Nervous System Anatomy and Physiology PDF

# Structural Classification of the Nervous System - **Central nervous system**(CNS) - Brain - Spinal cord - **Peripheral nervous system**(PNS) - Nerves outside the brain and spinal cord # Functional Classification of the Peripheral Nervous System - **Sensory (afferent) division**...

# Structural Classification of the Nervous System - **Central nervous system**(CNS) - Brain - Spinal cord - **Peripheral nervous system**(PNS) - Nerves outside the brain and spinal cord # Functional Classification of the Peripheral Nervous System - **Sensory (afferent) division** - Nerve fibers that carry information to CNS - **Motor (efferent) division** - Nerve fibers that carry impulses away from CNS - **Motor (efferent) division:-** - Somatic nervous system = voluntary - Autonomic nervous system = involuntary # Functions of the Nervous System 1. **Sensory input** - gathering information 2. **To monitor changes occurring inside and outside the body** - Changes = stimuli 3. **Integration** - To process and interpret sensory input and decide if action is needed. 4. **Motor output** - A response to integrated stimuli. - The response activates muscles or glands. # Nervous Tissue:- - The structural unit of CNS is the nerve cell (neuron). - Neurons:- Cells specialized to transmit messages - **Major regions of neurons:** - **Cell body** - nucleus and metabolic center of the cell. - **Processes** - fibers that extend from the cell body. # Cell body - **Nissl substance** - specialized rough endoplasmic reticulum. - **Nucleus** - Contain Large nucleolus. - **Extensions outside the cell body** - **Dendrites** - conduct impulses towards the cell body. - **Axons** - conduct impulses away from the cell body. - **A nerve is formed of a group of axons.** # Axons and Nerve Impulses - Axons end in axonal terminals. - **Axonal terminals:-** - Contain vesicles with neurotransmitters. - Are separated from the next neuron by a gap which is called **Synaptic cleft** (gap between adjacent neurons). - **Synapse** - junction between nerves. # Nerve Fiber Coverings - **Schwann cells** : produce myelin sheaths in jelly-roll like fashion. - **Nodes of Ranvier:** gaps in myelin sheath along the axon. - Some nerves have myelin sheath ( **myelinated nerves** ) and others have not and called **unmyelinated**. # Neuron Cell Body Location - Most are found in the central nervous system. - **Gray matter:** - cell bodies and unmylenated fibers . - **Nuclei:** - clusters of cell bodies within the white matter of the central nervous system. - **Ganglia** :- collections of cell bodies outside the central nervous system. # Sectional Organization of the Spinal Cord | | | | :---------------- | :----------------------------------------- | | Dorsal root | | | Dorsal root ganglion | | | Sensory nuclei | Somatic, Visceral | | Motor nuclei | Visceral, Somatic | | Ventral root | | | Anterior gray commissure | | | Anterior white commissure | | | Anterior median fissure | | | Posterior median sulcus | | | Posterior gray commissure | | | Posterior white column | | | Posterior gray horn | | | Lateral gray horn | | | Lateral white column | | | Anterior gray horn | | | Anterior white column | | # Functional Properties of Neurons - **Irritability** - Ability to respond to stimuli - **Conductivity** - ability to transmit an impulse. - The plasma membrane at rest is polarized. # Functional Classification of Neurons - **1-Sensory (afferent) neurons** - Carry impulses from the sensory receptors. - Cutaneous sense organs. - **2-Motor (efferent) neurons** - Carry impulses from the central nervous system. - **3-Interneurons (association neurons)** - Found in neural pathways in the central nervous system. - Connect sensory and motor neurons. # The action potential (nerve impulse):- (Starting a Nerve Impulse) - It is sudden reversal of polarity produced by a stimulus whatever this stimulus is electrical, chemical, mechanical, sound or light. - **Polarization** - At rest, fewer positive ions are inside the cell than outside the cell. - **Depolarization** - a stimulus depolarizes the neuron's membrane. - A depolarized membrane allows sodium(Na+) to flow inside the membrane. - The exchange of ions initiates an action potential in the neuron. - If the action potential (nerve impulse) starts, it is propagated over the entire axon. - Afterward, potassium ions rush out of the neuron after sodium ions rush in, repolarizing the membrane. - The sodium-potassium pump restores the original configuration. - This action requires **ATP**. # Resting Membrane Potential (RMP) - **Membrane potential** at which neuron membrane is at rest, i.e. does not fire action potential.