Animal Anatomy and Physiology 2 Assignment 2500 words March 2016 50% -Anatomy & physiology of a quadruped (horse) based upon structure and function of the endocrine and nervous system. -Importance of Homeostatic principles and the regulation of function of the two systems Exam @ university (after easter) 2 hours 50% Create a glossary Refer back to Learning Objectives find weak areas + example exam questions? Homeostasis Homeostasis: “The coordinated physiological processes which maintain most of the [constant] states in the organism” Hill et al (2012) Homeostasis: “the tendency for living things to attempt to maintain a state of relative stability. At either a whole-animal level or at a cellular level.” Frandson et al (2008) Homeokinesis: Mechanisms maintaining a relatively stable but constantly changing state of the body -Behavioural Freedom = allows mammal’s freedom to conduct their lives regardless of outside conditions -Healthy functioning = thermoregulation, osmoregulation, excretion, maintaining blood glucose levels, blood pressure, pH etc Regulation: Advantages Disadvantages Mammal independent of variations in outside climate Costs energy Conformity: Advantages Disadvantages Energetically Cheap Cells within body subject to change when outside conditions change Osmoregulation (physiology and metabolic) Thermoregulation (behaviour and physiology) Blood glucose Homeostatic Challenges: -Diabetes in companion animals -Thermoregulation in production pigs -Fevers -Dehydration -Blood pressure in stressed animals -Cell Death -Behavioural/Psych problems -Chronic stress in captives Thermoregulation Two types of organism: -Endotherms: Organisms that gain heat from their internal metabolism (warm blooded eg. mammals) -Ectotherms: Organisms that gain heat from their environment (cold blooded eg. Reptiles) Homeothermic 37⁰C: Optimal body temperature Higher above this: -Protein denaturation -Nerve malfunction -41⁰C can cause convulsion -43⁰C absolute limit in most animals – death? Heat Loss & Gain -Radiation (gain & loss) -Conduction (gain & loss) -Convection (gain & loss) -Evaporation (loss) Physiological Adaptions to Prevent Heat Loss -Hair/fur – eg. Polar Bears -Feathers – trap air -Adipose tissue – eg. Blubber (seals, whales) Behavioural Responses to Prevent Heat Loss -Curl up (reduction of surface area -Group together -Make nests -Seek heat Effector Mechanisms to Prevent Heat Loss/Increase Heat Generation -Vasoconstriction = decrease in blood vessel diameter -Reductions Effector Mechanisms to Prevent Heat Loss/Increase Heat Generation -Shivering and Increased voluntary activity -Increased secretion of epinephrine (adrenaline) -Increased food appetite Autonomic Nervous System Control Sympathetic (hot) -Contracts erector pili muscles -Increases sweat production -Increases ventilation rate Parasympathetic (cold) -Decreased sweat production -Decreased ventilation rate -Relaxes erector pili Anatomy and Physiology of Sweating/Sweat Glands -2-3 million sweat glands -2 litres of sweat can be lost by evaporation in 1 hr -Loss of water and electrolytes (Na+) Thermoregulatory Feedback Loop Homeostatic system: -Receptors to measure -Effectors to alter -Set point or target value -Integrating Centre sorts the info from the receptors to send to the effectors eg. Brain spinal cord Principles of Feedback -There needs to be feedback of information from the receptors to allow the current value to be compared to the set point and implement appropriate actions if they differ Negative Feedback Loops -Whenever there is a change in the system, this automatically causes and corrective mechanism to start -This reverses the original change -The bigger the original change the bigger the corrective change -Carried out by antagonistic effectors Perturbing factor Stimulus Sensor Integrating centre Effector Response Vasodilation (arteries/veins/capillaries open and close to hold or release heat) The Endocrine System Cushing’s Disease – Hyperadrenocorticism -Disease of the adrenal cortex excessive amount of gulcocorticoids (cortisol) Endocrine system -Part of the regulatory system of the body -Endocrine means ‘Internal Secretion’ -Endocrine Glands secrete directly into the bloodstream, not through the ducts (Ductless glands) Hormones -Chemical compounds produced in one part of animal body to produce response in another -Very small amount can have a large effect on