Neuroscience Chapter on Neurons and Action Potentials

The Neuron

• The functional unit of the nervous system.
• Composed of three main parts:
  • Cell Body: The central part of the neuron.
  • Dendrites: Branching extensions that receive signals from other neurons.
  • Axon: A single, long extension that transmits signals away from the cell body.
  • Synaptic Vesicles: Structures at the end of the axon that release neurotransmitters into the synaptic cleft.

Action Potential Conduction

• Depolarization: Caused by an influx of sodium (Na+) or calcium (Ca++) ions.
• Repolarization: Caused by an efflux of potassium (K+) ions.

Major Structures of the Nervous System

• Nerve: A bundle of axons in the Peripheral Nervous System (PNS), often containing both motor and sensory fibers. Examples include the sciatic nerve and the radial nerve.
• Tract: A bundle of axons in the Central Nervous System (CNS). Example: Lateral Spinothalamic Tract.
• Ganglion: A cluster of neuron cell bodies in the PNS. Example: Dorsal Root Ganglion (sensory input passes through these).

Neuroglia

• Supporting cells of the nervous system that protect, nourish, and remove debris from neurons.
• Six main types:
  • Astrocytes: Form the blood-brain barrier.
  • Oligodendrocytes: Form myelin in the CNS.
  • Schwann Cells: Form myelin in the PNS.
  • Microglia: Phagocytic cells that remove debris and pathogens.
  • Ependymal Cells: Line the ventricles of the brain and central canal of the spinal cord.
  • Satellite Cells: Surround neuron cell bodies in ganglia, providing support and protection.

Parts of the Brain

• Cerebrum: The largest part of the brain, responsible for higher-level functions.
  • Two Hemispheres: Left and right hemispheres, connected by the corpus callosum.
  • Four Lobes:
    • Frontal Lobe: Planning, decision-making, movement.
    • Temporal Lobe: Hearing, memory, language comprehension.
    • Parietal Lobe: Sensory information processing, spatial awareness.
    • Occipital Lobe: Visual processing.
  • Wernicke's Area: Responsible for language comprehension.
    • Damage leads to Fluent Aphasia: Difficulty understanding spoken and written language.
  • Broca's Area: Responsible for speech production.
    • Damage leads to Expressive Aphasia: Difficulty speaking fluently and finding the right words.
• Cerebellum: Coordinates movement, balance, and proprioception.
  • Responsible for skilled muscular activity (muscle memory).
  • Proprioception: Awareness of body part position and movement.
• Diencephalon: Located between the cerebrum and brainstem.
  • Thalamus: Relay center for sensory information.
  • Hypothalamus: Regulates body temperature, hunger, thirst, and sleep.
  • Pituitary Gland: Master endocrine gland.
  • Pineal Gland: Secretes the hormone melatonin.
• Brainstem: Connects the cerebrum to the spinal cord.
  • Midbrain (Superior): Sensory and motor functions, eye movements.
  • Pons: Relays signals between the cerebrum and cerebellum, controls breathing.
  • Medulla Oblongata (Inferior): Regulates vital functions like heart rate, breathing, and blood pressure.

Motor Pathway

• Primary Motor Cortex: Initiates voluntary movement.
• Pyramids of Medulla Oblongata: Where axons from the motor cortex cross to the opposite side of the body.
• Corticospinal/Pyramidial Tracts: Carry motor signals down the spinal cord.
• Anterior/Ventral Horn: Motor neurons in the spinal cord.
• Anterior/Ventral Root: Carries motor signals out of the spinal cord.
• Spinal Nerve: Mix of motor and sensory fibers.
• Peripheral Nerve: Conducts signals to muscle fibers.
• Muscle: Receives motor signals and contracts.

