Nervous System and Reflex Arc

The Nervous System

• The nervous system facilitates the body's response to internal and external stimuli, maintains homeostasis through nerve pathways governed by electrical and chemical signals.
• The nervous system consists of the brain, spinal cord, sensory organs, and nerves.

Reflex Arc

• The reflex arc is an automatic response to a stimulus considered a simple neural pathway from sensory to motor neurons
• Reflexes offer protection and maintain the body in a state of well-being.

Case Study

• A 23-year-old crashes his car and has a fracture dislocation of the 7th thoracic vertebra resulting in severe damage to the spinal cord and paralysis of the left leg.
• Testing reveals hyperesthesia on the left side of the abdomen at the umbilicus level, with anesthesia and analgesia just below.
• On the right, total analgesia, thermoanesthesia, and partial loss of touch occur below the umbilicus, involving the right leg.

Case Study Answer

• A fracture dislocation of the 7th thoracic vertebra may severely damage the 10th thoracic segment of the spinal cord due to the small vertebral foramen in the thoracic region.
• Unequal sensory and motor losses on each side can indicate a left hemisection of the spinal cord.
• Anesthesia and analgesia band is caused by cord destruction on the left side at the 10th thoracic segment, interrupting afferent nerve fibers.
• Loss of pain, thermal sensibilities, and light touch below the umbilicus on the right results from interruption of the lateral and anterior spinothalamic tracts on the cord's left side.

Superior Brain View

• Key structures include the frontal lobe, frontal pole, longitudinal fissure, precentral sulcus, precentral gyrus, central sulcus, postcentral gyrus, postcentral sulcus, parietal lobe, occipital lobe, and occipital pole.

Inferior Brain View

• Key structures include the frontal lobe, longitudinal fissure, olfactory tract, optic chiasma, right cerebral hemisphere, temporal lobe, oculomotor nerve, midbrain, pons, pyramid, cerebellum, and medulla oblongata.

Right Lateral Brain View

• Key structures include the postcentral sulcus, postcentral gyrus, central sulcus, precentral gyrus, precentral sulcus, parieto-occipital sulcus, occipital lobe, right cerebellar hemisphere, medulla oblongata, superior frontal gyrus, middle frontal gyrus, inferior frontal gyrus, lateral sulcus, superior temporal gyrus, middle temporal gyrus, and inferior temporal gyrus.

Medial View of the Right Side of the Brain

• Key brain structures include the corpus callosum, septum pellucidum, fornix, anterior commissure, optic chiasma, temporal lobe, pons, medulla oblongata, central sulcus, cingulate gyrus, cerebral aqueduct of the midbrain, parieto-occipital sulcus, cuneus, calcarine sulcus, vermis of cerebellum, and the cavity of the fourth ventricle.

Clinical Investigation

• Denise, suffering from depression and taking an MAO inhibitor, concealed her cocaine use and Xanax prescription.
• After eating fresh mussels at a restaurant claiming to serve only organic produce, Denise suffered a seizure.
• Paramedics used intravenous benzodiazepine to control the seizure, and Denise accused the manager of serving contaminated mussels, which he denied.
• Key terms include: Saxitoxin and voltage-gated channels, Acetylcholinesterase and cholinesterase inhibitors, Monoamine oxidase and MAO inhibitors, GABA and benzodiazepines.

Organization of the Nervous System

• The nervous system is organized into the central nervous system (CNS) and the peripheral nervous system (PNS).
• The CNS consists of the brain and spinal cord
• The PNS has afferent and efferent divisions
• Afferent division carries sensory input with somatic and special senses
• Efferent division carries motor output via somatic and autonomic nervous systems.
• Autonomic nervous system has sympathetic, parasympathetic, and enteric nervous systems which controls skeletal muscles, cardiac muscles, and glands

Spinal Cord

• The spinal cord arises from the brain and runs through the vertebral column.
• Key parts of the spinal cord include the cervical, thoracic, lumbar, sacral, and coccygeal segments.
• The spinal cord is protected by meninges known as dura mater, arachnoid mater, and pia mater.

Meninges of the Spinal Cord

• Space between the arachnoid and pial membranes is called the subarachnoid space, containing cerebrospinal fluid (CSF).
• In the cranium, fused periosteal and meningeal layers constitute the dura, which separates to create dural sinuses.

