UNIT 7 NOTES. STIMULI ê RECEPTOR CELL SENSORY NEURON INTERNEURON (CNS)

NERVOUS SYSTEM

UNIT 7 NOTES

Nervous System (NS) has 3 functions: 1. Sensory Input • Conduction of electrical signals from sensory receptors 2. Integration • Information is interpreted and response generated by central nervous system (CNS) 3. Motor Output • Signals from the CNS are conducted to effector cells (muscle or gland cells) which carry out the response • All electrical signals are conducted by nerves 2 main types of cells in NS 1. Neurons = nerve cells • Transmit messages in the form of nerve impulses 2. Supporting cells (glia) • Provides structure, protection, insulation and nourishment to neurons E.g. Schwann cells 3 parts of a neuron: 1. DENDRITES: receive signal & conduct an impulse towards the cell body. Usually branches 2. CELL BODY: contains the nucleus. Maintains the cell. 3. AXON: conducts impulse away from the cell body. 3 types of neurons: 1. Sensory Neurons 2. Interneurons 3. Motor Neurons

The 3 neurons together make up the REFLEX ARC.

 

Reflex Arc • An automatic response to large stimulus (ex. Hand on pin) -WHY? allows for immediate response without waiting for brain-. Threshold must be reached

STIMULI   ê

RECEPTOR  CELL   ê

SENSORY  NEURON   ê

INTERNEURON  (CNS)   Ã Brain

Ä Motor Neuron ê Effector (muscle or gland) ê Reaction to stimuli

 

Receptor: o Detects stimuli that surpasses a threshold o Initiates the impulse in the sensory neuron. Examples: • stretch receptor in the alveoli of the lungs • pain receptor in the skin • photoreceptor in the eyes • chemoreceptors in heart and brain Sensory Neuron: • Long dendrite, Short axon • Takes sensory info. from sensory receptors to cell body which is in DORSAL ROOT GANGLION of CNS Interneuron: • Short dendrite and short or long axon • Entirely in the spinal cord or CNS • Integrates sensory neuron info with the correct motor neuron to get correct response. Motor Neuron: • Short dendrites and very long axon • Dendrites are in the CNS, axon is outside the spinal cord • Takes the impulse to an effector. Effector: • Effector will be either a muscle or a gland. • When stimulated, muscle will contract. • When stimulated, gland will release a hormone Nerve Bundles • A NERVE is simply a bundle of long fibres from neurons. Myelin Sheath • The long fibres of the neurons are covered with a fatty sheath called MYELIN SHEATH. • This myelin sheath is composed of Schwann cells (a neuroglia cell) that wrap around the nerve fibre The myelin sheath has 2 functions: a) It insulates the neurons from each other as they pass through the nerve b) It helps to speed up the impulse Ø The points between the Schwann cells are called the NODES OF RANVIER. They speed the impulse as it jumps from node to node.

Threshold: = minimum amount of stimuli required to produce an impulse • a neuron will only fire if the threshold level has been reached – all the excitatory and inhibitory signals on the dendrites must be integrated (summed up) by the neuron to determine if it will fire or not

 

ACTION POTENTIAL • • •

Nerve impulses are electrical in nature. When a nerve impulse travels along a nerve fibre, there is actually a wave of ionic changes that occur that produces a membrane potential difference measured in millivolts (mV) This creates a very small shift in the electrical nature of the fibre.

Resting Potential: • At rest, the membrane is not permeable to these ions and they cannot move in or out. • Because of the distribution of the ions, the OUTSIDE of the neuron is slightly POSITIVE when compared to the inside. ∼ At rest, the membrane is not permeable to these ions. ∼ The sodium and potassium gates are closed. ∼ The ions cannot move in or out of the nerve cell. Depolarization: upswing (+40 mV at peak) • If a stimulus is strong enough to surpass threshold (-40mV), the membrane is suddenly made permeable to sodium. • The ‘gates’ for sodium are opened, and the sodium ions flood to the inside of the axon. Repolarization: downswing (-70 mV at lowest point) • When the voltage reaches +40mV, it causes the potassium gates to open, and the potassium floods to the outside. • This causes the initial polarity to be restored (at -70mV), but the ions are in the reverse positions. • At the same time the sodium gates are closing. Refractory Period: recovery + + • At -70mV, the K and Na voltage-gated channels are closed • Then the Na+/K+ pump (needs ATP) actively pumps the Na+ and K+ ions back to their original condition. • This re-establishes the resting condition so the neuron can conduct another impulse.