whole organism -Hormones are secreted by either: o Endocrine organs OR o Directly into the bloodstream -Carried around the bloodstream until they reach their target organs, which are structures respond to them (they are selective) Structure of hormones (3 categories): 1. Derived from amino acids Quick Acting eg. Adrenaline, noradrenalin and thyroid hormones 2. Derived from lipids, belong to one of two groups Slow Acting Steroid hormones or eicosanoid eg. Testosterone, oestrogens (steroid), prostaglandin (eicosanoid) 3. Peptide hormones Quick Acting Formed from peptides or polypeptides Eg. Antidiuretic hormone (ADH), oxytocin and follicle stimulating hormone (FSH) and luteinizing hormone (LH) Effect of hormones -Each hormone has a specialised effect upon the target cell, tissue or organ -Alters the function of the target cells -Can change the properties of the target cell -Specialised receptor sites on cells Hormones -Synthesized, stored and released by non-neural endocrine cells or neurons -Travel through the circulating blood and exert their effects on target tissues -Then are metabolically destroyed or excreted from the body Magnitude of effect depends on: -Abundance of receptor sites -Concentration in the blood Balance of rate between synthesis and degradation or excretion Hormone secretion -Released in areas that have a large capillary network -Either travel freely or with a carrier protein (steroid) -Release is stimulated by another hormones or a neurotransmitter -Controlled by a negative feedback mechanism (except oxytocin) Comparison of Hormonal and Neural Communication Hormonal Communication Neural Communication Chemical Slower Rapid Carried all around body = widespread Specific destination = localised Continue over long period Short-lived Unconscious Mostly conscious actions (pain, movements) Major Glands: -Pituitary -Thyroid -Parathyroid -Thymus -Adrenal -Pancreas -Ovaries -Testes -Pineal body The Hypothalamus -Integration centre of the body -Links the endocrine system to the nervous system via the pituitary gland -Located blow the thalamus, above the brainstem -Size of an almond (in humans) -Closely linked (geographically, physically and functionally) with the pituitary gland -Responsible for certain metabolic processes and some activities of the ANS -Controls body temperature, hunger, parenting and attachment behaviours, thirst, sleep, fatigue and circadian rhythms (physical, mental and behavioural changes that follow a 24 hour cycle) -Synthesizes and secretes neuro-hormones often know as releasing hormones or hypothalamic hormones stimulate/inhibit pituitary hormone secretion Relationship between hypothalamus and pituitary gland -Pituitary gland attached to hypothalamus -It is also known as the Hypophysis -Portal system (blood vessels) link hypothalamus to anterior pituitary gland The Pituitary Gland -Master endocrine gland -Size of small pea/bean (humans -lies within the cranium on ventral surface of midbrain -Controls the other endocrine glands Two areas: -Anterior lobe / adenohypophysis -Posterior lobe / neurohypophysis Divided into: -anterior pituitary (adenohypophysis) -posterior pituitary (neurophyphysis) The Anterior Pituitary Hormones 1. Growth Hormone (Somatotropin) Regulate metabolism of proteins, carbohydrates & lipids within body cells GH encourage anabolism/synthesis of proteins by body cells Breakdown lipids for energy production Discourage cells from using glucose (hyperglycemic effect) effect opposite pancreatic hormone insulin 2. Prolactin Triggers & maintains lactation Due to stimulation of teat nipple 3. Thyroid Stimulating Hormone Stimulate growth & development of thyroid glands 4. Adrenocorticotropic Hormone SEE POWERPOINT 4 5. Follicle Stimulating Hormone Stimulate growth and development of ovaries Stimulate lining of follicle to secrete oestrogens 6. Luteinizing Hormone Complete follicle development in ovaries In males LH (Interstitial cell stimulating hormone) stimulates interstitial cells to develop & produce testosterone [interstitial = fluid between the cells] 7. Melanocyte Stimulating Hormone Control colour changes in the pigment cells (melanocytes) The Posterior Pituitary -AKA. Neurohypophysis -Does not produce any hormone but stores (and releases) two hormones produced by the hypothalamus -Vasopressin/Antidiuretic hormone (ADH) o Increases water absorption by the kidneys o ADH release