Central Nervous System 2

The Association Areas

Motor, sensory, and language areas account for only about half of the total cerebral cortex

Remaining areas are called association areas:

• Prefrontal association cortex

• Parietal–temporal–occipital association cortex

• Limbic association cortex

1. Sensory input →

2. Primary sensory areas (somatosensory, 1^o visual, 1^o auditory cortices) →

3. Higher sensory areas →

4. Association areas →

5. Higher motor areas →

6. Primary motor areas →

7. Motor output

Sensory Input

Relayed from afferent neuronal receptors

Primary Sensory Areas (Somatosensory, 1^o Visual, 1^o Auditory Cortices)

Initial cortical processing of specific sensory input

Higher Sensory Areas

Further elaboration and processing of specific sensory input

Association Areas

Integration, storage, and use of diverse sensory input for planning of purposeful action

Higher Motor Areas

Programming of sequences of movement in context of diverse information provided

Primary Motor Cortex

Commanding of efferent motor neurons to initiate voluntary movement

Motor Output

Relayed through efferent motor neurons to appropriate skeletal muscles, which carry out desired action

Basal Ganglia

Consist of several nuclei located deep within the cerebral white matter

Primary functions:

• Associated with a variety of functions, including motor control, cognition, emotions, and learning

• inhibiting muscle tone throughout the body – balance of excitatory and inhibitory inputs to neurons that innervate skeletal muscles

• selecting, maintaining purposeful motor activity while suppressing unwanted patterns of movement

• coordinates slow, sustained contractions, especially those related to posture and support

Parkinson’s Disease (PD) - Diseases of the Basal Ganglia

Caused by the degeneration of dopaminergic neurons in Substantia Nigra in the midbrain which synapse onto neurons in the basal ganglia:

• This circuit is important for smooth movement.

• The loss of dopamine results in the characteristic features of PD:

1. Increased muscle tone, rigidity

2. Involuntary, useless movements at rest, tremors

3. Difficulty in initiating and carrying out movement

Huntington’s Disease (HD) - Diseases of the Basal Ganglia

• Caused by degeneration of the caudate nucleus

• This loss causes some motor circuits to become overactive resulting in the characteristic features of HD:

1. Chorea - rapid, uncontrolled jerky movements

2. Memory problems

Thalamus

Deep in the brain near the basal ganglia:

• Serves as “relay station” and synaptic integrating center for sensory input

• Helps direct attention to stimuli of interest

• Capable of crude awareness of sensations but cannot distinguish their location or intensity

Hypothalamus

• The integrating centre for homeostatic function

• Brain area most involved in directly regulating internal environment

  • Controls body temperature and food intake

  • Controls thirst and urine output

  • Controls anterior pituitary hormone secretion

  • Produces posterior pituitary hormones

Five functions of the brain stem:

1. Majority of cranial nerves arise from areas in brain stem

2. Contains centers that control cardiovascular, respiratory, and digestive function

3. Regulates postural muscle reflexes

4. RAS (reticular activating system) controls the overall degree of cortical alertness

5. Plays a role in the sleep–wake cycle

Cranial Nerves

• Optic (vision)

• Olfactory (smell)

• Oculomotor & trochlear (visual reflexes)

• Facial (taste, face muscles)

• Vestibulocochlear (hearing and equilibrium)

• Glossopharyngeal (taste)

Cerebellum

• Important in integration of motor output and sensory perception

• Important in balance and planning of movement

Vestibulocerebellum

balance and eye movement

Spinocerebellum

enhances muscle tone and coordinates skilled movements

Cerebrocerebellum

plans and initiates voluntary activity and stores procedural memories

Reticular Activating System (RAS)

• Behavioural state system - diffuse modulatory system

• Neurons originate in the RAS and project to various areas in the brain

• Influences attention, motivation, wakefulness, memory, motor control, mood and metabolic homeostasis

• Controls levels of consciousness and sleep-wake cycles (keeps the “conscious brain” awake)

• Controls the overall degree of cortical alertness

• General anaesthetics depress synaptic transmission in the reticular formation

• Blocking ascending pathways between the reticular formation and the cerebral cortex creates a state of unconsciousness.

