Nervous System Overview Quiz

The Nervous System

• The nervous system receives and processes sensory information from both internal and external environments.
• It regulates somatic (conscious) and autonomic motor control.
• It has two major divisions: the central nervous system (CNS) and the peripheral nervous system (PNS).
• The CNS consists of the brain and spinal cord.
• The PNS consists of nerves (axon bundles) and neuron cell bodies that are located outside the CNS.

Overview of Nervous System Functions

• Sensory: Sensory receptors respond to stimuli by generating nerve signals that travel via the PNS to the CNS (afferent information).
• Integration: The CNS sums up input from all over the body, stores memories, and creates motor responses.
• Motor: The CNS generates motor output, which travels via the PNS to muscles, glands, and organs (efferent information).

Nervous System Tissue

• Neurons transmit nerve impulses.
• Glial cells support and nourish neurons.
• Microglia are phagocytic cells that remove bacteria and debris in the CNS.
• Astrocytes provide metabolic and structural support directly to neurons.
• Oligodendrocytes form myelin sheaths in the CNS.
• Ependymal cells line the ventricles and produce cerebrospinal fluid (CSF).
• Schwann cells form myelin sheaths in the PNS.
• Satellite cells support neuron cell bodies in the PNS.

Types of Neurons

• Sensory neurons carry nerve signals from a sensory receptor to the CNS (afferent).
• Sensory receptors detect changes in the environment.
• Interneurons are found only within the CNS, receive input from sensory neurons and other interneurons, sum incoming signals, and communicate with motor neurons.
• Motor neurons carry nerve impulses away from the CNS (efferent) to an effector (muscle fiber, organ, or gland) which carries out the appropriate response to the environmental change.

Neuron Structure

• Neurons have three structures: a cell body, dendrites, and an axon.
• The cell body contains the nucleus and other organelles.
• Dendrites are short extensions off the cell body that receive signals from other neurons.
• The axon is the portion of the neuron that conducts nerve impulses.
• Individual axons are termed nerve fibers; collectively, they form a nerve.
• The axon hillock is the beginning of the axon where the electrical impulse (action potential) starts.

Myelin Sheath

• Myelin sheaths cover some axons, but not all.
• Myelin helps conduct electrical impulses (action potentials).
• Formed when Schwann cells (PNS) or oligodendrocytes (CNS) wrap around the axon.
• Nodes of Ranvier are spaces between myelin sheaths.
• Long axons typically have myelin sheaths; short axons usually do not.
• Gray matter of the CNS contains cell bodies and lacks myelin.
• White matter of the CNS contains myelinated axons and appears white.
• Multiple sclerosis (MS) is a disease where oligodendrocytes die and myelin breaks down preventing transmission in neurons.

Peripheral Nerve Regeneration

• Schwann cells secrete factors that promote axon growth.
• They form channels with the myelin sheath to help severed axons grow into them.

Basic Neurophysiology

• Neurons generally have a net negative internal charge relative to the outside.
• The resting membrane potential is typically -70mV.
• The sodium-potassium pump creates an electrical and chemical gradient to maintain the resting potential. (This 3 Na+ out and 2 K+ in process).

Membrane Potential

• The resting membrane potential of a neuron is -70mV.
• The cytosol (inside the cell) contains an abundance of negatively-charged proteins.
• The extracellular fluid contains relatively few proteins.
• The proteins cannot cross the plasma membrane to generate and/or retain the resting potential.

Ion Channels

• Ion channels in cell membranes allow specific ions to flow across.
• Flow is a result of concentration and charge gradients.
• "Excitable" cells (such as neurons) have ion channel gates that can open and close.
• Voltage-gated channels open when the membrane potential changes.
• Ligand-gated channels open when a neurotransmitter binds.
• Leak channels are always open.

Graded Potentials

• Graded potentials are local changes in the membrane potential.
• Caused by opening ligand-gated ion channels.
• Usually occur in dendrites or cell bodies.
• Can be summed like waves in the ocean.
• Hyperpolarization increases the negative charge inside the membrane which moves further away from the threshold (-50mv).
• Depolarization decreases the negative charge, which brings it closer to the threshold.
• Graded potentials occur in the dendrites and/or cell body, and either excitatory or inhibitory; to generate an action potential.

