maddie nervous system

Nervous System Outline

• Defines and describes the nervous system
• Considers its functional components of it

Neurons

• Afferent neurons, also known as sensory neurons, transmit signals from sensory receptors to the central nervous system (CNS)
• They facilitate the processing of external stimuli by conveying environmental information to the brain and spinal cord
• Efferent neurons, also known as motor neurons, carry signals from the CNS to muscles and glands
• Responsible for initiating movement and other bodily responses, such as muscle contraction or hormone secretion
• Interneurons located within the CNS serve as connectors between afferent and efferent neurons
• They process signals received from afferent neurons and communicate with efferent neurons, playing a critical role in reflexes and complex information processing

Glial Cells and Their Functions

• Astrocytes provide structural support to neurons, maintain the blood-brain barrier, and regulate nutrient and ion flow
• They aid in repairing brain tissue after injury
• Oligodendrocytes produce myelin, which insulates axons and enhances the speed of electrical signals in the CNS
• Schwann cells are similar to oligodendrocytes, but located in the peripheral nervous system and also produce myelin
• They aid in the regeneration of damaged nerves
• Microglia monitor the CNS environment for pathogens and debris and respond to injury or disease by clearing away dead cells and modulating inflammation, acting as the immune cells of the CNS
• Ependymal cells line the ventricles of the brain and the central canal of the spinal cord and produce and circulate cerebrospinal fluid to cushion and protect the brain and spinal cord

Parts of a Neuron

• Dendrites are tree-like structures extending from the soma that receive signals from other neurons
• Soma, is the cell body, integrates signals from dendrites and determines whether to send a signal down the axon
• Axon hillock acts as a trigger zone for action potentials; an action potential travels down the axon if incoming signals are strong enough
• Axon is a long, slender projection that sends electrical impulses away from the soma
• Axon terminal releases neurotransmitters into the synapse, enabling communication with other neurons, muscles, or glands

Types of Neurons

• Multipolar neurons: composed of one axon & multiple dendrites
• Most common type in the body, and most neurons in CNS
• Bipolar neurons: composed of one axon & one dendrite
• Relatively rare and primarily found in sensory organs, such as the retina of the eye and the olfactory system
• Unipolar neurons: single process leading away from cell body, splits into peripheral process and central process

Myelin Sheath

• In the CNS, oligodendrocytes form the myelin sheath
• They extend their processes to multiple axons, wrapping around them to create segments of myelin
• Saltatory conduction allows electrical impulses to jump from one node of Ranvier to another, speeding up signal transmission along the axon
• In the PNS, Schwann cells create the myelin sheath by wrapping around a single axon
• Similar to the CNS, the myelin sheath in the PNS facilitates saltatory conduction, improving nerve signal transmission efficiency

Ion Channels

• Ion channels are specialized proteins in the cell membrane that allow ions to pass in and out
• Crucial in maintaining the cell's electrochemical gradient and nerve impulse transmission
• Voltage-gated ion channels open or close in response to changes in membrane potential, critical for generating action potentials
• Ligand-gated ion channels open in response to the binding of neurotransmitters and play a key role in synaptic transmission
• Mechanically-gated ion channels open in response to mechanical stress or deformation and are important in sensory cells
• Sodium (Na+) and chloride (Cl-) ions typically have higher concentrations outside the cell, while potassium (K+) ions are more concentrated inside
• This distribution creates a resting membrane potential, with the inside of the cell being negatively charged compared to the outside

Resting Potential

• The resting potential is the electrical charge difference across the cell membrane when a neuron is not actively transmitting a signal
• It typically ranges from -60 to -70 millivolts (mV), with the inside negatively charged relative to the outside
• Sodium-Potassium Pump (Na+/K+ ATPase) actively transports sodium ions (Na+) out of the cell and potassium ions (K+) into the cell, maintaining concentration gradients
• Ion Channels, specific potassium channels, allow K+ to flow out of the cell more easily than Na+ can enter, contributing to the negative charge inside the cell
• Leak Channels are always open and allow ions, especially K+, to move across the membrane, which helps stabilize the resting potential

Action Potential

• The action potential involves the following steps
• 1: Resting State: The neuron is at its resting potential, around -60 to -70 mV
• Sodium channels are closed, and potassium channels are mostly closed
• 2: Depolarization: A stimulus strong enough to exceed the threshold (around -55 mV) opens voltage-gated sodium channels, causing sodium ions (Na+) to rush into the cell, making the membrane potential more positive
• 3: Repolarization: After reaching a peak around +30 mV, closing the sodium channels and opening voltage-gated potassium channels causes potassium ions (K+) to flow out of the cell, restoring the negative charge
• 4: Hyperpolarization: Sometimes, the membrane potential becomes even more negative than the resting potential
• 5: Return to Resting State: Eventually, the potassium channels close, and the sodium-potassium pump restores the resting potential

