The Spinal Cord (8) PDF - Notes on Reflexes

EXP 3422C Section 0001 10/3/24 Notes (Reflexes, Structure, & Function) Forebrain - Cerebrum - Thalamus - Hypothalamus - Pituitary gland Hindbrain - Pons - Medulla oblongata - Cerebellum The spinal cord descends from the hindbrain. Reflex: an action that is performed as a re...

EXP 3422C Section 0001 10/3/24 Notes (Reflexes, Structure, & Function) Forebrain - Cerebrum - Thalamus - Hypothalamus - Pituitary gland Hindbrain - Pons - Medulla oblongata - Cerebellum The spinal cord descends from the hindbrain. Reflex: an action that is performed as a response to a stimulus without conscious thought - These are hardwired into the nervous system and are present from birth - These are automatic, fast, and stereotypes responses to specific stimuli, so they do NOT require conscious thought or prior learning in order to be done - These are a type of innate behavior Both the spinal cord and brainstem are involved in mediating reflexes. The spinal cord mediates relatively simple reflexes. 1. Grasp (babies will automatically grasp whatever is touching their hand) 2. Patellar (knee jerk reflex) 3. Withdrawal (we will automatically move our limbs away from a source of pain) The brainstem is crucial for more complex reflexes, usually ones that involve the head and face. 1. Salivary (presence of food activates this) 2. Pupillary (eyes contract or expand based on light level) 3. Startle 4. Postural (for balance; some are mediated in the brainstem and some involve the cerebellum) More complex innate behaviors (not necessarily reflexes) are mediated by multiple brain regions, typically those within the forebrain. These are both innate and automatic, but they differ from reflexes because they are not fast. - Ex: migration and hibernation behaviors (triggered by environmental cues) While reflexes are a type of innate behavior, reflexes mediated by the brainstem and forebrain CAN BE MODIFIED through non-associative learning processes (habituation and sensitization) or associative learning (conditioning). EXP 3422C Section 0001 Habituation: a decrease in response to a repeated, harmless stimulus Sensitization: an increased response to a stimulus following a strong or noxious event Conditioning: the process by which an innate reflex can be modified through associations with other stimuli, such as classical conditioning - Ex: conditioning dogs to salivate when they hear bells (salivation is the reflex) - Ex: conditioning a baby to startle when he sees small, white animals A reflex is still considered a reflex even when it’s paired with a neutral stimulus through conditioning. Many phobias are created through associations that trigger innate reflexes. Spinal Cord 1. Major part of the central nervous system (CNS) that extends from the brainstem down through the vertebral column 2. Serves as a communication highway between the brain and the body, relaying sensory and motor information via nerves a. The nerves are axons that go to peripheral muscles with motor information and fibers from sensory receptors that bring information to the brain 3. Is involved in innate behaviors (reflexes and simple motor patterns) 4. It is small in diameter but is very long a. About 0.5 inches in diameter b. About 16 to 18 inches long Spinal nerves: pairs of nerves that originate from the spinal cord in four different segments - These nerves are protected by the vertebrae Spinal Cord Segments 1. Cervical (neck) 2. Thoracic (ribcage/chest area) 3. Lumbar (low back) 4. Sacral (pelvis) The spinal cord is made up of gray matter in the center (neuronal bodies) and white matter in the edges (axons). - Gray matter is divided into 10 layers with cells - Dorsal horns are made of sensory layers I-VI - Ventral horns are made of motor layers VII-IX - White matter is divided into roughly six general columns containing tracts - Ascending tracts relay information from the spinal cord towards the brain - Descending tracts carry information from the brain through the spinal cord EXP 3422C Section 0001 Spinal Gray Matter - Dorsal horns (facing the back of the body) - Sensory information comes in from these - Ventral horns (facing the front of the body) - Motor information output comes out from these Spinal White Matter - Ascending tracts - Sensory tracts - Descending tracts - Motor tracts that help us voluntarily control our limbs Sensory neurons: these cells receive signals from the body and transmit them to the brain - These receive things like touch, pain, and temperature signals - These are part of the peripheral nervous system, but they connect with the spinal cord (technically they aren’t part of the spinal cord) Motor neurons: these cells send signals from the spinal cord to the muscles, controlling movement - These are on the ventral horns Interneurons: these are excitatory or inhibitory cells that connect sensory and motor neurons, playing a crucial role in reflexes - If excitatory, they will use glutamate - If inhibitory, they will use GABA Sensory Neurons - These do NOT have dendrites - They have a single axon that splits into two branches - One goes to the periphery (skin, muscles, etc.) and collects sensory information and carries signals towards the soma - The central branch goes to the spinal cord and