Nervous System PDF - Seeley - Chapter 11-16

NERVOUS SYSTEM | FINAL | SEELEY | CHAPTER 11-16 1 CHAPTER 11: The Nervous System A bundle of axons that links the CNS to sensory and Nervous Tissue...

NERVOUS SYSTEM | FINAL | SEELEY | CHAPTER 11-16 1 CHAPTER 11: The Nervous System A bundle of axons that links the CNS to sensory and Nervous Tissue receptors, muscles, and glands. Consists of : cranial nerves (12 pairs from the brain) The brain, the nervous tissue contained within the skull, and spinal nerves (31 pairs from the spinal cord). appears as a dull gray, solid mass in the skull, attached Ganglion to the long, solid spinal cord, the extension of nervous A cluster of neuron cell bodies situated outside the tissue within the vertebral column. CNS. Plexus Function of Nervous System (NS) A large network of axons (and sometimes neuron cell 1. Maintaining homeostasis. bodies) outside the CNS, organized like a braid. 2. Receiving sensory input. 3. Integrating information. PNS (Peripheral Nervous System) 4. Controlling muscles and glands. 5. Establishing and maintaining mental activity. Two main parts Sensory (Afferent) Division Sensory Function (Sensation) Carries action potentials from sensory receptors to The nervous system receives information from the CNS. environment and transmits it to brain for processing. Sensory neuron cell bodies are located near the spinal Motor Function (Response) cord in dorsal root ganglia or near the origin of The nervous system generates responses to sensory certain cranial nerves. input by activating muscles or glands, resulting in Motor (Efferent) Division physical actions. Sends action potentials from the CNS to effector Integration (Cognition) organs (e.g., muscles and glands) to trigger a response. Sensory information is combined with memories, emotions, and learned information in association areas of the brain, allowing us to recognize objects, make decisions, and react appropriately. Association Areas Brain regions where sensory information is linked with past experiences and cognitive functions, enabling recognition and thoughtful responses. TWO MAJOR NERVOUS SYSTEM help us weigh options based on memories and emotions, leading to deliberate actions rather than automatic responses. Central Nervous System (CNS) brain and spinal cord, housed within the cranial cavity (skull) and vertebral Illustrates the process of sensory input and motor output cavity (spinal column), providing protection. within the nervous system Connect with each other at the foramen magnum of the skull. When a person sees a bear, their sensory system is Peripheral Nervous System (PNS) activated by the sight, which is sent to the brain and spinal all nervous tissue outside the brain and spinal cord cord (CNS) for processing. The brain quickly recognizes extending throughout the body to connect the CNS the bear as a threat and decides on a response. with muscles and organs. The CNS then sends signals to the motor system to carry ransmits sensory information to the CNS and out this response. The motor system has two main parts: conveys commands from the CNS to regulate body activities. Autonomic Nervous System (ANS): This part controls automatic body responses. It has two divisions: Neurons Sympathetic: Prepares the body for “fight or flight” by Primary cells of the nervous system that transmit increasing heart rate, dilating pupils, and activating electrical messages to other cells through extensions muscles and glands to help escape. called axons. Parasympathetic: Calms the body down after the Sensory Receptors: threat is gone, helping it to “rest and digest.” Endings of neurons or specialized cells that detect Somatic Nervous System (SNS): This part controls sensory stimuli like temperature, pain, touch, pressure, voluntary movements. It sends signals to the muscles, light, sound, and odor. Found in skin, muscles, joints, instructing the person to run. internal organs, and sensory organs. Nerve  NERVOUS SYSTEM | FINAL | SEELEY | CHAPTER 11-16 2 MOTOR DIVISION OF THE NERVOUS SYSTEM NEURON STRUCTURE Somatic Nervous System (voluntary): NEURON CELL BODY (SOMA) controls conscious movements Contains the nucleus and is responsible for protein lets you choose to move your skeletal muscles synthesis. Signals are sent from the brain (CNS) to somatic motor Surrounded by extensive rough endoplasmic reticulum neurons, which extend from the CNS to connect with (ER) and Golgi apparatus muscles through a "synapse," Composed neurofilaments and microtubules. ͦ a type of junction where signals are passed on. Nissl Bodies When the neuron connects with a muscle fiber, it’s ͦ Abundant rough ER within the cell body and called a neuromuscular junction. dendrites, serving as the primary sites of protein synthesis in neurons. Autonomic Nervous System (ANS) (involuntary): controls automatic functions, like heart rate and DENDRITES digestion, without conscious thought. Branching projections that receive signals from other neurons. Neuron Pathway Short, often highly branched cytoplasmic extensions The ANS uses two neurons in a series to connect the CNS that taper from the cell body to their tips. to target organs: Dendritic Spines: Small extensions on the dendrite The first neuron starts in the CNS and extends to a surfaces where axons of other neurons form synapses. cluster of nerve cells called an autonomic ganglion. When stimulated, dendrites generate small electric In the ganglion, it connects (or "synapses") with a currents conducted toward the neuron cell body. second neuron. AXON DIVISIONS OF THE ANS A single elongated projection that arises from the axon hillock, which is a cone-shaped area of the neuron cell The second neuron then sends signals from the body. ganglion to the target organs. transmits action potentials away from the cell body Sympathetic Division ("fight-or-flight"): to other neurons or effector organs. Prepares the body for action, especially during physical Maintains a constant diameter from few millimeters to activity or stress. over 1 meter. Parasympathetic Division ("rest-and-digest"): Axon Hillock: The region where the axon originates, Maintains the body’s resting functions, like digestion transitioning into the initial segment of the axon. and urination. Trigger Zone: The combination of the axon hillock Enteric Nervous System (ENS) and the initial segment, where action potentials are A subdivision of the PNS that consists of plexuses generated. within the wall of the digestive tract. Collateral Axons: Side branches that some axons form, can monitor and control the digestive tract allowing them to communicate with multiple targets. independently of the CNS through local reflexes. Axoplasm: The cytoplasm of the axon, and the plasma The CNS can override the functions of the ENS via membrane is referred to as the axolemma. parasympathetic and sympathetic actions. Presynaptic Terminals: Small extensions at the end The ENS is integrated with the autonomic nervous of axons that store secretory vesicles containing system (ANS), highlighting its role in regulating neurotransmitters. digestive processes. Action Potentials conducted along the axon to presynaptic terminals, stimulating the release of Two Cell Types That Make Up Nervous System neurotransmitters into the synapse. Neurons or nerve cell Neurotransmitters: Chemical signals that cross the primary cell type in the nervous system responsible synaptic cleft to stimulate or inhibit the postsynaptic for receiving stimuli, conducting action potentials, cell. and transmitting signals to other neurons or effector Axon Transport: Mechanisms that move proteins, organs. organelles, and vesicles within the axon; includes Glial Cells anterograde (toward terminals or away from cell body) Supportive cells that protect neurons and perform and retrograde (toward the cell body) transport. various functions in the nervous system. Infectious Agent Transport: Harmful substances, Glial cells account for over half of the brain's weight including viruses like rabies and herpes, can be and can outnumber neurons by 10 to 50 times in transported from the periphery to the CNS via axon different areas of the brain. transport mechanisms.  