Lec 1: The central nervous system Flashcards
Communication systems in
the body
- We have several communication systems in the body but there are 3 importanr ones:
- nervous system
- endocrine system
- immune systems - We will focus on the interactions between the endocrine system and the nervous system.
- All these systems are interconnected. For example, we have behaviors that elicit the release of hormones. We will also have hormones that will elicit the occurrence of behaviors.
Anotomical organization of the nervous system
Autonomic Nervous System
- The sympathetic and parasympathetic systems interact. For example, if you are often exposed to stress, meaning there is a lot of sympathetic activation, this will influence the parasympathetic axis of the PNS. A balance between these two activation are important.
- ie: in sexual arousal, the parasympathetic system is active, but for reaching an orgasm which is important for reproduction, the sympathetic innervation assumes. But if you are stressed and the sympathetic innervation is very active (the one responsible for digestion and sexual behavior) then you can have sexual dysfunction.
Somatic Nervous System
- Allows us to move and control muscles.
- Feeds information from all of our senses to the brain.
- Afferent pathway = carry sensory information to the brain. Info is carried through the afferent nerves and integrated in the thalamus. Thalamus then directs the info to different parts of the brain.
- Efferent pathway = carry motor information away from the CNS to the muscles of the body.
Integration of multiple nervous systems
Classical definition of behavior: behaving animals may be thought as being made of three interacting components: an input system (stimuli environment that can be external or an environment that can be internal), a central processing system (central system) and an output system (ie, muscles).
* Hormones may affect any or all of these three components (input,output system or integrator) when influencing behavior. Hormones can affect all of those pathways.
Review of neuroanatomical terminology
Review of brain fetal/neural development
- The entire central nervous system is derived from the neural tube.
- Neural tube is formed at a very early stage in embryonic development (3 weeks). The entire brain derives from these 3 primary vesicles of the neural tube: the forebrain, the midbrain and the hidbrain. Later in development, the forebrain will further divide in what we call the telencephalon and diencephalon.
- At the beggining, we have the forebrain, the midbrain and the hindbrain that come from the neural tube. The forebrain will be further divide in what we call the telencephalon which is basically the cortex (what we call the cerebrum).
Early Neural development
- The development of the nervous system beings at around the 3rd week of embryonic development, when an area of the ectoderm or the outer tissue layer of the embryo thickens and forms what is known as the neural plate.
- This plate begins to fold inward, forming a groove called the neural groove. At the end of the 3rd week, the neural folds begin to come together. By the end of the 4th week, they have completely fused together to form the neural tube which eventually become the brain and spinal cord.
- As the neural tube closes, bulges and bends begin to appear and they gradually become more noticeable.
- During the 4th week, there are 3 bulges present - the primary vesicles:
- Prosencephalon (which will eventually become the cerebrum)
- Mesencephalon (eventually becomes the midbrain)
- Rhombencephalon (eventually becomes the rest of the brainstem).
The end of the neural tube will form the spinal cord.
- As the brain continues to develop, the prosencephalon further divides to form the telencephalon (becomes the cerebral hemisphere) and the diencephalon (consist of the thalamus, hypothalamus…). The mesencephalon does not subdivide any further and will become the midbrain. The rhombencephalon will subdivide into the metencephalon (will become pons and cerebellum) and the myelencephalon (will become medulla).
- 7 weeks: the neural tuble will continue to develop to look more like a brain. the telencephalon will grow more rapoidlity than other parts of the tube.
- week 11: the brain will have a similar shape as to what it will at birth, although it will continue to develop after birth. The brain at birth is structurally similar to a fully developed brain.
Cerebral organization
Cerebral cortex organization
- Most of the brain is composed by the cerebral cortex and these are largely responsible for complex processing related to cognitive function like consciousness, thought, emotion, reasoning, language and memory.
- Each cerebral hemisphere can be subdivided into 4 lobes.
- The two hemispheres are commnected by axonal fibers called cerebral commisures (corpus callosum = largest commisure connection in the brain).
- Grey matter (body of the neurons) is 3mm wide and it covers the cortex, it is a layer of neuron cells.
Cerebral cortex - grey matter vs white matter
- Grey matter is outside in the thin layer of the cortex.
- White matter (the axons). White matter shows connections between brain regions. White matter is responsible for connectivity in the brain because we have axons and all of the cell bodies can be found in the gray matter of the brain.
- DTI is a new imaging technique that shows the motility of water. We can see the direction of the connections by following the motility of water. Inside the cytosol is basically water with salt and organelles. By keeping track of the direction of the flow of water, we can see which connections are more active than the other ones that are less active within the brain.
- The brain acts as an integrated organ.
Cerebral cortex and associated functions of lobes.
- In general, we have divisions of the brain.
- Certain functions are classically associated with a specific brain region.
Frontal lobe
- Anterior and largest portion of the brain.
