

paresis

partial paralysis, weakness

tonotopically

organized in terms of bass (lower notes or tones) and treble (higher notes or tones). The basilar membrane within the cochlea is organized such that the basal portion is associated with treble sounds and the apical portion is related to bass sounds. This tonotopic organization is reflected throughout the auditory system.

Facilitate Extensors Inhibit Flexors

pontine reticulospinal fibers

human cerebral cortex

the high development of the cerebral cortex and pyramidal tract means that this brain center is vitally important for voluntary motor activity, especially delicate movements involving the distal limb musculature; cortical damage can produce profound paralysis (stroke).

visual area

the most caudal portion of the occipital lobe of the cerebral cortex receives projection fibers from the lateral geniculate nucleus of the thalamus.

excitatory to flexor LMN

these pathways are important for control of non-antigravity skeletal muscles, especially those of the distal portion of the limbs. Such pathways are: 1. Pyramidal tract (location of UMN cell bodies) a. Corticobulbar fibers b. Corticospinal fibers (course in cord white matter). In domestic large animals these rarely descend past the cervical spinal cord so corticospinal fibers have minimal effects on movement in the horse, etc. 2. Rubrospinal tract (UMN cell bodies) a. Rubrobulbar fibers b. Rubrospinal fibers (funicular location) 3. Medullary reticulospinal tract (funicular location)

excitatory to extensor LMN

these pathways facilitate extensor (antigravity muscles, so are critical for support of the animal against gravity and for maintenance of balance and posture. 1. Pontine reticulospinal tract (funicular location) 2. Lateral vestibulospinal tract (UMN cell bodies; funicular location)