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C. BASIC ANATOMICAL STRUCTURES
The Spinal Cord
The spinal cord is divided into right and left sides by two grooves.
The anterior median fissure is a deep wide groove on the ventral surface.
The posterior median sulcus is a shallower, narrower groove on the dorsal surface.
The Spinal Cord
The spinal cord consists of both grey and white matter.
The grey matter forms an H-shaped area within the white matter and consists primarily of nerve cells bodies, dendrites, and unmyelinated axons of both motor neurons and interneurons.
The Spinal Cord
The white matter consists of bundles of myelinated axons of motor and sensory neurons that constitute the sensory and motor tracts of the spinal cord.
The cross-bar of the “H” is formed by the grey commissure.
Spinal Cord
In its center, is a small space called the central canal.
The canal runs the length of the spinal cord and is continuous with the 4th ventricle of the medulla.
It contains cerebrospinal fluid.
The Spinal Cord
Anterior to the grey commissure is the anterior white commissure, which connects the white matter of the right and left sides of the spinal cord.
The upright portions of the “H” are further subdivided into regions.
The Spinal Cord
Those portions closer to the front of the cord are called the ventral (anterior) grey horns.
The regions closer to the back of the cord are called the dorsal (posterior) grey horns.
The Spinal CordThe regions between the ventral and dorsal grey horns are the intermediate lateral grey horns.
The lateral grey horns are only present in the thoracic, upper lumbar, and sacral segments of the cord.
The Spinal Cord
The white matter, like the grey matter, is also organized into regions.
The anterior white columns, the posterior white columns, and the lateral white columns are regions that result from the division of the cord by the grey matter.
The Spinal Cord
Each column consists of bundles of myelinated fibers that run within the cord.
Theascending bundle tracts are called fasciculi.
They consist of sensory axons that conduct impulses that enter the spinal cord upward to the brain.
The Spinal Cord
The long descending tracts consists of motor axons that conduct impulses from the brain downward into the spinal cord where they synapse with other neurons whose axons pass out of the spinal cord to a muscle or gland.
Spinal Nerves
The 31 pairs of spinal nerves are the paths of communication between the periphery and the ascending sensory fasciculi as well as between the descending motor tracts and the periphery.
Spinal Nerves
The dorsal (posterior) root contains sensory nerve fibers only and conducts nerve impulses into the system from the periphery.
These fibers extend into the dorsal grey horn.
Spinal Nerves
Each dorsal root has a swelling called the dorsal root ganglion.
The dorsal root ganglion contains the cell bodies of the sensory neurons from the periphery.
Spinal Nerves
The ventral root contains motor axons only and conducts impulses away from the spinal cord toward the periphery.
The cell bodies of the motor neurons are located in the grey matter.
Spinal Nerves
If the motor neuron supplies a skeletal muscle, then its cell body is located in the ventral grey horn.
If the motor neuron supplies a smooth muscle, a cardiac muscle, or a gland through the autonomic nervous system, then its cells bodies are located in the lateral grey horns.
Spinal Cord Functions
The major function of the spinal tracts is to convey sensory impulses from the periphery to the brain and to conduct motor impulses from the brain to the periphery.
A secondary function is to provide reflex integration.
Reflexes are fast responses to certain stimuli in the internal or external environment that allow the body to maintain homeostasis.
Spinal Cord Functions
Reflexes that are carried out by the spinal cord alone are called spinal reflexes.
Reflexes that involve brain centers and cranial nerves are called cranial reflexes.
Reflexes that result in the contraction of skeletal muscles are called somatic reflexes.
Reflexes that result in the contraction of smooth or cardiac muscle, or the secretion by glands are called visceral reflexes.
The Reflex Arc
The reflex arc is the shortest route that can be taken by an impulse passing from a receptor to and effector.
The Reflex Arc
The basic components of the reflex arc are a receptor, a sensory neuron, an interneuron, a motor neuron, and an effector—skeletal muscle, smooth muscle, or gland.
The Brainstem and Cranial Nerves
The rhombencephalon is that portion of the brain that lies above the spinal cord and from which a number of cranial nerves emerge.
The longitudinal organization of the neural tube is still recognizable in the brainstem, although it’s altered by the enlargement of the central canal into the fourth ventricle.
The Brainstem and Cranial Nerves
In keeping with its embryonic organization, the ventral portion of the brainstem is motor in nature (red), while the dorsal portion is sensory in nature (blue).
Intermediate regions between these two areas are viscero-motor (white) and viscero-sensory (yellow).