NERVOUS SYSTEM | FINAL | SEELEY | CHAPTER 11-16 3 Ependymal Cells TYPES OF NEURONS Line the ventricles of the brain and the central canal of the spinal cord; involved in producing and Functional Classification of Neurons: circulating cerebrospinal fluid. ͦ Choroid Plexuses: Specialized structures Sensory Neurons (Afferent Neurons) formed by ependymal cells and blood vessels Conduct action potentials toward the CNS from that secrete cerebrospinal fluid (CSF) into the sensory receptors. brain's ventricles. Motor Neurons (Efferent Neurons) ͦ Cerebrospinal Fluid Circulation: Conduct action potentials away from the CNS to Ependymal cells have cilia that help circulate muscles or glands. Cerebrospinal Fluid Circulation (CSF) Interneurons through the brain's cavities. Conduct action potentials within the CNS, connecting Ependymal cells possess long processes at their basal one neuron to another. surfaces that extend into the brain and spinal cord, potentially serving functions similar to those of Structural Classification of Neurons astrocytes. Based on the number of dendrites Structure Function Location Microglia Multipolar multiple receive common in Immune cells of the CNS that act as phagocytes to dendrites and a impulses the CNS. clear debris and protect against pathogens. single axon from Increased numbers of microglia are found in areas of multiple the brain or spinal cord that are damaged due to neurons via infection, trauma, or stroke. dendrites Pathologists can identify damaged CNS areas during Bipolar one dendrite and Conduct sensory autopsies by observing the accumulation of microglia one axon action organs like in those regions. potentials the retina to the CNS. and nasal Oligodendrocytes cavity. Glial cells in the CNS that have cytoplasmic extensions Pseudo- One process that One branch sensory surrounding axons. unipolar splits into two; carries ganglia of Cells that produce myelin sheaths around multiple functionally acts sensory cranial axons in the CNS, facilitating faster signal as single axon. information nerves transmission. 1st branch from the When oligodendrocytes wrap their extensions multiple extends to CNS periphery to times around axons, they create an insulating layer and 2nd to the the CNS, known as the myelin sheath. periphery with typically A single oligodendrocyte is capable of forming myelin sensory found in sheaths around the axons of multiple neurons, receptors. sensory enhancing the efficiency of electrical signal neurons. transmission. 4 TYPE OF GLIAL CELLS OF CNS 2 TYPE OF GLIAL CELLS IN PNS Astrocytes Schwann Cells Star-shaped cells that maintain the blood-brain barrier, form myelin sheaths around axons; each Schwann has extensions that form foot processes covering blood cell encases only one axon. vessels, neurons, and the pia mater (the membrane Schwann cells contribute to the insulation of axons, surrounding the brain and spinal cord). facilitating faster electrical signal transmission. Contains microfilaments that provide structural support for blood vessels and neurons. Satellite Cells regulate the composition of extracellular brain fluid by surround neuron cell bodies in sensory and releasing chemicals that promote tight junctions autonomic ganglia. between endothelial cells of capillaries. Satellite cells provide nutritional support to neurons In the event of CNS tissue damage, it engage in and protect them from heavy metal toxins (e.g., lead reactive astrocytosis to isolate the injury and limit and mercury) by absorbing these substances, limiting inflammation spread, while also creating a reactive their access to the neuron cell bodies. scar that can hinder axon regeneration. promote synapse development and regulate synaptic activity by synthesizing, absorbing, and recycling neurotransmitters.  NERVOUS SYSTEM | FINAL | SEELEY | CHAPTER 11-16 4 Various stimuli (e.g., light, sound, pressure) activate MYELINATED AND UNMYELINATED AXONS sensory cells in organs like the eyes, ears, and skin, generating action potentials. These signals are Myelinated Axons transmitted to the spinal cord and brain, where they are Formed by Schwann cells in the PNS or processed. oligodendrocytes in the CNS, which wrap around responsible for muscle contractions and the secretion segments of axons multiple times to create a myelin of hormones from glands, highlighting their critical sheath. role in both voluntary and involuntary physiological Protects and electrically insulates axons, allowing for processes. faster action potential propagation These properties stem from: Composed of tightly wrapped membranes rich in Ionic Concentration Differences: Variations in phospholipids, resulting in a white appearance due to the concentration of ions across the plasma high lipid concentration. membrane. Gaps in the myelin sheath occurring every 0.3–1.5 mm where the axon is exposed, facilitating rapid IONIC CONCENTRATION DIFFERENCES signal conduction via saltatory conduction. ACROSS THE PLASMA MEMBRANE Saltatory Conduction: The process by which action potentials jump from one node of Ranvier to the next, enhancing the speed of nerve impulse transmission Unmyelinated Axons Contrary to what their name implies, unmyelinated axons are not without myelin. Instead, they are embedded within the invaginations of Schwann cells or oligodendrocytes. Each unmyelinated axon is encased by the plasma membrane of glial cells, Schwann cells can surround Sodium (Na⁺): Higher concentration outside the several unmyelinated axons at once. cell compared to the inside, resulting in a steep Myelin sheaths begin to form late in fetal development concentration gradient favoring inward movement. and accelerate until the first year of life, which is Potassium (K⁺): Higher concentration inside the crucial for the infant's ability to develop faster and cell, creating a steep concentration gradient more coordinated movements. favoring outward movement. Diseases such as multiple sclerosis and certain types Chloride (Cl⁻): Also has a higher concentration of diabetes mellitus can lead to the degradation of outside the cell than inside. myelin sheaths. The cytoplasm of cellscontains a high concentration of K⁺ , also has a significant amount Gray Matter of negatively charged molecules, such as proteins consists primarily of neuron cell bodies and and phosphates. dendrites contain little myelin, resulting in a darker PERMEABILITY CHARACTERISTICS OF THE appearance. PLASMA MEMBRANE In the CNS, gray matter is found in the cortex, which is the outer layer of the brain, as well as in deeper Sodium-Potassium Pump structures known as nuclei. The differences in ion concentrations across the In the PNS, gray matter is represented by ganglia, plasma membrane are primarily maintained by the which are clusters of neuron cell bodies. sodium-potassium pump. White Matter uses ATP to pump K+ against its concentration Composed of bundles of myelinated axons, appears gradient into the cell while at the same time lighter in color. pumping Na+ against its concentration gradient In the CNS, it forms nerve tracts that serve as out of the cell. conduction pathways, facilitating the propagation of This pump actively transports ions against their action potentials between different areas. In the PNS, the analogous structures are called nerves, concentration gradients using ATP: which are bundles of axons surrounded by connective Potassium (K⁺): The pump moves K⁺ into the tissue sheaths. cell Sodium (Na⁺): It also moves Na⁺ out of the cell, Action Potentials For every ATP molecule consumed, the pump electrical signals produced by specialized cells, functioning as a primary means of communication and transports three Na⁺ ions out of the cell and two information integration in the body. K⁺ ions into the cell.  