Nervous System Review PDF - Brain Anatomy, Function
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Summary
This document provides a detailed review of the nervous system, covering its organization, the structure and function of the brain, spinal cord, and different types of neurons. It also delves into the brain lobes, neuronal processes, and related functions. Useful for neuroscience students.
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Nervous System The master controller of the entire body. What does this system do? Regulates all function in the body Recieves all incoming stimuli that affect the body Cells communicate via electrical and chemical signals Said to be irritable (able to be stimulated) and conducti...
Nervous System The master controller of the entire body. What does this system do? Regulates all function in the body Recieves all incoming stimuli that affect the body Cells communicate via electrical and chemical signals Said to be irritable (able to be stimulated) and conductive (conducts signals such as action potentials) There are 3 overlapping functions of the nervous system. Describe each below: 1. Sensory input- monitoring stimuli 2. Integration- processing and interpretation of sensory input 3. Motor output- response to stimuli; activates effector (muscle or gland) organs to produce a response Organization of the Nervous system Describe the different branches of the nervous system below (CNS) central nervous system- Brain and spinal cord - Integration and command center (PNS) Peripheral Nervous System: With two divisions - Paired spinal cord and cranial nerves - Carries messages to and from the spinal cord and the brain Sensory (afferent) division 1. Sensory afferent fibers- carry impulses from skin, skeletal muscles, and joints to the CNS 2. (Organs) visceral afferent fibers- transmit impulses from visceral organ to the CNS Motor (efferent) division - Transmits impulses from the CNS to effector Somatic nervous syste- voluntary - Conscious control of skeletal muscle (ANS) Autonomic nervous system: With two divisions - Visceral motor nerve fibers - Regulates smooth muscle, cardiac muscle, and glands Sympathetic Nervous system- “Fight or flight” Parasympathetic Nervous system- Resting Central Nervous System- Brain and Spinal Cord Regions and Organization What are the 4 main adult brain regions 1. Cerebral hemispheres 2. Diencephalon 3. Brain stem (midbrain, pons, and medulla) 4. Cerebellum Describe the makeup of the Cerebral Cortex: Large area: 40% of brain mass - Outer layers of the brain - Grey matter, unmyelinated neurons - The cortex is arranged into folds of nerve tissue - Increases the surface area and number of nerves in an area - Underneath the cortex is white matter - Myelinated neurons - Association (memory) nerves and tracts (cluster of nerve cells that send one signal) - Divided into two hemispheres that are contralateral- Operate in two separate pieces - Composed of neuron cell bodies, dendrites, glial cells, and blood vessles, but no axons - Awareness, sensory perception, voluntary motor initiation, communication, memory storage, understanding Four general considerations of the cerebral cortex: outer portion of cerebrum- executive “suite” in brain- thin grey superficial layer 1. Contains three types of functional areas: Motor areas: control voluntary movement- skeletal muscle Sensory areas: conscious awareness of sensation Association areas: integrate diverse information 2. Each hemisphere is concerned with contralateral (opposite) side of body 3. Lateralization (specialization) of cortical function can occur in only one hemisphere 4. Conscious behavior involves entire cortex in one way or another Corpus callulosum: Sends signals faster because of color - Connects the hemispheres - White myelinated association fibers Each hemisphere of the cerebral cortex is divided into five lobes - The lobes are areas of common function Motor: these areas contain neurons that carry signal to PNS to control something in the body Sensory: these areas contain neurons receiving signals from the body Association: these areas contain neurons for communication and memory - Nerves in the CNS are divided into tracts. What is a tract? Groups of nerves that travel to the same area and are transmitting in the same direction - Some will be sensory and some will be motor Brain Lobes and their Functions Frontal Lobe- Mostly motor Personality and intellect: takes longer to develop- longer for men than women Broca’s area: Motor cortex to your speech- no language just sound Somatic motor cortex: located in the precentral gyrus, all skeletal muscle movements, pyrimidal