Central and Peripheral Nervous Systems Comparison

• Central Nervous System (CNS) includes the brain, forebrain, cerebrum, diencephalon, midbrain, hindbrain, medulla oblongata, pons, cerebellum, spinal cord, cervical segments, thoracic segments, lumbar segments, sacral segments, and coccygeal segments.
• Peripheral Nervous System (PNS) cranial nerves along with their ganglia (12 pairs),exit the skull through the foramina
• Spinal nerves and their ganglia (31 pairs) exit the vertebral column through the intervertebral foramina.
• Spinal cord consists of 8 Cervical, 12 Thoracic, 5 Lumbar, 5 Sacral, and 1 Coccygeal segments.

Cerebral Cortex

• Motor areas control voluntary movements, while sensory areas receive and process information from senses.
• Frontal lobe is for motor control
• Parietal lobe is for somatosensory processing.
• The central sulcus divides the motor and somatosensory cortices.

Cerebral Lobes

• Functions of the frontal lobe include voluntary motor control of skeletal muscles, personality, higher intellectual processes (e.g., concentration, planning, and decision making), and verbal communication.
• Functions of the parietal lobe include somatesthetic interpretation (e.g., cutaneous and muscular sensations); understanding speech and formulating words to express thoughts and emotions; interpretation of textures and shapes.
• The temporal lobe's function includes interpretation of auditory sensations and storage (memory) of auditory and visual experiences.
• Functions of the occipital lobe is the integration of movements in focusing the eye, the correlation of visual images with previous visual experiences and other sensory stimuli, and conscious perception of vision.
• The insula lobe handles memory, sensory(pain), and visceral integration

Techniques to Visualize Brain Function

• Electroencephalogram (EEG) measures neuronal activity using scalp electrodes.
• Functional Magnetic Resonance Imaging (fMRI) detects increased neuronal activity via changes in cerebral blood flow and oxygen consumption, reflected by blood oxyhemoglobin/deoxyhemoglobin ratios.
• Magnetoencephalography (MEG) measures neuronal magnetic activity using magnetic coils and mathematical plots.
• Positron Emission Tomography (PET) measures increased neuronal activity through cerebral blood flow and metabolite consumption, using radioactively labeled deoxyglucose.
• Single-Photon Emission Computed Tomography (SPECT) measures increased neuronal activity via cerebral blood flow, using emitters of single photons, such as technetium.
• Computerized Tomography (CT) uses X-ray beams and detectors to produce images of brain slices through computer processing.

The Nervous System Cells

• The nervous system primarily consists of neurons and supporting cells (neuroglia).
• Neurons transmit electrical signals.
• Neuroglia support and protect neurons.

Neuron Anatomy

• Neurons have three main regions: a cell body, dendrites, and an axon.
• The cell body contains the nucleus and produces macromolecules.
• Nissl bodies are large stacks of rough endoplasmic reticulum in the cell body and larger dendrites.
• In the CNS, cell bodies are clustered into nuclei.
• In the PNS, cell bodies are clustered into ganglia.
• Dendrites are branched processes that receive electrochemical impulses.
• The axon transmits action potentials away from the cell body.
• The axon hillock is an expanded region where the axon originates.

Axonal Transport

• Axons vary in length from a millimeter to over a meter.
• Axons produce axon collaterals that synapse with other neurons.
• A single CNS axon can synapse with 30,000 to 60,000 other neurons.
• Axonal transport is energy-dependent, divided into fast and slow components.
• The fast component transports membranous vesicles at 200-400 mm/day.
• The first slow component transports microfilaments and microtubules at 0.2-1 mm/day.
• The second slow component transports proteins for synaptic function at 2-8 mm/day.
• Kinesin proteins move cargo along microtubules for anterograde transport.
• Dynein and dynactin move cargo along microtubules for retrograde transport

Anterograde vs Retrograde Transport

• Anterograde transport moves materials from the cell body to the axon which involves kinesin proteins.
• Retrograde transport moves materials from the axon to the cell body involves dynein and dynactin proteins.
• Retrograde transport is also able to transport viruses such as herpes, rabies, and tetanus into the cell bodies.
• Retrograde transport can be responsible for movement of herpes virus, rabies virus, and tetanus toxin from the end of the axon into cell bodies.