The Brainstem and Cranial Nerves
The secondary divisions of the rhombencephalon include the myelencephalon from which the medulla oblongata and 4th ventricle arise and the metencephalon from which the pons, cerebellum, are differentiated.
As we ascend in a rostral direction, the first structure upon which we will focus is the medulla oblongata.
Medulla
The medulla oblongata is the most inferior portion of the brainstem.
The inferior portion is closed and continuous with the spinal cord at roughly the level of the foramen magnum.
The superior portion of the medulla is open and contains the 4th ventricle.
Ventral Medulla
Just below the pons, on the ventral (anterior) side of the medulla, are two roughly triangular structures called the pyramids.
The pyramids are composed of the largest motor tracts coming from the cerebral cortex, through the cerebral peduncles, on their way to the spinal cord.
Pyramidal Decussation
In the area just above the transition from brainstem to spinal cord, most of the fibers contained in the pyramids decussate—cross—to the contralateral side.
The motor fibers which undergo this decussation in the pyramids belong to the lateral corticospinal tracts.
Lateral Corticospinal Tract
After they decussate, they descend in the lateral columns of the spinal cord, terminating in the ventral grey horns.
Cortico=origin of the motor tracts;
Spinal=termination point of the motor tracts;
Lateral=the area of the spinal cord in which the fibers pass.
Lateral Corticospinal Tract
In the ventral grey horns, the motor fibers synapse with motor neurons that terminate in skeletal muscles.
Lateral Corticospinal Tract
As a result of the crossing, fibers that originate in the left hemisphere activate muscles on the right side of the body, and fibers that originate in the right hemisphere, activate muscles on the left side of the body.
Dorsal Medulla
The dorsal portion of the medulla contains two pairs of prominent nuclei: the right and the left nucleus gracilis and nucleus cuneatus.
These nuclei receive sensory fibers from ascending white matter tracts of the posterior column of the spinal cord.
Posterior Column
Lumbar and thoracic fibers from the lower limb area ascend in a bundle called the fasciculus gracilis (2) to end in the nucleus gracilis (4).
Cervical fibers from the upper limb area ascend in another bundle called the fasciculus cuneatus (3) to end in the nucleus cuneatus (5).
Posterior Column
The second order fibers arising from these nuclei (6, 6’, 6”) cross the midline (7) forming the medial lemniscus (6), and then continue their ascent through the brainstem to the thalamus.Because of this crossing over, nearly all sensory impulses received on one side of the body are perceived in the opposite hemisphere of the cerebral cortex.
Posterior Column
In other words, we have contralateral sensory perception of upper and lower body stimuli leading to conscious appreciation of touch, pressure, vibration, and joint position/movement because of this decussation of the fasciculi gracilis and cuneatus.
Lateral Medulla
On each lateral surface of the medulla, an oval shaped projection called the olive contains the inferior olivary nucleus.
Its fibers project to the cerebellum ensuring the efficiency of voluntary movements, especially precision movements.
Lateral Medulla
Also contained within each olive is the accessory olivary nucleus which has fibers that project to the cerebellum to monitor maintenance of equilibrium, postural changes, and locomotion.
Medullary Cranial Nerves
The nuclei of origin for cranial nerves VIII, IX, X, XI, and XII attach at various locations along the length of the medulla.
Within the substance of the medulla is an area through which the cranial nerves pass and in which certain nuclear centers are found.
Reticular Formation
This substance is known as the reticular formation (3, 3’, 3”).
It occupies the anterior and lateral districts of the medulla oblongata and is situated behind the pyramid (5) and olive (7).
Reticular Formation
The reticular formation is caused by the intersection of bundles of fibers running at right angles to each other, some being longitudinal, others more or less transverse in direction.
Reticular Formation & Cranial Nerves
In the substance of the reticular formation are the nuclei of origin or termination of several cranial nerves.
Specifically, the nucleus of origin of the motor hypoglossal (XII) nerve, the nucleus of termination of the Vagus (X) nerve, and the nuclei of origin of motor fibers of the Glossopharyngeal (IX), vagus (X), and accessory (XI) nerves.
Medullary Reflex Centers
Also contained within the medullary reticular formation are three vital reflex centers: the cardiac center, which regulates the rate of
heartbeat and the force of contraction; the medullary rhythmcity center, which
regulates/adjusts the basic rhythm of breathing; and
the vasomotor center, which regulates the diameter of the blood vessels.
Other centers in the medulla coordinate swallowing, vomiting, coughing, sneezing, and hiccoughing (hiccupping).
PonsThe metencephalon is the other secondary division of the rhombencephalon.