is affected by alcohol and caffeine o Deficiency results to Diabetes insipidus Polyuria (excessive urination) Polydipsia (excessive drinking) -Oxytocin (positive feedback systems) o Involved in parturition and lactation o Causes contractions during labour Glands of the Endocrine System Thyroid Gland -Found below the larynx -Secretes: o Thyroxine regulates metabolic rate o Tri-iodothyronine regulates metabolic rate o Thyrocalcitonin or Calcitonin decreases levels of plasma calcium -Consists of two lobes located either side of the larynx -Shape – differs in several animal spp -Composed of tiny follicles (a sac of pouch like depression or cavity) where thyroid hormone is produced -Absorbs material from bloodstream including iodine to produce thyroxine -2 types of hormone producing cells: o Follicular cells o Parafollicular cells (C cells) Calorigenic effects of thyroid hormones (TH) Calorigenic: producing or increasing production of heat or energy increasing oxygen consumption -Regulate metabolic rate of the body cells – rate at which they burn nutrients to produce energy (Controls weight gain/loss) -TH production increases with exposure to cold temperatures -TH production is inhibited by stress Follicular Cells -Hollow secretory cells -Secretions of thyroxine regulated by TSH (thyroid stimulating hormone) – released by anterior pituitary gland Functions: -Regulate metabolic rate -Influence absorption of sugars by intestines – how much sugar is absorbed, how much sugar you need -Stimulate energy production in cells -Influences blood – cholesterol levels Parafollicular Cells -2nd type of secretory cells in thyroid glands -Secrete calcitonin -Regulates Ca2+ levels along with parathyroid hormone Calcitonin -Produced by C cells located between the thyroid follicles -Maintain homeostasis of blood sugar levels -Body functions: o Muscle contraction o Blood clotting o Milk secretion o Formation and maintenance of skeleton -Hypercalcaemic effect – encourages excess Ca to be deposited in the bones Control of Thyroid Hormones -Hypothalamus produces the thyrotropin-releasing hormone (TRH) -TRH stimulates pituitary gland to produce Thyroid stimulating hormone (TSH) -TSH regulates the rate of T4 and T3 release -Thyroid hormones are active only if they are free (unbound) in the blood to transport proteins Tropins Hormones that influence the functions of other endocrine glands Imbalance in thyroid production -Hypothyroidism – under activity of thyroid gland -Hyperthyroidism – enlargement of gland and over activity Hypothyroidism -Shortage of iodine can result in under secretion of thyroxine -Can lead to: o Stunted growth o Lower resistance to cold o Lack of energy Hyperthyroidism -Over production increases metabolic rate: o Rapid Heart Rate o High Blood pressure o Rapid weight loss The Parathyroid Glands -Produces parathormone or parathyroid hormone (PTH) -Several small pale nodules on or near thyroid gland -Regulates metabolism and distribution of calcium in blood o Increases absorption of calcium from digestive tract o Increases absorption from bone o Decreases excretion in urine -4 tiny glands -Parathyroid Hormone (PTH) -Metabolism of Ca2+ & P -Low blood Ca2+ triggers secretion of PTH -Releases Ca2+ from bones -Acts of kidneys to ↓ Ca2+ and ↑P excretion -Promotes Ca2+ absorption from intestine Hyperparathyroidism -Overactivity of the parathyroid glands resulting in excess production of PTH -Harmful to bone The Adrenal Glands -Two parts: o Inner medulla (adrenal medulla) o Outer cortex (adrenal cortex) -Both from different embryonic development structures & functions Medulla -Produces Adrenalin & Noradrenalin -Secretion directly controlled by sympathetic nervous system (SNS) stimulated by nervous impulses in the Brain -Fight/flight response Hormones of the Medulla -Adrenaline/epinephrine -Noradrenaline/norepinephrine -‘fight or flight’ hormones -Not equivalent but similar in action on tissues -Adrenalin o Increases heart rate o Increases blood pressure o Vasoconstrictors for most tissues o Vasodilators for muscles and bronchus blood vessels in lungs and muscles increase in diameter = SA ↑ = ↑ in gaseous exchange = ↑ in efficiency (get more out of each breath) o Increases blood sugar levels more energy Fight/Flight -Frontal lobe of brain is stimulated by nerve impulses -Hypothalamus & adrenal medulla stimulated -Adrenal medullary secretion permits