tract (motor) Premotor association area: learned movements Frontal eye field: eye movements ½ of olfactory cortex: sense of smell (shared with temporal lobe) Parietal Lobe- Mostly sensory Somatosensory cortex: in postcentral gyrus; sensory input from the body surface tissues Somatosensory association area: memory and understanding of the areas where information is coming in from Wernicke’s area (shared with temporal lobe): The ability to understand and form speech Temporal Lobe- Ears Auditory cortex: ability to hear sounds Auditory association area: memories of sounds ½ of olfactory cortex: sense of smell (shared with frontal lobe) Wenicke’s area: the ability to understand and form speech (shared with parietal lobe) Occipital Lobe- Eyes Visual cortex: all visual input Visul association area: understanding and memory of the visual input Insula- Deep to temporal lobe Vestibular cortex: balance and position of the body Visceral association area: monitors all visceral input, like stomach aches, full bladder, etc. Gustatory cortex: taste and memory of tastes Diencephalon Area below the cortex Formed by the: - Thalamus: Circular area in center: makes sure signal goes to the right area - Post office of the brain - Sorts and sends signals to the right area - Hypothalamus: Below thalamus, above pituitary: - Controls the ANS - Regulates hunger, thirst, temperature* - Controls most endocrine function - Limbic system (emotions) - Mammilary bodies; contain smeel reflexes - Epithalamus- pineal gland - Pineal gland - Melatonin; regulates sleep cycle Brain Stem Part that attaches the brain to the spinal cord. Made up of: - Midbrain: Tracts: below pineal gland - Corpora quadrigemina ~ Reflex for hearing and vision (startle reflex) - Pons: Tracts - Respiration center (thythm of breathing) ~ Exhalation center - Houses vitals- ***Not temperature (hypothalamus) Medulla Oblongata: - Connection to the spinal cord - Regulates the vitals ~ Heart rate ~ Respiration rate (inhalation) ~ Blood pressure ~ Deglutition (swallowing) ~ Emesis (vomiting reflex) - Reticular formation: Controls consciousness - Grey matter in the brainstem that regulates visceral activity and consciousness - Coma Cerebellum ★ Functions? Coordinates skeletal muscle activity Monitors joint position ★ Describe the structure: - Contains both white and grey matter ~ White matter called the Arbr Vitae (tree of life) - Damage to the area can cause ataxia ~ Drunken and uncoordinated movements Meninges Coverings around the CNS. It is made up of three layers: 1. Dura mater- 2. Arachnoid mater- 3. Pia mater- Meningitis Cerebral Spinal Fluid: 4 Ventricles of the Brain Protection of the brain: 1. Meninges 2. Cerebrospinal fluid 3. Cranium (bones around brain) 4. Blood brain barrier - Selective barrier between the blood vessels of the brain and the nerve cells - Allows glucose, essential amino acids, and electrolytes - Blocks metabolic waste, proteins, some toxins, most drugs - Ineffective against: alcohol nicotine, anaesthetics - Barrier is lacking around part of brainstem and hypothalamus Problems in the Brain CVA– Cerebral Vascular Accident (stroke) – TIA– Transient ischemic attack- temporary episodes of reversible cerebral ischema – Ischemia– Loss of blood flow to any tissue Concussion– a traumatic brain injury that changes the way your brain functions- can progressively get worse Coma– A prolonged state of unconsciousness– reticular formation affected Alzheimer’s disease (AD)-- Progressive degernerative disease of brain that resuts in dementia Key proteins appear to be misfolded and malfuntion ➔ Memory loss, short attentions span, disorientation, eventual language loss, irritability, moodiness, confusion, hallucinations ➔ Plaques of beta-amyloid peptides form in brain ➔ Neurofibrillary tangles inside neurons interfere with transport mechanisms, eventually killing neuron ➔ As brain cells die, brain shrinks Spinal Cord Describe the overall structure of the spinal cord: ★ Made of grey and white matter - White matter is …. ★ Protected by bone, meninges, and CSF (Cerebral spinal fluid) ★ Spinal dura mater is one layer thick - Does not attach to vertebrae ★ Epidural space- alleviate pain for childbirth: cusion of fat and network of veins in space between vertebrae and spinal dura mater ★ CSF fills subarachnoid space between arachnoid and pia maters ★ Dura and arachnoid membranes extend to sacrum, beyond end of cord at L1 or L2 - Site of lumbar puncture or tap Describe the following spinal ord structures and their locations: - Conus medullaris: The terminal portion of the spinal cord extends to the level of L1, where it ends - Caude equina- The