It further differentiates into the pons and the cerebellum.
If we move rostrally, the pons is found superior to the medulla.
The word “pons” means bridge, and it is the portion of the brainstem located immediately inferior to the midbrain and anterior to the fourth ventricle.
Pons
It measures about 1” in length, and like the medulla, its basic anatomy can be organized into anterior (motor) and posterior (sensory) divisions.
Ventral Pons
The anterior portion of the pons is called the pontine bulb.
It contains large numbers of descending motor fibers from the corticospinal tracts.
Dorsal Pons
In the dorsal pons, we have the medial lemniscus (blue), the site of ascending sensory information tracts.
The reticular formation of the pons contains a collection of pigmented cells containing neuromelanin and the neurotransmitter norepinephrine.
Pontine Reticular FormationThis collection of
pigmented cells is termed the locus coeruleus, with projections to the hypothalamus, cerebellum, cerebral cortex, and the spinal cord.
This “blue spot” has been implicated in maintaining arousal, dreaming, and the regulation of mood.
Pontine Reticular Formation
Other important nuclei in the reticular formation of the pons comprise the pontine respiratory center.
Specifically, the pneumotaxic area is found in the upper pons; the apneustic area is found in the lower pons.
Together they work with the medullary rhythmicity area in the medulla to inhibit (PA) or facilitate (AA) respiration.
Reticular Formation & Cranial Nerves
The cranial nerve nuclei and tracts that are located in the reticular formation are the Trigeminal (Vth), and the Abducens (VIth) nerves.
The Facial (VIIth) and the Vestibulococchlear (VIIIth) are located at the junction of the pons and medulla.
CerebellumThe second largest portion of the brain is the cerebellum.
It lies behind the pons and medulla oblongata.
Between its central portion and these structures is the cavity of the fourth ventricle.
Cerebellum
The cerebellum also lies inferior to the occipital lobe of the cerebrum.
It is separated from the cerebrum by the transverse fissure.
Cerebellum The cerebellum is
somewhat oval in form, but constricted medially and flattened.
Its greatest diameter is from side.
Shaped like a butterfly, the central constricted area is the vermis, which means “worm-shaped.”
The lateral projections are referred to as hemispheres.
Cerebellar Hemispheres
Each hemisphere consists of lobes that are separated by deep and distinct fissures.
The anterior lobe plays a prominent role in postural adjustment.
The posterior lobe plays a major role in coordinating voluntary movement.
Cerebellar Lobes
The flocculonodular lobe is concerned with the sense of equilibrium.
Eye movements and postural adjustments to gravity are also mediated in this lobe.
Cerebellar Cortex
The surface of the cerebellum is called the cortex, and it consists of grey matter in a series of slender parallel ridges called folia.
They are less prominent than the convolutions (gyri) of the cerebral cortex.
Cerebellar White & Grey Matter
Beneath the grey matter are white matter tracts termed arbor vitae that resembles the branches of a tree.
Deep within the white matter are masses of grey matter—the cerebellar nuclei.
These nuclei give rise to nerve fibers that convey information out of the cerebellum to other parts of the nervous system.
Cerebellum
In the adult the proportion between the cerebellum and cerebrum is about 1 to 8.
In the infant, the proportion is about about 1 to 20.
Functionally, the cerebellum is a motor area of the brain concerned with coordinating subconscious movements of skeletal muscles.
Cerebellar Peduncles
The cerebellar peduncles are the fiber tracts that permit information to pass into and out of the cerebellum.
The three major bundles of fibers are the superior, middle, and inferior cerebellar peduncles.
Cerebellar Peduncles
The superior cerebellar peduncles (#4) connect the cerebellum with the midbrain.
They contain mostly efferent fibers that bring information out of the cerebellum into the midbrain.
Cerebellar Peduncles
Fibers constituting the superior cerebellar peduncle arise from several deep cerebellar nuclei.
These fibers pass rostrally for a while and then cross at the level of the midbrain to form the decussation of the superior cerebellar peduncle.
These fibers then continue rostrally to terminate in the red nucleus and the motor nuclei of the thalamus.
Cerebellar Peduncles
The middle cerebellar peduncles (#5) connect the cerebellum with the pons.
These peduncles contain only afferent fibers and thus bring information into the cerebellum.
Cerebellar Peduncles
Pontine grey nuclei are involved in conveying information from the motor cortex to the cerebellum regarding intended movement(s) of contralateral arm and leg.
Because there are lots of pontine grey neurons, the cortico-ponto-cerebellar circuit in very important in overall motor function.