a rapid response to stressful stimuli -Adrenalin causes pupils and bronchioles to dilate, heart rate & blood pressure increases -Bladder and bowels may evacuate Noradrenalin -Also a nerve transmitter substance -Released at the synapses of sympathetic nerve cells and has the same effect in preparing a horse for fight or flight Adrenal Cortex Produces corticosteroids: -Cortisone produced in response to stress -Adrenocorticotropic hormone (ACTH) released from pituitary gland -Aldosterone – electrolyte and fluid balance -Glucocorticoids – metabolism of carbs – raising the blood sugar level -Androgens – sex hormones – responsible for sexual development and function in males Aldosterone -Maintains correct mineral balance (electrolytes) – sodium and potassium -Increases extracellular volume by increasing absorption of sodium from the urine and colon Cortisol -Stress related -Increases blood sugar level by mobilising energy reserves -Stimulates protein breakdown -Inhibits inflammation Adrenal sex hormones -Oestrogen in both male and female -Control body hair growth in human females but levels too low to have substantial effect on males -Possibly involved in castrated males trying to mount Hormones of the Cortex -Produces more than 24 different hormones -All steroid-based -Carried in circulation by carrier proteins -Corticosteroids – 3 groups o Mineralocorticoids eg. Aldosterone o Glucocortoids eg. Cortisol o Adrenal sex hormones Diseases of the Adrenal Cortex -Hypoadrenocorticism o Addison’s disease adrenal cortices can’t produce adequate levels of aldosterone, resulting in the increase in retention of potassium in circulation (usually young to middle age dogs but can occur in any age) The Pancreas -Located near duodenum -Exocrine glandular tissue produces digestive enzymes -Endocrine tissue produce – Insulin -Involved in the maintenance of blood sugar levels -Islet of Langerhans cells produce: o Glucagon (from alpha cells) increases BSL o Insulin (from beta cells) decrease BSL o Somatostatin (from delta cells) suppresses surges of insulin or glucagon production o Pancreatic peptide (from F cells) related to production of pancreatic enzymes – more research required (little known about it) Regulation of blood sugar level -Glucose conc. Is controlled within the range 0.8 – 1g per dm3 of blood & very < levels (hypoglycaemia) or very > levels (hyperglycaemia) can be fatal -Controlled by pancreas -Glucose receptor cells monitor the concentration of glucose in the blood -Islets of Langerhans o α cells glucagon & β-cells - insulin. Antagonistic (works together) and have opposite effects on blood glucose o Insulin uptake of glucose by cells for respiration. In the liver – conversion of glucose to glycogen (glycogenesis). It therefore decreases blood sugar level o Glucagon breakdown of glycogen to glucose in the liver (glycogenesis) synthesis of glucose from pyruvate therefore > blood glucose -After a meal – glucose (G) is absorbed from the gut into the hepatic portal vein, increasing the blood G concentration -Pancreas -secretes insulin from its β-cells in response -Insulin -G to be taken up by the liver and converted to glycogen -< blood G, which causes the pancreas to stop secreting insulin -If G level < too far, pancreas detects this & releases glucagon from α-cells -Glucagon causes the liver to break down some of its glycogen store to G - diffuses into the blood -> blood G - causes pancreas to stop producing glucagon The Gonads -Male and female gonads = testes & ovaries -Ovaries o Oestrogens Produced in ovary by the developing cells of follicle, prior to ovulation Increases attractiveness and receptiveness to males Prepares reproductive tract for fertilisation o Progesterone Produced by the corpus luteum of the ovary following ovulation (↑LH) Maintains pregnancy o Relaxin Produced by corpus luteum, placenta and uterus in later stages of pregnancy Prepares female body for birth Relaxes ligaments around birth canal Production is stimulated by LH (early pregnancy) and chorionic gonadotropin (later pregnancy) -Testes o Produces testosterone and oestrogen o Testosterone Produced by Cells of Leydig Involved in sperm production Maintains secondary sexual characteristics Maintains sex drive o Oestrogen Produced by Serotoli cells Maturation of spermatozoa Secretion is controlled by FSH Introduction to the Central Nervous System Functions -Communication system -Sends rapid electrical and chemical messages around the body to allow mammals an efficient response to their surroundings -Help maintain homeostasis by coordinating the function of internal organs The Brain -Coordinates body activities -Made up of approx. 