NERVOUS SYSTEM | FINAL | SEELEY | CHAPTER 11-16 5 The plasma membrane is selectively permeable, Other Gated Ion Channels: meaning it allows certain substances to pass while These channels respond to stimuli other than restricting others. ligand binding or voltage changes. Cells synthesize negatively charged proteins, Establishing the Resting Membrane Potential which are large and cannot easily diffuse across higher proportion allows K⁺ ions to diffuse more the membrane. freely across the membrane. Due to the presence of negatively charged K⁺ is more concentrated inside the cell than proteins, Cl⁻ is repelled and tends to exit the cell. outside,. As K⁺ ions diffuse out, they carry positive charges This results in a higher concentration of Cl⁻ in the with them, resulting in a loss of positive charge extracellular fluid compared to the cytoplasm. inside the cell. The inside of cell becomes negatively charged due Ion Channels to the loss of K⁺, while the outside of membrane Ions must move across the plasma membrane via becomes positively charged as K⁺ accumulates specialized structures known as ion channels. there. This creates an electrochemical gradient where the positively charged K⁺ ions are attracted Two Types Of Ion Channels back into the cell by the negatively charged Leak Ion Channels proteins and other molecules that cannot cross the also known as nongated ion channels membrane. always open, allowing specific ions to move passively across the membrane according to their Equilibrium of Forces resting membrane potential represents an concentration gradients. equilibrium state. This equilibrium is reached This contributes to the resting membrane potential. when the outward diffusion of K⁺ due to its Higher Permeability to K⁺ and Cl⁻: This is due concentration gradient is balanced by the inward to the presence of a significantly larger number of electrostatic force attracting K⁺ back into the cell. K⁺ and Cl⁻ leak ion channels in the membrane. At this point, the net movement of K⁺ across the membrane is effectively zero, even though K⁺ Lower Permeability to Na⁺: There are relatively continues to move back and forth across the fewer Na⁺ leak ion channels, resulting in much membrane. lower permeability for sodium ions. The higher permeability to K⁺ allows potassium to Outward Diffusion flow out of the cell, while the limited permeability K⁺ ions diffuse out of the cell due to the concentration gradient. to Na⁺ prevents significant inward movement of Electrostatic Attraction sodium. The inside of the cell becomes more negative, creating a pull on K⁺ ions back into the cell due to Gated Ion Channels the attraction to negatively charged proteins. open or close in response to specific stimuli, allowing for controlled ion flow and enabling the generation and propagation of action potentials. By controlling the flow of ions, these channels play a crucial role in changing the permeability of the membrane and facilitating action potentials. Three Types Of Gated Ion Channels Ligand-Gated Ion Channels open in response to the binding of a specific molecule, known as a ligand, to the receptor site located on the extracellular side of the channel. can be neurotransmitters, hormones, or other signaling molecules. Voltage-Gated Ion Channels: open and close in response to changes in the electrical charge (voltage) across the plasma membrane. The resting membrane potential is typically negative inside the cell compared to the outside. When a cell is stimulated, the influx or efflux of ions alters the membrane potential, which can cause voltage-gated channels to open or close.  NERVOUS SYSTEM | FINAL | SEELEY | CHAPTER 11-16 6 Changing the Resting Membrane Potential negative charge increases due to the loss of positively charged K⁺ ions. Depolarization Depolarization process in which the membrane potential becomes ͦ occurs when the extracellular concentration more positive, moving closer to zero from a of K⁺ increases, resulting in a more positive negative value. resting membrane potential. typically occurs when there is an influx of Hyperpolarization positively charged ions, primarily sodium ions ͦ occurs when the extracellular concentration (Na⁺), into the cell. of K⁺ decreases, leading to a more negative Membrane potential becomes more positive resting membrane potential. (e.g., from -70 mV towards 0 mV) due to Na⁺ influx. Sodium Ions (Na⁺) In a resting cell, the plasma membrane is Hyperpolarization relatively impermeable to Na⁺ due to a limited occurs when the membrane potential becomes number of Na⁺ leak ion channels. This means that more negative, moving further away from zero. the concentration of Na⁺ inside the cell remains can occur through several mechanisms, including low compared to the outside. the efflux of K⁺ ions or the influx of negatively As Na⁺ enters, the inside of the membrane charged ions, such as chloride ions (Cl⁻). becomes more positive, leading to depolarization. Membrane potential becomes more negative This rapid influx of Na⁺ is a crucial step in the (e.g., from -70 mV to -80 mV or lower) due to generation of action potentials, which are the K⁺ efflux or Cl⁻ influx. electrical signals that neurons use to communicate. Potassium Ions (k⁺) Calcium Ions (Ca²⁺) When the concentration of K⁺ outside the cell Ca²⁺ plays a critical role in stabilizing voltage- increases, the concentration gradient between the gated Na⁺ channels. These channels remain closed inside and outside of the cell decreases. under normal conditions but can be affected by Depolarized state the concentration of Ca²⁺ in the extracellular fluid. ͦ With a reduced concentration gradient, K⁺ When the extracellular Ca²⁺ concentration will tend to stay inside the cell more than it decreases, Ca²⁺ ions diffuse away from the normally would. The electrical pull (negative voltage-gated Na⁺ channels, leading to their charge inside the cell) that attracts K⁺ back opening. Conversely, an increase in extracellular into the cell does not need to be as strong Ca²⁺ results in binding to these channels, causing because the concentration driving K⁺ out has them to close. weakened. The influx of Ca²⁺ contributes to depolarization, ͦ This leads to a decrease in the negative making the inside of the membrane more positive. charge inside the cell, resulting in a This mechanism is particularly important at the depolarized state of the membrane potential. axon terminal of neurons, where Ca²⁺ triggers ͦ The resting membrane potential shifts to a neurotransmitter release. more positive value than normal (e.g., moving from -70 mV toward 0 mV). Chloride Ions (Cl⁻) Hyperpolarized state The permeability of Cl⁻ can also influence the ͦ As K⁺ diffuses out, the inside of the cell resting membrane potential. If gated Cl⁻ channels becomes less positive. A larger negative open, Cl⁻ ions will diffuse into the cell, as there is charge is needed to attract K⁺ back into the typically a higher concentration of Cl⁻ outside the cell due to the increased diffusion tendency. cell compared to the inside. ͦ This leads to an increase in the negative The entry of negatively charged Cl⁻ leads to an charge inside the cell, resulting in a increase in negative charge inside the cell, hyperpolarized state of the membrane resulting in hyperpolarization. This shift makes potential. the membrane potential more negative than its ͦ The resting membrane potential shifts to a resting state. more negative value than normal (e.g., moving from -70 mV to -80 mV or lower). Summary K⁺: High extracellular concentration leads to depolarization; Gated K⁺ Channels low concentration causes hyperpolarization. Opening of Gated K⁺ Channels: If gated K⁺ Na⁺: Low permeability at rest, but influx during stimulation channels open (in response to stimuli), the causes depolarization. permeability of the membrane to K⁺ increases. Ca²⁺: Stabilizes Na⁺ channels and contributes to This allows more K⁺ to diffuse out of the cell. depolarization upon entering the cell. With more K⁺ leaving the cell, the inside becomes Cl⁻: Increased permeability leads to hyperpolarization when even more negative, further contributing to Cl⁻ diffuses into the cell. hyperpolarization. This occurs because the  NERVOUS SYSTEM | FINAL | SEELEY | CHAPTER 11-16 7 Graded Potentials Threshold small, localized changes in the membrane membrane potential at which voltage-gated Na+ potential that can influence the generation of channels open action potentials. Depolarization phase the membrane potential moves away from the resting state and becomes more positive Repolarization phase the membrane potential returns toward the resting state and becomes more negative. Afterpotential After the repolarization phase, the plasma membrane may be slightly hyperpolarized for a short period, called the afterpotential All-or-None Principle describes how action potentials are generated Causes of Graded Potentials in neurons Chemical Signals For an action potential to occur, a stimulus must Changes in Voltage produce a depolarizing graded potential that is Mechanical Stimulation strong enough to reach a critical level known as Temperature Changes the threshold Spontaneous Opening of Ion Channels Depolarization Phase Summation of Graded Potentials crucial part of the action potential process in Temporal Summation neurons. If multiple stimuli occur in rapid succession at the begins when a stimulus causes a graded potential same location, the resulting graded potentials can that reaches the threshold level (typically around - combine to produce a larger depolarization. 