Synapses: Electrical and Chemical

• Somatic Motor Pathways involve lower motor neurons (LMNs) that extend from the CNS to skeletal muscles in the PNS.
• LMNs are peripheral nerves and are also termed the final common pathway as regulatory mechanisms summate before the signal is sent.
• LMNs extend as cranial nerves to skeletal muscles of the face and head and as spinal nerves to skeletal muscles of the limbs and trunk; output occurs from the CNS to skeletal muscles only via LMNs.

Control of Body Movement

• Motor portions of the cerebral cortex initiate and control precise movements.
• Basal ganglia help regulate muscle tone, integrate semi-voluntary automatic movements, and inhibit excess movement.
• The cerebellum coordinates smooth movement and helps maintain posture and balance.

Local Circuit Neurons (Interneurons)

• Local Circuit Neurons, also known as aka Interneurons, are located close to LMN cell bodies in the brain stem and spinal cord.
• They receive input from somatic sensory receptors (nociceptors and muscle spindles) and higher centers in the brain.
• These neurons coordinate rhythmic activity in specific muscle groups, such as alternating flexion and extension of the lower limb during walking.

Local Circuit Neurons & Lower Motor Neurons

• They receive input from upper motor neurons (UMNs).
• UMNs from the cerebral cortex are essential for planning, initiating, and directing sequences of voluntary movements.
• Other UMNs originate in motor centers of the brain stem and regulate:
  • Muscle tone.
  • Control of postural muscles.
  • Balance.
  • Orientation of the head and body, influenced by the basal ganglia and cerebellum.

Neurons of the Basal Ganglia

• They provide input to UMNs.
• They assist movement by initiating and terminating movements, suppressing unwanted movements, and establishing a normal level of muscle tone.

Cerebellar Neurons

• They also control the activity of UMNs.
• Cerebellar neurons interconnect the cerebellum with motor areas of the cerebral cortex (via the thalamus) and the brainstem.
• These neurons monitor differences between intended movements and actual movements performed.
• They issue commands to UMNs to reduce errors in movement, coordinate body movements, and maintain normal posture and balance.

Axons of UMN

• The UMN axons extend from the brain to LMNs via two types of descending somatic motor pathways:
  • Direct Pathways: cerebral cortex to the spinal cord and then out to muscles via LMN.
  • Indirect Pathways: includes synapses in motor centers in the brain stem from the basal ganglia, thalamus, reticular formation, and cerebellum, then to the LMNs.

Paralysis

• Damage to LMNs causes flaccid paralysis, whereas damage to UMNs usually causes spastic paralysis.

Direct Motor Pathways

• Axons from UMN extend from the brain to LMNs via two descending somatic direct motor pathways that provide input to LMNs via axons extending directly from the cerebral cortex.
  • Lateral and Anterior Corticospinal Tracts.
  • Corticobulbar Tracts.

Pyramidal Pathways

• There are approximately 1 million upper motor neurons in the cerebral cortex.
• Nerve impulses for voluntary movements propagate from UMN in the primary motor area and premotor area of the cerebral cortex to LMNs.
• Axons descend from the cerebral cortex to the medulla oblongata, where axon bundles form the ventral bulges known as pyramids.
• 90% of UMN fibers decussate (cross over) in the medulla, and the right side of the brain controls the left side muscles.
• 10% remain on the same side and eventually decussate at the spinal cord, synapsing with interneurons or LMNs in either the nuclei of cranial nerves or the anterior horns of the spinal cord.

Lateral Corticospinal Tract

• 90% of the fibers decussate in the medulla and make up this tract.
• This tract controls muscles in limbs for precise, agile, and highly skilled movement (hands and feet).
  • Used for tasks like buttoning a shirt or playing the piano.
• It supports finer movement but also requires more nervous control to maintain dexterity.

Anterior Corticospinal Tract

• These are the 10% of axons that do not cross in the medulla.
• They decussate in the spinal cord then synapse with interneurons or LMN in the anterior gray horn.
• Axons of LMNs exit the cervical and upper thoracic segments of the spinal cord in the anterior root of spinal nerves.
• Controls movement of the neck, trunk, and girdle muscles, and takes up little muscle fibers.

Corticobulbar Tracts

• The tracts control skeletal muscles of the head.
• They form in the left and right cerebral peduncles, descending from the cerebral cortex to the brain stem.
• Some fibers decussate; some do not.
• UMNs synapse with LMNs of the CNS, exiting the brainstem and go to nuclei of the CNS including III, IV, V, VI, VII, IX, X, XI, and XII.
• They control precise and voluntary movements of the eyes, tongue, chewing, facial expressions, and speech.

Primary Motor Cortex

• Located in the precentral gyrus of the frontal lobe.
• Upper motor neurons plan and initiate voluntary movements.
• Different muscles are represented unequally in the primary motor units.
• More cortical area is needed if the number of motor units in a muscle is high.
  • Examples include vocal cords, tongue, lips, fingers, and thumb.

Paralysis (LMN vs UMN)

• LMN Damage results in Flaccid paralysis where there are no voluntary movements on the same side as the damage, no reflex actions, muscle limp & flaccid, and decreased muscle tone.
  • UMN damage is variable but often results in spastic paralysis on the opposite side of the injury, increased muscle tone, and exaggerated reflexes.