- Executive function, reasoning - contains the prefrontal cortex (higher level cognitive function. ie, working memory)
- Motor control - contains the motor cortex
- Emotion
- Language - contains broca’s area (language production).
Phineas Gage
- Railroad foreman that worked in vermont.
- Phineas P. Gage (1823 – 1860)
- Railroad construction survival of an accident in which a large iron rod was driven completely through his head.
- Widespread lesion of his brain’s left frontal lobe.
- Reported effects on his personality and behavior.
- This is one of the first evidence that the prefrontal cortex is important for fine behavior (changed his personality and fine traits).
- No longer has social and appropriate relations. He turned into a person that was completely different - his behaviour completely changed.
Parietal lobe
- Processes mainly sensory information of the body.
- Contains the somatosensory cortex.
- The somatosensory cortex is topographically organized in terms of spatial relationship - cortical homuculus created by Doctor Penfield.
- Used electrophysiology to identify which regions in the brain were related to sensory information and somatosensory activity.
- Cortical homunculus illustrates the number of neurons associated with each function. Ie, we have higher sensitivity in our hands and this is associated woith a larger part in the parietal lobe.
Subcortical organization
Subcortical regions include:
* limbic system
* Amygdala (fear/anxiety)
* basal ganglia
* Hippocampus (learning and memory)
* thalamus (relay center of the brain for sensory information - directs information to different parts of the brain to process it)
* hypothalamus (homeostasis - connection with endocrine system - hormone relay of the body)
There is not a consensus of if the basal ganglia is part of the limbic system or not (consider it is for this class).
* Amygdala is especially involved with fearful and anxious emotions.
* In terms of hormones, the amygdala has a lot of receptors for glucocorticoids (cortisol is a glucocorticoid). The amygdala reacts to stress via hormones.
* Hippocampus is interconnected with the amygdala and it is an essential structure for learning and memory.
* Hypothalamus communicates with the hippocampus. Lots of functions are organized and orchestrated by the hypothalamus and its mainly a gland that makes other glands release hormones (ie, HPA axis, HPG axis)
* If you have a stressful situation, the learning process is heightened by hormonal activity like the cortisol hormone.
Midbrain
Midbrain is comprised of structures located deep within the brain:
* Substantia nigra
* Ventral tegmental area
**Involved in regulation of mood, reward and addiction. **
ie: the relevance of reward in developing gambling addiction is mediated by neurons that secrete dopamine (dopaminergic neuron = neuron that produces mainly dopamine).
* The VTA has axonal projections throughout the brain and uses this kind of communication with dopamine that is related to those addictive behaviors.
Hindbrain
- medulla - controls the autonomic nervous system (breathing, blood pressure, heart rate)
- pons (connect hindbrain to rest of brain = bridge)
- cerebellum - receives message from muscles, joints and structures in our ear to control balance, coordination and some types of memories (like procedural memories).
Glial cells
- Glia comes from the Greek for “glue,” and initially it was thought that glia served only to hold neurons in place and act as supportive cells. However, glia are recognized as having a variety of functions:
- Providing support to neurons (positioning neurons)
- Repair damage
- Fight infections
- Influencing neurotransmission.
Neurons
- Neurons are essentially excitable cells. They can be excited when receiving certain electroimpulses.
- Neurons are not the only cells that are excitable. Muscle cells can also be excited and will contract and make movements.
- Cells get excited = means they can perform some sort of activity or function.
- In the case of neurons, when they are excited, they send signals and communicate.
- They have dendrites that serve as an input site where the signals are received from another neuron. The morphology of the dendrites can be different depending on the situation. ie, dendritic spines are qualitativelly different across ages. Yound individuals have more dendrites and more dendritic spines. As we age, we lose some of these spines.
- If we think about the effects of stress (cortisol hormone), it impacts some cells in the brain. Some neurons are less branched in terms of numbers of dendrites and also impacts the spines. Cells can cause chronicle cases of anxiety and some research shows enlargement in the amygdala due to long term exposure to stress.
- In children with intellectual disability, we can see difference in the morphology of the dendritic spines.
- Each spine is post synaptic to one or two axon terminala.
- Each spine can receive a connection from an axon. .
Axons
- Axons are the longest projections that come from the soma (body of the neuron) and it will end at multiple terminal buttons - this is where the action potential will finish. The buttons will communicate with another neuron using neurotransmitters.
Axon
- Longest projections from the soma
- Ends at multiple terminal buttons, this is where the
signal, called the action potential, finishes and the neuron
communicates with another neuron or other cells.
Dendrite
- Serve as input sites where signals are received from other neurons
Terminal buttons of axons contain synaptic vesicles that house neurotransmitters
Soma
Soma
* Central part of the neuron. It is the largest portion of the neuron called the soma.