55 mV) at the axon hillock. Spatial Summation Once the threshold is reached, voltage-gated If stimuli occur simultaneously at different sodium (Na⁺) channels begin to open rapidly. locations, their effects can add together, further During the depolarization phase of an action increasing the likelihood of reaching the threshold potential, a graded potential that reaches threshold for an action potential. triggers a rapid opening of voltage-gated Na⁺ channels, resulting in a massive influx of sodium Decremental fashion ions. This creates a positive feedback loop that Where graded potentials spread, or are conducted, leads to significant depolarization of the over the plasma membrane membrane. Graded Potentials Graded potentials are critical for initiating action Repolarization Phase potentials. In the resting state, the activation gates of the Important because they can summate to generate voltage-gated Na⁺ channels are closed, while the action potentials. inactivation gates are open. This configuration prevents Na⁺ from entering the cell, maintaining Action Potentials the resting membrane potential. essential for the transmission of signals in neurons, the dynamics of voltage-gated Na⁺ and K⁺ and their generation is a complex process that channels during the action potential are critical for begins with graded potentials. the process of depolarization. Afterpotential phase of hyperpolarization that occurs following an action potential. Characterized by a transient increase in the negativity of the membrane potential beyond its resting level. results from the behavior of voltage-gated K⁺ channels, which open and close more slowly than voltage-gated Na⁺ channels: Essential for restoring the resting membrane potential through the action of the sodium- potassium pump. The consistent nature of action potentials, regardless of the frequency, allows neurons to effectively transmit signals throughout the nervous system.  NERVOUS SYSTEM | FINAL | SEELEY | CHAPTER 11-16 8 Refractory Period Maximal Stimulus a time during which a specific region of the Strong enough to elicit the maximum plasma membrane becomes less responsive to frequency of action potentials, limited by the further stimuli following an action potential. absolute refractory period. essential for maintaining the one-way propagation Submaximal stimuli of action potentials and ensuring that each action produce action potential frequencies proportionate potential is a discrete signal. to stimulus strength Supramaximal stimuli Absolute Refractory Period though stronger than maximal stimuli, cannot the initial phase during which the neuron is increase the frequency beyond the maximum set completely insensitive to any additional stimuli, by the neuron’s absolute refractory period. regardless of their strength. lasts from the onset of depolarization through  According to the all-or-none principle, the amplitude almost the entire repolarization phase. of action potentials remains the same regardless of ensures that each action potential is a distinct stimulus strength. Therefore, stimulus strength is event. communicated by the frequency of action potentials It allows for full repolarization before the next rather than their size. action potential can occur, preventing prolonged For ex. depolarization of the membrane. A weak pain stimulus might trigger a low During depolarization, voltage-gated Na⁺ channels frequency of action potentials, interpreted as mild have their activation gates open, allowing Na⁺ to pain. enter the cell. A strong pain stimulus could result in a high As depolarization peaks, the inactivation gates of frequency of action potentials, signaling intense the Na⁺ channels close, preventing any further Na⁺ pain. entry. Propagation of Action Potentials Relative Refractory Period allows the nervous system to transmit signals over a phase during which a stronger-than-normal distances, from one part of the neuron to another or stimulus can initiate another action potential. from neuron to target cells. limits the frequency of action potentials, ensuring typically initiated in the trigger zone of a neuron and a controlled response to stimuli. propagate in one direction along the axon. provides the neuron with a short window to generate another action potential if necessary, but IN UNMYELINATED AXON (CONTINUOUS only if the stimulus is particularly strong. CONDUCTION) controls the frequency and direction of action potential propagation along the neuron. It ensures unidirectional flow propagate in one direction along the axon. occurs during the final stages of repolarization and An action potential (orange part of the membrane) the afterpotential phase. generates local currents (black arrows) that tend to During this phase, many voltage-gated K⁺ depolarize the membrane immediately adjacent to the channels are still open, leading to increased K⁺ action potential. permeability. When depolarization caused by the local currents The membrane potential is closer to the reaches threshold, a new action potential is produced hyperpolarized afterpotential than to the resting adjacent to where the original action potential occurred. state, so a larger-than-normal stimulus is required The movement of positively charged ions is called a to reach the threshold. local current, or an ionic current. When the depolarization reaches threshold, an action Action Potential Frequency potential is produced. This type of action potential number of action potentials produced per unit conduction in unmyelinated axons is called continuous of time in response to a stimulus, and it serves as conduction. a way for the nervous system to convey information about stimulus intensity. IN MYELINATED AXON (SALTATORY CONDUCTION) Thresholds Subthreshold Stimulus occurs through a unique process called saltatory Too weak to produce a graded potential that conduction, where the action potential effectively reaches threshold, so no action potential occurs. "jumps" from one node of Ranvier to the next. Threshold Stimulus An action potential (orange) at a node of Ranvier Just strong enough to produce a graded generates local currents (black arrows). When the potential that reaches threshold, leading to a depolarization caused by the local currents reaches single action potential. threshold at the next node of Ranvier, a new action potential is produced (orange).  NERVOUS SYSTEM | FINAL | SEELEY | CHAPTER 11-16 9 Action potential propagation is rapid in myelinated The membrane of the receiving cell, which contains axons because the action potentials are produced at receptors for the neurotransmitter is the postsynaptic successive nodes of Ranvier (1–5) instead of at every membrane. Postsynaptic cells are typically other part of the membrane along the axon. neurons, muscle cells, or gland cells. NERVE FIBERS (AXONS) Neurotransmitter Release in Chemical Synapses Are classified according to their size and degree of In chemical synapses, action potentials do not pass myelination. directly from the presynaptic terminal to the postsynaptic membrane. Instead, the action potentials Types of Nerve Fibers in the presynaptic terminal cause the release of Type A fibers neurotransmitters from its terminal. Large-diameter heavily myelinated axons conducting action potentials Presynaptic terminals at 15–120 m/s. specialized to produce and release neurotransmitters. found in motor neurons that supply skeletal muscles contains essential organelles like mitochondria and and in most sensory neurons, enabling rapid responses synaptic vesicles. These vesicles store to the external environment. neurotransmitters, ready to be released upon Type B fibers stimulation. Medium-diameter 1. Action potentials arriving at the presynaptic terminal lightly myelinated axons conducting at 3–15 m/s. cause voltage-gated Ca2+ channels to open. Type C fibers 2. Ca2+ diffuses into the cell and causes synaptic vesicles Small-diameter to release neurotransmitter molecules. unmyelinated axons conducting at 2 m/s or less. 3. Neurotransmitter molecules diffuse from the presynaptic terminal across the synaptic cleft. THE SYNAPSE 4. Neurotransmitter molecules combine with their junction between two cells where information is receptor sites and cause ligand-gated Na+ channels to transferred. open. Na+ diffuses into the cell or out of the cell and The presynaptic cell (the sending cell; before) causes a change in membrane potential. transmits the signal toward the synapse, while the postsynaptic cell (the receiving cell; after) responds to Neurotransmitter Removal the signal. The interaction between a neurotransmitter and a receptor Each presynaptic neuron can connect with about 1,000 represents an equilibrium: other neurons, while a single postsynaptic neuron can Neurotransmitter + Receptor Neurotransmitter – have up to 10,000 synaptic connections. Certain Receptor complex neurons in the cerebellum can have as many as 100,000 synapses, allowing complex and extensive Neurotransmitters like acetylcholine are broken down communication networks within the brain. by specific enzymes after they bind to receptors. Acetylcholine, is broken down by acetylcholinesterase TYPES OF SYNAPSES into acetic acid and choline. Electrical Synapses Choline is transported back into the presynaptic allow direct passage of ions and electrical currents terminal, where it combines with acetyl-CoA to reform between cells via gap junctions, enabling rapid acetylcholine. communication. Acetic acid can either be absorbed back into the commonly found in cardiac and smooth muscle tissues. presynaptic cell or diffuse out and be taken up by other At electrical synapses, adjacent cells are separated by a cells. 