Terminology used in the Nervous System

• Central nervous system (CNS): Brain and spinal cord
• Peripheral nervous system (PNS): Nerves, ganglia, and nerve plexuses (outside of the CNS)
• Interneuron: Multipolar neuron located entirely within the CNS
• Sensory neuron (afferent neuron): Neuron that transmits impulses from a sensory receptor into the CNS
• Motor neuron (efferent neuron): Neuron that transmits impulses from the CNS to an effector organ; for example, a muscle
• Nerve: Cablelike collection of many axons in the PNS; may be “mixed” (contain both sensory and motor fibers)
• Somatic motor nerve: Nerve that stimulates contraction of skeletal muscles
• Autonomic motor nerve: Nerve that stimulates contraction (or inhibits contraction) of smooth muscle and cardiac muscle and that stimulates glandular secretion Ganglion: Grouping of neuron cell bodies located outside the CNS
• Nucleus: Grouping of neuron cell bodies within the CNS
• Tract: Grouping of axons that interconnect regions of the CNS

Interneurons

• Interneurons or association neurons are located entirely within the CNS between sensory and motor neurons
• Interneurons perform the process of integration by processing incoming sensory information from sensory neurons and eliciting a motor response by activating the appropriate motor neurons
• Interneurons usually have numerous dendrites and one main axon extending from their cell bodies
• Interneurons make up 99% of all neurons in the body.

Functional Classification of Neurons

• Motor/Efferent Neurons - Conduct impulses out of the CNS to effector organs (muscles and glands)
• Sensory/Afferent Neurons - Conduct impulses from sensory receptors into the CNS

Types of Motor Neurons

• Somatic motor neurons are responsible for both reflex and voluntary control of skeletal muscles.
• Autonomic motor neurons innervate the involuntary smooth muscle, cardiac muscle, and glands, with cell bodies located outside of the CNS.
• Sympathetic and parasympathetic nervous systems are two subdivisions of autonomic neurons that innervate involuntary effectors, with their cell bodies outside the CNS in autonomic ganglia.

Structural Classification of Neurons

• Pseudounipolar neurons have a single short process: a form a pair of longer processes.
• Bipolar neurons end with two processes, found in the retina of the eye.
• Multipolar neurons are the most common with multiple dendrites and one axon mainly for motor neurons
• A nerve is defined as a bundle of axons outside the CNS, innervating a particular body region, and is composed of both motor and sensory fibers (mixed nerves).
• Cranial nerves may include only sensory fibers, serving the special senses of sight, hearing, taste, and smell.

Neuroglia

• Schwann cells (neurolemmocytes) form myelin sheaths around peripheral axons.
• Satellite cells support neuron cell bodies within the ganglia of the PNS
• Oligodendrocytes form myelin sheaths around axons of the CNS.
• Microglia migrate through the CNS and phagocytose foreign and degenerated material.
• Astrocytes help regulate the external environment of neurons in the CNS.
• Ependymal cells line the ventricles (cavities) of the brain and the central canal of the spinal cord, consisting of an epithelial cells.

Myelin Sheath

• All axons in the PNS (myelinated and unmyelinated) are surrounded by a continuous living sheath of Schwann cells known as the neurilemma.
• The axons of the CNS, by contrast, lack a neurilemma (Schwann cells are found only in the PNS).
• In the PNS, this insulating covering is formed by successive wrappings of the plasma membrane of Schwann cells; in the CNS, it is formed by oligodendrocytes.
• Myelinated axons conduct impulses more rapidly than those that are unmyelinated.
• Axons smaller than 2 micrometers (2µm) in diameter are usually unmyelinated, versus larger axons are likely to be myelinated

PNS Myelin Sheath Formation

• Schwann cells attach to and roll around the axon during the myelin formation process in the PNS.
• Cytoplasm is forced into the outer region of the Schwann cell.
• Schwann cell axon wrap about a millimeter of axon, leaving open gaps of exposed axon between adjacent Schwann cells and known as the nodes of Ranvier
• Axons are surrounded by a living sheath of Schwann cells, called neurilemma.
• Unmyelinated axons are also surrounded by a neurilemma, but they lack the multiple wrappings of Schwann cell plasma membranes that compose the myelin sheath.

