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Chapter 9 (PART ONE)

9.1: Introduction to the Nervous System

  • Major Aspects of Nervous System:

    • Sensory Input

    • Integration and Processing (Decision-Making)

    • Motor Output (Response)

  • Functions of the Nervous System:

    • Thinking

    • Movement

    • Internal Processes of Physiology

  • Main Cell Types in the Nervous System:

    • Neurons:

    • Cells that communicate via electrical impulses with other neurons or cells.

    • Characteristics:

      • Extreme longevity

      • Amitotic (most lose mitotic activity after fetal development)

    • Neuroglia:

    • Support, nourish, protect, and insulate neurons

  • Neurotransmitters:

    • Chemical messengers in a synapse that convey an electrical impulse from a neuron to another cell.

  • Control and Regulation:

    • The nervous system controls the endocrine system, which regulates body functions and helps maintain homeostasis via hormone secretion.

9.2: Nervous System Organization

  • General Functions of the Nervous System:

    • Sensory

    • Integrative

    • Motor

  • Organs of the Nervous System:

    • Central Nervous System (CNS):

    • Consists of brain and spinal cord

    • Responsible for integration of information and decision-making

    • Peripheral Nervous System (PNS):

    • Made up of cranial and spinal nerves connecting CNS to the body

    • Contains:

      • Sensory (afferent) division

      • Motor (efferent) division

  • Motor Functions:

    • Somatic Nervous System: Controls voluntary skeletal muscles.

    • Autonomic Nervous System: Controls involuntary effectors (smooth and cardiac muscles, and glands).

General Functions of the Nervous System

  • Sensory Function:

    • Provided by sensory receptors that detect internal or external changes.

    • Information travels from receptors to sensory neurons, which transport it into the CNS.

  • Integrative Function:

    • Coordination of sensory information in the CNS.

    • Processing this information is the basis for decision-making.

  • Motor Function:

    • Nerve impulses from the CNS are conducted along motor neurons to effectors.

    • Effectors are muscles or glands responding to decisions made in the CNS.

9.3: Neurons

  • Neuron Structure:

    • Contains a cell body, tubular cytoplasm-filled dendrites, and a tubular, cytoplasm-filled axon.

    • Cell Body (Soma):

    • Contains organelles: mitochondria, lysosomes, Golgi apparatus, Nissl bodies (similar to rough ER), neurofilaments, and a large nucleus with a nucleolus.

    • Dendrites:

    • Conduct impulses toward the cell body; short and branching; provide receptive surfaces for communication with other neurons.

    • Axon:

    • Conducts impulses away from the cell body; arises from axon hillock.

    • Each neuron has only one axon.

Neuron Structure: Myelin Sheath

  • Myelin Sheath:

    • Encloses larger axons, resulting in myelinated fibers.

    • Smallest axons lack myelin sheath (unmyelinated fibers).

    • Narrow gaps in the myelin sheath are called nodes of Ranvier.

    • Myelin sheath increases conduction speed of nerve impulses.

Structural Classification of Neurons

  • Types of Neurons:

    • Multipolar Neurons:

    • Many dendrites and one axon from cell body; most neurons with cell bodies in CNS (interneurons and motor neurons) are multipolar.

    • Bipolar Neurons:

    • Two processes (a dendrite and an axon) from the cell body; found in special senses (eyes, nose, ears).

    • Unipolar Neurons:

    • One process from cell body, which splits into two parts that function as one axon; peripheral process has dendrites near peripheral body parts, central process runs into CNS; cell bodies found in ganglia outside CNS; these are sensory neurons.

Functional Classification of Neurons

  • Sensory (Afferent) Neurons:

    • Conduct impulses from peripheral receptors to the CNS; usually unipolar, with some bipolar.

  • Interneurons (Association or Internuncial Neurons):

    • Multipolar neurons in CNS linking other neurons; cell bodies may aggregate in nuclei.

  • Motor (Efferent) Neurons:

    • Multipolar neurons that conduct impulses from CNS to peripheral effectors (muscles or glands).

9.4: Neuroglia

  • Neuroglia (Glial Cells, 'Nerve Glue'):

    • Support neurons by: filling spaces, structural support, protecting and insulating neurons, aiding synapse formation, capable of mitosis.

    • Do not generate or conduct nerve impulses.

  • Types in CNS:

    • Microglia:

    • Small cells functioning as phagocytes for bacteria and debris; produce scar tissue in injury sites.

    • Oligodendrocytes:

    • Form myelin sheath around CNS axons.

    • Ependymal Cells:

    • Produce cerebrospinal fluid (CSF).

  • Types in PNS:

    • Schwann Cells:

    • Produce myelin sheath around PNS axons.