2 nm gap that is bridged by connexons: made up of six Neurotransmitters like norepinephrine are often taken tubular proteins, known as connexins, that form a back into the presynaptic cell through reuptake channel between the two cells mechanisms. Once reabsorbed, norepinephrine can be repackaged Chemical Synapses into vesicles for future use, or partially broken down Occurs where a chemical messenger, called a by monoamine oxidase (MAO). neurotransmitter, is used to communicate a message Some neurotransmitters diffuse away from the synaptic to an effector or carry signals across a small gap cleft, reducing the concentration of neurotransmitter between cells. molecules available to bind to receptors. essential components of a chemical synapse are the presynaptic terminal, the synaptic cleft, and the Receptor molecules at synapses postsynaptic membrane. are highly specific, typically binding only to particular presynaptic terminal consists of the end of an axon neurotransmitters or closely related substances. transmitting neuron or presynaptic cell. usually membrane-bound and ligand-activated, The space separating the axon ending and the cell with meaning they only respond to specific molecules like which it synapses is the synaptic cleft. The small gap neurotransmitters. (usually 20–40 nm wide) between the presynaptic The effect of a neurotransmitter on a target cell terminal and the target cell. depends on the type of receptor it binds to  NERVOUS SYSTEM | FINAL | SEELEY | CHAPTER 11-16 10 For example, norepinephrine can cause depolarization help regulate neuronal activity by preventing excessive (stimulation) in one cell type by binding to a certain firing of action potentials. norepinephrine receptor, while it can cause Hyperpolarizing, inhibitory, move the membrane hyperpolarization (inhibition) in another cell type by potential farther from threshold, and decrease the binding to a different receptor. likelihood of an action potential. Some neurotransmitter receptors are also found on Many of the synapses of the CNS are axoaxonic presynaptic membranes. For ex. Norepinephrine synapses, meaning that the axon of one neuron binds to receptors on both presynaptic and postsynaptic synapses with the presynaptic terminal (axon) of membranes. another. When norepinephrine binds to presynaptic receptors, it Presynaptic Inhibition can reduce the release of further neurotransmitters, the release of neurotransmitters from the presynaptic thereby creating a feedback mechanism. terminal is reduced, thus decreasing the stimulation of the postsynaptic cell. Neurotransmitters This can decrease or block the transmission of signals. chemicals that transmit signals across synapses Decreases neurotransmitter release, often through the between neurons or between neurons and other cells, inhibition of calcium channels, reducing neuronal such as muscle or gland cells. communication. They can have excitatory or inhibitory effects on the postsynaptic cell, depending on the type of receptor Presynaptic Facilitation they bind to. the release of neurotransmitters from the presynaptic directly cause signals to be transmitted terminal is enhanced, thereby increasing the Neuromodulators communication between neurons. This occurs when substances that modulate the activity of neuromodulators promote neurotransmitter release. neurotransmitters, influencing the likelihood that an Presynaptic facilitation: Increases neurotransmitter action potential will be generated in the postsynaptic release, typically by enhancing calcium influx, cell. boosting the excitability of the postsynaptic cell. alter the strength or probability of these signals. Neuromodulators can affect neurotransmitter release, Spatial and Temporal Summation reuptake, or receptor binding, thereby enhancing or inhibiting signal transmission across synapses. Spatial Summation Occurs when multiple presynaptic neurons contribute Drugs can affect neurotransmitter systems to the depolarization of a single postsynaptic neuron at Cocaine and amphetamines the same time. increase the release and block the reuptake of Temporal Summation norepinephrine, leading to elevated levels of Occurs when multiple action potentials from one norepinephrine in synapses. presynaptic neuron arrive closely together, allowing can result in overstimulation of postsynaptic neurons, the graded potentials to add up in time. contributing to harmful effects such as increased heart rate, blood pressure, and anxiety. In the nervous system, excitatory and inhibitory Selective serotonin reuptake inhibitors (SSRIs), neurons can synapse with same postsynaptic neuron, which block the reuptake of serotonin, used to treat and the final response depends on balance between the depression and certain behavioral disorders by excitatory and inhibitory inputs. increasing serotonin levels in the synapse, thus EPSPs are depolarizing graded potentials that make enhancing mood regulation. the postsynaptic membrane more positive, increasing the likelihood of an action potential being generated. Excitatory Postsynaptic Potential (EPSP) IPSPs are hyperpolarizing graded potentials that make occurs when the neurotransmitter binding to its the postsynaptic membrane more negative, decreasing receptor causes depolarization of the postsynaptic the likelihood of an action potential being generated. membrane. If EPSPs (excitatory signals) dominate, the sensory occur when the membrane becomes more permeable to signal will be transmitted, and the brain will perceive Na+ ions. the sensation. essential for processes like pain perception, where If IPSPs (inhibitory signals) dominate, the sensory sensory neurons must generate action potentials that signal will not be transmitted to the brain, and no stimulate neurons in the(CNS). perception of the stimulus will occur. Depolarizing, stimulatory, move the membrane potential closer to threshold, and increase the Neuronal Pathways and Circuits likelihood of an action potential. Serial Pathways These are the simplest, where the input travels along a Inhibitory Postsynaptic Potential (IPSP) single, direct pathway from one neuron to another. occurs when neurotransmitter binding leads to Parallel Pathways hyperpolarization of the postsynaptic membrane. more complex and involve multiple pathways. IPSPs occur due to an increase in the membrane's permeability to Cl− or K+ ions  NERVOUS SYSTEM | FINAL | SEELEY | CHAPTER 11-16 11 Axons of neurons can branch repeatedly to form regions where their corresponding nerves enter and synapses with many other neurons, allowing multiple exit. signals to be transmitted simultaneously. Spinal Nerves Four Types Of Parallel Pathways gives rise to 31 pairs of spinal nerves, each exits the Convergent Pathways vertebral column through intervertebral and sacral multiple neurons synapse onto a smaller number of foramina. Each spinal nerve is a bundle of axons, neurons. This allows different parts of the nervous Schwann cells, and connective tissue sheaths. system to influence or regulate the activity of the target neurons. Spinal Cord Enlargements enables the integration of signals from different Cervical Enlargement sources, allowing for more complex and coordinated Located in the inferior cervical region, it corresponds responses. to where nerves supplying the upper limbs enter and Ex. Several neurons can simultaneously excite or exit the spinal cord. inhibit motor neurons that control muscle contraction. Lumbosacral Enlargement The output will depend on the balance of (EPSPs) and Located in the inferior thoracic, lumbar, and superior (IPSPs) produced by the converging neurons. sacral regions, it is where nerves supplying the lower Divergent Pathways limbs enter and exit the spinal cord. a smaller number of presynaptic neurons synapse with Conus Medullaris a larger number of