Propagation of Action Potential

• Local Depolarization: Influx of sodium ions at the axon hillock causes local depolarization, opening nearby voltage-gated sodium channels
• Wave of Depolarization: Opening sodium channels causes sodium ions to rush in, making the membrane potential positive
• This depolarization spreads to adjacent sections, triggering more sodium channels to open
• Repolarization: After the action potential peak, sodium channels close and potassium channels open, allowing potassium ions to exit, restoring the negative charge
• Saltatory Conduction: In myelinated axons, the action potential jumps from one node of Ranvier to another, greatly increasing signal transmission speed compared to unmyelinated axons
• Continuous Conduction: In unmyelinated axons, the action potential propagates continuously along the axon, as each segment undergoes depolarization and repolarization

Steps at the Chemical Synapse

• Arrival of Action Potential: The action potential change in the axon terminal causes voltage-gated calcium channels to open
• Calcium Influx: Calcium ions (Ca2+) rush into the axon terminal from outside
• Neurotransmitter Release: Calcium triggers synaptic vesicles to move toward the presynaptic membrane, fuse, and release neurotransmitters into the synaptic cleft through exocytosis
• Binding to Receptors: Neurotransmitters diffuse across the cleft and bind to specific receptors on the postsynaptic neuron's membrane
• Post-Synaptic Response: Neurotransmitter binding leads to either depolarization (generating a new action potential) or hyperpolarization (inhibiting the neuron)
• Termination of Signal: Neurotransmitters are broken down by enzymes taken back into the presynaptic neuron, or diffuse away, ending the signal

Events at the Neuromuscular Junction and Muscle Contraction

• 1: Stimulus Initiation: A signal from the central nervous system travels down a motor neuron toward the neuromuscular junction
• 2: Action Potential Arrival: The action potential change to the motor neuron causes voltage-gated calcium channels to open
• 3: Calcium Influx: Calcium ions flow into the axon terminal from the extracellular fluid, increasing intracellular calcium concentration
• 4: Neurotransmitter Release: Calcium triggers synaptic vesicles filled with acetylcholine (ACh) to fuse with the presynaptic membrane, releasing ACh into the synaptic cleft through exocytosis
• 5: Binding to Receptors: ACh diffuses across the synaptic cleft and binds to nicotinic acetylcholine receptors on the muscle fiber
• 6: Muscle Fiber Depolarization: ACh binding opens ligand-gated ion channels, allowing sodium ions (Na+) to enter the muscle cell
• 7: Action Potential Propagation: The action potential travels along the muscle fiber membrane and into T-tubules
• 8: Calcium Release: The action potential triggers calcium ions to release from the sarcoplasmic reticulum
• 9: Muscle Contraction: Calcium ions bind to troponin, causing tropomyosin to move
• 10: Cross-Bridge Cycling: Myosin heads pull actin filaments, causing muscle contraction
• 11: Relaxation: The nerve signal stops, ACh is broken down, and calcium is pumped back, leading to muscle relaxation

Peripheral (Spinal) Nerve Anatomy

• Epineurium is the outermost layer of connective tissue surrounding the entire nerve, offering protection and support, and contains blood vessels
• Fascicles are bundles of nerve fibers within the nerve that the epineurium encloses
• Perineurium is the connective tissue layer that surrounds each fascicle and serves as a protective barrier the internal environment of the nerve fibers
• Endoneurium surrounds individual axons inside each fascicle and provides support and insulation, helping to maintain the proper environment for nerve conduction
• Axon is the nerve fiber itself, which transmits electrical impulses away from the nerve cell body, often covered by a myelin sheath to enhance speed through saltatory conduction

Sensory Tracts

• Dorsal Columns carry fine touch, vibration, and proprioceptive information
• Spinothalamic Tract transmits pain, temperature, and crude touch sensations
• Spinocerebellar Tracts assist with proprioception and balance by conveying muscles and joints position information to the cerebellum
• Trigeminal Tract transmits sensory information from the face, sending touch, pain, and temperature sensations from these areas to the thalamus and then to the sensory cortex

Motor Tracts

• Corticospinal Tract is the primary pathway for voluntary motor control and starts in the motor cortex and descends to the brainstem and spinal cord
• Corticobulbar Tract carries motor signals to the cranial nerves that control facial muscles, chewing, and swallowing. It synapses with cranial nerve nuclei in the brainstem
• Extrapyramidal Tracts regulate involuntary movements and muscle tone and include pathways, such as the rubrospinal, vestibulospinal, and reticulospinal tracts
• Spinothalamic Tract also plays a role in motor functions by providing feedback to the brain about pain and temperature