Cerebellar Peduncles Cortico-pontine fibers arise in the
ipsilateral motor association cortex and provide the pontine grey neurons with information regarding intended movement(s) of the contralateral side of the body.
Specifically, neurons in the left pontine grey nuclei project to the right side of the cerebellum via the right middle cerebellar peduncle.
Neurons in the right pontine grey nuclei project to the left side of the cerebellum via the left middle cerebellar peduncle.
Cerebellar Peduncles
The inferior cerebellar peduncles (#7) connect the cerebellum with the medulla.
These peduncles contain both efferent and afferent fibers bringing information into and out of the cerebellum.
Cerebellar Peduncles
Specifically, the inferior cerebellar peduncle contains fibers that arise from cells in the ipsilateral spinal cord.
The largest component of the inferior cerebellar peduncle consists of fibers that arise from the contralateral inferior olive (medulla) and from the vestibular nuclei in the reticular formation on the border of the pons and medulla.
Cerebellar Peduncles
With information coming into the cerebellum from the spinal cord (bottom up) and cortex (top down), the cerebellum is constantly kept apprised of the status of the body in space.
Specifically, it receives input signals from proprioceptors in muscles, tendons, and joints, and from receptors for equilibrium, and from visual receptors (e.g., the eyes).
Cerebellar Functions
In conjunction with other motor areas of the brain, such as the motor cortex of the cerebrum and the basal ganglia, the cerebellum receives a duplicate set of signals regarding the motor plan.
The cerebellum compares input information regarding the actual status of the body with the intended movement determined by the motor areas of the brain.
Cerebellar Functions
If the intent of the motor plan is not being attained by the skeletal muscles, the cerebellum is going to detect the variation and send feedback signals to the motor areas to either stimulate or inhibit the activity of the skeletal muscles.
The interaction of the cerebellum with the motor cortex and basal ganglia produces smooth, coordinated movements of the body’s skeletal muscles.
The Mesencephalon
The midbrain extends from the lower portion of the diencephalon to the pons.
It is about 1” in length and is organized around the cerebral aqueduct, which passes down through the midbrain, connecting the third ventricle with the fourth ventricle.
Anterior Midbrain
The area anterior to the cerebral aqueduct is called the tegmentum.
The tegmentum is continuous with the reticular formation of the pons, and consists of longitudinal and transverse fibers, together with a considerable amount of gray substance.
Ventral Midbrain
The principal gray masses of the tegmentum are the red nuclei.
The red nuclei are found in the reticular formation and are the site of termination for fibers from the cerebellum and cerebral cortex.
Ventral Midbrain
The red nuclei are important relay centers for cortico-motor impulses that govern muscle tone, body posture, and walking.
Damage to the red nucleus results in tremor at rest, alteration in muscle tone, and choreiform-athetoid movements.
Ventral Midbrain
The midbrain reticular formation also contains cranial nerve nuclei and tracts for the oculomotor nerve (III) and the trochlear nerve (IV).
CN III mediates some movements of the eyeball and changes in pupil size and lens shape.
CN IV is involved also in eyeball movement.
Ventral Midbrain
The substantia nigra (6) runs the vertical length of the anterior midbrain between the cerebral peduncles (d, f, g).
It is important for the control of involuntary coordinated movements and the rapid onset of movement.
When damaged, tremor at rest, loss of coordinated movements, and a mask-like face result.
Cerebral Peduncles
The cerebral peduncles (d, f, g) are a pair of fiber bundles on the ventral midbrain.
They consist mostly of descending motor tracts connecting the cortex and the pons, and the cortex and the spinal cord.
Dorsal Midbrain
The area of the midbrain found posterior to the cerebral aqueduct is termed the tectum (roof).
On the midbrain tectum we fine an important landmark referred to as the quadrageminal plate.
Dorsal Midbrain
It is comprised of the paired superior colliculi (#2) and the paired inferior colliculi (#3).
The superior colliculi serve as reflex centers for movements of the eyeballs and head and neck in response to visual and other stimuli.
The inferior colliculi serve as reflex centers for movements of the head and trunk in response to auditory stimuli.
Dorsal Midbrain Colliculi
Visual information taken in by the eye is carried by the optic nerve and then forwarded to the superior colliculus.
From the superior colliculus, this stimulus is relayed on to the lateral geniculate body in the thalamus.
From the lateral geniculate body, it is relayed to the primary visual cortex in the occipital lobe for recognition and perception.
A similar relationship exits with the medial geniculate bodies of the thalamus and the inferior colliculi of the midbrain.
They are relay centers for auditory information as it travels upstream to the primary auditory cortex.