postsynaptic neurons. This allows the tapering cone-shaped end of the spinal cord that information from one neuron to spread across multiple occurs immediately inferior to the lumbosacral pathways. enlargement. enables a single stimulus to affect multiple targets, It extends to the level of the second lumbar vertebra. amplifying the impact of the original signal. The conus medullaris is the inferior tip of the spinal Ex. Sensory information can be transmitted to both the cord. brain and spinal cord. Cauda Equina Reverberating Circuits Below the conus medullaris involve a chain of neurons that form a positive- meaning "horse’s tail"; is a bundle of nerve roots feedback loop. Neurons in the circuit synapse with that resemble a horse's tail. previous neurons in the chain, leading to repeated These nerve roots arise from the lumbosacral stimulation. enlargement and conus medullaris, extending inferiorly involved in controlling rhythmic activities, such as through the vertebral canal before exiting the spinal respiration and the sleep-wake cycle. Ex. Once column through the intervertebral and sacral foramina. activated, these circuits can continue to discharge These roots supply the lower limbs and other inferior action potentials, even after the initial stimulus has structures of the body. ended. This phenomenon is called after-discharge and prolongs the response to the original stimulus. Meninges of the Spinal Cord Parallel After-Discharge Circuits The meninges are connective tissue membranes that multiple neurons stimulate several neurons arranged in surround and protect the spinal cord and brain. parallel, all of which converge on a common output cell. Three Primary Layers Of The Meninges contribute to the processing of complex information, Dura Mater integrating inputs from multiple sources and refining most superficial and thickest layer. responses based on the combined activity. forms a protective sac around the spinal cord, often Ex: involved in complex cognitive functions, tasks referred to as the thecal sac. such as mathematical or chemical conversions. The thecal sac attaches to the foramen magnum at the base of the skull and extends down to the second sacral CHAPTER 12: SPINAL CORD AND SPINAL vertebra. NERVES SPINAL CORD It is continuous with the dura mater surrounding the brain and the connective tissue around the spinal Spinal cord nerves. the major communication link between the brain and Between the dura mater and the periosteum lies the the PNS inferior to the head. It integrates incoming epidural space. This space contains spinal nerve roots, information and produces responses through reflex blood vessels, areolar connective tissue, and adipose mechanisms. tissue. Arachnoid Mater General Structure of the Spinal Cord thin, wispy layer that lies beneath the dura mater. extends from the brainstem at the foramen magnum It is named after its spiderweb-like appearance. down to the level of the second lumbar vertebra. The space between the dura mater and the arachnoid Shorter than the vertebral column, because the spinal mater is called the subdural space, which contains a cord does not grow as rapidly during development. small amount of serous fluid. divided into cervical, thoracic, lumbar, and sacral segments, which are named based on the vertebral  NERVOUS SYSTEM | FINAL | SEELEY | CHAPTER 11-16 12 Pia Mater Dorsal roots (posterior side): Contain axons of deepest layer, tightly bound to the surface of the spinal sensory neurons, with cell bodies located in the dorsal cord. root ganglion. It follows all the contours of the spinal cord. Denticulate ligaments and the filum terminale help Dorsal Root Ganglion (Spinal Ganglion) hold spinal cord in place within the thecal sac: houses the sensory neuron cell bodies, where axons  Denticulate Ligaments: Paired ligaments enter from various parts of the body but do not synapse extending from the lateral sides of the spinal cord here. to the dura mater. These ligaments limit lateral They pass through the dorsal root and either synapse movement and are named for their toothlike with interneurons or continue in the spinal cord for processes. further processing.  Filum Terminale: A connective tissue strand that anchors the conus medullaris (tip of the spinal Ventral Root Contains The Axons Of Motor Neurons cord) and the thecal sac to the first coccygeal Somatic motor neurons: Their cell bodies are located in vertebra, limiting their superior movement. the anterior (motor) horn. Autonomic motor neurons: Their cell bodies are found in Subarachnoid Space the lateral horn. lies between the arachnoid mater and pia mater. This space contains: The spinal cord is composed of white matter Weblike strands of the arachnoid mater. (myelinated axons forming tracts) and gray matter Blood vessels. (containing neuron cell bodies and synaptic Cerebrospinal fluid (CSF), which provides connections). cushioning and nourishment to the spinal cord. REFLEXES Cross Section of the Spinal Cord Spinal Cord Two Regions Spinal nerves are formed by the union of dorsal and White Matter ventral roots, which carry sensory and motor located outside of the spinal cord and consists of information, respectively. myelinated axons, which form nerve tracts. Reflex Arc basic functional unit of the nervous system, responsible White Matter Three Main Columns Or Funiculi for producing automatic responses to stimuli.  Ventral (anterior) column Reflexes  Dorsal (posterior) column occur without conscious thought and are essential for  Lateral column maintaining homeostasis and protecting the body. Each column contains tracts (or fascicles), which are bundles of axons transmitting specific types of information. Reflex Arc Five Components Sensory Receptor Gray Matter Detects a stimulus from the environment, such as pain located centrally in the spinal cord and contains neuron or stretch. cell bodies, dendrites, and axons. Sensory Neuron Transmits the action potential from the sensory Organized Into Horns receptor to the central nervous system (CNS). Posterior (dorsal) horn: Receives sensory Interneuron information from the body. Processes the information within the CNS. In simpler Anterior (ventral) horn: Contains motor neurons reflexes, interneurons are absent, and the sensory that control muscles. neuron synapses directly with the motor neuron. Lateral horn: Found only at specific levels of the Motor Neuron spinal cord, associated with the autonomic Transmits the action potential from the CNS to an nervous system. effector organ (muscle or gland). Effector Organ Central Canal Carries out the response, such as muscle contraction or Located in the gray commissure, the central canal is gland secretion. part of the ventricular system and helps circulate cerebrospinal fluid (CSF). Types of Reflexes categorized based on their complexity Spinal Nerves and Roots Spinal Nerves arise from the dorsal and ventral rootlets Monosynaptic Reflexes along the spinal cord Involve a direct synapse between the sensory neuron Ventral roots (anterior side): Contain axons of motor and the motor neuron. neurons that send signals to muscles and glands. Ex. The stretch reflex, such as the knee-jerk response, is a monosynaptic reflex that helps maintain posture and muscle tone.  NERVOUS SYSTEM | FINAL | SEELEY | CHAPTER 11-16 13 Polysynaptic Reflexes Modulation of the Stretch Reflex Involve multiple synapses, including interneurons Gamma Motor Neurons: innervate the ends of the muscle between sensory and motor neurons. spindles. These neurons control the sensitivity of the Ex. The withdrawal reflex involves multiple neurons muscle spindle to stretching. and helps protect the body from harmful stimuli, such The sensory neurons also send collateral axons that as pulling a hand away from a hot surface. synapse with neurons contributing to ascending nerve Autonomic Reflexes tracts, which help the brain perceive muscle stretch. Regulate involuntary functions like heart rate, blood Additionally, descending neurons from the brain pressure, and digestion. They help maintain modulate the activity of the stretch reflex, influencing homeostasis in the body posture and coordination. Somatic Reflexes The knee-jerk reflex is often used by clinicians to test Control voluntary movements or responses to stimuli the functional integrity of the nervous system, that could harm body, such as pulling away from a particularly the spinal cord and the brain's influence on painful stimulus or maintaining balance. it. An exaggerated stretch reflex may indicate that the MAJOR SPINAL CORD REFLEXES gamma motor neurons and the pathways