Pathways and Integrative Functions

The CNS communicates with peripheral body structures through pathways. 

These pathways conduct either sensory or motor information; processing and integration occur continuously along them. 

These pathways travel through the white matter of the brainstem and/or spinal cord as they connect various CNS regions with cranial and spinal nerves.

A pathway consists of a tract and nucleus. Tracts are groups or bundles of axons that travel together in the CNS. Each tract may work with multiple nuclei groups in the CNS. A nucleus is a collection of neuron cell bodies located within the CNS.

Nervous system pathways are sensory or motor. 

• Sensory pathways are also called ascending pathways because the sensory information gathered by sensory receptors ascends through the spinal cord to the brain.

• Motor pathways are also called descending pathways because they transmit motor information that descends from the brain through the spinal cord to muscles or glands.

Sensory pathways are ascending pathways that conduct information about limb position and the sensations of touch, temperature, pressure, and pain to the brain.

• Somatosensory pathways process stimuli received from receptors within the skin, muscles, and joints.

• Viscerosensory pathways process stimuli received from the viscera.

The multiple types of body sensations detected by the somatosensory system are grouped into three spinal cord pathways, each with a different brain destination: 

• Discriminative touch permits us to describe textures and shapes of unseen objects and includes pressure, touch, and vibration perception. 

• Temperature and pain allow us to detect those sensations, as well as the sensation of an itch.

• Proprioception allows us to detect the position of joints, stretch in muscles, and tension in tendons.

Functional Anatomy of Sensory Pathways

• Sensory pathways utilize a series of two or three neurons to transmit stimulus information from the body periphery to the brain.

• The first neuron in this chain is the primary neuron (or first-order neuron). 

  • The dendrites of this sensory neuron are part of the receptor that detects a specific stimulus.

  •  The cell bodies of primary neurons reside in the posterior root ganglia of spinal nerves or the sensory ganglia of cranial nerves. 

  • The axon of the primary neuron projects to a secondary neuron within the CNS.

• The secondary neuron (or second-order neuron), the second neuron in this chain, is an interneuron.

  •  The cell body of this neuron resides within either the posterior horn of the spinal cord or a brainstem nucleus. 

  • The axon of a secondary neuron projects either to the thalamus for conscious sensations or to the cerebellum for unconscious proprioception. 

  • The axon of the secondary neuron arriving in the thalamus synapses with the tertiary neuron, the third neuron in the chain. 

• The tertiary neuron (or third-order neuron) is an interneuron whose cell body resides within the thalamus. Recall that the thalamus is the central processing and coding center for almost all sensory information.

• The three major types of somatosensory pathways are:

  • Posterior Funiculus–Medial Lemniscal Pathway

    • The posterior funiculus–medial lemniscal pathway (or posterior column pathway) projects through the spinal cord, brainstem, and diencephalon before terminating within the cerebral cortex.

    • Its name derives from two components: the tracts within the spinal cord, collectively called the posterior funiculus; and the tracts within the brainstem, collectively called the medial lemniscus. 

    • This pathway conducts sensory stimuli concerned with proprioceptive information about limb position and discriminative touch, precise pressure, and vibration sensations.

    • The posterior funiculus–medial lemniscal pathway uses a chain of three neurons to signal the brain about a specific stimulus. Axons of the primary neurons traveling in spinal nerves reach the CNS through the posterior roots of spinal nerves. 

    • Upon entering the spinal cord, these axons ascend within a specific posterior funiculus, either the fasciculus cuneatus or the fasciculus gracilis. The fasciculus cuneatus houses axons from sensory neurons originating in the upper limbs, superior trunk, neck, and posterior region of the head, whereas the fasciculus gracilis carries axons from sensory neurons originating in the lower limbs and inferior trunk. 

    • The sensory input into both posterior funiculi is organized somatotopically that is, there is a correspondence between a receptor’s location in a body part and a particular location in the CNS. Thus, the sensory information originating from inferior regions is medially located within the fasciculus, and the sensory information originating at progressively more superior regions is located more laterally.

    • Sensory axons ascending within the posterior funiculi synapse on secondary neuron cell bodies housed within a posterior funiculus nucleus in the medulla oblongata. 

    • These nuclei are either the nucleus cuneatus or the nucleus gracilis, and they correspond to the fasciculus cuneatus and fasciculus gracilis, respectively.

    • These secondary neurons then project axons to relay the incoming sensory information to the thalamus on the opposite side of the brain through the medial lemniscus. Decussation occurs after secondary neuron axons exit their specific nuclei and before they enter the medial lemniscus. 

