The Nervous System PT.2 (March 13, 2025) (In Class)

• The Brain

  • The sensory, motor, and association areas are in the cerebral cortex.

  • The primary somatosensory area receives sensory information from the body while the primary motor area controls the skeletal muscles.

  • Just in front of the motor cortex is the premotor cortex which coordinates learned motor skills.

• Cerebral Cortex

  • Is highly convoluted

    • An elevated fold is called a gyrus.

    • A depressed grove is called a sulcus.

  • Each hemisphere has 5 lobes: frontal, parietal, temporal, occipital, and insula.

  • Insula plays role in memory encoding.

    • Integrates sensory info with visceral responses.

    • Coordinates cardiovascular response to stress.

• Basal Nuclei (Basal ganglia)

  • Are distinct masses of cell bodies located deep inside cerebrum.

• Cerebral Lateralization

  • Refers to specialization of each hemisphere for certain functions.

  • Left hemisphere possesses language and analytical abilities

    • Olfaction

    • Speech, writing

    • Left ear

    • Main language center

    • Calculation

    • Right visual half field

  • Right hemisphere best at visuospatial tasks.

    • Olfaction

    • Right ear

    • Simple language comprehension

    • spatial concepts

    • Left visual half field

• Language

  • Language areas of brain known mostly from aphasias.

    • = speech and language disorder due to brain damage.

  • Broca’s area necessary for speech.

  • Wernicke’s area involved in language comprehension.

• Emotion and Motivation

  • Originate largely in hypothalamus and limbic system.

    • Include aggression, fear, feeding, sex, and goal-directed behaviors.

• Thalamus and epithalamus

  • Are located at base of cerebral hemispheres.

  • Thalamus is relay center thru which all sensory info (except olfactory) passes to cerebrum

    • And plays role in level of arousal.

  • Epithalamus contains choroid plexus which secretes CSF.

• Hypothalamus

  • Is most important structure for homeostasis.

  • Contains neural centers for hunger, thirst, body temperature.

  • Regulates sleep, emotions, sexual arousal, anger, fear, pain, and pleasure.

• Pituitary Gland

  • Is divided into anterior and posterior lobes.

  • Posterior pituitary stores and releases ADH (vasopressin) and oxytocin.

    • Both made in hypothalamus and transported to pituitary.

  • Hypothalamus produces releasing and inhibiting hormones that control anterior pituitary hormones.

• Circadian Rhythms

  • Are body’s daily rhythms

  • Regulated by SCN (suprachiasmatic nucleus) of hypothalamus

    • The master clock

    • Adjusted daily by light from eyes.

    • Controls pineal gland secretion of melatonin which regulates circadian rhythms.

• Midbrain

  • Contains:

    • Superior colliculi — involved in visual reflexes

    • Inferior colliculi —- relays for auditory information.

  • Red nucleus and substantia nigra — involved in motor coordination.

    • Nigra dopamine neurons degenerate in Parkinson’s

  • Mesolimbic dopamine neurons are involved in reward and addiction.

• Hindbrain - Pons

  • Contains several nuclei of cranial nerves.

  • And two important respiratory control centers.

    • Apneustic and pneumotaxic centers.

• Reticular Formation

  • Is complex network of nuclei and fibers spanning medulla, pons, midbrain, thalamus, and hypothalamus.

  • Functions as reticular activating system.

    • Set level of arousal of cerebral cortex to incoming sensory information.

• The Brain

  • The thalamus

    • Serves as the relay station of the brain for all sensory information except smell.

    • Also directs motor activity, cortical arousal, and memory.

  • Thalamus

    • Processes all sensory information (except olfaction)

    • Relays information to appropriate higher brain centers.

  • Hypothalamus

    • Controls heart rate, blood pressure, breathing rate, body temperature, food intake.

  • The hypothalamus

    • Maintains homeostasis by regulating blood pressure, heart rate, breathing rate, digestion and body temperature.

  • The hypothalamus coordinates the nervous and endocrine systems through its connection to the pituitary gland.

    • It is a center for emotions and serves as the master biological clock.

  • The cerebellum

    • Integrates information from the motor cortex and sensory pathways to produce smooth, well-timed voluntary movements.

    • Controls equilibrium and posture.

    • Stores memories of learned motor skills.

  • The medulla oblongata

    • Contains reflex centers to regulate the rhythm of breathing, forced and rate of the heartbeat, and blood pressure.

    • Serves as the pathway for all sensory messages to the higher brain centers and motor messages leaving the brain.

• The Midbrain

  • Processes information about sights and sounds and controls simple reflex responses to these stimuli.

• The Pons

  • Means “bridge”

  • Connects the spinal cord and cerebellum with the cerebrum, thalamus, and hypothalamus.