    • Satellite Cells:

    • Provide protective coating around neuron cell bodies in PNS.

Regeneration of Neurons

  • Mature Neurons:

    • Do not divide; injury to the cell body typically results in neuron death.

  • PNS Neurons:

    • Able to regenerate axons due to Schwann cell neurilemma guiding growth to original connection.

  • CNS Axons:

    • Usually do not regenerate since oligodendrocytes lack a neurilemma.

9.5: Charges Inside a Cell

  • Polarity:

    • Neuron membranes have different internal and external charges due to uneven ion distribution; inside is more negative than outside.

  • Excitability:

    • Neurons and muscle cells can respond to stimuli by adjusting internal charge (+).

  • Communication Sequence:

    • Change in internal charge enables inter-neuronal communication.

Membrane Potential and Distribution of Ions

  • Membrane Potential:

    • The charge inside a cell.

  • Resting Membrane Potential:

    • Charge in a rest state (~ -70 mV in neurons); due to unequal sodium and potassium ion distribution.

  • Ionic Concentration:

    • Higher sodium concentration outside cells; higher potassium inside cells. Large negatively charged ions and proteins are inside cells.

Stimulation and the Action Potential 1

  • Neuron at Rest:

    • Waits until stimulated. A stimulus can affect resting potential in either direction.

  • Excitatory Stimulus:

    • Opens chemically-gated sodium channels; sodium flows into cell, decreasing negativity.

  • Threshold Stimulus:

    • Strong enough to raise potential from -70 mV to -55 mV (threshold potential).

  • Action Potential Initiation:

    • Upon reaching threshold potential, voltage-gated sodium channels open, moving potential to ~ +30 mV, inducing an action potential.

  • Depolarization:

    • Change from negative to positive charge inside the neuron.

Stimulation and the Action Potential 2

  • All-or-None Response:

    • Action potential occurs or does not; if reached at -55 mV, the action potential is of uniform strength.

  • Returning to Resting Potential:

    • Following action potential, cell undergoes repolarization, returning to resting potential (-70 mV) via potassium outflow.

  • Hyperpolarization:

    • A slight overshoot occurs where potential dips below -70 mV.

  • Na/K Pump:

    • Restores sodium and potassium ions back to original positions post-action potential.

9.6: Impulse Conduction

  • Action Potential Effect:

    • Triggers an electrical current flowing down axon’s membrane to the next region, stimulating next areas.

  • Impulse Conduction:

    • The process of action potentials triggering subsequent action potentials.

  • Refractory Period:

    • Following an action potential, this period prevents additional action potentials from occurring; ensures one-way impulse transmission.

The Process of Impulse Conduction

  • Steps of Impulse Conduction:

    1. Neuron membrane maintains resting potential.

    2. Threshold stimulus is received.

    3. Sodium channels at the axon trigger zone open.

    4. Sodium ions diffuse inward, causing depolarization.

    5. Potassium channels in the axon open.

    6. Potassium ions diffuse outward, repolarizing the axon membrane.

    7. Action potential causes local current stimulating adjacent axon portions.

    8. Series of action potentials occurs along the axon.

Types of Impulse Conduction

  • Continuous Conduction:

    • Occurs in unmyelinated axons; conduct impulses sequentially along the membrane.

  • Saltatory Conduction:

    • Occurs in myelinated axons; myelin sheath insulates axons from ion movements; impulses jump from one Node of Ranvier to the next.

  • Speed of Impulse Conduction:

    • Proportional to axon diameter; thicker myelinated motor axons conduct at 120 m/s; thinner unmyelinated sensory axons conduct at 0.5 m/s.

9.7: The Synapse

  • Definition of Synapse:

    • A junction between two communicating neurons; consists of:

    • Synaptic Cleft:

      • Small gap between neurons across which impulses are conveyed.

    • Presynaptic Neuron:

      • Neuron sending the impulse.

    • Postsynaptic Neuron:

      • Neuron receiving the impulse.

  • Synaptic Transmission:

    • Neural communication across synaptic cleft via neurotransmitters.

    • Neurotransmitters stored in synaptic vesicles and released from the presynaptic neuron’s synaptic knob upon impulse arrival; they diffuse across cleft and bind to postsynaptic neuron receptors.

9.8: Synaptic Transmission

  • Excitatory and Inhibitory Actions:

    • Excitatory Neurotransmitters:

    • Increase + Na entry into postsynaptic neuron; makes action potential more likely.

    • Inhibitory Neurotransmitters:

    • Increase - Cl entry or + K flow out, making charge inside more negative, reducing action potential likelihood.