remnants of the spinal cord that is made up of spinal nerves - Filum terminale- the last part of the pia mater that attaches to the sacrum Spinal Nerves 31 pairs exit off of the spinal cord through the intrevertebral foramen (holes between vertebrae) List the different pairs below: 8 Cervical 12 Thoracic 5 Lumbar 5 Sacral 1 Coccygeal The spinal nerves are constructed from nerve fibers that extend from the ventral and dorsal aspect of the cord - Describe how the dorsal root diffes from the ventral root of the spinal nerves Dorsal root–Towards Ventral root–Away Big bulge: Called Motor nerves ganglion Carries sensory nerves Most common to get towards spinal cord pinched when a disk bulges Cross-Sectional Anatomy of the Spinal Cord Spinal Cord Tracts Name and describe the main tracts found within the spinal cord: - Ventral side: Pyramidal-Motor ↓ - Dorsal sde: Fasciculus-Sensory ↑ ~Cuneatus: Arms ~Gracialis: Legs PNS (Peripheral nervous system) Made up of all the nerves that extend off the CNS -These nerves are made from nerve fibers (axons) -These nerves can be sensory, motor, or both Describe how cranial nerves differ from peripheral nerves: - Cranial: - 12 paired nerves extending off teh brain to innervate the head and neck - Peripheral: - Main divisions of spinal neres - These will innervate the body Reflexes Describe what a reflex is: - A reflex is a set pathway from a sensory nerve to the CNS and back to a motor nerve - This pathway is for a preventative, predictable response to a threatening stimulus ***Reflex arc Pic - Sensory nerve - Interneuron (integration center) - Motor nerve - Effector (muscle or gland) Autonomic Nervous System What is the autonomic nervous system? The division of the PNS that is involuntary What are some body activities that it controls? Blood flow, heart rate, digestion, respiration rate, hormones Centered in teh hypothalamus of the brain Two divisions (antagonists to each other): parasympathetic and sympathetic Sympathetic - Thoracolumbar (T1 to L3) - Describe this branch and some of the changes that occur in the body when in this system: - Prepares for action (“fight or flight”) - Uses norepinephrine at the main neurotransmitter ~ Enhanced by epinephrine (adrenaline) - Changes in the body include: ~ Heart rate rises ~ Respiration rate goes up ~ Blood flow increases ~ Pupil dialation ~ Decrease in digestive activity Parasympathetic-Rest - Cranial–Sacral - Describe this branch of the ANS and some of the changes that occur in the body when in this system: Causes decrease of body activity Our “rest and digest” system Increases the visceral activity of digestio and metabolism Main neurotransmitter is ACh Changes in the body include: ~ Respiration rate goes down ~ Heart rate goes down ~ Blood flow decreases ~ Pupil constriction ~ Increased digestive activity Histolgy of nerve Tissue ❖ Highly cellular, little extracellular space ❖ Tightly packe ❖ The two principle cell types of teh nervous system are: - Neurons (nerve cells): - Excitable cells that transmit electrical signals - Irritable (can be stimulated) and conductive - Carries impulses (action potentials) - Amitotic (can’t make new ones) - Supporting cells (Neuroglia): - Cells that surround and wrap neurons - Connective tissue of the nervous system - Connects, protects and supports the nerves Glial cells: Any of the cells that hold nerve cells in place and help them work as they should: 1. Astrocytes: Most abundant, versatile, nd highly branched neuroglia cells. They cling to neurons and their synaptic endings, and cover capillaries Functions: a. Support and brace neurons b. Anchor neurons to their nutrient supplies -Help to regulate waste and nutrients c. Guide migration of young neurons d. Control the chemical environment -Reabsorb neurotransmitters e. Respond to nerve impulses and neurotransmitters f. Influence neural funtioning - Participate in information processing in brain 2. Microglia: Small, ovoid cells with thorny processes that touch and monitor neurons Migrate toward injured neirons Function: Can transform to phagocytize microorganisms and neuronal debris (defensive cells in the CNS) 3. Ependymal cells: Range in shape from squamous to columnar May be ciliate - Cilia beat to circulate CSF Line the central cavities of the brain and spinal column 4. Oligodendrocytes Branched cells that wrap CNS nerve fibers - Creates the insulating myelin sheath in CNS - Large cells 5. Schwann Cells (neurolemmocytes): Surround peripheral nerve fibers of the PNS Creates myelin sheath in PNS Vital to regeneration fo damaged peripheral nerve fibers Small cells 6. Satellite cells: Cushions and protects nerves in PNS