    • As the sensory information travels toward the thalamus, the same classes of sensory input (touch, pressure, and vibration) that have been collected by cranial nerves CN V (trigeminal), CN VII (facial), CN IX (glossopharyngeal), and CN X (vagus) are integrated and incorporated into the ascending pathways, collectively called the trigeminothalamic tract.

• Anterolateral Pathway

  • The anterolateral pathway (or spinothalamic pathway) is located in the anterior and lateral white funiculi of the spinal cord.

  • It is composed of the anterior spinothalamic tract and the lateral spinothalamic tract. 

  • Axons projecting from primary neurons enter the spinal cord and synapse on secondary neurons within the posterior horns. 

  • Axons entering these pathways conduct stimuli related to crude touch and pressure as well as pain and temperature.

• Spinocerebellar Pathway

  • The spinocerebellar pathway conducts proprioceptive information to the cerebellum for processing to coordinate body movements. 

  • The spinocerebellar pathway is composed of anterior and posterior spinocerebellar tracts; these are the major routes for transmitting postural input to the cerebellum. 

  • Sensory input arriving at the cerebellum through these tracts is critical for regulating posture and balance and for coordinating skilled movements.

  • Anterior spinocerebellar tracts conduct impulses from the inferior regions of the trunk and the lower limbs. Their axons enter the cerebellum through the superior cerebellar peduncle. 

  • Posterior spinocerebellar tracts conduct impulses from the lower limbs, the trunk, and the upper limbs. Their axons enter the cerebellum through the inferior cerebellar peduncle.

Functional Anatomy of Motor Pathways

• Motor pathways are formed from the cerebral nuclei, the cerebellum, descending projection tracts, and motor neurons.

• Descending projection tracts are motor pathways that originate from the cerebral cortex and brainstem. 

• Motor neurons within these tracts either synapse directly on motor neurons in the CNS or on interneurons that, in turn, synapse on motor neurons.

• There are at least two motor neurons in the somatic motor pathway: an upper motor neuron and a lower motor neuron. These neurons are involved in voluntary movements.

  • The cell body of an upper motor neuron is housed within either the cerebral cortex or a nucleus within the brainstem. 

  • Axons of the upper motor neuron synapse either directly on lower motor neurons or on interneurons that synapse directly on lower motor neurons.

  • The cell body of a lower motor neuron is housed either within the anterior horn of the spinal cord or within a brainstem cranial nerve nucleus. 

  • Axons of the lower motor neurons exit the CNS and project to the skeletal muscle to be innervated.

• The two types of motor neurons perform different activities:

  • The activity of the upper motor neuron either excites or inhibits the activity of the lower motor neuron, but the activity of the lower motor neuron is always excitatory because its axon connects directly to the skeletal muscle fibers. 

  • The cell bodies of motor neurons and most interneurons involved in the innervation and control of limb and trunk muscles reside in the spinal cord anterior horn and the gray matter zone between the anterior horn and the posterior horn.

• Motor neuron axons form two types of somatic motor pathways: direct pathways and indirect pathways. 

  • The direct pathways are responsible for conscious control of skeletal muscle activity, and the indirect pathways are responsible for unconscious control of skeletal muscle activity.

• Direct Pathway

  • The direct pathway, also called the pyramidal pathway or corticospinal pathway, originates in the pyramidal cells of the primary motor cortex. 

  • The name pyramidal is derived from the shape of the upper motor neuron cell bodies, which have a tetrahedral, or pyramid-like, shape. 

  • Their axons project either into the brainstem or into the spinal cord to synapse directly on lower motor neurons.

  • The axons from pyramidal cell upper motor neurons descend through the internal capsule, enter the cerebral peduncles, and ultimately form two descending motor tracts of the direct pathway:

    • Corticobulbar Tracts

      • The corticobulbar tracts originate from the facial region of the motor homunculus within the primary motor cortex. 

      • Axons of these upper motor neurons extend to the brainstem, where they synapse with lower motor neuron cell bodies that are housed within brainstem cranial nerve nuclei.

      • The corticobulbar tracts transmit motor information to control the following movements:

        • Eye movements (via CN III, IV, and VI)

        • Cranial, facial, pharyngeal, and laryngeal muscles (via CN V, VII, IX, and X)

        • Some superficial muscles of the back and neck (via CN XI)

        • Intrinsic and extrinsic tongue muscles (via CN XII)

    • Corticospinal Tracts

      • The corticospinal tracts descend from the cerebral cortex through the brainstem and form a pair of thick anterior bulges in the medulla oblongata called the pyramids. 

      • Then they continue into the spinal cord to synapse on lower motor neurons in the anterior horn of the spinal cord. 

      • The corticospinal tracts are composed of two components: lateral and anterior corticospinal tracts.

• Indirect Pathway

  • Several nuclei within the mesencephalon initiate motor commands for activities that occur at an unconscious level.