Sensory Pathway

• Peripheral Receptor: Detects sensory information.
• Peripheral Nerve: Conducts sensory signals to the spinal cord.
• Spinal Nerve: Mix of motor and sensory fibers.
• Posterior/Dorsal Root: Carries sensory signals into the spinal cord.
• Dorsal Root Ganglion: Contains cell bodies of sensory neurons.
• Posterior/Dorsal Horn: Sensory neurons in the spinal cord.
• Spinothalamic Tract: Carries sensory information up the spinal cord.
• Thalamus: Relays sensory information to the cerebral cortex.
• Somatosensory Cortex: Processes sensory input from the body.

Tract Crossing Points

• Corticospinal Tracts: Cross/decussate in the Pyramids of the Medulla Oblongata.
• Lateral Spinothalamic Tract (Pain and Temperature): Cross at the spinal cord level.

### Meninges

• Protective connective tissue layers that surround the CNS (brain and spinal cord).
• Three Layers:
  • Pia Mater (Deepest): Thin, delicate layer that adheres to the surface of the brain and spinal cord.
  • Arachnoid Mater: Web-like layer located between the pia mater and dura mater.
  • Dura Mater (Most Superficial): Tough, fibrous outer layer.

Cerebrospinal Fluid (CSF)

• Clear fluid that circulates within the ventricles of the brain and the subarachnoid space.
• Formation: Produced by the choroid plexus, a network of capillaries in the ventricles.
• Circulation: Flows through the ventricles and into the subarachnoid space.
• Reabsorption: Absorbed back into the bloodstream by the arachnoid villi.

### Naming Spinal Nerves

• Cervical Region: Nerves are numbered above their corresponding vertebrae (i.e., C1 nerve is above C1 vertebra).
• Thoracic and Lumbar Regions: Nerves are numbered below their corresponding vertebrae (i.e., nerve between C7/T1 is C8).
• Dermatome: An area of skin that is innervated by a single spinal nerve.

Cranial Nerves

• Twelve pairs of nerves that originate from the brainstem.
• I - Olfactory: Sense of smell.
• II - Optic: Sense of vision.
  • Vision Pathway:
    • Light Enters: Pupil.
    • Focusing: Lens.
    • Light Strikes: Retina (photoreceptor cells).
    • Action Potentials: Triggered along CN II.
    • Visual Processing: Occipital lobe.
• III - Oculomotor: Eye movements, pupil constriction, eyelid raising.
• IV - Trochlear: Moves the eye down and inward.
• V - Trigeminal: Sensory for the face, motor for chewing (mastication).
• VI - Abducens: Moves the eyeball laterally.
• VII - Facial: Motor for facial expressions, sensory for taste on the anterior 2/3 of the tongue.
• VIII - Vestibulocochlear: Hearing and balance.
• IX - Glossopharyngeal: Motor for swallowing, sensory for taste on the posterior 1/3 of the tongue.
• X - Vagus: Longest cranial nerve, controls parasympathetic functions of many organs.
  • Parasympathetic Actions: Slows heart rate, stimulates digestion, promotes peristalsis.
• XI - Accessory: Controls the sternocleidomastoid and trapezius muscles (shoulder shrug and head turning).
• XII - Hypoglossal: Controls tongue movement.

Mnemonic for Cranial Nerves

• "On Occasion, Our Trusty Truck Acts Funny, Very Good Vehicle Any How."

Autonomic Nervous System

• Controls involuntary functions.
• Two Divisions:
  • Sympathetic Nervous System: Fight-or-flight response.
  • Parasympathetic Nervous System: Rest-and-digest response.

Autonomic Nervous System Actions

The Neuron: Functional Unit of the Nervous System

• The neuron is the fundamental cell responsible for the nervous system's functions.
• It comprises three main parts:
  • Cell Body: The neuron's central hub.
  • Dendrites: These branching extensions receive incoming signals.
  • Axon: A single, long projection that transmits action potentials away from the cell body.
• Action potentials travel down the axon and are released at the axon terminal, where they are converted into chemical signals via neurotransmitters.

Depolarization and Repolarization

• Depolarization: Caused by the influx of sodium (Na+) or calcium (Ca++) ions into the neuron.
• Repolarization: Caused by the efflux of potassium (K+) ions out of the neuron.