Surround neuron cell bodies in the PNS Function similar to astrocytes of CNS Neurons (Nerve Cells) Structural units of the nervous system ○ Describe the characteristics of a neuron: ○ Composed of a body, axons, and dendrites ○ Long life (100 years or more) amitotic, and have a high metabolic rate ○ All have cell body and one or more processes Their plasma membrane function in: ○ Electrical signaling ○ Cell-to-cell signaling during development Neuron Process – Armlike processes extend from body ○ Tracts - Bundles of neuron processes in CNS ○ Nerves - Bundles of neuron processes in PNS ○ Two types of processes: - Dendrites: short - Axon: long Neuron Model ★ Four major parts: 1. Dendrites: Receptive areas (antennae) 2. Cell body: Main large part - where organelles are found 3. Axon: Sends electric signal (action potential) and only goes one way: away from body 4. Axon terminals: Attach to other ells and release neurotransmitters to go to another cell - dendrites or muscle cell ★ Five minor parts: 1. Dendrites: Funnel shape 2. Nucleus: Control center of cell 3. Nissl bodies: Membrane next to nucleus RER that makes the transmitter that get sereted 4. Schwann cells: Signal goes faster – form myelin sheath in the PNS 5. Node of Ranvier: Gaps between schwann cells Structural Classification of Neurons Grouped by number of processes Three types: 1. Multipolar neurons: 3 or more processes One axon and one dendrite Most common; major neuron in CNS 2. Bipolar neurons: 2 processes One axon and one dendrite Rare, locate in retina and olfactory mucosa 3. Unipolar neurons: 1 short process Divides T-like- both branches now considered axons Distal (peripheral) process - associated with sensory receptor Proximal (central) process - enters CNS Neuron Classification Functional: ○ Sensory (afferent) – transmit impulses toward the CNS ○ Describe: ~ May be myelinated or unmyelinated ~ Almost all are unpolar ~ Contain specialized dendritic receptors for each stimuli received Examples: ○ Mechanoreceptors= Touch, vibrations, pressure, stretch ○ Thermoreceptors= Temperature (hot vs. cold) ○ Chemoreceptors= monitors chemical levels (taste buds, Ca2+ receptors, smell receptors ○ Photoreceptors= Rods and cones in eyes to detect color or black/white light ○ Nociceptors= Pain receptors Motor (efferent) – carry impulses away from the CNS ○ Describe: ○ Stimulate change in an effector (muscle or gland) ~ Ex. Muscle contraction, gland secretion ○ Multipolar ○ Very large and myelinated Interneurons (association neurons) – shuttle signals through CNS pathways ○ Describe each and how they differ: ○ Lie between motor and sensoy neurons ○ In brain (association neurons) an spinal cord (interneurons) ○ 99% of body’s neurons ○ Act as relay point for sensory nerves to motor nerves, and from the sides of the brain to each other Neurophysiology Neurons are highly irritable (they can be stimulated easily) Action potentials, or nerve impulses, are: ~ Electrical impulses carried along the length of axons ~Always the same regardless of stimulus – Does not matter how hard or how soft you touch ~ The action potential is the underlying functional feature of the nervous system Role of membrane Ion Channels Large proteins serve as selective membrane ion channels Two main types of ion channels - Leakage (non gated) channels - Always open; ions can float through - Gated channels - Part of protein changes shape to open/close channel ➔ 3 types of gated channels: 1. Chemical (ligand-gated) channels- Open with binding of a specific neurotransmitter (Dendrites) 2. Voltage channels - Open and close in response to changes in membrane potential (Axon) 3. Mechanical channels - Open and close in response to physical deformation of receptors, as in sensory receptors (Dendrites-Na+ goes in) The Resting Membrane Potential Potential difference across membrane of resting cell - slightly negative Generated by: Differential permeability of the plasma membrane Differences in ionic makeup of ICF and ECF Membrane potential Changes Used as Communication Signals What causes changes in membrane potential? - Concentrations of ions across membrane change - Membrane permeability to ions changes Changes produce two types of signals: - Graded potentials: incoming signal operating over short distance - Action potentials: ong distance signals of axons Changes in membrane potential is used as signals to receive, integrate, and send information Changes in Membrane Potential 1. Describe what Depolarization is: - A reversal of the membrane potential - Causes the cell membrane to become less negative (Na+ in) 2. Describe what Hyperpolarization is: - An increase in membrane