CENTRAL NERVOUS SYSTEM

Summaries:

1. Control of Posture and Movement Animal preparations used to study the regulation of posture include:
2. Spinal preparation: spinal cord transection at midthoracic level
3. Decerebrate preparation: transection of the neuraxis at the superior border of pons
4. Midbrain preparation: transection of the neuraxis on top of the midbrain
5. Decortication: removal of cerebral cortex alone leaving subcortical structures intact

With regard to control of posture and movement, upper motor neurons may be classified as:

• Posture regulating upper motor neurons
• Movement regulating upper motor neurons

Posture regulating upper motor neurons:

Origin:	Brain stem
Termination:	On alpha and gamma motor neurons in spinal cord
Controlled by:	Inhibitory inputs from cerebral cortex (suppressor strip – 4s) in the anterior edge of the precentral gyrus and basal ganglia.

Origin: Corticospinal tract (layer V, pyramidal cell axons) from primary motor cortex, sensory cortex, and premotor area. Termination: On alpha-motor neurons / interneurons innervating alpha motor neurons in spinal cord Lesions: limited to the corticospinal and corticobulbar tracts produce “weakness” rather than paralysis and the affected musculature is generally “hypotonic”. Isolated involvement of

Movement mediating / regulating upper motor neurons:

Level of integration of principal postural reflexes:

Level	Postural reflexes
Spinal cord	Stretch reflexes, withdrawal reflexes
Medulla	Labyrinthine righting reflexes, tonic neck reflexes
Midbrain	Righting reflexes
Cerebral cortex	Hopping and placing reactions, optokinetic reflexes

Descending motor pathways are classified as lateral system and medial system pathways, and other pathways.

Lateral system pathways

• Lateral corticospinal tract
• Rubrospinal tract

Medial system pathways

• Ventral corticospinal tract
• Reticulospinal tract
• Vestibulospinal tract
• Tectospinal tract

Lateral system pathways are movement regulating pathways. If we could manage to cut only lateral corticospinal tract axons (for example, by sectioning pyramids in monkeys), posture would not be affected but the animal would lose its ability to perform fine skilled movements. The affected muscles would become hypotonic.

Medial system pathways are posture-regulating pathways.

Other descending monoaminergic pathways

• Raphespinal pathways
• Ceruleospinal pathway

One way of classifying descending motor pathways is to classify them as lateral system pathways and medial system pathways.

The lateral system pathways include the lateral

corticospinal tract and the rubrospinal tract (which originates in the red nucleus). The neurons of the lateral corticospinal tract typically (though not necessarily) end straight on alpha-motor neurons innervating skeletal muscles of the distal musculature used for fine, skilled voluntary movements; otherwise, they end on interneurons that regulate the alpha-motor neurons innervating these muscles. The lateral system pathways also course laterally (i.e., in the lateral funiculus) in the spinal cord.

The medial system pathways course medially in the spinal cord and end (at various levels) typically on interneurons in the medial aspect of the ventral horn of the spinal cord. Furthermore, these interneurons, on which the medial system pathways end, connect with their counterparts on the contralateral aspect of the spinal cord contributing to balance and posture. They mainly control axial muscles. The ventral corticospinal tract, the tectospinal tract, the reticulospinal tract and the vestibulospinal tracts are all medial system pathways.

Functional divisions of the cerebellum

Division	Function
Spinocerebellum (midline & paravermal zone)	Maintenance of posture
Vestibulocerebellum (flocculonodular lobe)	Maintenance of posture and equilibrium
Neocerebellum (lateral cerebellar hemispheres)	Coordination of voluntary movement

• Output from the Purkinje cells to the deep cerebellar nuclei is always inhibitory.
• Output from the deep cerebellar nuclei to the thalamus is excitatory.
• Cerebellum facilitates stretch reflexes and usually hypotonia occurs in cerebellar lesions.
• Selective ablation of the flocculonodular lobe of the cerebellum has been shown to abolish the symptoms of motion sickness. The symptoms of motion sickness occur due to excessive stimulation of the vestibular system via its connections to the brain stem and the flocculonodular lobe of the spinal cord

• Thalamocortical Projections, EEG, Sleep

Types of thalamocortical projections: Specific: function to transmit information to specific areas in the cortex; results in sensory perception; they end in layer IV of the cortex Nonspecific: function to arouse the individual; end in layers I-IV of the cortex.

Notes on EEG: In an adult human at rest with mind wandering and eyes closed, the most prominent component of the EEG is a fairly regular pattern of waves at a frequency of 8-12 Hz, and amplitude of about 50 microvolts when recorded from the scalp. This pattern is the alpha rhythm. It is most prominent in the parieto- occipital area.