CNS Myelin Sheath Formation

• Oligodendrocytes have extensions to form myelin sheaths around several axons.
• The myelin sheaths around CNS axons give the tissue a white color; areas of the CNS with higher concentration of axons thus forming the white matter
• Grey matter of the CNS has high concentrations of cell bodies and dendrites which lack myelin sheaths.

Clinical Applications of the Myelin Sheath

• Demyelinating diseases involve specific loss of the myelin sheath
• Immune system attacks either Schwann cells (PNS myelin) or oligodendrocytes (CNS myelin) in autoimmune diseases
• Multiple sclerosis (MS) is an autoimmune attack by T lymphocytes
• Lymphocytes and monocyte-derived macrophages target myelin sheaths
• MS causes inflammation, demyelination, scarring, and axon degeneration
• MS is often diagnosed in people 20-40 years old, twice as common among women
• MS is a chronic remitting and relapsing disease
• MS symptoms are sensory impairments, motor dysfunction, spasticity, bladder and intestinal problems, and fatigue
• Treatment for MS includes drugs reducing autoimmune activity, inflammation, and entry of autoreactive T cells into the CNS, however the drugs do not cure the disease.

Neurotrophins

• Neurotrophins are Nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin 3, neurotrophin 4, and glial-derived neurotrophic factor (GDNF)
• Neurotrophins are required for mature sensory neurons to regenerate after injury.
• Neurotrophins regulate the survival and differentiation of adult neural stem cells as needed for learning and memory.
• NGF is required for the maintenance of sympathetic ganglia.
• GDNF promotes the survival of dopaminergic neurons and spinal motor neurons.

Functions of Astrocytes

• Astrocytes take up K+ from the extracellular fluid to maintain the proper ionic environment for neurons.
• Astrocytes take up neurotransmitters like Glutamate, can be used for other neurons to produce GABA
• Astrocytes releases lactate which aids the maintenance of neuron functions
• Astrocytes assist in the release of some neurotransmitters from the terminal boutons of axons
• Astrocytes end-feet surround blood capillaries to take up glucose from the blood
• Astrocytes are needed for synapse formation, maturation, and maintenance.
• Astrocytes regulate neurogenesis in the adult brain.
• Astrocytes secrete glial-derived neurotrophic factor (GDNF).
• Astrocytes induce the formation of the blood-brain barrier.
• Neurons communicate with astrocytes which lead to glutamate release.
• Gliotransmitters for the release, can stimulate or inhibit the inflow of Ca2+, which can be released in Glia

Blood-Brain Barrier

• Blood-brain barrier presents difficulties in treating disease in the chemotherapy of brain diseases as drugs that can be used in other organs cannot enter the brain.
• With Parkinson's Disease to treat in the brain to make sure dopamine can enter, patients need to be administered levodopa in l-dopa format.
• Some antibiotics are restricted from entering as well Capillaries in the brain do not have pores between adjacent endothelial cells but joined together by tight junctions.
• Restricts paracellular movement molecule known as - Molecules within brain capillaries and require the molecules The molecules take the transcellular route and pass through the epithelial cells.
• • Nonpolar O2 and CO2, alcohol and barbiturates, can pass through the phospholipid components of plasma membranes.
• Ions and polar molecules require ion channels and carrier proteins to move between the blood and brain
• Plasma glucose can pass into the brain using specialized carrier proteins known as GLUT1, does not require insulin stimulation than GLUT4.
• metabolic component of the blood-brain barrier can be effective for detoxifying substances for entering the brain
• two-way communication can be created through the regulation of some regulatory molecules

Neuron-glia Crosstalk

• Neuronal activity triggers Ca2+ flow into neuroglia, which in turn stimulates the release of gliotransmitters that affect nearby neurons
• A rise in the Ca2+ concentration within astrocytes in response to the increased activity of surrounding neurons can also promote the production of prostaglandin E2
• Prostaglandin E2 is released from the astrocyte end-feet surrounding cerebral blood vessels to cause these vessels to dilate
• An increase in neuronal activity within a brain region is accompanied by an increased blood flow to that region