Chapter 16: Pathways and higher order function
Pathways: exactly as it sounds, a way to get from point A to point B; ways for the CNS and PNS to communicate.
Pathways in the spinal cord are paired and grouped according to the body region innervated.
All tracts involve the brain and spinal cord and the name typically starts with the origin, i.e. the spinocerebellar tract originates in the spine and terminates in the cerebellum
Ascending pathways are sensory
Descending pathways are motor
Ascending/sensory pathways: Detect changes in the body or external environment and relay it to the CNS
Most sensory input is processed in the spinal cord or brainstem. About 1% makes it to the cerebrum and our conscious awareness.
First order neurons: sensory neurons that deliver sensation to CNS and synapse on . . .
Second order neurons: which carry the information to the thalamus and synapse on . . .
Third order neurons: which may carry information on to cerebral cortex
Most sensory information crosses over at a decussation point so that the right hemisphere processes information from the left side of the body
Posterior column pathway: fine touch, pressure and proprioception
Decussates in pyramids. Anterior and lateral spinothalamic decussates in the cord.
Location of any stimulus is known because the information projects to a specific location of the primary sensory cortex via projection fibers. Homunculus
Spinothalamic pathways carry poorly localized sensation of touch, pressure, temperature and pain. Decussates in cord and ascends in anterior and lateral spinothalamic tracts.
Spinocerebellar pathway carries sensation to cerebellum concerning position of muscles, tendons and joints.
Motor Pathways:
Autonomic: visceral motor system
Preganglionic neurons: nucleus in CNS
Ganglionic neurons: nucleus in peripheral ganglion
Somatic: skeletal muscle: Pathways always involve . . .
Upper motor neurons: nucleus lies in CNS processing area
Lower motor neurons: nucleus is in brain stem or spinal cord
Conscious and subconscious motor command travel over three motor pathways:
Corticospinal (pyramidal system)
Medial pathway
Lateral pathway
Corticospinal:
a) corticobulbar tracts synapse in brain stem to control many cranial nerves
b) corticospinal tracts: 85% decussate in the pyramids and descend in the lateral corticospinal tracts, 15% don’t decussate and descend in the anterior corticospinal tracts
Medial and lateral pathways: centers in the cerebrum, diencephalon and brainstem issue somatic motor commands as a result of subconscious processing which descend along medial and lateral pathways.
Medial pathway: gross movement of the trunk and proximal limbs
Lateral pathway: more precise movement of distal limbs
Basal Nuclei: caudate nucleus, putamen, globus pallidus and others. Motor control (modification) from the basal nuclei represent the extrapyramidal system.
Basal Nuclei and Cerebellum: responsible for coordination and feedback control over muscle contractions, consciously and subconsciously directed. Do not exert direct control over lower motor neurons but fine tune input from upper motor neurons.
Cerebellum: Monitors proprioception (position) sensations, visual information and balance sensation (vestibular) as movement is underway. Motor pathways bring information to the cerebellum which compares this information with experience from previous movements and fine tunes the motor response.
Levels of Somatic Motor Control:
Ascending information can be acted upon at many levels; in the cord, medulla, thalamus, cortex or all of these. For instance, a finger that has touched a hot surface is quickly removed via a command from the spinal cord long before the cortex receives and initiates a more complex response like cursing and sticking the finger in the mouth.
Anencephaly: I have never actually heard of a doctor routinely shining a light into an infant’s skull. (This rare condition is usually diagnosed prior to delivery). Nevertheless, many reflexes and movements are arranged infratentorially.
1. Are performed by the cerebral cortexes
2. Involve complex interconnections and communication between areas within the cerebral cortex and between the cerebral cortex and other areas of the brain
3. Involve both conscious and unconscious information processing
4. Are not part of the preprogrammed wiring of the brain and therefore the functions are subject to modification and adjustment over time (plasticity).
General Interpretive Area (GIA): Chief analytical area present in only one hemisphere, usually the left. Damage here results in difficulty in interpreting what is read or heard.
Speech center: In same hemisphere as GIA. Damage to this area may result in difficulty with speech or constantly talking with the wrong words.
Prefrontal cortex: Complicated learning and reasoning, estimates future based on past events. Generates tension and anxiety before anatomy exams. Disconnected with lobotomy.
Most areas of higher order function are present in both hemispheres, but are not equally distributed. The dominant hemisphere (categorical hemisphere) is typically opposite of handedness. The non-dominant (representational hemisphere) is involved with abstract thinking such as music appreciation.
The hemispheres communicate through over 200 million axons carrying 4 billion impulses per second.
Memory:
Short term memory lasts seconds to hours
Long term memory last for years
Memory consolidation is the conversion of short term memory to long term memory.
Reticular Activating System: Poorly defined network of neurons extending from medulla to the Mesencephalon. Primary importance is keeping one alert and attentive. Rarely functional in college students.
Aging and the CNS:
Reduction in brain size and weight
Reduction in number of neurons
Decrease in blood flow to brain
Changes in synaptic organization of the brain
Intracellular and extracellular changes; plaques, amyloid deposits, neurofibrillary tangles, etc.