 

All or None Response: • As long as the threshold stimulus of -40mV has been reached, there will be an impulse. • Each impulse is equal to all other impulses. • This is called the “ALL OR NONE” response. µA stronger stimulus does not mean a bigger impulse. Rather it means a greater number of impulses (more nerves involved or a single nerve conducting a series of impulses) will give bigger results. ¯Most long fibres have myelin sheath around them, which allows the impulse to move faster (as if it ‘jumps’ from node to node). • The action potential can only occur at the nodes of Ranvier due to the insulation of ion loss along myelinated membrane

Propagation of an Action Potential along a neuron: Ø This entire process occurs along sections of the neuron like dominoes. – As action potential occurs, it “shocks” neighbouring protein ion channels to open causing the propagation of the action potential down the axon

SYNAPTIC TRANSMISSION The Synapse: • • • • • •

How does the impulse travel from one neuron to another? How does an impulse travel from a neuron to a muscle or gland (effector)? How can they do this when they are not physically connected (there’s a GAP)?

When an impulse arrives at the end of an axon, it must make a connection to the next nerve cell, or to the muscle or gland. BUT neurons do NOT directly contact each other There is a small space, termed the SYNAPTIC GAP / CLEFT, which the impulse must cross. Ø

How does this work?

PRESYNAPTIC MEMBRANE

POSTSYNAPTIC MEMBRANE

Encloses synaptic vesicles filled with neurotransmitters = n.t. (a chemical message manufactured by the axon).

Contains protein receptor sites within the membrane to recognize specific neurotransmitters

 

Synaptic transmission: STEP 1: Ø When an impulse arrives at the end of an axon, the sodium gates open and sodium floods into the axon bulb / terminal. STEP 2: 2+ Ø At the same time, the CALCIUM GATES OPEN and calcium (Ca ) also moves into the axon bulb / terminal of the presynaptic neuron. STEP 3: Ø The calcium binds with CONTRACTILE PROTEINS (microfilaments) attached to the vesicles and this causes them to contract, thus pulling the vesicles towards the pre-synaptic membrane o This requires ATP STEP 4: Ø EXOCYTOSIS occurs as the vesicles release neurotransmitters into the synaptic gap. The neurotransmitter diffuses across the gap. STEP 5: Ø Neurotransmitters bond with the receptor sites on the postsynaptic membrane.

STEP 6: Ø When an excitatory neurotransmitters attaches to the receptors, the voltage of the post-synaptic membrane changes to cause the sodium gates to open. o This depolarizes the membrane. Ø

If an inhibitory neurotransmitter is released and attaches to the receptors, the post-synaptic membrane will be hyperpolarized which makes it more difficult for the threshold to be reached (less likely an action potential will occur).

STEP 7: Ø If the synapse is between an axon and dendrite, then the Action Potential will continue down the next neuron Ø If the synapse is between an axon and a muscle cell, then the muscle will contract. Ø If the synapse is between an axon and a gland, then the gland will release a hormone. STEP 8: Ø The synaptic gap contains enzymes that will destroy the neurotransmitters, thus returning the synapse to its original condition prior to the arrival of the impulse STEP 9: Ø The calcium ions are returned to the synaptic gap by active transport. Ø Because only the axon bulb has the neurotransmitters and only the dendrite has the receptors, synaptic transmission may only occur in one direction. Ø The energy for this entire process comes from the mitochondria that can be found in abundance in the synaptic / axon bulb.

