Nervous System PDF

NERVOUS SYSTEM Nervous and Endocrine Systems: These are the two primary systems for body communication. The nervous system sends rapid signals for immediate responses, while the endocrine system controls slower, hormonal changes. Purpose of the Nervous System The nervous system controls all...

NERVOUS SYSTEM Nervous and Endocrine Systems: These are the two primary systems for body communication. The nervous system sends rapid signals for immediate responses, while the endocrine system controls slower, hormonal changes. Purpose of the Nervous System The nervous system controls all bodily functions, including voluntary and involuntary actions (e.g., breathing, walking, thinking). It detects information, processes it in the brain, and coordinates appropriate responses Two Nervous Systems Central nervous system (CNS): Consists of the brain and spinal cord. It integrates sensory information and directs motor responses. Peripheral nervous system (PNS): Comprises nerves outside the CNS. It acts as a communication pathway between the CNS and the rest of the body, divided into the somatic and autonomic systems. Functions of the Nervous System Sensing: The nervous system gathers information from inside the body and the environment via sensory neurons. (afferent pathway) Integration: Information is processed and interpreted in the CNS. Recalls past experiences relation to similar situations using these to determine responses Motor Function: The CNS sends messages away from the CNS to muscles and glands through motor neurons (efferent pathway). Afferent vs Efferent Pathways Afferent Pathway: Carries sensory signals from the body to the CNS. Efferent Pathway: Carries motor signals from the CNS to muscles and glands. Somatic and Autonomic Nervous System Somatic System: Involves sensory and motor nerves responsible for voluntary movements and responses to external stimuli (touch, temperature, pain). Autonomic System: Involuntary control of physiological functions (heart rate, smooth muscle, gland activity), further divided into sympathetic (activates fight or flight) and parasympathetic (controls rest and digest functions) depending on environment NERVOUS SYSTEM Sympathetic and Parasympathetic Systems Sympathetic: Prepares the body for intense physical activity by increasing heart rate, blood flow to muscles, and energy release while reducing non-essential functions like digestion. Parasympathetic: Returns the body to a resting state by promoting functions like digestion, energy storage, and waste elimination. Intrinsic (Enteric) Nervous System Enteric Nervous System: Operates independently within the gastrointestinal tract, controlling peristalsis and secretion of digestive enzymes without requiring input from the CNS. Anatomy of a Nerve Nerves consist of neurons (electrically excitable cells) and glial cells (support and protect neurons). Neurons: Send and receive electrical impulses. Different types of neurons include: Sensory neurons (respond to stimuli - touch / sound) Motor neurons (control muscles & glands) Relay neurons (connect neurons within the CNS). Neuron Structure Neurons have long axons and dendrites. The dendrites receive information, and the axon transmits the signal. Some axons are over a meter long (e.g., sciatic nerve). How Messages Travel Nerve impulses travel from neuron to neuron via electrical and chemical signals. The direction is usually from the dendrites to the cell body (soma), down the axon to the synapses (can be electrical / chemical) Dendrites: Act as the "feelers" of the neuron, with a large surface area to receive and transmit signals into the cell body (soma). Soma and Nucleus: The soma is the main body of the neuron, housing the nucleus (containing DNA) and storing nutrients. It functions like a factory to fuel and build the neuron. Axon: A long, cable-like structure that extends from the soma to transmit electrical signals to other neurons. Axons can be very long (up to 90 cm) and their width determines signal efficiency. Myelination, Nodes of Ranvier, and Schwann Cells: Myelin sheaths, produced by Schwann cells in the peripheral nervous system (PNS), insulate the axon and increase the speed of nerve signals. Gaps between myelin sheaths, called Nodes of Ranvier, help signals "jump" quickly along the axon through saltatory conduction. NERVOUS SYSTEM Nodes of Ranvier: Unmyelinated gaps in the axon that assist in rapid signal transmission through jumping of action potentials. Axon Terminals: The ends of the axon, which form connections (synapses) with the dendrites of other neurons, allowing communication between nerve cells. Neuroglia Cells Glial cells (neuroglia) are helper cells in the nervous system. There are more glial cells than neurons. Their primary function is to support neurons. They act as "glue" in the central nervous system (CNS). They support, protect, insulate, and nourish neurons. Astrocytes: Transfer nutrients from the blood to neurons and form part of the blood-brain barrier. Microglia: Act as immune cells within the CNS, detecting and eliminating damage or disease. Schwann Cells: Located in the PNS, Schwann cells produce the myelin sheath and aid in nerve regeneration if damaged. Helps to create insulin layer around the axon cells Ependymal cells: Line brain cavity forming ependymal layer. Important for pressure & nutrition Communication Between Neurons Synapses: Neurons communicate via synapses, which can be electrical (direct ion flow) or chemical (release of neurotransmitters). The synaptic transmission is key to neuron-to-neuron communication. Action Potentials and Ions Banana View of Action Potential: Depicts how sodium (Na+) ions enter the neuron during depolarization, and potassium (K+) ions exit during repolarization. Synapse Junction where signals are transmitted between neurons or between a neuron and a gland/muscle cell. Can be chemical or electrical. Chemical Synapses Use neurotransmitters to