🎯 Objectives
The main objective of this lesson is to study:
- The Brain and the Peripheral systems 🧠🔌
- Brain: Forebrain 🔝, Mid brain 🔄, Hind Brain 🔙 functioning of each anatomical location in the CNS. Starting from the posterior located areas up to the anterior-most 📍
- Cerebral Cortex 🧠: its layers 📚, Corpus Callosum 🤝 and the two hemispheres 🌐
📚 Main Purpose
Students will become familiarized with the location of various brain areas 📍, their functions ⚙️ and their relationship to other areas 🔗. The students would also understand how control and modulation of behavior related to each neuroanatomical site takes place 🎮.
In lesson 27 we discussed some parts of the Mesencephalon 🔄, and now we continue with the remaining areas which fall under Mesencephalon.
🔄 Tegmentum
Tegmentum: This lies between the Tectum 🎯 and Substantia Nigra ⚫. It contains nuclei and relay fibers of ascending ⬆️ and descending ⬇️ tracts. It also contains the motor cranial nerves for eye movements (oculomotor 👁️) and the trochlear cranial nerves.
🔌 Three Major Fiber Bundles
- The ML (Medial Lemniscus) 🔵: Lies above the Substantia Nigra (SN) and conveys kinesthetic and discriminative touch sensory information to the Thalamus 🎯.
- The TL (Trigeminal Lemniscus) 🟢: Also are tracts which travel up to the Thalamus 🎯
- The STT (Spino-thalamic tract) 🔴: Conveys the pain 😣 and temperature 🌡️ sensations from the contralateral hemisphere (e.g., from the left arm to the right side of the brain) ⬅️➡️.
🧠 PAG: Peri-Aqueductal Gray
PAG (Peri-Aqueductal Gray): This is the grey matter which surrounds the cerebral aqueduct 💧: This has the neural circuitry for sequence of movements for species specific behaviors (fight 🥊, flight 🏃, and mating ❤️). Research has also shown this to be an important area for pain sensations 😣. If opiates 💊 are injected in this area, they reduce the sensitivity to pain (raise the tolerance threshold ⬆️).
🔌 Cerebral Peduncles
In the basal portion of tegmentum, we have the cerebral peduncles which are large fiber bundles which are placed in the ventral region of the mesencephalon 🔄. These carry fibers of the cortico-spinal tract 🦴, and the cortico-pontine tract 🌉. There are also large projections such as the parieto-occipito-tempero-pontine projections which carry projection from cerebral cortex 🧠 down through pontine area into the spinal cord 🦴.
⚫ Substantia Nigra
Substantia Nigra: dark pigmented mass of neurons ⚫, between cerebral peduncles and tegmentum, zona compacta rich in Dopamine 🧪 and brain opiate receptors 💊. This is part of the brain's motor control and modulation system 🎮, and is involved in movement 🏃 and balance ⚖️. If we create an imbalance in the bilaterally located SN areas by a unilateral lesion, we see asymmetric body posture 🚶❌. The body turns from the high to the low region (lopsided posture). If the Dopaminergic neurons degenerate in this area it leads to Parkinson's disease 🤕.
⚠️ Parkinson's Disease Characteristics
This is characterized by:
- Tremor 🫨
- Rigidity 💪🔒
- Slowness of motor activity 🐌
- Stiffness in muscles 💪⚠️
- Pin rolling movement 🔄
- Loss of adaptation (facial expression 😐 and gait 🚶)
Inputs come from the Neostriatum, cerebral cortex 🧠, Globus Pallidus 🌐, and other parts of the Tegmentum 🔄
🔴 Red Nucleus
Red Nucleus: neurons with a pinkish hue 🔴 are an important area in Tegmentum. Inputs come in from deep cerebellar nuclei 🏀 and the cerebral motor cortex 🧠. If the deep cerebellar input is damaged, tremor is manifested when the hand 🤚 or foot 🦶 is in motion (reaching out) 🫳. The gross movements of body are controlled by this system 🎮. If there is unilateral electrical stimulation, it leads to circulatory motion 🔄, and lesions result in disturbances of gait (walking 🚶).
🐟 Evolutionary Perspective
The Medulla 🦴, Pons 🌉, and the lower areas had evolved earlier on the evolutionary scale 🧬 and are similar from fish 🐟 to man 👨. The fish don't have a cerebral cortex 🧠, but have large inferior 👂 and superior 👁️ colliculi, as this is what fishes would need for determining direction while swimming in water 💧.