(Vr) - It is the **potential difference** across the cell membrane. - It equal (-70mV) in nerve and (-90mV) in muscle. # Causes of RMP:- 1. **Selective permeability** of the membrane:- - The membrane is permeable to K , not Na. - This leads to unequal distribution of ions across membrane. 2. **Due to open potassium channels and closed sodium and chloride channels**. 3. **Membrane is 50 more permeable to K than to Na.** So, more +ve charge leak out. 4. The **non-diffusible ions** (protein, sulphate, phosphate ions) cannot leave the cell. - Proteins are -ve charged and cannot leave the cell. 5. **Action of ion pumps 3Na/2K ATPase.** - This is the most active transport mechanism in the body. - It transport 3 Na ions to outside and 2 K ions to inside. # A- Impulses are able to cross the synapse to another nerve by the following ways:- 1. **Neurotransmitter** is released from a nerve's axon terminal. 2. The **dendrite** of the next neuron has receptors that are stimulated by the neurotransmitter. 3. An **action potential** is started in the dendrite. # Nerve Impulse Propagation - The impulse continues to move toward the cell body. - Impulses travel faster when fibers have a **myelin sheath**. - **Two ways to increase AP propagation speed:-** 1. Increase internal diameter of axon which decreases the internal resistance to ion flow. 2. Increase the resistance of the plasma membrane to charge flow by insulating it with **myelin**. # Nervous System - **Peripheral Nervous System** - **Afferent (Sensory)** - **Efferent (Motor)** - Somatic Nervous System - Autonomic Nervous System - **Parasympathetic** - **Sympathetic** - **Central Nervous System** - **Brain** - **Spinal Cord** # Autonomic Nervous System - ANS controls: - glands - smooth muscles - cardiac muscle # Autonomic Nervous System (ANS) - **Sympathetic (SNS)** - UP (eyes, heart, lungs, liver, stomach) - DOWN (stomach) - **Parasympathetic (PSNS)** - DOWN (eyes, heart, lungs) - UP (stomach, liver) # Major Functions: - Maintain the performance of visceral organs, glands, smooth muscle, and cardiac muscle. - **Involuntary nervous system:-** - Regulates heart rate, blood pressure, respiratory rate, body temperature, smooth muscle contraction, glandular function, and secretions - **General visceral motor system:-** - Most "effectors" are visceral (organs & tissues regulated) - opposed to somatic the effectors (skeletal muscle & joints) # The Autonomic Nervous System - **Sympathetic** - **NorEpi** - Mydriasis - Reduced saliva flow - Increased SV & HR - Vasoconstriction - Glycogen -> glucose - Reduced peristalsis & secretion - Inhibition of bladder contraction - B. bronchodilation - **Parasympathetic** - **ACh** - Ganglia (N) - Miosis - Stimulated saliva flow - Decreased HR - Bronchoconstriction - Stimulates peristalsis & secretion - Stimulates bile release - Bladder contraction # Somatic vs. Autonomic Nervous Systems 1. **Effectors**(Target of Motor Neuron) - **SOMATIC** - skeletal muscle - **AUTONOMIC** - organs, glands, smooth & cardiac muscle 2. **Efferent (Motor Neuron) Pathway** - **SOMATIC** - one motor neuron from CNS to skeletal muscle - **AUTONOMIC** - two neurons - Pre-ganglionic neuron: - Located in the CNS. - Sends its axon to an autonomic ganglion located outside the CNS. - Slightly myelinated, thin axons. - Post-ganglionic neuron: - Located in the autonomic ganglion. - Sends its axon to the effector (tissue to be effected) - Unmyelinated, very thin axons. 3. **Coordination of Somatic and Autonomic Systems** - Both systems send their motor neurons to effectors via the 31 pairs of spinal nerves and 12 pairs of cranial nerves. - When skeletal muscle is active, autonomic must regulate other body activities for sufficient blood, oxygen, waste removal. 