 

The 2 neurotransmitters you need to know: Acetylcholine: • This is responsible for promoting all responses in a relaxed state. • Also involved in controlling skeletal muscles. ⇒ it is destroyed by the enzyme acetylcholinesterase Noradrenalin: • This is the excitatory transmitter (also known as norepinephrine). • It almost always increases the activity of the receiving cell / tissue / organ. • It is involved in ‘fight or flight’ situations (stress). ⇒ it is destroyed by the enzyme monoamine oxidase

How Drugs Affect the Synapse: Drugs have different ways of acting on the synaptic transmission system: 1. Some drugs hold the receptors open for a longer time. § Alcohol causes the GABA neurotransmitters to work for a longer amount of time, thus quieting the brain more than normal. 2. Some drugs block the enzymes from destroying the neurotransmitters. § When you are depressed, your serotonin is usually reabsorbed before it can do its job. v Prozac stops this from happening. 3. Some drugs cause increased secretions of neurotransmitters • Cocaine increases dopamine secretion and causes pleasure sensations. • Ecstasy increases serotonin secretion and produces a sense of intimacy with others and diminished feelings of fear and anxiety. 4. Some drugs imitate or mimic the neurotransmitters and take their place on the receptors. • Morphine binds to the receptors that endorphins naturally would and cause a sense of well being (like you normally get after exercise). • Nicotine binds to the receptors for acetylcholine and cause arousal and reward sensations. • Caffeine counters inhibitory neurons to increase alertness. 5. Some drugs stop the neurotransmitters from joining the receptors. Ø Pain killers occupy the receptor sites so that the sensation of pain cannot be transmitted between the nerve cells.

Drug Addiction (ex. Cocaine) — — — — —

Normally at a synapse, the neurotransmitter is broken down and recycled so receptors do not continually fire. Some drugs, like cocaine, bind to “recyclers” so the neurotransmitter cannot be removed from the synapse which causes neurons to increase their rate of depolarization. To compensate, the postsynaptic membrane can decrease the number of protein receptors. — If you remove the drug, the level of neurotransmitter returns to normal BUT with fewer receptors. Result with usage? Decreased number of receptors will make it difficult to depolarize the neurons which makes the “high” more difficult to get. You crave the drug! And can experience withdrawal symptoms without it.

PARTS OF THE NERVOUS SYSTEM The nervous system can be divided into numerous parts:

NERVOUS SYSTEM í

Peripheral Nervous system

í Somatic ê Voluntary í î Motor Sensory

î

Central Nervous system

î í Autonomic Brain ê Involuntary í î Sympathetic Parasympathetic

î Spinal Cord

1. Peripheral Nervous System (PNS) -all nerves outside the brain and spinal cord -carries information between the body and the CNS -further broken down into: i)Somatic Nervous System (voluntary) -may be consciously controlled = voluntary muscles - only takes one neuron from CNS to muscle -nerves carry impulses between receptors and effectors Eg.) Reflex Arc – burning hand →moving hand away →screaming ii)Autonomic Nervous System (ANS) (involuntary) -functions automatically = involuntary muscles Eg.) internal organs, smooth & cardiac muscles, glands -takes 2 neurons with a synapse and ganglion -ANS further broken into 2 types of autonomic nerves: a) Sympathetic Nervous System -prepares body for stress I.e. “fight or flight response” in emergency situations (requires the parasympathetic system to return to normal) →inhibits digestion, dilates pupils, ↑ heart rate, more blood to skeletal muscles, ↑ breathing -adrenal medulla (adrenal glands near kidney) release norepinephrine as main neurotransmitter 9 similar chemical structure to adrenaline b) Parasympathetic Nervous System -promotes a relaxed state (vegetative body functions - rest) →pupils contract, food digested, $ heartbeat -main neurotransmitter released is acetylcholine

2. Central Nervous System (CNS) -brain and spinal cord -all completely enclosed in bone and wrapped in 3 protective membranes (MENINGES) 9 space in between are full of cerebrospinal fluid which cushions and protects the spinal cord and brain -these are tested during a spinal tap -control centre of nervous system →receives sensory input from PNS and formulates responses -both areas contain 2 types of nerve tissue: Grey matter = unmyelinated fibres (cell bodies, short fibres) White matter = myelinated fibres