* The watery fluid inside the cell, called the cytosol , is a salty, potassium rich solution that is separated from the outside by the neuronal membrane. Also sodium.
* Within the soma are a number of membrane-enclosed
structures called organelles (mitochondria, ribosomes…) .
* The cell body of the neuron contains the same organelles
found in all animal cells.
Nucleus
- It produces proteins and it contains the genetic material.
- Spherical, centrally located part of the neuron cell. It is contained within a double membrane called the nuclear envelope.
- The nuclear envelope is perforated by pores. Product of the neurons will be produced and transported through those pores.
- Some hormones acts within the neuron nucleus and have transcriptional properties.
- Some hormones, when it binds to the receptors it will act as gene transcription factors. This will be important to produce proteins.
Gene transcription.
1 - RNA molecules are synthesized by RNA polymerase and then processed into mRNA to carry the genetic instructions for protein assembly from the nucleus to the cytoplasm.
2- Transcription is initiated at the promoter region of the
gene and stopped at the terminator region. The initial RNA must be spliced to remove the introns that do not code for protein and are exported from the nucleus.
Membrane
Serves as a barrier to enclose the cytoplasm inside the neuron and to exclude certain substances that float in the fluid that bathes the neuron.
Various types of proteins are embedded within the neuron cell membrane.
o Ion Channels: Allow the passage of ions (e.g., sodium, potassium, calcium) in and out of the cell, facilitating nerve impulse transmission.
o Transporters: Assist in the movement of molecules and ions across the membrane.
o Receptors: Bind to specific neurotransmitters or signaling molecules, initiating cellular responses.
o Enzymes: Catalyze specific chemical reactions crucial for cellular functions
The function of neurons cannot be understood without
understanding the structure and function of the membrane
and its associated proteins.
Membrane and neural communication
- There is a difference in voltage in neuronal cells. This is important because for neuro impulses to happen, we need to have a difference in this electrocharge.
- Neurons are in a state of rest when they are not producing a neural impulse.
- There is an electric charge difference in the neuron between the inside of the cell (-) and the outside of the cell (+)
How do neurons obtain this difference in electric charge?
- Ion channels
- They pump positive ions out making the inside of the cell negative.
- Na-K channels: they have a different proportion of Na and K to reach the resting potential of -70mV.
Sodium-potassium pumps
- uses energy via ATP to constantly pump 3 sodium ions out of the cell and 2 potassium ions into the cell.
- Transport protein inside the membrane that uses ATP to constantly maintain this difference in electrical charge.
- More + is being pumped out than in, it keeps the membrane potential more - inside and more + outside.
- This process (rest) is very energy demanding in terms of energy. Around 60-70% of the energy consumed by the brain is used to maintain this state of equilibrium.
The action potential
- The resting state: -70mV
- When the neuron receives a stimulus, the sodium channels located in the cell membrane will open and Na will enter the cell (because Na is more concentrated outside the cell than inside).
- 3 Na out, 2 K in
- Membrane starts to depolarize (become more +), other channels will be activated and this depolarization will become very fast. Reaches a peak of +50mV.
- Then we reach a refractory period where the neuron starts to go back to its resting state (-70).
- There is a period where the neuron becomes too negative and cannot start another process of transmission because it became too hyperpolarized (refractory period).
- Na - K pump restores the basal amount of Na and K inside the cell.
- AP only happens in the axon of the neuron and in that specific sequence of events.
- Myelination is very important because where there is myelin, there is no exchange of those ions. Therefore, the electric inputs will jump from node to node and makes the transmission of the AP much faster = saltatory conduction.
- Multiple sclerosis = demyelination of axon.
Action potential
- The action potential is a rapid reversal of resting
membrane potential, such that, for an instant the inside of
the membrane becomes positively charged in relation to
the outside
Action potentials properties:
* When generated by a patch of membrane, they are all similar in size and duration, and they do not diminish as they are conducted down the axon;
* The frequency and pattern of action potentials constitute the code used by neurons to transfer information from one location to another.
* Can occurs very rapidly—100 times faster than the blink of an eye, and the action potential lasts about 2 milliseconds
Synaptic transmission
- When the AP reaches the end of the neuron, the neurotransmitters are released and by summation can trigger another AP in another cell.
Neurotransmitter release
- voltage gated calcium channels - allows the release of NT into the synaptic cleft.
- Happens very fast = more than 100 times in a millisecond.
Types of channels in membrane
- When NT attaches to receptor, it can make conformational changes that will allow the entrance of this ions.
Neurotransmitter pathways
- We have different types of NT (dopamine, acetylcholine, serotonin).
- They are not randomly spaced through the brain, but instead they occur in specific well delineated pathways (ie, dopaminergic pathways from the VTA to PFC, parietal lobe, subcortical regions.
- Serotonin - will have axonal projections that will go through different regions of the brain