Indirect Motor Pathways

• This includes all somatic motor tracts other than the corticospinal and corticobulbar tracts.
• They involve the motor cortex, basal ganglia, thalamus, cerebellum, reticular formation, and nuclei in the brain stem.
• Indirect tracts include the:
  • Rubrospinal (red nucleus).
  • Tectospinal.
  • Vestibulospinal.
  • Lateral Reticulospinal.
  • Medial Reticulospinal.
• Complex polysynaptic circuits include basal ganglia, thalamus, cerebellum, and reticular formation.
• All 5 tracts descend in the spinal cord as major tracts and terminate upon interneurons or LMNs.

Rubrospinal/Red Nucleus

• This conveys nerve impulses from the red nucleus to contralateral skeletal muscles for gross and precise movements of the upper limbs.

Tectospinal

• This conveys nerve impulses from the superior colliculus to contralateral skeletal muscles that move the head and eyes in response to visual stimuli.

Vestibulospinal

• This conveys nerve impulses from the vestibular nucleus in the pons and medulla to regulate ipsilateral muscle tone for maintaining balance in response to head movements.

Lateral Reticulospinal

• This conveys nerve impulses from the reticular formation to facilitate flexor reflexes, inhibit extensor reflexes, and decrease muscle tone in the muscle of the axial skeleton and proximal parts of the limb.

Medial Reticulospinal

• This conveys nerve impulses from the reticular formation to facilitate extensor reflexes, inhibit flexor reflexes, and increases muscle tone of the axial skeleton and proximal parts of the limbs.

Final Common Pathway

• LMNs receive signals from both direct and indirect UMNs.
• The sum total of all inhibitory and excitatory signals determines the final response of the LMNs and the skeletal muscles.

Integrative Functions of the Cerebrum

• The integrative functions include sleep, wakefulness, and memory.

Sleep & Wakefulness

• This is controlled by the Reticular Activating System (RAS): Sleep and wakefulness are integrated functions that are controlled by the reticular activating system.
• Arousal, or awakening from sleep, involves increased activity of the RAS: when the RAS is activated, the cerebral cortex is also activated, and arousal occurs, which results in a state of wakefulness called consciousness.
• The RAS has connections to the cortex and spinal cord and can be activated by pain, light, noise, muscle activity, and touch.
• Coma is a sleep-like state defined in a number of different ways, and the most common objective indicator used is the Glasgow Coma Scale (GCS).
• Circadian Rhythm is any biological process that cycles on a daily schedule and is established by the hypothalamus. Some research has suggested it cycles for a longer period than 24 hours.
• EEG recording shows large amount of activity in cerebral cortex when awake.
• EEG is our best tool for defining sleep/wake cycles
• Sleep is a state of altered consciousness or partial unconsciousness from which an individual can be aroused by different stimuli
  • During sleep activity in the RAS is very low
• NREM consists of 4 stages, each defined by the activity on an EEG
  • A regular night consists of phasing from one stage to the next in a typical sleep cycle
    • Stage 1 - Person drifting off with eyes closes (first few minutes)
      • Hypnic jerk occurs here
    • Stage 2 - Fragments of dreams
      • Eyes may roll side to side
    • Stage 3 - Very relaxed, moderately deep
      • 20 minute after stage 1, body temp & BP drops
    • Stage 4 - Parasomnia occurs here
      • Bed-wetting, sleep walking, night terrors, etc
• Sleep cycles occur over 90 minutes
  • There are 2 types of sleep
    • Non-rapid eye movement sleep (NREM) with 4 stages, each defined by the activity on an EEG
    • Rapid eye movement sleep (REM)
      • Most dreams occur during REM sleep (lucid dreams)
      • Cycle go from stage 1 to 4 of NREM and up to stage 2 of NREM to REM.
      • The 1st REM of the nights is very short
      • Every time REM sleep is entered, it lasts longer
        • Cycles repeat. until REM sleep totals 90 – 120 minutes
      • Neuronal activity & oxygen use is highest in REM sleep
      • Total sleeping & dreaming time decreases with age
• Sleep problems:
  • Parasomnia
  • Delayed sleep phase disorder
  • REM sleep behaviour disorder
  • Narcolepsy

Learning & Memory

• Learning is the ability to acquire new knowledge or skills through instruction or experience, and memory is the process by which that knowledge is retained over time.
• For an experience to become part of memory, it must produce persistent functional changes that represent the experiences in the brain.
• The capability for change (with learning) is called "plasticity." Example: LTP & LTD
• Memory occurs in stages over a period and is described as immediate memory, short term memory, or long term memory.
  • Immediate memory- the ability to recall for a few seconds
  • Short term memory- lasts only seconds or hours and is the ability to recall bits of information; it is related to electrical and chemical events
  • Long term memory lasting from days to years and related to anatomical and biochemical changes at synapses
• Amnesia refers to the loss of memory.
  • Anterograde amnesia is the loss of memory for events that occur after the trauma ( the inability to form new memories).
  • Retrograde amnesia is the loss of memory for events that occurred before the trauma; the inability to recall past events.