Bats 🦇 are nocturnal creatures which fly and hunt at night 🌙. For bats audition 👂 becomes more important for survival more than vision 👁️, therefore the bats have bigger inferior than superior colliculi 🎯.
The brain stem areas serve as a connection between upper and lower areas 🔗, that is they connect the Telencephalon 🔝 and Diencephalon 🎯 to the Spinal Cord 🦴
🧠 MID BRAIN AREAS: Corpus Striatum
Corpus Striatum: Striated bodies (corpus) This group of structures includes the Basal Ganglia 💎 the Caudate (Tail Like) Nucleus 🦎, Globus Pallidus (Pale Globe) 🌐, Putamen 🧠. These areas lie underneath the cerebral cortex 🔽.
The Corpus Striatum receives input from and sends output to the cerebral cortices 🧠, especially the frontal areas where the motor lobe is located 🏃. They also send fibers to the Thalamus (output) 🎯. There are numerous interconnections between different areas of the corpus striatum 🔗. These areas appear to be involved in the modulation of motor movement 🎮, especially the movement/modulation of contralateral aspect of the body ⬅️➡️. This also has the controls of initiation of movement 🚀, contralateral head turning 🔄, circling 🔁, licking 👅, chewing 😋, gnawing 🦷 etc. This is also important part of the Dopaminergic pathway involved in Parkinson's disease 🤕. Bilateral lesions of pallidus lead to hypoactive and sleepy animal 😴. Thus, this appears to be the basis of decreased movement and hypokinesia of Parkinson's disease. On the other hand, the Caudate promotes arousal of motor system ⚡. So, they balance the controls of motor movement ⚖️.
🎮 Hierarchy of Movement
If we look at the hierarchy of movement starting from the highest control 🔝, the cerebral cortex 🧠, we see that the Motor cortex sends commands to the striatum 📤, from there the commands go through the nigral system ⚫ and then the action takes place 🏃. As motor skills are learned the basal ganglia takes over role of executing learned strategy 📚. So, the commands go from the striatum to the nigral for preparation of action to take place. This becomes more automated and programmed 🤖.
Think about how a child 👶 learns to walk 🚶, slow deliberate balancing takes place initially ⚖️ and then gradually locomotion becomes a routine 🔄. However, when basal ganglia are damaged the individual reverts to slower less automatic cortical response 🐌. Basically, the preparation of execution of movement is the main function ⚡: We could call it a Gating function 🚪: This means gating of sensory-motor processing (controlling the gates of channels which provide information 📊). Thus, it limits the access of information to motor systems 🔒.
💃 Huntington's Chorea
Huntington's chorea is due to degeneration of Basal ganglia ⚠️. This degeneration results in reduction of inhibitory output of BG 🔽 which leads to increased access of sensory information 📊⬆️, which leads to increased activity (hyperkinesis ⚡, sudden jerks 🫨, tics 😖, and jerky movements of head 🔄, trunk and extremities, facial grimaces 😬, repetitive dancing movement 💃).
🤕 Parkinson's Disease Mechanism
On the other hand, in Parkinson's disease, there is decreased Dopaminergic input 🧪⬇️ which inhibits action in Basal Ganglia 💎. As you have seen earlier, if the basal ganglia are not working the cortex would take over 🧠. Therefore, the reduced Dopamine in Basal Ganglia then allows cortical areas to stimulate the motor system 🔄. There is an increased inhibition of inhibitory BG output ⬆️, (stimulates the inhibitory hypokinesia 🐌). The damage to Basal Ganglia also results in deficits in Cognitive functions 🧠 such as deficits in spatial memory 🗺️❌, and inability to switch to appropriate behavior 🔄❌.
💚 Limbic System
Limbic system: It borders the Telencephalon 🔝, and Diencephalon 🎯, in appearance looks like a ring 💍 around the Thalamus. It was first described by Broca in 1878 👨🔬. This is also known as the Circuit of Papez, as Papez (1937) first identified it as a reverberating circuitry which was important in emotions 💖. Limbic areas are spread into parts of the Frontal 🔝, Parietal 🔙 and Temporal lobes ⏰. Therefore, it appears to have diverse connections and functions 🔗. These are mainly emotions 💖 but also memory 🧠, homeostatic and survival functions 🏥 (fight 🥊, flight 🏃, feed 🍽️, and mating ❤️).