potential (away from zero) - Inside of cell more negative than resting membrane potential (K+ out or Cl- in) - Reduces probability of producing a nerve impulse Graded Potentials Short lived, localized changes in membrane potential ○ Magnitude varies with stimulus strength ○ Stonger stimulus= more voltage changes; farther current flows What are the two types of graded potentials? ○ Depolarization–Less or Hyperpolarization–More Where do graded potential occur in the neuron? ○ Dendrites What triggers a graded potential? ○ Stimulus that opens gated ion channels The Action Potential When a nerve is stimulated, it will undergo a process of being excited and creating an impulse called an Action Potential - Where do action potentials occur? Muscle cells and axons of neurons - Where are they used for in neurons? Long distance neural communication - Action potential don’t decay like graded potential do. Why? Changes in Membrane Potential Changes are caused by three events 1. Depolarization – The inside of the membrane becomes less negative (Na+ in) 2. Repolarization – The membrane returns to its resting membrane potential (K+ out) 3. Hyperpolarization – The inside of the membrane becomes more negative than the resting potential 1. Depolarization Starts with stimulation. What are some ways the neuron can be stimulated? - The nerve needs to be stimulated in the appropriate way - Ex. touch, pressure, hot/cold - Nerves can be signaled by another nerve at the synapse What happens to the neuron once it is stimulated? - The membrane becomes very permeable to Na+, elevating the resting potential - After stimulation, Na+ rushes into the cell - When a nerve depolarizes, polarities … Not every stimulation will result in depolarization. Why? ○ Stimulus is too small As the elevreic harge elevates, it will cross a level called Threshold. Define threshold. ○ Required level of stimulation to create an impulse Can happen at once or in successive attempts. EPSP and IPSP- excitatory post synaptic potential, inhibitory post synaptic potential What is Summation? The adding of signals together resulting in threshold All or nothing action–Action potentials are all the same size As the membrane depolarizes, the action potential will travel down the nere to the axon terminals. ○ The action potential ALWAYS travels in what direction in a neuron? From cell body to the axon terminal–Towards the synapse What happens in the neuron when the action potential reaches the axon terminals? It releases neurotransmitters As depolarization peaks, it will lead to the recovery phase. 2. Repolarization After the membrane depolarizes, the membrane potential has to be restored to its resting state. Why must this occur? So the muscles can rest before they are used again Repolarization begins as depolarization peaks. Describe what happens in the neuron to restore the membrane potential: ○ Na+ gates will close ○ K+ gates will open–K+ leaves cell and brings charge back to normal ○ Na+/K+ pumps start to get rid of Na+, and bring in K+ ○ Membrane potential is restored 3. ***Refractory Period= This is the amount of time it takes to repolarize Absolute refractory period – neuron can’t perform another action potential at this time ○ When does this occur during an action potential? From the opening of the Na+ channels until the resetting of the channels ○ Why is this period of time important? Ensures that each AP is all-or-none event, enorces one way transmission of nerve impulses Relative refractory period ○ When does this occur during an action potential? Follows the absolute refractory period Most Na+ channels have returned to their resting state Some K+ channels are still open Repolarization is occurring Threshold for AP generation is elevated Exceptionally strong stimulus may generate an AP during this period Hyperpolarization ★ What is it? - Some K+ channels open, allowing excessive K+ efflux - This causes after hyperpolarization of the membrane (undershoot) ★ What causes this to occur during an action potential? - Excess of open K+ channels and Na+ efflux from the cell Propogation of an Action Potential Once initiated, an action potential is self-propagating ○ In non-myelinated axons, each successive segment of membrane depolarzes, then repolarizes ○ Propagation in myelinated axons differs Conduction Velocity ❖ Conduction velocities of neurons vary widely ❖ Rate of AP propagation depends on: thickness of axon–Thicker=faster Axon diameter – Larger diameter fibers have less resistance to local current flow so faster impulse conduction Degree of myelination – Continuous conduction in unmyelinated axons is… Effects of myelination: ➔ Why do