The Limbic System

• Surrounds the brain stem and is not a separate structure

• It is an interconnected ring of forebrain structures:

  • Includes portions of cerebral lobes, the basal nuclei, the thalamus, and the hypothalamus

  • Utilizes the neurotransmitters norepinephrine, dopamine, and serotonin

  • Depression is associated with defects in limbic system neurotransmitters

• Complex interacting network is associated with emotions, basic survival, sociosexual behavior, motivation and learning

Limbic system functions:

• Emotion: subjective feelings and moods and the physical responses associated with these feelings (amygdala)

• Basic behavioural patterns: aimed at survival and perpetuation of the species

• Motivation: directing behaviour toward goals

• Learning: acquiring knowledge or skills as a result of experience and/or instruction

• Memory: storage of acquired knowledge for later use

Medial Prefrontal Cortex

Executive function: decision making, control of emotion & impulses

Cingulate Cortex

Motivation, drive, mood: decreased activity correlates with depression

Medial Temporal Lobes

Episodic memory formation of recent event sequences

Subcortical Structures of Limbic System

Hippocampus (within medial temporal lobe)

• Episodic memory formation, context & location

Hypothalamus

• Homeostasis & basic drives: food, water, sex, aggression

Amygdala

• Involved in fear & affective learning.

• Activation causes anxiety, (ablation eliminates fear recognition & learning)

Learning

Is the acquisition of knowledge or skills as a consequence of experience, instruction, or both

• It is widely believed that rewards and punishments are integral parts of many types of learning

Memory

The storage of acquired knowledge for later recall:

• Short-term: seconds to hours

• Long-term: days to years

• Working memory: temporarily holds and interrelates various pieces of information relevant to a current mental task

Memory Trace

A neural change responsible for retention or storage of knowledge. These traces are present across multiple regions of the brain (sites of original auditory, visual, etc. experience)

Declarative memory (“what” memories of people, places etc):

Important for remembering facts and events – involves Hippocampus

Procedural memory (“how to” memories):

The learning of new motor skills – involves Cerebellum

Consolidation of short-term declarative memory into long term memory:

The function of the medial temporal lobe (hippocampus and adjacent areas of temporal lobe), occurs largely during sleep

• Requires gene activation leading to protein synthesis and synaptic changes

Short-term Memory

Involves transient changes in synaptic activity.

• Habituation: decreased responsiveness to a repetitive and indifferent stimulus

• Sensitization: increased responsiveness to mild stimuli following a noxious stimuli

  • Both involve changes in ion channels and currents

Long-term Memory

Involves formation of new, permanent synaptic connections.

• Activation of specific genes that control synthesis of proteins needed for lasting structural or functional changes in pre- or postsynaptic membranes

• Long term potentiation (LTP) is important for initial storage into long term memory

Working Memory

• Concept that working memory temporarily holds and interrelates various pieces of information that are relevant to a current mental task (7 +/- 2 objects)

• Hold and process data for immediate use - can include newly acquired and previously stored knowledge that is transiently accessed

• Critical for ability to reason, plan and make judgments

• By comparing and manipulating new and old information within your working memory you can, for example, find your way home, carry on a conversation etc

• Takes place within prefrontal cortex

Alzheimer’s Disease

• Characterized by short-term memory loss in early stages

• Followed eventually by loss of long-term memory - eg, recognition of family members

• Confusion, disorientation, personality changes

• Loss of ability to read, write, calculate

• Language ability and speech also impaired

• Brain alterations - neurofibrillary tangles, amyloid plaques

• Characteristic loss of cholinergic neurons in the basal forebrain - cells that project to hippocampus

• Neuronal loss in hippocampus & temporal lobes → cortex (cholinergic neurons – neurons that use acetylcholine as their neurotransmitter.)

Language

Localized in one hemisphere – left hemisphere (95% of right-handers, 60-70% of left-handers)

• Broca’s and Wernicke’s area

Broca’s Area

Speaking ability:

• Damage results in an inability to send the proper commands to the motor cortex to form the words.

Wernicke’s Area

Language comprehension:

• Damage results in an inability to attach meaning to words or choose the appropriate words

Language Disorders

• Aphasia – is a defect in language processing caused by dysfunction of the dominant cerebral hemisphere (i.e. stroke)(Broca’s aphasia, Wernicke’s aphasia)

• Speech impediments – defects in the mechanical aspect of speech

• Dyslexia – difficulty in learning to read because of inappropriate interpretation of words. Due to developmental abnormalities in connections between the visual and language areas of the cortex. (independent of intelligence)

Cortical Pathway for Speaking a Written Word or Naming a Visual Object

1. 
   