🧠 Areas of the Limbic System
- Cingulate Gyrus 🔄
- Septal Nuclei 📊
- Hippocampus 🌊
- Amygdala 🔺
- Hypothalamus 🎯
- Anterior Thalamus 🔝
- Mammillary Bodies 🧠
🌊 Hippocampus
Hippocampus like a sea horse 🌊 lesion is important in the formation of memories 🧠 especially long term 📚. If lesioned bilaterally there is damage to learned emotional response 💔 and memory is severely affected (both recent and long term) ⚠️. Hippocampus is thus involved in Emotions 💖, memory 🧠, homeostatic responses ⚖️ fight/flight 🥊🏃, motivational states 🎯
🌉 Fornix
Fornix: This is a large fiber bundle shaped in the form of an arch 🌉 which connects the hippocampal formation to subcortical areas such as thalamus 🎯, hypothalamus 🏥, and septum 📊. It runs directly under the corpus callosum (bands of fibers which connect the two lobes 🤝).
🔺 Amygdala
Amygdala: (almonds: Greek 🥜) major part limbic system: located at the tip of temporal lobe ⏰ beneath the cortex and rostrally to hippocampus 🌊. Have connections with hippocampus, septum 📊, medial dorsal thalamus 🎯 and the prefrontal areas 🧠. It is because of these connections that the amygdala is important in emotional responses love ❤️, friendship 🤝 fear 😨, and rage aggression 😡. This is involved in physiological response of emotions: heart rate ❤️ pulse 💓 etc. orienting to novel stimuli 🆕, déjà vu 🔄. If stimulated of olfactory 👃 and gustatory 👅 hallucination (temporal lobe epilepsy ⚡). Bilateral lesions of amygdala lead to the Kluver-Bucy syndrome ⚠️: hyperorality 👄, hypersexuality (animate or inanimate objects) ❤️🔥, docility 🐑, learned fears (and aggression) such as fear of snakes 🐍 gone as the animal put snakes in their mouth 👄🐍. This accompanied by lack of affect 😐, apathy 😶 and blunted expression. This syndrome was first described by Kluver Bucy in 1939 after bilateral lesions in monkeys 🐵.
On the other hand, electrical stimulation of Amygdala lead to a rage reaction 😡⚡ the amygdala involved in identification of danger ⚠️ therefore is important for self-preservation 🛡️. When triggered, it gives rise to fear 😨 and anxiety 😰 which lead the animal into a stage of alertness ⚡, getting ready to flight 🏃 or fight 🥊.
📊 Septum
Septum: lesion in the septum leads to intense rage reaction (called the Septal Rage 😡) as does stimulation of the amygdala. Septum leads to increased activity in a novel situation 🆕, increase reward feeling with stimulation 🎁. This also plays an important role in motivational states such as feeding 🍽️ and drinking 🥤 etc.
🔄 Cingulate Gyrus
Cingulate Gyrus: (Cingulate: encircling 🔄) located between the cingulate sulcus and the corpus callosum 🤝. The anterior area gives rise to déjà vu that is smells 👃 and sights 👁️ with pleasant memories of previous emotions 💭, also important role in emotional reaction to pain 😣 and aggression 😡 cingulotomy tames unruly and wild animals 🐆. If a single bundle of this gyrus is cut (cingulotomy ✂️) it interrupts the limbic areas communications with each other (reverberating circuitry affected 🔄) thus leading to reduced depression 😔 and anxiety levels 😰 (which preexisted in the patients). (Points to ponder 🤔 - looks like a good option for treatment—but who is to decide? ❓)
📚 References
- Carlson, N. R. (2005). Foundations of physiological psychology. Pearson Education New Zealand.
- Pinel, J. P. (2003). Biopsychology. (5th ed). Allyn & Bacon Singapore.
- Bloom, F., Nelson., & Lazerson. (2001), Behavioral Neuroscience: Brain, Mind and Behaviors. (3rd ed). Worth Publishers New York
- Bridgeman, B. (1988). The Biology of Behavior and Mind. John Wiley & Sons, New York
- Brown, T.S. & Wallace, P.S. (1980). Physiological Psychology. Academic Press, New York