Structures within the Nervous System

• Nerve: A bundle of axons in the peripheral nervous system (PNS). Can contain both motor and sensory axons. Examples include the sciatic nerve and radial nerve.
• Tract: A bundle of axons in the central nervous system (CNS), which includes the brain and spinal cord. Examples include the lateral spinothalamic tract.
• Ganglion: A cluster of neuron cell bodies located in the PNS. Example: The dorsal root ganglion, responsible for relaying sensory input.

Neuroglia: Supportive Cells of the Nervous System

• Neuroglia are the supportive cells of the nervous system, providing protection, nourishment, and debris removal.
• Six types of neuroglia:
  • Astrocytes: Form the blood-brain barrier, protecting the brain from harmful substances.
  • Oligodendrocytes: Form myelin sheaths around axons in the CNS, increasing the speed of nerve impulse transmission.
  • Schwann Cells: Form myelin sheaths around axons in the PNS.
  • Microglia: Act as immune cells in the nervous system, removing debris and pathogens.
  • Ependymal Cells: Line the ventricles of the brain and spinal cord, producing cerebrospinal fluid.
  • Satellite Cells: Support neuron cell bodies in the PNS.

Parts of the Brain

• Cerebrum: The largest part of the brain, responsible for higher-level functions. It has two hemispheres and four lobes:

  • Frontal Lobe: Responsible for planning, reasoning, motor control, and speech.
  • Temporal Lobe: Involved in auditory processing, memory, and language comprehension.
  • Parietal Lobe: Responsible for sensory perception, spatial awareness, and attention.
  • Occipital Lobe: Houses the visual cortex, responsible for processing visual information.

• Wernicke's Area vs. Broca's Area:

  • Broca's Area: Involved in speech production, including the formation of words and their articulation. Damage to this area can lead to expressive aphasia (difficulty speaking).
  • Wernicke's Area: Responsible for understanding written and spoken language. Damage to this area can lead to fluent aphasia (difficulty comprehending language, but fluent speech is often maintained).

• Cerebellum: Plays a crucial role in coordination, balance, and proprioception (awareness of body position). Contributes to smooth, coordinated muscular activity and muscle memory.

• Diencephalon: Located beneath the cerebrum and contains the thalamus, hypothalamus, pituitary gland, and pineal gland.

  • Thalamus: Acts as a relay station, processing and transmitting sensory information to the cortex.
  • Hypothalamus: Responsible for regulating essential bodily functions such as body temperature, hunger, thirst, and sleep-wake cycles.

• Brainstem: Connects the cerebrum to the spinal cord and regulates involuntary functions through the autonomic nervous system.

  • Its three parts:
    • Midbrain (superior): Involved in visual and auditory reflexes, and eye movements.
    • Pons: Relays signals between the cerebrum and cerebellum, and controls breathing.
    • Medulla Oblongata (inferior): Controls vital functions such as heartbeat, respiration, and blood pressure. It contains the respiratory center, cardiac center, and vasomotor center.

Motor Pathway

• Initiation: The primary motor cortex in the cerebrum sends signals down.
• Decussation: Axons cross over, or decussate, at the pyramids of the medulla oblongata.
• Projection: Signals continue down the spinal cord via corticospinal/pyramidal tracts.
• Transmission: They reach the anterior/ventral horn of the spinal cord, then travel through the anterior/ventral root, exiting the spinal cord.
• Final Destination: The signal continues through the spinal nerve, peripheral nerve, and ultimately activates the muscle.
• Neurotransmitter: Acetylcholine is the primary neurotransmitter involved in the motor pathway.

Sensory Pathway:

• Signal Origin: Sensory receptors in the periphery detect stimuli.
• Transmission: The signal travels through peripheral nerves, then spinal nerves, and enters the spinal cord via the posterior/dorsal root.
• Relay: It passes through the dorsal root ganglion and reaches the posterior/dorsal horn of the spinal cord.
• Ascent: The signal ascends the spinal cord via spinothalamic tracts.
• Relay Station: The signal reaches the thalamus, which relays sensory information to the somatosensory cortex.