  • These nuclei and their associated tracts constitute the indirect pathway, so named because upper motor neurons originate within brainstem nuclei.

  • The axons of the indirect pathway take a complex, circuitous route before finally conducting the motor impulse into the spinal cord.

  • The indirect pathway modifies or helps control the pattern of somatic motor activity. This is accomplished by 

    • Altering motor neuron sensitivity to incoming impulses to control muscles individually or in groups.

    • Activating feedback loops that project to the primary motor cortex.

  • The different tracts of the indirect pathway are grouped according to their primary functions. 

  • The lateral pathway regulates and controls precise, discrete movements and tone in flexor muscles of the limbs.

  • This pathway consists of the rubrospinal tracts that originate in the red nucleus of the mesencephalon. The medial pathway regulates muscle tone and gross movements of the muscles of the head, neck, proximal limb, and trunk.

  •  The medial pathway consists of three groups of tracts: 

    • The reticulospinal tracts originate from the reticular formation in the mesencephalon. They help control more unskilled automatic movements related to posture and maintaining balance

    • The tectospinal tracts conduct motor commands away from the superior and inferior colliculi in the tectum of the mesencephalon to help regulate positional changes of the arms, eyes, head, and neck as a consequence of visual and auditory stimuli.

    • The vestibulospinal tracts originate within vestibular nuclei of the brainstem. Impulses conducted within these tracts regulate muscular activity that helps maintain balance during sitting, standing, and walking.

• Role of the Cerebral Nuclei

  • Cerebral nuclei interact with motor pathways in important ways.

  • The cerebral nuclei receive impulses from the entire cerebral cortex, including the motor, sensory, and association cortical areas, as well as input from the limbic system.

  • Cerebral nuclei provide the patterned background movements needed for conscious motor activities by adjusting the motor commands issued in other nuclei.

• Role of the Cerebellum

  • The cerebellum plays a key role in movement by regulating the functions of the motor pathways such as:

    • Voluntary Movements

    • Assessment of Voluntary Movements

    • Integration and Analysis

    • Corrective Feedback

Levels of Processing and Motor Control

• Simple reflexes that stimulate motor neurons represent the lowest level of motor control. The nuclei controlling these reflexes are located in the spinal cord and the brainstem.

• Highly variable and complex voluntary motor patterns are controlled by the cerebral cortex and occupy the highest level of processing and motor control.

Higher-order mental functions encompass learning, memory, reasoning, and consciousness. 

These functions occur within the cortex of the cerebrum and involve multiple brain regions connected by complicated networks and arrays of axons.

Both conscious and unconscious processing of information are involved in higher-order mental functions, and they may be continually adjusted or modified.

Development and Maturation of Higher-Order Processing

• From infancy on, our motor control and processing capabilities become increasingly complex as we grow and mature. 

• The maturation of the control and processing pathways is reflected in increased structural and functional complexity within the CNS.

• As the CNS continues to develop, many neurons expand their number of connections, providing the increased number of synaptic junctions required for increasingly complex reflex activities and processing.

Hemispheric Lateralization

• Anatomically, the left and right cerebral hemispheres appear identical, but careful examination reveals a number of differences.

• Humans tend to have shape asymmetry of the frontal and occipital lobes of the brain, called petalias. 

• Right-handed individuals tend to have right frontal petalias, meaning that the right frontal lobe projects farther than the left frontal lobe, and left occipital petalias, meaning that the left occipital lobe projects farther than the right occipital lobe.

• Each hemisphere tends to be specialized for certain tasks, a phenomenon called hemispheric lateralization

• The left hemisphere is the categorical hemisphere. 

  • It usually contains the Wernicke area and the motor speech area. 

  • It is specialized for language abilities, and is also important in performing sequential and analytical reasoning tasks, such as those required in science and mathematics.

• The other hemisphere (the right in most people) is called the representational hemisphere, because it is concerned with visuospatial relationships and analyses. 

  • It is the seat of imagination and insight, musical and artistic skill, perception of patterns and spatial relationships, and comparison of sights, sounds, smells, and tastes.

• Both cerebral hemispheres remain in constant communication through commissures, especially the corpus callosum, which contains hundreds of millions of axons that project between the hemispheres.

Language

• The higher-order processes involved in language include reading, writing, speaking, and understanding words.

Cognition

• Mental processes such as awareness, knowledge, memory, perception, problem solving, decision making, information processing, and thinking are collectively called cognition.

Memory

• Memory is a versatile element of human cognition involving different lengths of time and different storage capacities.

Consciousness

• Consciousness includes an awareness of sensation, voluntary control of motor activities, and activities necessary for higher mental processing.