carries signals from the cell body to the spinal cord via the dorsal horn - Their somas are located inside ganglia (cell bodies in the PNS), just outside of the spinal cord - They are not part of the central nervous system; they just connect to it Motor Neurons - These are multipolar neurons that extend a long axon to innervate muscles and initiate muscle contraction - They can receive multiple different inputs - These form synapses with muscle fibers, AKA neuromuscular junctions - These receive input from the brain and spinal cord sensory neurons and interneurons EXP 3422C Section 0001 - The brain controls voluntary movement - Sensory neurons and interneurons control reflexes Neuromuscular junctions: synapses made between motor neurons and muscle fibers Sensory neurons can either go straight into their respective motor neurons or they can first connect with an interneuron that relays the signal. Monosynaptic components: involves ONE synapse between the sensory neuron and the motor neuron in the spinal cord - Important for rapid and automatic information transfer Polysynaptic components: involves MULTIPLE synapses between the sensory neuron and the motor neuron in the spinal cord - Polysynaptic components help relax antagonistic muscles, facilitating the main reflex - When dealing with antagonistic muscle pairs, one will always need to be relaxed when the other contracts - Also helps coordinate the activity of contralateral muscles - Polysynaptic circuits will contract muscles in our bodies to make sure that our reflexes don’t throw us off balance (ex: reflex happening in one leg, so these circuits deal with the other leg’s muscles) - These send information to the forebrain in order to mediate our conscious perception of the stimulus that is triggering our reflexes Central pattern generators (CPGs): neuronal circuits that (when activated) can produce rhythmic motor patterns (like walking, breathing, flying, and swimming), in the absence of descending inputs - These are NOT reflexes, but they share some similarities (like automatic actions) - CPGs generate intrinsic rhythmic activity WITHOUT needing ongoing sensory input - Reflexes are triggered by a specific stimulus and involve a simpler, immediate response - These rhythmic motor patterns alternate between muscles on each side of the body - Ex: left leg, right leg, left leg, right leg, etc. These CPGs are present in the spinal cord at every level. If these circuits are present even after spinal cord injury, then it may be possible for them to be activated in order to facilitate spinal cord healing. - The movements activated by CPGs aren’t voluntary; they just happen Properties of CPGs 1. Rhythmogenesis a. Ability to produce rhythmic outputs 2. Reciprocal inhibition EXP 3422C Section 0001 a. Mechanism where the activation of one set of neurons leads to the inhibition of an opposing set, allowing for alternating activation of muscle groups After spinal cord injury, neurons and axons have a limited ability to regenerate. Despite this, synaptic plasticity can help rehabilitate victims of spinal cord injury in some ways. - Existing neurons can only modify their remaining connections and strengthen surviving synapses in response to stimulation or rehabilitation - Synaptic plasticity can potentially compensate for lost functions There is increasing evidence that magnetic or electrical stimulation can promote neural functioning by stimulating synapses. In one study, researchers used electrical impulses to stimulate the motor cortex (these neurons are presynaptic to motor neurons in the spinal cord that control the targeted muscle). - After a delay of 1-2ms, researchers stimulated the peripheral nerve electrically to achieve a depolarization of the motor neuron - The researchers used Hebbian principles (stimulating the presynaptic neurons just before the postsynaptic neuron) to strengthen connections necessary for motor activity - They manipulated spike-timing dependent plasticity rules - It worked, and it enhanced synaptic plasticity in the spinal cord In another study, researchers studying CPGs selectively stimulated the appropriate muscles in order to help animals initiate voluntary walking. - Electrodes were implanted in the motor cortex to measure electrical output - A stimulation device on the spine would alternate stimulation of muscle groups in order to facilitate voluntary walking in conjunction with the electrical output measured in the motor cortex The study of CPGs can also help with building humanoid robots. If we can understand how WE control muscles, we can build robots that accurately mimic our movements. 