• To speak about something seen, the brain transfers the visual information from the primary visual cortex to the angular gyrus of the parietal-temporal-occipital association cortex, which integrates inputs such as sight, sound, and touch

• To speak about something heard, the brain transfers the auditory information from the primary auditory cortex to the angular gyrus

2. The information is transferred to Wernicke’s area, where the choice and sequence of words to be spoken are formulated

3. This language command is then transmitted to Broca’s area, which translates the message into a programmed sound pattern

4. This sound program is conveyed to the precise areas of the primary motor cortex that activate the appropriate facial and tongue muscles for causing the desired words to be spoken

Electrical activity patterns define arousal states:

• Electrical activity in neurons can be measured by EEGs (Electroencephalograms)

• Surface electrodes placed on the scalp can detect depolarizations of the cortical neurons in the region under the electrode

• Represents Excitatory Post Synaptic Potentials (EPSPs) and Inhibitory Post Synaptic Potentials (IPSPs) in cell bodies of the cortical layers

• Patterns of activity can be detected

• In awake states - neurons are firing but not always in a coordinated fashion

• This desynchronization may be a result of ascending signals coming from the Reticular Activating System (RAS)

What is the value of the EEG?

• Often used as a clinical tool in diagnosis of cerebral dysfunction

  • Epilepsy – shows distinctively abnormal traces

  • Seizures occur when collections of neurons undergo synchronous action potentials that produce stereotypical, involuntary spasms and alterations in behaviour

  • Neuronal excitability coupled with compromised inhibitory activity or prolonged activation of excitatory transmitters

• Legal determination of death

  • Electrocerebral silence – flat EEG

  • Needs to be coupled with other stringent criteria

• Also used to distinguish various stages of sleep

Alpha & Beta Waves

• Waking brain activity

• The alpha rhythm in the EEG is 8-13 Hz: index of cortical inactivity

  • Present in adult who is awake but relaxed with eyes closed

  • Amplitude is negatively correlated with cortical activity

• Beta rhythm in the EEG is 13-30 Hz: decreased synchronization with cortical activity

  • In individuals who are alert and attentive to external stimuli or exert specific mental effort

Maximum Alertness

depends on sensory input that stimulates the RAS and subsequently the activity levels of the CNS as a whole

Sleep-wake Cycle

• normal cyclic variation in awareness

• brain’s overall activity is not reduced during sleep (sleep is an active process)

Two types of sleep characterized by different EEG patterns and different behaviours:

• Slow-wave sleep – delta-wave; non-rapid eye movement, NREM

• Paradoxical sleep – rapid eye movement, REM

Stages of Sleep

• Slow wave sleep occurs in 4 stages – each displaying a lower frequency, but higher amplitude EEG wave

• At onset of sleep, you move through the stages, from light sleep (1) to deep sleep (4) in a 35-45 min period

• Then it reverses through the same stages in same amount of time

• A short period of REM sleep occurs at the end of each slow-wave sleep cycle

• Paradoxical sleep – because the EEG wave form resembles that of an awake alert individual

• 90 minutes/cycle

• Time in REM stage increases whereas time in stages 3 and 4 decreases over the cycles

Slow-Wave Sleep

• Displays slow waves

• Considerable muscle tone; frequent shifting

• Minor reductions in heart rate, respiratory rate, and blood pressure

• Dreaming is rare

• Sleeper is easily awakened

• 80% of sleeping time

• Has 4 stages

Paradoxical Sleep

• Similar to the EEG of an alert, awake person

• Abrupt inhibition of muscle tone; no movement

• Heart rate, respiratory rate, and blood pressure are irregular

• Dreaming is common

• Sleeper is hard to arouse but apt to wake up spontaneously

• Rapid eye movements occur

The best-supported evidence suggests that main functions of sleep include:

1. Conservation of energy, defense from predation

2. Restorative function for the brain and body: healing, growth, immune function

3. An important role in the cerebral changes that underlie consolidation of long-term memory in cortex