controlling Stretch Reflex them are excessively active, suggesting hyperactivity A monosynaptic reflex that helps maintain muscle in the higher CNS centers. tone and posture. When a muscle is stretched, it Absence of the stretch reflex may indicate a automatically contracts to resist further stretching. disruption in the reflex pathway or suppression of Golgi Tendon Reflex gamma motor neuron activity. This could signal A polysynaptic reflex that prevents muscle damage potential issues within the spinal cord or brain. by causing the muscle to relax when tension on the tendon is too great. GOLGI TENDON REFLEX Withdrawal Reflex a type of polysynaptic reflex that involves both A protective reflex where the body pulls away from sensory and inhibitory pathways to modulate muscle harmful stimuli (e.g., touching something hot). This activity and prevent overexertion. reflex involves multiple neurons and interneurons to essential in preventing muscle and tendon injuries due process the response. to excessive tension. MAJOR SPINAL CORD REFLEXES Mechanism Of Gtr specialized sensory receptors located at the muscle- STRETCH REFLEX tendon junction. simplest type of reflex and is classified as a When a muscle contracts, the associated tendons are monosynaptic reflex. stretched, leading to an increase in tension. The tension crucial role in maintaining posture and muscle tension stimulates the Golgi tendon organs to generate action by providing automatic, rapid muscle contractions in potentials in the sensory neurons. response to stretching. The action potentials travel along sensory neurons This reflex occurs when a muscle contracts in response through the dorsal root of the spinal cord and enter the to a stretching force. posterior gray matter of the spinal cord. A classic example of the stretch reflex is the knee-jerk Within the spinal cord, the sensory neurons synapse reflex (patellar reflex), which is elicited by a clinician with inhibitory interneurons. tapping the patellar ligament. The inhibitory interneurons synapse with alpha motor neurons that innervate the muscle from which the Mechanism Of Sr Golgi tendon organ is located. The patellar ligament is tapped, which stretches the In athletes, the Golgi tendon reflex plays a protective quadriceps femoris muscle group and the tendons role during activities that exert high levels of muscle associated with it. tension, such as running… However, intense physical The muscle spindle, located within the muscle, detects activity, such as that in football players or sprinters, the stretch. may sometimes cause the reflex to be insufficient in The sensory neurons transmit signals from the protecting against injury. stretched muscle spindle to the spinal cord. This is why athletes in high-tension sports are more These sensory neurons synapse directly with alpha prone to injuries like hamstring pulls or Achilles motor neurons in the spinal cord, making the stretch tendon strains. reflex a monosynaptic reflex (no interneuron involved). WITHDRAWAL REFLEX The alpha motor neurons transmit action potentials to known as the flexor reflex the muscle, causing it to contract in response to the is an automatic, protective response that serves to stretch. remove a body part from a painful stimulus. The stretch reflex is vital for maintaining posture. It is a polysynaptic reflex that involves multiple neurons and synapses within the spinal cord.  NERVOUS SYSTEM | FINAL | SEELEY | CHAPTER 11-16 14 Mechanism Of Wr The interneurons on the opposite side synapse with responsible for detecting the painful stimulus are alpha motor neurons that innervate the extensor nociceptors (pain receptors). muscles of the opposite limb. This causes the extensor Once activated, the nociceptors generate action muscles on the opposite side to contract. potentials that are transmitted via sensory neurons. As the flexor muscles of the affected limb contract to These neurons travel through the dorsal root of the withdraw the limb, the extensor muscles of the spinal cord to synapse with interneurons. opposite limb contract, causing the opposite limb to In the spinal cord, the sensory neurons synapse with extend. excitatory interneurons, which relay the signal to alpha motor neurons. Interactions with Spinal Cord Reflexes The alpha motor neurons then stimulate the contraction of flexor muscles (the muscles responsible Divergent Pathways for pulling the limb toward the body) refer to sensory neurons or interneurons in a reflex arc In addition to activating the motor neurons for the that send action potentials to different areas of the withdrawal, collateral branches from the sensory nervous system. neurons synapse with ascending fibers that travel to the allow sensory information to be perceived consciously. brain. For ex. When a painful stimulus is detected, it not only Typically, flexor muscles are activated to withdraw the triggers a withdrawal reflex to remove the affected limb. At the same time, the opposing extensors are body part from the painful stimulus but also sends inhibited to prevent them from resisting the withdrawal signals to the brain to perceive the pain. motion (this is called reciprocal inhibition). Convergent Pathways Convergent pathways involve descending tracts Reciprocal innervation from the brain, which carry action potentials to motor crucial neural mechanism that ensures the efficient neurons in the spinal cord. These descending pathways functioning of certain reflexes, particularly the converge with the neurons involved in reflex arcs, withdrawal reflex and stretch reflex, by facilitating thereby influencing the reflex response. coordinated muscle movements. essential for the proper functioning of reflexes, Integration with Brain Functions enabling coordinated muscle activity by ensuring that The brain's influence on reflexes allows for opposing muscles work in harmony during reflex modulation of reflexive actions. actions. allow brain inputs to modify reflex actions, either Function in the Withdrawal Reflex enhancing or suppressing them. In the withdrawal reflex, when a painful stimulus is Ex. If you touch a hot object, the spinal cord reflex detected, the flexor muscles contract to withdraw the will withdraw your hand, but the brain will perceive body part from the stimulus. the pain and may lead to a response like "moving away Reciprocal innervation ensures that the extensor faster" or possibly even "shouting," depending on the muscles (the muscles that would normally oppose the situation. flexors) are inhibited and relax. Function in the Stretch Reflex Spinal Nerves Reciprocal innervation also plays a critical role in the Structure of Nerves stretch reflex, such as the patellar reflex Nerves in the Peripheral Nervous System (PNS), When the stretch reflex causes a muscle to contract, including spinal nerves, are composed of axons, reciprocal innervation causes opposing muscles to Schwann cells, and various connective tissue layers. relax. These components work together to protect and organize the nerve fibers, ensuring efficient CROSSED EXTENSOR REFLEX transmission of signals between the body and the key protective mechanism that complements the central nervous system (CNS). withdrawal reflex by ensuring balance and stability when a painful stimulus affects one side of the body. Structural Layers Of a Nerve helps to shift body weight from the affected limb (the Endoneurium one that withdrew) to the unaffected limb. innermost layer of connective tissue. It surrounds each essential for maintaining balance and preventing falls individual axon (nerve fiber) and its associated Schwann cell sheath. Mechanism of CER provides support and protects the delicate axons, When a painful stimulus causes a withdrawal reflex in ensuring they are insulated and maintained in the one limb, sensory neurons transmit action potentials proper environment for effective signal transmission. through the dorsal root to the spinal cord. Surrounds individual axons and their Schwann cells. The sensory neurons synapse with interneurons in the Perineurium spinal cord, which not only trigger the withdrawal thicker layer that surrounds a bundle of axons, known reflex on the same side but also send collateral axons as a nerve fascicle. It is made up of several layers of through the white commissure to the opposite side of connective tissue, providing an additional level of the spinal cord. protection.  