10/10/24 Notes (Brainstem & Forebrain) Neuromodulators 1. Motivation a. Serotonin b. Dopamine 2. Attention a. Acetylcholine b. Noradrenaline Forebrain - Cerebrum - Thalamus EXP 3422C Section 0001 - Hypothalamus - Pituitary gland Hindbrain - Pons - Medulla oblongata - Cerebellum The first brain region coming from the spinal cord is the brainstem (literally the stem of the brain). The brainstem includes the medulla oblongata, the pons, and the midbrain. Brainstem: located at the base of the brain and connected the brain to the spinal cord - Composed of the midbrain (closest to forebrain), pons, and medulla (closest to spinal cord) - Regulates essential autonomic functions while also coordinating many reflexes - Heart rate, breathing, and blood pressure - Contains key neuromodulatory centers, which are crucial for regulating a wide array of brain functions - Sleep-wake cycles, attention, motivation, etc. Internal Organization of the Spinal Cord (Review) - Gray matter (neuron bodies) in the center - White matter (axons) surrounding the gray matter - Peripheral nerves contain axons from sensory and motor neurons that have their somas (neuron bodies; gray matter) in the spinal cord Structure of the Brainstem - Complex internal structure, with areas of white and gray matter intermingled - Unlike the spinal cord, which has clear-cut separations between white and gray matter, the brainstem has white and gray matter all throughout itself, criss-crossing each other and going to other areas of the brain - Contains cranial nerve nuclei (contain the somas of neurons that form the cranial nerves) and neuromodulatory centers - The spinal cord is where the nerves originate, and the brainstem does the same (with some nerves, not all of them) Cranial nerves: twelve pairs of nerves that emerge directly from the brain and brainstem, and are responsible for transmitting sensory and motor information to and from the head and neck - They help regulate vital functions like breathing, heart rate, and blood pressure - Some are primarily sensory while others are primarily motor - Some have mixed functions and are involved in reflex actions (reflexes that involve the neck and head) EXP 3422C Section 0001 For example, the facial nerve is primarily motor (for expressing facial movements) while the optic nerve is primarily sensory (for vision) and the olfactory nerve is primarily sensory (for smell). Other nerves may be involved in reflexes like the pupillary light reflex or the gag reflex. - Olfactory (I), sensory - Optic (II), sensory - Facial (VII), motor Neuromodulatory centers: contain neurons that influence membrane potential and intracellular molecular cascades - These centers are small clusters of neurons that use the neuromodulator neurotransmitters - They might not contribute directly to encoding information, but they regulate the neurons that DO - They make them either more or less likely to fire spikes Cholinergic centers use acetylcholine and impact attention. Dopaminergic centers use dopamine and impact reward and motivation (ventral tegmental area, VTA, in the midbrain). Serotonergic centers use serotonin and impact mood regulation and motivation (Raphe nuclei, in all sections of the brainstem). - You don’t need to memorize this, but you should know the general locations of each center group Cholinergic Centers - These increase arousal and focus, facilitating sensory perception - There are two main brain regions that use acetylcholine as a neurotransmitter and have widespread projections to other brain regions - Brainstem cholinergic nuclei (project extensively to the thalamus) - Involved in general alertness and arousal - Ex: waking up - Basal forebrain (projects extensively to the neocortex/cerebral cortex) - Critical for focused, goal-directed attention - Ex: studying EXP 3422C Section 0001 - In Alzheimer’s disease, the basal forebrain is one of the places that deteriorates. Dopaminergic Centers - These signal the value of sensory stimuli and their potential to lead to reward - Ventral tegmental area (VTA), in the midbrain (project to forebrain structures) - Contains neurons that send their axons to forebrain structures involved in making decisions based on the potential value of stimuli - VTA neurons respond to rewards (increase their firing rates), both innately rewarding stimuli (like food or water) and stimuli that have been learned to be associated with reward (like Pavlov’s dogs, food, and the bell) - They increase their firing rates when rewarding stimuli are present - Serotonergic Centers - These facilitate motivated behavior, and are known as the raphe nuclei - They are distributed throughout all sections of the brainstem (midbrain, pons, and medulla) and project widely to various regions of the cortex - They serve as the primary source of serotonin in the brain EXP 3422C Section 0001 - - It is unclear exactly how serotonin affects motivation - Lack of motivation is an important symptom of depression, and most medications for depression target serotonin in order to increase its level in the brain Expectancy theory of motivation: suggests that motivation is driven by the expectation that efforts will lead to rewarding outcomes - This theory is trying to explain how serotonin affects motivation - It’s possible that serotonin influences how individuals evaluate effort and reward - More serotonin could increase motivation by reducing the perceived cost of putting in more effort, or by making people value the reward more The cerebral cortex and thalamus are important for the encoding of sensory information (the sensory content of memories) Thalamus: transmits sensory content of memories to the cortex and other brain areas (like the hippocampus) where a memory trace can be formed based on that sensory information - Highly connected to the cerebral cortex - Located deep in the center of the forebrain - There are two thalami, one per