4. **Action of Neurotransmitters** - **SOMATIC** - acetylcholine (ACh); stimulatory - **AUTONOMIC** - post ganglionic neurons may release either ACh or Norepinephrine (NE) - whether response is stimulatory or inhibitory depends on the effector # Divisions of the Autonomic Nervous System - 1. **Sympathetic Division** - Classic "flight-or-fight" response in scary situations. - Increased heart rate. - Rapid, deep breathing pattern. - Cold, sweaty skin. - Dilated eye pupils. - Large burst of energy available (glucose) - Constriction of blood vessels. - Increased blood pressure. - Dilation of bronchioles (more air). 2. **Parasympathetic Division** - The "resting and digesting" system keeps body calm. - Stimulates digestion, urination, and defecation. # Acetylcholine (cholinergic) receptors 1. **Nicotinic receptors** - ACh receptors that can also be activated by nicotine-----STIMULATE - At neuromuscular junction of skeletal muscle. - Located on ALL postganglionic nerve cell bodies of ANS. - Hormone-producing cells of adrenal medulla. 2. **Muscarinic receptors** - ACh receptors which are activated by mushroom toxin muscarine. - STIMULATE or INHIBIT depending of target cell. - Located on the target cells of the body receiving ACh from the parasympathetic division. - Cardiac muscle ---> INHIBITORY. - Smooth muscle of GI tract- -> STIMULATORY. # Physiology of Autonomic Nervous System - **Autonomic Neurotransimitters and Their Receptors** 1. **Acetylcholine(ACh)** - The same neurotransmitter at the neuromuscular junction - Is used by autonomic fibers in two places (cholinergic fibers): - Secreted by **ALL preganglionic neurons** of the ANS. - Secreted by **MOST postganglionic neurons** of PARASYMPATHETIC onto target cells. 2. **Norepinephrine (noradrenaline)**- Released by most postganglionic neurons of the SYMPATHETIC system (adrenergic fibers). - **Exceptions:**: postganglionic sympathetic fibers to some sweat glands of skin, some blood vessels to skeletal muscle and external genitalia. # norepinephrine (noradrenergic) receptors - **Alpha adrenergic receptors** - Generally STIMULATE the effector organ/tissue - **Beta adrenergic receptors** - Generally INHIBIT the effector organ/tissue # Drug Effects on the Autonomic NS - **1. Synaptic cleft enzyme inhibitors** - Neostigmine - Blocks acetylcholinesterase for accumulation of ACh (myasthenia gravis). - **2. Tricyclic antidepressants** - Prolong the binding of norepinephrine to postsynaptic receptors - **3.beta-blockers** - Block binding of norepinephrine to beta-l receptors; reduce heart rate. # Antagonistic Control with Sympathetics & Parasympathetics | Target | Parasympathetic | Sympathetic | | :--------- | :---------------- | :---------------- | | Eye pupil | constricts | dilates | | Heart | decreases heart rate | increases heart rate | | Lung | constricts bronchioles | dilates bronchioles | | GI Tract | increases digestion | decreases digestion | | Urinary | promotes urination | inhibits urination | | Most glands | stimulate secretion | inhibits secretion | # Other Roles of the Autonomic Nervous System 1. **Cooperative Effects** - Occurs when both divisions produce different effects that work together to promote a single action. As seen in the control of external genitalia during sexual activities. - Parasympathetic causes vasodilatation of penis and clitoris; Sympathetic causes ejaculation and peristalsis of vagina. 2. **Sympathetic ONLY** - Stimulation of adrenal medulla to secrete norepinephrine and epinephrine. - Stimulates sweat glands to secrete. - Blood vessels - causes to constrict. - Thermoregulation - vasodilatation of capillaries in skin and sweat gland stimulation. - Release of renin from kidney - the release of hormone increases blood pressure # Central Nervous System Regulation of the Autonomic NS - **Control by Brain Stem and Spinal Cord** - Medulla - can regulate heart rate (cardiac center). - Blood vessel constriction (vasomotor center), respiration, and some gastrointestinal activity. - Pons - can regulate respiration (respiratory center). - The pupils (ocularmotor nuclei). - Spinal cord - can regulate defecation/urination. - **Hypothalamus** - Can coordinate the regulation of heart rate, blood pressure, body temperature, water/electrolyte balance, hormonal activity, pleasure, rage, thirst, hunger, and sex drive. - Sympathetic - lateral & posterior hypothalamus. - Parasympathetic - medial & anterior hypothalamus.