Action Potentials

• Action potentials occur in axons only, and begins when voltage-gated Na+ channels open.
• Na+ diffuses inward leading to depolarization.
• Then voltage -gated K+ channels open causing repolarization as K+ diffuses outward.
• Action potentials are typically "all or none", meaning that they occur entirely or not at all at full amplitude; once reached, there is no change in size or amplitude with increasing stimulus strength.
• Action potentials can be summed in frequency, meaning the strength of action potential increase with increasing strength of the stimulus. This is also known as increasing the number/frequency of action potentials. (example)

Propagation of Action Potentials

• In unmyelinated axons, action potentials stimulate adjacent parts of the axon membrane to produce action potentials, but conduction can be slow (1 m/s).
• In myelinated axons, the action potentials only occur at nodes of Ranvier, leading to conduction that is much faster (100 m/s). This type of conduction is called saltatory conduction.
• Action potentials continuously propagate along the entire length of the axon.

The Synapse

• Axons branch into axon terminals near the dendrite or cell body of another neuron.
• This region of close proximity is the synapse.
• The synaptic cleft is a small gap between the sending and receiving neurons.
• Neurotransmitters transmit information across the synapse.
• Neurotransmitters are stored in synaptic vesicles within the axon terminals.

Events at a Synapse

• Action potentials travel along the axon and reach the axon terminal.
• Voltage-gated calcium channels open, and calcium ions enter the terminal.
• Calcium stimulates synaptic vesicles to fuse with the pre-synaptic membrane and exocytose their contents (neurotransmitters).
• Neurotransmitters diffuse across the synaptic cleft to the post-synaptic membrane.
• Neurotransmitters bind to receptor proteins on the post-synaptic membrane, causing a graded potential or some metabolic event.

Removal of Neurotransmitter

• After initiating a response, neurotransmitters can diffuse away from the synaptic cleft and/or be inactivated by enzymes.
• For example, acetylcholinesterase (AChE) breaks down acetylcholine in some synapses,.
• In other synapses, the pre-synaptic membrane reabsorbs the neurotransmitter with transport proteins.
• The short existence of neurotransmitters at a synapse prevents continuous stimulation of receiving membranes.

Neurotransmitters

• About 30 different neurotransmitters exist. Examples include acetylcholine (ACh), norepinephrine, dopamine, serotonin, glutamate, and GABA.
• Nerve-muscle, nerve-organ, and nerve-gland synapses all use neurotransmitters.
• ACh is excitatory in skeletal muscle but inhibitory in cardiac muscle.
• Norepinephrine is excitatory in smooth muscle.

Common CNS Neurotransmitters

• Acetylcholine plays a role in memory circuits.

• Norepinephrine is important for dreaming, waking, and mood.

• Dopamine regulates mood, addiction, and coordinate movements.

• Serotonin is involved in thermoregulation, sleep, emotions, and perception.

• GABA is inhibitory in the CNS.

• Neuromodulators, like substances P and endorphins, block the release of other neurotransmitters and/or produce other effects like pain killing.

• Synaptic Integration*

• Many different types of receptors exist for each neurotransmitter.

• Each synapse can exhibit different kinds of receptors.

• Some receptors are excitatory, some inhibitory.

• Integration is the summing of multiple incoming excitatory and inhibitory signals.

• If sufficient excitatory signals outweigh the inhibitory signals, the threshold is reached and an action potential is fired.

• If the inhibitory signals outweigh the excitatory signals, there is insufficient stimulation to reach threshold, and no action potential is fired.

The Central Nervous System

• The CNS consists of the spinal cord and the brain.
• It receives sensory information and initiates motor control.
• Both are protected by bone (vertebrae and skull) and within membranes called meninges.
• Meningitis is an infection of the meninges.
• Cerebrospinal fluid (CSF) cushions and protects the CNS. The CSF is in the 4 ventricles of the brain and central canal of the spinal cord.