  • Has a region that assists the medulla in regulating respiration.

• The Brain

  • Midbrain

    • Relays information between the cerebellum or spinal cord and the cerebrum.

    • Integrates sensory input.

  • Pons

    • A bridge between higher and lower brain centers.

  • Medulla oblongata

    • Contains autonomic centers for heart rate and digestiven activities.

    • Relays sensory information to thalamus.

  • Cerebellum

    • Coordinates sensory-motor voluntary movement.

    • Stores memory of learned motor patterns.

  • The limbic system, which includes several brain structures is largely responsible for emotion.

  • It is defined on the basis of function rather than anatomy.

  • It includes parts of several brain regions and the neural pathways that connect them.

• Memory

  • The limbic system plays a role in forming memory. The storage and retrieval of information takes place in two stages:

    • Short-term memory, which holds a small amount of information for a few seconds or minutes.

    • Long-term memory, which stores limitless amounts of information for hours, days or years.

• The Brain

  • The reticular activating system (RAS)

    • An extensive network of neurons that runs through the medulla and projects to the cerebral cortex.

    • Filters sensory input and keeps the cerebral cortex in an alert state.

• Spinal Cord Tracts

  • Sensory information from body travels to brain in ascending spinal tracts.

  • Motor activity from brain travels to body in descending tracts.

• Ascending Spinal Tracts

  • Ascending sensory tracts decussate (cross) so that brain hemispheres receive info from opposite side of body.

  • Same for most motor tracts from brain.

• Descending Spinal Tracts

  • Are divided into 2 major groups:

  • Pyramidal or corticospinal tracts descend directly without synaptic interruption from cerebral cortex to spinal cord.

    • Function in control of fine movements.

  • Reticulospinal or extrapyramidal tracts descends with many synapses.

• Peripheral Nervous System (PNS)

  • Consists of nerves that exit from CNS and spinal cord, and their ganglia (= collection of cell bodies outside CNS).

• Cranial Nerves

  • Consist of 12 pairs of nerves.

    • 2 pairs arise from neuron in forebrain.

    • 10 paris arise from midbrain and hindbrain neurons.

    • Most are mixed nerves containing both sensory and motor fibers.

• Spinal Nerves

  • Are mixed nerves that separate next to spinal cord into dorsal and ventral roots.

    • Dorsal root composed of sensory fibers.

    • Ventral root composed of motor fibers.

  • There are 31 pairs:

    • Cervical: 8 pairs (C1-C8)

    • Thoracic: 12 pairs (T1-T12)

    • Lumbar: 5 pairs (L1-L5)

    • Sacral: 5 pairs (S1-S5)

    • Coccygeal: 1 pair (Co1)

• Reflex Arc

  • Is a simple sensory input, motor output circuit involving only peripheral nerves and spinal cord.

  • Sometimes arc has an association neuron between sensory and motor neuron.

• The Spinal Cord

  • (a) Spinal nerves conduct sensory and motor information between the central nervous system and a specific region of the body.

  • (b) Pairs of spinal nerves leave through openings between the vertebrae.

  • (c) A micrograph of a cross section of the spinal cord. The white matter transmits messages to and from the brain. The gray matter (shaped like a butterfly) functions as a reflex center.

  • A reflex action is an automatic response to a stimulus in a pre-wired circuit called a reflex arc.

  • Step 1: A stimulus initiates a pain sensation.

  • Step 2:Sensory messages are carried to the spinal cord by a sensory neuron.

  • Step 3: Interneurons in the spinal cord integrate information from sensory neurons and stimulate the appropriate motor neurons.

• The Peripheral Nervous System

  • The peripheral nervous system consists of spinal nerves and cranial nerves.

  • The body has 31 pairs of spinal nerves, each of which originates in the spinal cord and services a specific region of the body.

• Nervous System (NS)

  • Consists of 2 kind of cells:

    • Neurons and supporting cells (= glial cells)

• Neurons

  • Gather and transmit information by:

    • Responding to stimuli

    • Sending electrochemical impulses.

    • Releasing chemical messages.

  • Have a cell body, dendrites, and axon

    • Cell body contains nucleus.

• Functional Classification of Neurons

  • Sensory/Afferent neurons conduct impulses into CNS.

  • Motor/Efferent neurons carry impulses out of CNS

  • Association/Interneurons integrate NS activity

    • Located entirely inside CNS.

• Structural CLassification of Neurons

  • Pseudounipolar:

    • Cell body sits along side of single process

    • e.g. sensory neurons

  • Bipolar:

    • Dendrite and axon arise from opposite ends of cell body.

    • e.g. retinal neurons

  • Multipolar

    • Have many dendrites and one axon.

    • e.g. motor neurons.