Decussation Points of Neural Tracts:

• Corticospinal Tracts: Decussate at the pyramids of the medulla oblongata.
• Lateral Spinothalamic Tract (Pain and Temperature): Decussate at the spinal cord level.

Meninges: Protective Layers of the Central Nervous System

• The entire CNS is enveloped by a three-layered protective sheath:
  • Dura Mater (most superficial & thickest): The tough outer layer.
  • Arachnoid Mater: The middle layer with a spiderweb-like appearance.
  • Pia Mater (deepest): The delicate inner layer that directly adheres to the brain and spinal cord.

Cerebrospinal Fluid (CSF):

• CSF is produced by the choroid plexus, a network of capillaries within the ventricles (hollow spaces) of the brain.
• CSF circulates through the ventricles and the subarachnoid space between the meninges.
• It is reabsorbed back into the bloodstream via arachnoid villi.

Naming Spinal Nerves:

• Cervical Region: The spinal nerve is located above the corresponding vertebra (e.g., C1 nerve above C1 vertebra).
• Thoracic and Lumbar Regions: The spinal nerve is located below the corresponding vertebra (e.g., the nerve between C1/C2 is C2).

Dermatome:

• Each spinal nerve is associated with a specific area of sensation on the skin, known as a dermatome.

Cranial Nerves:

• The cranial nerves are 12 pairs of nerves that originate from the brain and control sensory and motor functions in the head, neck, and visceral organs.

• Cranial Nerve I (Olfactory): Sense of smell.

• Cranial Nerve II (Optic): Sense of vision. Light enters the eye through the pupil, is focused by the lens, and strikes the retina, stimulating photoreceptor cells that send signals along the optic nerve (CN II). The occipital lobe processes visual information.

• ** Cranial Nerve III (Oculomotor):** Controls most eye movements, raises the eyelid, and constricts the pupil.

• Cranial Nerve IV (Trochlear): Controls the movement of the eye downwards and inwards.

• Cranial Nerve V (Trigeminal): Responsible for facial sensation and motor control of chewing.

• Cranial Nerve VI (Abducens): Controls the movement of the eye outwards.

• Cranial Nerve VII (Facial): Controls facial expressions and conveys taste sensations from the anterior two-thirds of the tongue.

• Cranial Nerve VIII (Vestibulocochlear): Controls hearing and balance and receives signals from the inner ear.

• Cranial Nerve IX (Glossopharyngeal): Contributes to swallowing, and taste sensation on the posterior one-third of the tongue.

• Cranial Nerve X (Vagus): The longest cranial nerve, responsible for parasympathetic control of internal organs, slowing heart rate, promoting digestion, and regulating other functions.

• Cranial Nerve XI (Accessory): Controls the sternocleidomastoid and trapezius muscles, which are responsible for head movements and shoulder shrugging.

• Cranial Nerve XII (Hypoglossal): Controls tongue movements.

Autonomic Nervous System (ANS):

• The ANS is responsible for controlling involuntary bodily functions, such as heart rate, respiration, digestion, and blood pressure.

• Two main branches:

  • Sympathetic Nervous System (Fight-or-Flight): Prepares the body for action in stressful situations.
  • Parasympathetic Nervous System (Rest-and-Digest): Promotes relaxation and conserves energy.

• Sympathetic Effects:

  • Pupils dilate.
  • Heart rate and force increase.
  • Blood vessels constrict (elevating blood pressure).
  • Bronchiole diameter increases.
  • Digestion, bowel movements, and urination are suppressed.

• Parasympathetic Effects:

  • Pupils constrict.
  • Heart rate and force decrease.
  • Blood vessels dilate (lowering blood pressure).
  • Bronchiole diameter decreases.
  • Digestion, bowel movements, and urination are promoted.

Test your knowledge of the structure and function of neurons, including their major parts and the process of action potential conduction. This quiz covers fundamental concepts in neuroscience, including the roles of depolarization and repolarization, as well as the different structures of the nervous system.