THE BRAIN Embryonic Brain Regions • Hindbrain • Midbrain • Forebrain Brain Stem / Hindbrain: • The brain stem is the smallest and the oldest and most primitive part of the brain. • The brain stem is continuous with the spinal cord, and is composed of: 1. Midbrain 2. Medulla oblongata 3. Pons Brain Stem: Medulla Oblongata • The ‘unconscious’ part of the brain. • Is closest to the spinal cord • Involved with the regulation of heartbeat, breathing, vasoconstriction (blood pressure) • Has reflex centers for vomiting, coughing, sneezing, swallowing, and hiccupping. Brain Stem: Pons • Latin for “bridge” • Carries on activities of the medulla oblongata Brain Stem: Midbrain • Connects the hindbrain and forebrain. • Contains reflex centers for vision, hearing and touch

 

Cerebellum • Leafy, butterfly-shaped near brain stem • Impulses for movements are coordinated here • Coordinates small, smooth movements (as in fine motor control) • Maintains normal muscle tone & posture, and coordinates balance FOREBRAIN Cerebrum • Wrinkled to increase surface area • Divided into a right (visual) and left (verbal) cerebral hemispheres, which are connected by the corpus callosum. • Hemispheres are covered by a thin layer known as the cerebral cortex (responsible for intelligence) • The cerebrum coordinates sensory data and voluntary motor functions. • Where memory is kept; where conscious thought processes are made • Where impulses requiring some processing go before responses are made(ie: responding to a verbal question). • Overall, the cerebrum governs intelligence and reasoning, planning, learning, memory, and personality. • Has mostly white matter (myelinated axons) • To a less extent, grey matter (cell bodies, dendrites & unmyelinated axons) • Integrates motor commands The cerebrum is divided into 4 lobes according to function. 1. Frontal Lobe - conducts 3 functions: a) Motor control b) Speech production c) Thought processes 2. Parietal Lobe - associated with sensations (processes information about touch, taste, pressure, pain, and heat/cold) and understanding speech 3. Occipital Lobe - receives and processes visual information 4. Temporal Lobe - receives auditory signals, olfactory signals, processing language and the meaning of words. v No region of the brain functions alone Corpus Collosum: • •

Dense tissue that holds the two hemispheres of the cerebrum together. Conducts impulses from one side of the brain to the other; therefore, coordinates the activities of the two sides of the brain.

Thalamus • Directs impulses that travel up the spinal cord to the correct region of the cerebrum • Often referred to as the ‘sorting and relay station’ • Filters out extraneous and unimportant sensory stimuli

 

Hypothalamus – under thalamus • Regulates homeostasis. • Has regulatory areas for thirst, hunger, body temperature, water balance, and blood pressure. • Links Nervous System to Endocrine System (neuroendocrine control center) • This part of the brain also has control over the internal organs. • It samples the blood that travels through it and responds, either through the initiation of nerve impulses or by causing the pituitary gland to release hormones. Pituitary Gland • Also known as the ‘Master Gland’. • Small gland with two parts: the anterior and posterior lobes. • Produces a large number of hormones, many of which control the release of hormones from other glands in the body. Posterior Pituitary: • This part of the gland releases the hormones that are made in the Hypothalamus, but are stored and secreted by the Posterior Pituitary. Examples of hormones: (eg. ADH and Oxytocin) Anterior Pituitary: • This part of the gland makes and releases its own hormones. • It is stimulated to release its hormones by the hypothalamus hormones. • There is a blood connection between the hypothalamus and the anterior pituitary. Examples of hormones: Growth hormone, Prolactin, FSH & LH, Thyroid Stimulating Hormone (TSH), Adrenal Cortex Stimulating Hormone (ACTH), Melatonin Hormones that control homeostasis are controlled by: 1. Negative feedback (self-regulating) • hormone reaches a certain level it will stop the release of more of that hormone. 2. Other hormones • hormone with opposite function is released to stop the initial hormone release