Somatic vs. Autonomic Nervous Systems

• The Somatic Nervous System is responsible for voluntary control of skeletal muscles
• It uses a single motor neuron that extends from the CNS directly to the skeletal muscle, releasing acetylcholine at the neuromuscular junction, leading to muscle contraction
• Autonomic Nervous System regulates involuntary functions automatically
• It has preganglionic neurons which originate in the CNS and synapse with a second neuron in a ganglion, and postganglionic neurons, which extend to the target organ releasing neurotransmitters that can vary

Divisions of the Autonomic Nervous System

• The Sympathetic Division, or the "fight or flight" system, prepares the body for stressful situations to increase heart rate, dilate airways - its' neurons originate in the thoracic and lumbar regions
• The Parasympathetic Division, or the "rest and digest" system, promotes relaxation, constricts airways, stimulates digestion - its' neurons originate in the brainstem and sacral regions

Sympathetic Trunk Ganglia

• The sympathetic trunk ganglia are involved in the "fight or flight" response
• Signals travel to these ganglia, where they synapse with postganglionic neurons, which project to various target organs, initiating a rapid response

Meninges

• Dura Mater is the outermost layer which protects against impacts and contains the cerebrospinal fluid
• Arachnoid Mater is the middle layer which cushions the brain and spinal cord
• Pia Mater is the innermost layer, containing blood vessels and helping in the production of cerebrospinal fluid

Cerebrospinal Fluid (CSF)

• CSF is a clear, colorless fluid that surrounds and protects the brain and spinal cord
• Provides protection, buoyancy, nutrient transport, homeostasis, and circulation
• CSF acts as a cushion, cushioning the brain and spinal cord and reducing pressure

Blood-Brain Barrier (BBB)

• The BBB is a selective permeability barrier that protects the brain from potentially harmful substances in the bloodstream
• The BBB also allows essential nutrients to pass through while blocking larger molecules and the entry of immune cells

Spinal Cord Anatomy

• The spinal cord extends from the brain down to the lower back
• It has cervical, thoracic, lumbar, sacral, and coccygeal regions, each with distinct functions
• The denticulate ligament anchors the spinal cord, stabilizing it and preventing movement

Information Flow in Spinal Cord and Neurons

• Sensory information enters the spinal cord through sensory neurons via the dorsal roots, processed in the brain
• Motor commands originate in the brain, travel down the spinal cord, exit via the ventral roots, and ultimately allow muscle and gland responses

Brain Parts

• Brain Stem controls heart rate, breathing, and blood pressure as well as reflexes, at the base of the brain and composed of gray and white matter
• Diencephalon relays sensory information and regulates autonomic functions - located above the brain stem and surrounding the third ventricle
• Cerebellum coordinates voluntary movements is at the back of the skull and beneath the cerebrum
• Medulla Oblongata controls life-sustaining processes and involved in reflex actions found in the brain stem
• Pons regulates breathing and sleep cycles, located in the brain stem above the medulla and below the midbrain
• Cerebral Peduncles connect the cerebrum to the brain stem and spinal cord, in the midbrain
• Substantia Nigra helps with movement control
• Superior and Inferior Colliculi processes vision and auditory information, located in the dorsal aspect of the midbrain that helps
• Reticular Formation regulates arousal and alertness, mixed gray and white matter within the brain stem
• Hypothalamus regulates autonomic functions, hormone release, located below the thalamus
• Thalamus is situated above the hypothalamus transmits sensory information to the cortex, acting as the brain's relay station
• Epithalamus includes the pineal gland, regulating sleep-wake cycles
• Cerebrum is the largest part of the brain, responsible for higher functions
• Cerebral Cortex is the outer layer of cerebrum and involved in complex brain functions
• Postcentral Gyrus processes sensory information from the body
• Frontal Lobe handles higher functions
• Temporal Lobe: auditory processing
• Occipital Lobe visual processing
• Parietal Lobe: sensory information
• Corpus Callosum: communication between two brains
• Basal Ganglia: coordinates voluntary motor control
• Limbic System: located in the temporal lobe is essential for processing emotions

Functional Areas of the Cerebrum

• Vestibulocerebellum: balance and eye movement
• Spinocerebellum coordinations
• Cerebrocerebellum: plan and timing, helps new motor tasks

Cranial Nerves and their functions

• There are 12 Cranial Nerves
• Olfactory Nerve deals with smell
• Optic Nerve has controls Vision
• Oculomotor Nerve controls eye movements
• Trochlear Nerve rotates the eye
• Trigeminal Nerve gives sensation
• Abducens Nerve moves eyes outward
• Facial Nerve controls facial and taste
• Vestibulocochlear Nerve controls hearing and balance
• Glossopharyngeal Nerve controls swallowing
• Vagus Nerve controls heart functions
• Accessory Nerve controls head moments
• Hypoglossal Nerve helps tongue movement