myelin sheaths speed up an action potential? ◆ They insulate and prevent leakage of charge ➔ Review – What cells form the myelin sheath in the CNS? Oligodendrocytes. In the PNS? Schwann cells ➔ Saltatory conduction*** (possible only in myelinated axons) is about 30 times faster. Describe this process: The rapid method by which nerve impulses move down a myelinated axon with excitation occurring only at the Nodes of Ranvier Importance of Myelin Sheaths: Multiple Sclerosis Describe what multiple sclerosis is: ○ Autoimmune disease affecting primarily young adults ○ Myelin sheaths in CNS destroyed Immune system attacks myelin Turns it to hardened lesions called scleroses ○ Symptoms? Visual disturbances, weakness, loss of muscular control, speech disturbances, and urinary incontinence ○ What are some treatmens? Drugs that modify immune system’s activity improve lives ○ Prevention? High blood levels of vitamin D reduce risk of development Nerve Fiber Classification Nerve fibers are classified according to: Diameter, degree of myelination Descrube the following types of fibers: ★ Group A fibers – Large diameter, myelinated somatic sensory and motor fibers of skin, skeletal muscles, and joints ○ Transmit at 150 m/s ★ Group B fibers – Intermediate diameter; lightly myelinated fibers ○ Transmit at 15 m/s ★ Group C fibers – Smallest diameter, unmyelinated ANS fibers ○ Transmit at 1 m/s Synapses What is a Synapse? A connection between the nerve cells ❖ Nervous system works because information flows from neuron to neuron ❖ Helps to relay signals. Describe the following types of synapses: Chemical synapses – Specialized for release and reception of chemical neurotransmitters Typically composed of two parts Axon terminal of presynaptic neuron (contains synaptic vesicles filled with neurotransmitter (exocytosis)) Neurotransmitter receptor region on postsynaptic neuron’s membrane Usually on dendrite or cell body Electrocal synapses – Less common than chemical synapses – more abundant in embryonic nervous tissue Neurons electrically couples (joined by gap junctions that connect cytoplasm of adjacent neurons) Communication very rapid May be undirection or bidirectional Synchronize activity Nerve impulse remains electrical Synapse Classification Describe the different types of synapses (how nerve cells connect) below: ○ Axodendritic – between axon terminals of one neuron and dendrites of others ○ Axosomatic – between axon terminals of one neuron and soma of others ○ Less common types: Axoaxonic – axon to axon Dendrodendritic – dendrite to dendrite Somatodendritic – dendrite to soma Important Neurotransmitters Acetylcholine- excitatory to NMJ, used in somatic nervous system (PNS); excitatory to parasympathetic nervous system (rest and digest) Norepinephrine – Found in CNS and PNS, excitatory to sympathetic NS (fight or flight), inhibitory to parasympathetic NS Dopamine – predominantly CNS; excitatory to hypothalamus; “feel good” neurotransmitter; addictive Serotonin – Antagonist to dopamine; predominantly CNS; inhibitory (depressant) Postsynaptic Potentials Neurotransmitter receptors cause graded potentials that vary in strength with: - Amount of neurotransmitter released - Time neurotransmitter stays in area Types of postsynaptic potentials ○ EPSP – Excitatory PostSynaptic Potentials Describe: Trying to send a signal Neuotransmitter binding opens chemically gated channels ○ Allows simultaneous flow of Na+ and K+ in opposite directions Na+ influx greater than K+ efflux= depoarization called EPSP (not AP) EPSPs help trigger AP if EPSP is of threshold strength ○ Can spread to axon hillock, trigger opening of voltage gated channels, and cause AP to be generated ○ IPSP – Inhibitory PostSynaptic Potentials Describe: Reduces postsynaptic neuron’s ability to produce an action potential ○ Makes membrane more permeable to K+(out) or Cl-(in) If K+ channels open, it moves out of ell If Cl- channels open, it moves into cell ○ Therefor neurotransmitter hyperpolarizes cell Inner surface of membrane becomes more negative Synaptic Integration: Summation A single EPSP cannot induce AP Summation: Adding upp graded potential to reach threshold Most neurons receive both excitatory and inhibitory inputs from thousands of other neurons Processing ➔ Processing is the way the nerves recognize information and the path that it takes ➔ Two types: 1. Serial processing - A very predictable pathway that elicits the same response each time - A reflex 2. Parallel processing - Higher level awareness that will stimulate a response - Logical reasoning - Learned behavior of a habit - Memories ~Caused by repetition, or very stron stimulation