The Spinal Cord

• White matter surrounds gray matter , which contains cell bodies and short, nonmyelinated axons.
• The spinal cord contains sensory fibers and motor fibers.
• Dorsal root contains sensory fibers; dorsal root ganglia contain bodies of the sensory neurons.
• Ventral root contains motor fibers.
• The dorsal and ventral roots join to form a mixed nerve.
• The spinal cord is essential for thousands of reflex arcs.
• Reflex arcs have 3 parts:
  1. Sensory receptors generate signals that travel via sensory axons to the spinal cord.
  2. Interneurons integrate the incoming data and relay signals to motor neurons.
  3. Motor axons cause skeletal muscles to contract;.
• Reflex arcs can be somatic or autonomic.

The Brain

• The brain is composed of the cerebrum, diencephalon, cerebellum, and brain stem.
• The cerebrum is the largest part of the human brain. It is responsible for receiving sensory input and carrying out integration before commanding voluntary motor responses.
• The cerebral cortex covers the cerebral hemispheres. It is a thin outer layer of gray matter, responsible for sensation, voluntary movement, thought processes, and consciousness.

Brain Regions

• Cerebral hemispheres: The left and right sides are separated by the longitudinal fissure; they communicate via the corpus callosum.

• Lobes: Divided into lobes; frontal, parietal, occipital(posterior to parietal), and temporal(inferior to frontal and parietal).

• Diencephalon: Processes sensory input, except smell; Hypothalamus: integrates center, regulating hunger, sleep, thirst, and body temperature and water balance, regulating pituitary gland; Thalamus: Relay center, regulating the sense of smell

• Cerebellum: Under the occipital lobe, separated from the brain stem by the fourth ventricle. Primarily white matter arbor vitae, with overlying gray matter. It is responsible for maintaining posture and balance and producing smooth coordinated, voluntary movements.

• Brain stem: Connects the brain to the spinal cord and includes the midbrain, pons, and medulla oblongata.

• Midbrain: Relay station, and reflex centers.

• Pons: Communicates between the cerebellum and other parts of the CNS. Reflex centers for head movement in response to visual or auditory stimuli.

• Medulla oblongata: Regulates heartbeat, breathing and vasoconstriction(blood pressure). Reflex centers for vomiting, coughing and sneezing. Contains tracts that ascend or decend between CNS and higher brain centers.

• Basal nuclei: Masses of gray matter deep within the white matter, which integrate motor commands to coordinate movements.

• Parkinson disease is a result of degeneration of dopaminergic neurons in the substantia nigra.

• Limbic system: Integrates emotions with higher mental functions like reasoning and memory. It includes the amygdala (fear) and hippocampus (learning and memory).

The Peripheral Nervous System

• Ganglia= clusters of nerve cell bodies outside the CNS.
• Cranial nerves=12 pairs attached to the brain, involved in the head, neck and facial regions.
• The vagus nerve (X) is a major part of the parasympathetic nervous system , which relates to neck and internal organs. . Spinal nerves= emerge from both sides of spinal cord, 31 pairs of spinal nerves. Dorsal root contains sensory fibers; dorsal root ganglia contain bodies of the sensory neurons. Ventral root contains motor fibers, and the dorsal and ventral roots join forming spinal nerves, These nerves are "mixed nerves".

Autonomic Nervous System

• Involuntary. Regulates cardiac, smooth muscles, organs and glands.
• Divided into sympathetic(emergency situations) and parasympathetic (relaxed states) divisions.
• Creates antagonistic responses.
• Innervates all internal organs; uses two neurons with a ganglion between them for one motor output, to ensure proper coordination.

Sympathetic Division (fight or flight)

• Active during emergency situations.
• Increases heart rate and dilates airways.
• Inhibits digestive and urinary organs.
• Norepinephrine is the neurotransmitter released by postganglionic axon. (structurally similar to epinephrine(adrenaline)).

Parasympathetic Division (rest-and-digest)

• Promotes responses associated with a relaxed state.
• Promotes digestion of food and slows heart rate.
• Acetylcholine is the neurotransmitter released by postganglionic axon.

Test your knowledge on the nervous system with this quiz. Explore key concepts such as the roles of interneurons, the structure of neurons, and the function of various glial cells. Perfect for students studying biology or neuroscience.