100 billion neurons -Divided into 3 major parts: o The cerebrum o The cerebellum o The brain stem Cerebrum -Largest part of the brain -Memory is stored -Movements are controlled -Impulses from the senses are interpreted Cerebellum -Interprets stimuli from eyes, ears, muscles -Controls voluntary muscle movements -Maintains muscle tone -Helps maintain balance Brain Stem -Connects brain to spinal cord - controls involuntary actions -Made up of: o The midbrain, the pons act as pathways connecting various parts of the brain with each other o The medulla The Spinal Cord -Extension of the brain stem -Bundles of neurons carries impulses from all parts of the body to the brain and from the brain to all parts of the body Nerve Cells Neurons -Basic functioning units of the nervous system -Carries messages called an impulse -Made up of: o Cell body o Branches called dendrites and axons o Dendrites receive messages from other neurons and send them to the cell body o Axons carry messages away from the cell body Axons Take information away from the cell body Dendrites Bring information to the cell body Smooth Surface Rough Surface (dendritic spines) Generally only 1 axon per cell Usually many dendrites per cell No ribosomes Have ribosomes Can have myelin No myelin insulation Branch further from the cell body Branch near the cell body Types of Neurons: -Sensory Receive information and send impulses to the brain or spinal cord -Interneuron send impulses from sensory neurons to motor neurons -Motor Neuron Conduct impulses from the brain or spinal cord to muscles or glands throughout the body Sensory Neuron Lengt Long dendrites and short h of axons Fibre s Locati on Cell body and dendrite are outside of the spinal cord. The cell body is located in a dorsal root ganglion Funct Conduct impulses to the ion spinal cord Interneuron Motor Neuron Short dendrites and short or long axon Short dendrites and long axons Entirely within the spinal cord or GNS Dendrites and the cell body are located in the spinal cord. The axon is outside of the spinal cord Interconnect the sensory neuron with Conduct impulse to an effector (muscle or gland) appropriate motor neuron Membrane Potentials -Intracellular fluid contains o High concentrations of K+ o Ionized non-diffusible molecules particularly proteins with negatively charged side chains and phosphate compounds o Distribution of these charged particles results in the electrical phenomena of plasma membranes -Significant role in: o Signal integration o Cell-to-cell communication Electrical Potential -Aka. Potential Difference -Units = Volts (or millivolts) -Movement of electrical charge = Current -I = V/R Resting Potential -In a resting neuron there is a difference in electrical charges of the outside and inside of the plasma membrane -Outside = pos+ charge -Inside = neg— charge Na+/K+ Pump (electrogenic pump) -Keeps an ion gradient across the cell membrane -Three Na+ to the outside of the cell and two K+ ions to the inside -The sodium-potassium pump is an important contributor to action potential produced by nerve cells -Called a P-type ion pump because of ATP interactions phosphorylating the transport protein (loses a phosphorus which causes a change in its conformation) Contribution of Active Transport -Actively transports Na+ molecules out and K+ cells in to ensure there are 3 Na+ outside and 2 K+ inside -Occurs via the sodium-potassium pump Contribution of Facilitated Diffusion -Sodium-potassium pump creates a concentration and electrical gradient for Na+ and K+ -K+ tends to diffuse out of the cell and Na+ tends to diffuse in Membrane is much more permeable to K+ (so K+ diffuses out along its concentration gradient) Gradient Potentials and Action Potentials -Changes in membrane potential from resting potential produce electrical signals -Such changes are the most important way nerve cells process and transmit information Changes in Membrane Potential -Depolarized when potential less –ve (closer to zero) than resting level (-30mV) -Overshoot Reversal of the membrane potential polarity = when inside the cell becomes +ve relative to outside -Repolarized when a membrane has been depolarised returns to resting value -Hyperpolarized potential is more –ve than resting level Action Potential -Self-regenerating wave of electrochemical activity (in response to stimuli) that allows excitable cells (nerve/muscle) to carry signal over a distance -(All or nothing) -When the cell membranes are stimulated, there is a change in the permeability of the membrane to sodium ions (Na+) so Na+ ions diffuse into the cell down a concentration gradient -The membrane becomes more permeable to Na+ and K+ 1. The first step of the action potential is that the Na+ channels open allowing a flood of sodium ions into the cell. This causes the membrane potential to become positive. 2. At some positive membrane potential the K+ channels open allowing the potassium ions to flow out of the cell. Next the Na+ channels close. This stops inflow of positive charge. But since the K+ channels are still open it allows the outflow of positive charge so that the membrane potential plunges. 3. When the membrane potential begins reaching its resting state the K+ channels begin to close. Now the sodium/potassium pump starts working again and starts transporting sodium out of the cell, and potassium into the cell so that it is ready for the next action potential. The action potential travels down the length of the axon as a voltage spike. It does this using the steps outlined above. As a section of the axon undergoes the above process it increases the membrane potential of the neighbouring section and causes it to spike. This is like a mini chain reaction that proceeds down the length of the axon until it reaches the synapse. Myelinated Axons -Axon = a single long, thin extension that sends impulses to another neuron -Surrounded by a many-layered lipid & the myelin sheath formed by Schwann cells The Central Nervous System (CNS) Spinal Cord Made up of: o Gray matter – in the centre of the spinal cord and is densely packed with cell bodies and dendrites o White Matter – mostly of myelinated axons that carries information from the gray matter to the brain or other areas of the spinal cord -Each segment sends sensory information to the brain and receives motor commands (movements) Meninges -Connective tissue layers that surround brain and spinal cord -Contain blood vessels – nutrients & O2 -Dura mater – outermost -Arachnoid – delicate & spiderlike -Pia mater – thin layer Cerebral Spinal Fluid -Baths and protects brain and spinal cord -Circulates between layers of meninges -May also be involved in autonomic functions like respiration and vomiting Blood-brain barrier -Functional barrier separating capillaries in brain from nerve tissue -Prevents many drugs, proteins and other molecules from passing from blood to brain -Protects brain from poisons circulating in blood Synapses An anatomically specialized junction between two neurons Point at which the electrical activity in one neurone, the presynaptic neuron, influences the electrical of metabolic activity in the second, the postsynaptic neuron Activity at synapses can increase or decrease the likelihood that the postsynaptic neuron will fire action potentials (by producing a brief, graded potential in the postsynaptic membrane) Neurotransmitter -Chemical messenger used by neurons to communicate with each other or with effectors eg. Serotonin, dopamine, noradrenaline In brief: 1. Stored and released by the presynaptic membrane at synapse of first neuron when stimulated by Action Potential 2. Neurotransmitters diffuse across synapse and bind to postsynaptic receptors on membrane of second neuron 3. Stimulates or inhibits second-messenger activity within post-synaptic cell The Peripheral Nervous System (PNS) Comprised of: 1. Somatic Nervous System 2. Autonomic Nervous System -Efferent neurons transport signals from the brain to these systems via the efferent division of the nervous system -Neuron in the PNS transmit signals between the CNS and the receptors and effects in all other parts of the body Somatic Nervous System -Consists of nerves that: o Convey sensory information to the CNS o Transmit messages for motor movement from the CNS to the body Autonomic nervous system -Regulates the automatic behaviours of the body (heart rate, blood pressure, respiration, digestion etc.) -Innervates (supplies) smooth and cardiac muscle, glands and neurons in the gastrointestinal tracts -3 Subsystems 1. Sympathetic Nervous System 2. Parasympathetic Nervous System 3. (Enteric Nervous System) nerve network in the gastro-intestinal tract Sympathetic Nervous System Network of nerves that prepares the organs for rigorous activity -Increases heart rate, blood pressure, respiration etc. (“fight or flight” response) -Comprised of ganglia on the left and right of the spinal cord -Mainly uses noradrenaline as a neurotransmitter at the postganglionic synapses Parasympathetic Nervous System Facilitates vegetative, nonemergency responses by the organs (“Rest and Digest”) -Decreases functions increased by the sympathetic nervous system -Comprised of long preganglion axons extending from the spinal cord and short postganglionic fibres that attach to the organs themselves -Dominant during our relaxed states Enteric Nervous System -Aka. Intrinsic Nervous System -One of the main divisions of the nervous systems -Consists of a mesh-like system of neurons that govern the function of the gastrointestinal tract -Usually referred to as separate from the ANS since it has its own independent reflex activity PNS Nerves -Peripheral nerves contain nerve fibres that are the axons of both efferent and afferent neurons or both Efferent neurons = transport signals away from the CNS to the effectors Afferent neurons = transport signals from sensory receptors to the CNS -43 pairs of nerves o 12 pairs of cranial nerves Some only afferent eg. Optic o 31 pairs that connect with the spinal cord as spinal nerves Both afferent and efferent Spinal Nerves -31 pairs -Categories for each of the pairs of spinal nerves -8 cervical spinal nerves -12 thoracic spinal nerves -5 lumbar spinal nerves -5 sacral spinal nerves -1 coccygeal spinal nerve Reflexes and the reflex arc Reflexes Rapid, automatic response to stimuli designed to protect the body and maintain homeostasis -Maintain balance and posture (eg. Spinal reflexes control trunk and limb muscle) -Brain reflexes involve reflex centre in brainstem (eg. Eye movement) -Somatic Reflexes involve contraction of skeletal muscles -Autonomic reflexes regulate smooth muscle, cardiac muscle and endocrine glands -All reflexes have basic structure called a reflex arc Reflex Arc -Originate from a sensory receptor – detect changes in internal/external environment -Reflex arc – a response to a perturbing stimulus that acts to return the body to homeostasis -Action Potential ( nerve impulse) go through sensory neurons to gray matter of spinal cord to brain Classification Sensory Receptors are classified by 3 methods: 1. Receptor complexity 2. Classification by location 3. Classification by stimulus detected Parts of the Reflex arc 1. Receptor -detects the stimulus and sensitive to a specific type of internal or external change 2. Sensory Neuron -conveys the sensory info to brain or spinal cord -Transmit nerve impulses from the receptor into the brain or spinal cord 3. Interneuron (relay neurons) -Serves as processing centre, conducts nerve impulses from the sensory neuron to a motor neuron 4. Motor Neuron -Transmits nerve impulse from the brain or spinal cord out to an effector (muscle or endocrine gland) Monosynaptic Reflex A two neuron reflex arc -Involve only a sensory neuron & motor neuron – no interneuron -Only one synapse between them -Relies on a specialised structure within the muscle (muscle spindle) Stretch Reflex -Stretching the muscle activates the muscle spindle -Excited motor neurons of the spindle cause the stretched muscle to contract -Afferent impulses from the spindle result in inhibition of the antagonist -Example: Patellar Reflex o Tapping the patellar tendon stretches the quadriceps and starts the reflex action o The quadriceps contract and the antagonistic hamstring relax Stretch Reflex Monosynaptic Stretch Reflex -Simplest reflex because it only has one synapse in the path of its arc -Muscles spindles contain the sensory receptors for the stretch reflex -Each spindle contains modified muscle fibers called spindle of intrafusal fibers (inside spindle), innervated by efferent fibers -The middle segment of each spindle fiber acts as a mechanical stretch receptor that is connected to a sensory afferent nerve to the spinal cord -Stretching of the muscle stretches the spindle fibers activating the muscle spindle stretch receptors and the associated sensory fibers Afferent = Sensory Efferent = Motor
