Case 20- Physiology Flashcards
Consciousness
Awareness of self (internal) and surrounding (external) with the ability to react to these
Reduced or altered consciousness
Partial or complete loss of awareness of self and surroundings and/or your ability to respond to it
Loss of consciousness
Suggests a global dysfunction of the brain (other than sleep)
What does consciousness require
- Wakefulness- a state in which the eyes are open and there is a degree of motor arousal/response i.e. spontaneous movement, reflexes such as coughing
- Awareness- the ability to have an experience of any kind. Knowledge and understanding of things happening. Complex though processes. Hard to test
Disorders of consciousness
- Coma- absent wakefulness and awareness. Unrousable responsiveness lasts >6 hours.
- Vegetative state- wakefulness but absent awareness
- Minimally conscious state- wakefulness with minimal awareness
Criteria for a coma
- Cannot be awakened
- Lacks a normal sleep-wake cycle
- Fails to respond normally to painful stimuli, light or sound
- Does not initiate voluntary actions
Criteria for vegetative state
- The patient has no awareness- no recognition of self or environment
- But shows signs of being awake- Spontaneous or stimulus induced response, sleep wake cycle, reflex behaviour
Criteria for minimally conscious state
- Minimal or inconsistent awareness
- May enter this state after being in a coma or vegetative state
- Behavioural evidence of self or environmental awareness is demonstrated
- May have periods where they communicate or follow small commands i.e. moving hand
Other conditions that mimic a coma
- Locked in syndrome- fully conscious aware and alert but paralysed. Can see, hear, feel pain. Often eyes are unaffected so can communicate using movement of eyes and eyelids. Caused by occlusion of the basilar artery
- Brainstem death- loss of brainstem function and reflexes. Spontaneous respiratory effort in response to rising carbon dioxide levels
Reticular formation
A complex matrix of neurons that extend throughout the brainstem. They have widespread projections into the cerebral hemispheres and spinal cord. Some aspects have well defined nuclei. Some portions are more dispersed but can be grouped together by function i.e. the respiratory and cardiac centres.
The different projections of the reticular formation
- Ascending projections for arousal and attention
- Intrinsic connections for control of eye movement, swallowing and brainstem reflexes
- Descending projections for control of muscle tone, respiration and arterial pressure. Such as the rubrospinal nucleus or the red nucleus
Reticular formation- Afferent connection (inputs)
- Input to the reticular formation comes from all parts of the CNS
- For example, Cerebral cortex, Spinal cord, Anterolateral system, Cerebellum, Cranial nerves (V, VII, VIII, IX, X), Forebrain centres motor and visceral)
Reticular formation- Efferent (outputs)
- The RF has connections to visually all other nuclei in the brainstem
- Descending fibres project to the spinal cord
- Ascending fibres influence widespread areas of the cerebral cortex
Basic functions of the reticular formation
- Sleep and consciousness- reticular activating system
- Somatic motor control- balance posture
- Cardiovascular control- respiratory centres
- Pain modulation
The 4 groups of nuclei which form the major part of the ascending activating system
- Locus Coeruleus
- Raphe nucleus
- Ventral tegmental area
- Basal forebrain (Nucleus of Meynert)
Noradrenaline
1) The locus coeruleus nuclei are found in the lateral tegmentum of the pons and medulla.
2) It functions in attention, cortical arousal, sleep-wake cycle, learning and memory, mood.
3) Noradrenaline starts in the locus coeruleus and terminate in the cortex also goes towards the limbic system and Hypothalamus.
4) The locus coeruleus is part of the reticular activating system.
Serotonin
1) The raphe nucleus throughout the brainstem. It functions in sleep-wake, control of mood, certain types of emotional behaviour i.e. aggression, association with modulation of slow pain (descending pathways).
2) Low levels associated with depression. SSRI’s prevent reuptake. Goes towards the cerebellum and cortex, links to limbic system and hypothalamus.
Dopamine pathways- Mesolimbic and Mesocortical
Ventral Tegmental area of brainstem. Mesolimbic and mesocortical pathways linked with reward behaviour, attention and mood. Goes towards the limbic system and deep brain nuclei i.e. the nuclear accumbens which is involved with the reward pathway. Goes towards the frontal areas of the cortex which is associated with attention and the desire to do things. Also goes towards the Hippocampus, frontal lobe and Amygdala.
Acetylcholine
Pontomesencephalic (brainstem) and nuclei located at the basal forebrain. It functions in memory and learning, arousal. Degeneration neurones that use acetylcholine as their transmitter is thought to be linked to Alzheimer’s. AChE inhibitor inhibits reuptake. Goes towards the limbic system, cortex and hypothalamus. Involves the Forebrain basal nuclei and the Dorsolateral pons/tegenum goes outwards the cerebellum and down the spinal cord.
Centres and nuclei associated with sleep
Rem sleep is primarily regulated by centres in the pontine reticular formation. Involved are the Locus ceruleus, Dorsolateral pontine R.F, Raphe nucleus which feed into the Hypothalamus to regulate the sleep wake cycle. Connections from the pontine reticular formation to the spinal cord are essential to prevent the ‘acting out’ of dreams. They inhibit the lower motor neurones (paralysing you).
Descending motor effects of reticular formation- medial/lateral
- Medial (pontine) reticulospinal tract- enhances antigravity reflexes of the spinal cord, helps maintain a standing posture by resisting the effects of gravity
- Lateral (medullary) reticulospinal tract- liberates antigravity muscles from reflex control
- Go down the spinal cord, have alpha and gamma motor neurones
- Collectively the reticulospinal fibres modulate muscle tone, regulate posture and participate in automatic reflexes involving the extensor muscles
Structure of the reticular formation
1) The reticular formation is in the brainstem.
2) Neurones project to the cortex and spinal cord.
3) The descending motor pathways are involved in balance and equilibrium.
4) The Ascending pathways is the reticular activating system.
5) Functions to influence mood, learning and memory and arousal.
6) The ascending reticular activating system (ARAS) is a network of neurons originating in the tegmentum of the upper pons and midbrain, believed to be integral to inducing and maintaining alertness