NERVOUS SYSTEM | FINAL | SEELEY | CHAPTER 11-16 15 protects the fascicles and acts as a barrier, maintaining Dermatomes the internal environment and preventing the spread of region of skin that is supplied by sensory fibers from a harmful substances to the axons. specific spinal nerve. All spinal nerves, except for C1, Encloses bundles of axons (fascicles). have a designated cutaneous sensory distribution. Epineurium Dermatomal map outermost layer of connective tissue that surrounds and shows the areas of skin that each spinal nerve binds together all the fascicles to form a complete innervates, and is helpful for understanding the sensory nerve. It is made of dense connective tissue and is distribution across the body. continuous with the dura mater of the CNS. Ramus (Ramus and Communicating Rami) provides overall protection, structural integrity, and A ramus is a major branch of a spinal nerve. elasticity to the entire nerve. It also acts as a conduit for blood vessels that supply the nerve fibers with Spinal Nerve Branches oxygen and nutrients. Dorsal Ramu Encases the entire nerve, binding the fascicles together. Innervates most of the deep muscles of the dorsal trunk that are responsible for moving the vertebral column. ORGANIZATION OF SPINAL NERVES Also innervates the connective tissue and skin near the midline of the back. Ventral Ramus: The 31 pairs of spinal nerves are distributed along the In the thoracic region, the ventral rami form vertebral column and emerge from specific regions to intercostal nerves, which run along the inferior supply various body parts. margin of each rib and innervate the intercostal muscles and the skin over the thorax. Cervical Region (C1–C8): In the remaining spinal nerves, the ventral rami 8 pairs of spinal nerves. contribute to the formation of five major plexuses. The first cervical nerve (C1) exits between the skull and the first cervical vertebra, and the remaining Communicating Rami: cervical nerves exit through intervertebral foramina These are additional rami found in the thoracic and between the cervical vertebrae. upper lumbar spinal cord regions. Thoracic Region (T1–T12): Carry axons that are part of the sympathetic division 12 pairs of spinal nerves. of the autonomic nervous system. These nerves exit the vertebral column through the Plexuses intervertebral foramina located between the thoracic network or intermingling of nerve fibers from multiple vertebrae. spinal nerves. Lumbar Region (L1–L5): The ventral rami from different spinal nerves form 5 pairs of spinal nerves. these plexuses, which ensure that the nerves supplying These nerves exit between the lumbar vertebrae. a particular area of the body are often derived from Sacral Region (S1–S5): more than one spinal cord level. This helps to 5 pairs of spinal nerves. minimize loss of control or sensation in case of These nerves exit through the sacral foramina in the spinal cord injury. sacrum. Coccygeal Region (Co): Five Major Plexuses Formed By The Ventral Rami 1 pair of spinal nerve, often referred to as the coccygeal nerve. Cervical Plexus (C1–C4): This nerve exits through the sacral foramina and is Supplies muscles and skin of the neck, shoulders, and usually designated as Co. part of the head. The phrenic nerve (from C3–C5) is crucial for diaphragm innervation. Spinal Nerve Designation Each spinal nerve is given a letter and a number: Letter: Indicates the region of the vertebral column Brachial Plexus (C5–T1): from which the nerve emerges. Supplies the shoulder, arm, and hand. C = Cervical (1-8) This plexus is essential for motor control and sensory T = Thoracic (1-12) perception in the upper limb. L = Lumbar (1-5) Lumbar Plexus (L1–L4): S = Sacral (1-5) Innervates the lower abdomen, pelvis, and anterior Co = Coccygeal (when designated) thigh. Number: Indicates the specific nerve in that region, Important nerves include the femoral nerve, which with the smallest number representing the most controls muscles in the thigh. superior origin. Sacral Plexus (L4–S4): Supplies the pelvic region, buttocks, genitals, and parts of the legs.  NERVOUS SYSTEM | FINAL | SEELEY | CHAPTER 11-16 16 The sciatic nerve, which innervates the posterior thigh, Brachial Plexus leg, and foot, is derived from this plexus. complex network of nerves formed by the ventral rami Coccygeal Plexus (S5 and Coccygeal Nerve): of spinal nerves C5–T1. Provides sensory and motor innervation to the skin primarily responsible for the motor and sensory around the coccyx (tailbone) and perineum. innervation of the upper limb. provides motor innervation to the muscles of the Importance of Plexuses upper limb and sensory innervation to the skin of the it prevents the loss of function in an area of the body arm, forearm, and hand. due to the injury of a single spinal nerve. Smaller branches from the brachial plexus also In addition to the major plexuses, there are smaller innervate the shoulder and pectoral muscles. somatic plexuses, such as the pudendal plexus, which vital structure for upper limb function, providing both is located in the pelvis and supplies the pelvic muscles motor and sensory innervation to the arm, forearm, and and skin. hand. Autonomic Plexuses Structure of the Brachial Plexus which provide innervation to organs in the thorax and The brachial plexus is organized into five ventral rami, abdomen, coordinating involuntary functions such as which combine to form three trunks: heart rate, digestion, and respiration. Superior trunk (C5 and C6) Middle trunk (C7) Cervical Plexus Inferior trunk (C8 and T1) relatively small network of nerves originating from the The trunks separate into six divisions (three anterior and spinal nerves C1–C4. three posterior), which then regroup into three cords: primarily responsible for sensory and motor Posterior cord (formed by the posterior divisions innervation to the neck and parts of the head. of all three trunks) plays a critical role in both motor control of the neck Lateral cord (formed by the anterior divisions of and hyoid muscles and in sensory perception in the the superior and middle trunks) neck and posterior head. Medial cord (formed by the anterior division of the inferior trunk) Branches of the Cervical Plexus From these three cords, five major branches emerge to Superficial Neck Structures: The cervical plexus innervate the upper limb. innervates muscles attached to the hyoid bone (important for movements related to swallowing and Major Nerves Emerging from the Brachial Plexus speech) and other superficial neck structures. Axillary Nerve: Innervates the shoulder and part of the Skin of the Neck and Posterior Head: The plexus deltoid muscle. Provides sensation to the shoulder joint supplies sensory innervation to the skin of the neck and and the skin over part of the shoulder. the posterior part of the head, as shown in the Radial Nerve: Innervates the posterior arm, forearm, dermatomal map of the body. and hand (including triceps and extensors of the Ansa Cervicalis forearm). Provides sensation to the posterior part of unique feature of the cervical plexus, often referred to the upper limb, including the posterior surface of the as the “bucket handle”. It is a loop formed by the hand. union of C1–C3. Musculocutaneous Nerve: Innervates the anterior arm, provides motor innervation to the infrahyoid muscles, primarily the biceps and brachialis muscles. which are important for movements of the neck and responsible for arm flexion. Provides sensation to part larynx, crucial for swallowing and speech. of the forearm. Phrenic Nerve Ulnar Nerve: Innervates two forearm muscles and essential for breathing, making it one of the most vital most of the intrinsic muscles of the hand, except for structures of the cervical plexus. those associated with the thumb.Provides sensation to one of the most important branches of the cervical the ulnar side of the hand. plexus, originates from spinal nerves C3–C5 and is Median Nerve: Innervates most of the flexor muscles essential for breathing. of the forearm and hand muscles in the thenar region derived from both the cervical and brachial plexuses. (thumb area).Provides sensation to the radial portion of The phrenic nerve descends on each side of the neck, the palm of the hand. travels into the thorax, and continues along the sides of the mediastinum to innervate the diaphragm. Other Nerves from the Brachial Plexus: Contraction of the diaphragm is largely responsible for Pectoral Nerve: Innervates muscles of the pectoral a person’s ability to breathe; therefore, damage to the region. phrenic nerve during surgery or compression of the Long Thoracic Nerve: Innervates the serratus nerve by a tumor at the base of the lung severely limits anterior muscle, which is involved in the movement breathing. of the scapula.