hemisphere - Contains more than 15 subregions (nuclei) that perform different functions - Contains excitatory glutamatergic neurons (relay cells) that project to the cortex, including the neocortex and hippocampus - There are a lot of these - They relay information to the cortex - Contains inhibitory interneurons in the thalamic reticular nucleus (TRN) that surround the main part of the thalamus - These establish reciprocal connections with the relay cells of the thalamus (they project from the thalamus AND receive input from the thalamus) - These circuits can generate rhythmic activity in the thalamus - Sleep rhythms that help the consolidation of memory EXP 3422C Section 0001 EVERY cortical region receives input from a subregion of the thalamus. In other words, for EVERY sensory system, there is a region in the thalamus (a thalamic nucleus) that relays that information to the cortex. - You don’t need to memorize this Sleep spindles are neural oscillations (brain rhythms) that occur during non-REM sleep in all types of mammals. They don’t occur in REM sleep. - These rhythms are generated by reciprocal circuits in the thalamus (thalamus reticular nucleus, TRN) - The projections from relay cells to the cortex entrain cortical cells in the oscillation - The thalamus (TRN, GABAergic cells) is the spindle pacemaker - GABAergic and glutamatergic cells interact in order to create spindles - The GABAergic cells are the pacemakers of sleep spindles, and because they project to glutamatergic relay cells, and those cells project to cortical cells in the cortex, the spikes from GABAergic cells can result in rhythms throughout the brain Sleep Spindles (MEMORIZE THIS) - Oscillations observed during non-REM sleep - Occur at 10-15 cycles per second - Last between 0.1 to 2 seconds - Are important for memory consolidation Cortical Lobes 1. Frontal a. Motor control, working memory, and cognitive flexibility 2. Parietal a. Somatosensory 3. Temporal EXP 3422C Section 0001 a. Audition and olfaction 4. Occipital a. Vision 5. Insular (behind the lateral sulcus, between the temporal and frontal lobes) a. Complex, and its functions are largely unclear b. Involved in taste and emotion 6. Limbic (deep in the brain) a. Motivation, emotion, learning, and memory The gray matter of the cerebral cortex is on the outside of the brain. The white matter of the cerebral cortex is farther within the brain. Cortical Cell Types - Pyramidal cells (excitatory cells in the cortex) - Triangular-shaped soma - Basal dendrites are deep in the brain and the apical dendrites reach up towards the surface of the cortex - They receive most of the excitatory inputs, and there are thousands of spines per pyramidal neuron - They are crucial for encoding and processing information in the brain - Interneurons (inhibitory cells in the cortex) - Most inhibitory interneurons don’t have spines on their dendrites - Plasticity in interneurons involves functional changes, rather than large structural remodeling (can’t have much structural plasticity because of this lack of spines) - They control the activity of pyramidal cells and their pattern of activation Perceptual learning: the process by which the brain improves its ability to interpret sensory information through experience or practice - This learning enhances one’s ability to distinguish between stimuli EXP 3422C Section 0001 - Ex: wine tasting, reading Braille (or other symbols), etc. Perceptual learning demonstrates that our visual cortex has adapted over years of evolution and through experience to better discriminate between different types of sensory stimuli. - Ex: vertical lines vs. tilted lines (we’re better at distinguishing between vertical lines and slightly tilted vertical lines, rather than tilted lines and slightly more tilted tiled lines) In the thalamus and cortex, excitatory neurons will respond to specific sensory stimuli. Neurons will change their rate code to transmit information about the stimulus that they are reacting to. Receptive field: a region of sensory space in which the presence of a stimulus can elicit a response from a neuron - Receptive fields allow neurons in the brain to represent specific sensory stimuli (they’ll only respond a certain way if they’re presented with the stimulus that they represent) - Ex: neurons in the visual region of the thalamus respond to spots of light - Ex: neurons in the visual cortex respond to edges For a neuron to respond, the stimulus has to be present in a specific field of the environment (the neuron’s receptive field). - Neurons respond more strongly when the stimulus in its receptive field has certain properties - Ex: a neuron in the visual cortex will respond to edges, but it will respond more strongly to certain types/angles of edges These receptive fields allow cells in the visual cortex to represent the outlines of objects and shapes. As you get further into the visual system, the represented stimuli become increasingly complex. Orientation tuning curve: mathematical function that describes the response of a cell to stimuli presented on the receptive field that vary gradually in one or more features - The firing rate of the cell is on the Y axis - The orientation of the stimulus being presented is on the X axis - Ignore the red dot