• Supporting/Glial Cells

  • PNS has Schwann and satellite cells

    • Schwann cells myelinate PNS axons.

  • CNS has oligodendrocytes, microglia, astrocytes, and ependymal cells.

  • Each oligodendrocyte myelinates several CNS axons.

• Myelination

  • In PNS each Schwann cell myelinates 1mm of 1 axon by wrapping round and round axon.

  • Uninsulated gap between adjacent Schwann cells is called node of Ranvier.

• Nerve Regeneration

  • Occur much more readily in PNS than CNS.

    • Oligodendrocytes produce proteins that inhibit regrowth.

  • Neurotrophins

  • Promtoe fetal nerve frowth.

  • Required for survival of many adult neurons.

  • Important in regeneration.

  • Occurs much more readily in PNS than CNS.

  • When axon in PNS is severed:

    • Distal part of axon degenerates

    • Schwann cells survive; form regeneration tube.

      • Tube releases chemicals that attract growing axons.

      • Tube guides regrowing axon to synaptic site.

• Astrocytes

  • Most common glial cell.

  • Involved in:

    • Inducing capillaries to form blood-brain barrier.

    • Buffering K⁺ levels

    • Recycling neurotransmitters.

    • Regulating adult neurogenesis.

    • Maintain interstitial fluid

• Blood-Brain Barrier

  • Allows only certain compounds to enter brain.

  • Formed by capillary specializations in brain.

Membrane Potential

• Resting Membrane Potential (RMP)

  • At rest, all cells have a negative internal charge and unequal distribution of ions:

    • Results from:

      • Large anions being trapped inside cell.

      • Na⁺/K⁺ pump and limited permeability keep Na⁺ high outside cell.

      • K⁺ is very permeable and is high inside cell.

        • Attracted by negative charges inside.

• Excitability

  • Excitable cells can discharge their RMP quickly.

    • By rapid changes in permeability to ions.

    • Neurons and muscles do this to generate and conduct impulses.

• Membrane Potential (MP) Changes

  • Measured by placing 1 electrode inside cell and 1 outside.

  • Depolarization occurs when MP becomes more positive.

  • Hyperpolarization: MP becomes more negative than RMP.

  • Repolarization: MP returns to RMP

• Membrane Ion Channels

  • MP changes occur by ion flow through membrane channels.

    • Some channels are normally open; some closed.

    • Closed channels have molecular gates that can be opened.

      • Voltage-gated (VG) channels are opened by depolarization.

      • 1 type of K⁺ channel is always open; other type of VG and is closed in resting cell.

      • Na⁺ channels are VG; closed in resting cells.

• The Action Potential (AP)

  • Is a wave of MP change that sweeps along the axon from soma to synapse.

  • Wave is formed by rapid depolarization of the membrane by Na⁺ influx; followed by rapid repolarization by K⁺ efflux.

  • Depolarization causes more channels to open (positive feedback loop)

• Mechanism of Action Potential

  • Depolarization and repolarization occur via diffusion.

    • Do not require active transport.

    • After an Ap, Na⁺/K⁺ pump extrudes Na⁺, recovers K⁺.

• Refractory Periods

  • Absolute refractory period:

    • Membrane cannot produce another AP because Na⁺ channels are inactivated.

  • Relative refractory period occurs when VG K⁺ channels are open, making it harder to depolarize to threshold.

• Conduction in an Unmyelinated Axon

  • After aon hillock reaches threshold and fires AP, its Na⁺ influx depolarizes adjacent regions to hreshold.

    • Generating a new AP

      • Process repeats all along axon.

      • So APA amplitude is always same.

    • Conduction is slow.

• Conduction in Myelinated Axon

  • Ions can’t flow across myelinated membrane

    • Thus no APs occur under myelin and no current leaks.

      • Increases current spread.

  • Gap sin myelin are called Nodes of Ranvier

    • APs occur only at nodes.

    • Current from AP at 1 node can depolarize next node to threshold.

      • Fast because APs skip from node to node.

      • Called Saltatory conduction.

• Electrical Synapse

  • Depolarization flows from presynaptic into postsynaptic cell through channels called gap junctions.

    • Formed by connexin proteins.

    • Found in smooth and cardiac muscles, brain and glial cells.

• Chemical Synapse

  • Synaptic cleft separates terminal bouton of presynaptic from postsynaptic cell.

  • NTs are in synaptic vesicles.

  • Vesicles fuse with bouton membrane; release NT by exocytosis.

  • Amount of NT released depends upon frequency of APs.

• Synaptic Transmission

  • APs travel down axon to depolarize bouton.

    • Open VG Ca²⁺ channels in bouton.

    • Ca²⁺ driven in by electrochemical gradient.