transmit signals. Can be excitatory or inhibitory. Involve neurotransmitter release into the synaptic cleft via exocytosis. Neurotransmitters bind to receptors on the receiving cell to relay the signal. Control communication between the nervous system and other body systems. Electrical Synapses NERVOUS SYSTEM Use ions to transmit signals through gap junctions. Allow fast communication as ions move directly between neurons. Changes in membrane potential in one neuron trigger ion movement to the next. Dopamine: Synthesized in the brain and kidneys. Involved in reward-motivated behavior, motor control, and executive function. In the peripheral nervous system (PNS), it affects noradrenaline, vasodilation, sodium excretion, and reduces insulin production and gastrointestinal activity. Noradrenaline: Mobilizes the brain and body for action, especially during stress (fight-or-flight response). Increases arousal, alertness, and vigilance. A key neurotransmitter in the sympathetic nervous system. Adrenaline: Produced by the adrenal glands. Involved in the fight-or-flight response, respiration, energy homeostasis, and stress response. Neuropeptide Y (NPY): A 36-amino acid neuropeptide found in the CNS. Increases food intake, fat storage, reduces anxiety, stress, and pain perception. GABA (γ-Aminobutyric acid): The most common inhibitory neurotransmitter in the CNS. Exists in two classes (GABAA, GABAB) and is targeted by many drugs. Glutamate: The most abundant excitatory neurotransmitter in the nervous system. Involved in learning, memory, and is a precursor for GABA. Serotonin (5-HT): Regulates mood, cognition, reward, learning, memory, sleep, and pain perception. Found in the GI tract and the CNS. Acetylcholine (ACh): Involved in neuromuscular junctions, activating skeletal muscles. Plays a role in attention, memory, arousal, and motivation. Neuromuscular Junction: Acetylcholine is released from motor neurons to stimulate muscle contraction. Acetylcholinesterase breaks down ACh to reset the process. Inhibition of Neurotransmitters: Botox blocks ACh release, preventing muscle contraction. Nicotine is an ACh agonist, increasing its activity and enhancing attention. NERVOUS SYSTEM Enhancement of Neurotransmitters: Caffeine stimulates ACh release, acting as a CNS stimulant to increase alertness and reduce fatigue. Spinal Cord and Peripheral Nerves Spinal Cord Overview The spinal cord is a long, thin structure made of nervous tissue, extending from the base of the skull to L1. It is continuous with the medulla oblongata and is protected by bone, meninges, and cerebrospinal fluid (CSF). It transmits motor signals from the brain to the body and sensory signals from the body to the brain. Spinal cord functions Sensory pathway - Providing pathways for sensory information from the periphery to the brain Motor pathway - Providing information from the brain to the motor neurons Reflex center - Receiving involuntary responses from stimuli Conus Medullaris and Cauda Equina The conus medullaris is the tapered end of the spinal cord, usually at L1-L2. The cauda equina, a bundle of nerves resembling a horse's tail that extends from T12-L1 to the coccyx. Spinal Cord Structure The spinal cord is surrounded by three protective layers: 1. Dura Mater: Outer tough layer 2. Epidural space: Blood vessel network filled with adipose 3. Arachnoid Mater: Middle layer with a web-like structure, Subarachnoid space: contains CSF 4. Pia Mater: Innermost layer White and Grey Matter The spinal cord has white and grey matter: ○ Grey matter: Central, butterfly-shaped region containing cell bodies, interneurons, and synapses. ○ White matter: Surrounding region, consisting of myelinated and unmyelinated axons. Spinal Nerves There are 31 pairs of spinal nerves grouped by spine regions: Cervical (8), Thoracic (12), Lumbar (5), Sacral (5), and Coccygeal (1). Each spinal nerve has a mixed function: ○ Dorsal root: Carries sensory (afferent) fibers. ○ Ventral root: Carries motor (efferent) fibers. NERVOUS SYSTEM Dermatomes and Spinal Injury Dermatomes are skin areas supplied by specific spinal nerves. Damage to the spinal cord or nerves can result in sensory and motor impairments, with severity depending on the level of injury. Cranial Nerves There are 12 pairs of cranial nerves connected under the brain (leaving foramina) and extending into the periphery each with sensory, motor, or parasympathetic functions. Olfactory (I): ○ Sensory (afferent) ○ Involved in smell ○ Innervates the olfactory mucosa and sends information to the brain Optic (II): ○ Sensory (afferent) ○ Responsible for vision ○ Innervates the retina and carries visual information to the brain Oculomotor (III): ○ Motor and parasympathetic (efferent) ○ Controls eye movements and pupil constriction ○ Innervates most extraocular muscles Trochlear (IV): ○ Motor (efferent) ○ Innervates the superior oblique muscle of the eye Trigeminal (V): ○ Sensory and motor (afferent & efferent) ○ Divides into ophthalmic (sensory), maxillary (motor for mastication), and mandibular (sensory) Abducens (VI): ○ Motor (efferent) ○ Innervates the lateral rectus muscle, involved in eye movement Facial (VII): ○ Sensory, motor, parasympathetic (afferent & efferent) ○ Six branches that control facial expressions, taste, and salivary glands Vestibulocochlear (VIII): ○ Sensory (afferent) NERVOUS SYSTEM ○ Divided into vestibular (balance) and cochlear (hearing) Glossopharyngeal (IX): ○ Sensory, motor, parasympathetic (afferent & efferent) ○ Involved in taste, motor control of pharyngeal muscles, and parotid gland function Vagus (X): Sensory, motor, parasympathetic (afferent & efferent) Longest cranial nerve, controls visceral organs, motor control of palate and pharynx, and leaves the head/neck region Accessory (XI): Motor (efferent) Innervates laryngeal muscles, sternocleidomastoid, and trapezius Hypoglossal (XII): Motor (efferent) Controls tongue muscles, essential for speech and swallowing