The alpha rhythm is replaced by a fast irregular low voltage beta rhythm (18–30 Hz). This phenomenon is called alpha block. A breakup of the alpha rhythm is produced by any kind of sensory stimulation such as mental arithmetic.

Large amplitude, regular 4–7 Hz waves called the theta rhythm occurs in children and is generated in the hippocampus in experimental animals.

Stages of sleep: Stages I – IV of NREM (slow wave sleep) followed by rapid eye movement (REM) sleep

Sleep spindles (occur in stage II sleep) Frequency: 10–14 Hz; amplitude: 50 microvolts.

EEG waves	Frequency
Delta	0.5–4 Hz
Theta	4–7 Hz
Alpha	8–13 Hz
Beta	14–30 Hz
Gamma	30–80 Hz

Delta waves occur in deep sleep (stages III and IV).

REM sleep is called paradoxical sleep because it is marked by rapid, low voltage, irregular EEG activity. The threshold for sensory arousal from REM sleep is sometimes greater than NREM sleep presumably because the brain is actively

processing information during REM sleep (dreams).

EEG during sleep:

Stage	Characteristics
Stage 1	Alpha rhythm
Stage 2	Sleep spindles and K complexes (beginning of slowing; reduction in alpha content)
Stages 3 & 4	Characterized by slow waves (theta and delta)
REM sleep	High frequency, low amplitude rhythm similar to the wakeful state

Hypothalamus and food intake:

Satiety center works by inhibiting feeding center

Feeding center located in	Lateral hypothalamus
Satiety center located in	Ventromedial hypothalamus
Lesions in ventromedial nucleus lead to	Obesity
Lesions in lateral hypothalamus lead to	Anorexia

• Learning and Memory

Types of learning:

Associative learning	Nonassociative learning
Associating a conditioned stimulus with a neutral stimulus	Learning about a single stimulus
Examples include classical (Pavlovian) conditioning, and operant / instrumental conditioning	Examples include habituation and sensitization

Classical conditioning

(Pavlovian conditioning)

• Conditioned stimulus (food)
• Conditioned response (salivation)
• Conditioned reflex
• Neutral stimulus (bell ringing)

• Pairing the neutral stimulus with the conditioned stimulus several times…
• The neutral stimulus (bell ringing) eventually elicits the same response as the conditioned stimulus (food).
• Conditioned reflexes (or behavior) are classic examples of associative learning.

Instrumental conditioning

(Synonymous with operant conditioning)

• This phenomenon was described by B.F.Skinner.
• Here the animal learns by operating on the environment.
• The main learning point here is that animal behavior is determined by the likely consequences of that behavior. From experiments conducted with the Skinner box arose concepts such as the presence in the limbic system of “reward centers” and “punishment centers”
• “We may apply this concept to reform society” – Skinner

Habituation: The response to a benign stimulus repeated over and over gradually decreases.

This is due to decreased release of excitatory neurotransmitter from the presynaptic neuron. The opposite response where the response to a noxious stimulus repeated over and over gradually increases is termed sensitization. Both habituation and sensitization are examples of nonassociative learning because the organism learns about a single stimulus.

Memory is retention and storage of learnt information.

• Implicit memory is reflexive (nondeclarative). It is nondeclarative in that one is not aware of its execution. Examples include skills one has perfected.
• Explicit memory, which is declarative, requires conscious recall of events or facts. It requires processing in the hippocampus.
• Working memory is a form of short term memory used to plan actions; example, dialing a phone number one has just seen on the phone book. It is stored in prefrontal cortex.

• Short-term memory is prone to erasure; long- term memory (‘true memory’) is resistant to erasure.
• Consolidation refers to the formation of new long term memories from short-term memories, and it occurs in the hippocampus. The hippocampus is rich in NMDA receptors. NMDA receptors are a type of glutamate receptors.

Mechanisms implicated in memory:

• Post-tetanic potentiation
• Long-term potentiation
• Changes in synaptic strength
• Increases in synaptic contacts
• Formation of new neurons from stem cells in the olfactory bulb and hippocampus may play a role.

Post-tetanic potentiation: Enhanced postsynaptic potentials after a brief tetanizing train of stimuli in the presynaptic neuron.

Long-term potentiation: Long-lasting facilitation of transmission in neural pathways following a brief period of high-frequency stimulation. This process is important for consolidation, i.e. the formation of long term memories.

Conditions which influence consolidation (formation of new memories from short-term memory)

• Repetition of stimulus (facilitates)
• Sleep (facilitates)
• Convulsions, electroconvulsive therapy (inhibit)
• Anesthetics, tranquilizers (inhibit)
• Hypoxia (inhibits)
• Inhibitors of protein synthesis (inhibit)