    • Postsynaptic neuron sums inputs from multiple presynaptic neurons, determining net response.

Neurotransmitters

  • Production of Neurotransmitters:

    • Over 100 types produced in synaptic knobs and stored in synaptic vesicles, including:

    • Acetylcholine

    • Monoamines (norepinephrine, dopamine, serotonin)

    • Amino acids and neuropeptides.

    • Actions depend on receptor types; some neurons may produce one or multiple neurotransmitter types.

Events Leading to Release of a Neurotransmitter

  1. Action potential travels along axon to synaptic knob.

  2. Synaptic knob membrane becomes permeable to calcium ions, allowing entry.

  3. Calcium presence causes synaptic vesicles to fuse with knob membrane.

  4. Neurotransmitter is released into synaptic cleft.

Neurotransmitter Recycling

  • Effect Cancellation:

    • Neurotransmitter effects must be eliminated post-action to prevent continuous stimulation.

  • Decomposition:

    • Enzymes in synaptic clefts swiftly break down neurotransmitters; example: acetylcholinesterase degrades acetylcholine.

  • Reuptake:

    • Some neurotransmitters are absorbed back into the presynaptic neuron for reuse.

9.9: Impulse Processing

  • Processing Mechanism:

    • Depends on neuron organization within the CNS; arranged into neuronal pools that process input from afferent neurons based on pool characteristics.

  • Net Input:

    • A neuron in a pool receives both excitatory and inhibitory input. If net excitatory input does not reach threshold potential, no impulse occurs; if it does, an impulse occurs.

Facilitation, Convergence, and Divergence

  • Facilitation:

    • Increased neurotransmitter release from presynaptic neuron following repeated stimulation enhances postsynaptic likelihood of reaching threshold.

  • Convergence:

    • Multiple fibers transmit impulses to a single neuron within a pool, allowing impulse summation from various sources.

  • Divergence:

    • One neuron transmits impulses to multiple output fibers, serving to amplify one impulse.

9.10: Types of Nerves

  • Definition of Nerve:

    • A bundle of nerve fibers (axons) in the PNS.

  • Types of Nerves:

    • Sensory (Afferent) Nerves:

    • Conduct impulses to CNS; axons termed sensory or afferent fibers.

    • Motor (Efferent) Nerves:

    • Carry impulses from CNS to effectors; axons termed motor or efferent fibers.

    • Mixed Nerves:

    • Carry both sensory and motor fibers, constituting most nerves.

  • Connective Tissue Coverings:

    • Epineurium:

    • Outer nerve covering.

    • Perineurium:

    • Covering around bundles (fascicles) of nerve fibers.

    • Endoneurium:

    • Covering around individual nerve fibers (axons).

9.11: Neural Pathways

  • Neural Pathways Definition:

    • Routes through which nerve impulses travel, exemplified by reflex arcs.

  • Components of Reflex Arc:

    • Sensory Receptor:

    • Detects changes.

    • Sensory Neuron:

    • Carries information from receptor to CNS.

    • Interneuron:

    • Processes information within CNS.

    • Motor Neuron:

    • Carries command to effectors.

    • Effector:

    • Muscle or gland responding to initial change.

Parts of a Reflex Arc

Part

Description

Function

Receptor

Receptor end of a dendrite or receptor cell in a sensory organ.

Senses specific type of internal or external change.

Sensory Neuron

Dendrite, cell body, and axon of a sensory neuron.

Carries information from receptor to brain or spinal cord.

Interneuron

Dendrite, cell body, and axon of CNS neuron.

Carries info from sensory neuron to motor neuron.

Motor Neuron

Dendrite, cell body, and axon of motor neuron.

Carries instructions from CNS to effector.

Effector

Muscle or gland.

Responds to stimulation by motor neuron; produces reflex.

Reflex Behavior 1

  • Reflexes:

    • Automatic responses to stimuli maintaining homeostasis; involved in heart rate, blood pressure, etc.

    • Example:

    • Patellar Reflex (Knee-Jerk Reflex):

      • Contains only 2 neurons (sensory and motor) and lacks an interneuron:

      • Striking the patellar ligament stretches quadriceps femoris, activating stretch receptors.

      • Sensory neurons transmit impulses to spinal cord where they synapse with motor neurons, issuing commands.

      • Motor neurons send impulses to quadriceps femoris causing knee extension; helps maintain posture.

Reflex Behavior 2

  • Withdrawal Reflex:

    • Responds to painful stimuli (e.g., stepping on a tack):

    • Sensory receptors transmit pain messages through sensory neurons to spinal cord.

    • Sensory neurons activate interneurons coordinating info.

    • Interneurons send commands to motor neurons leading to contraction of flexor muscles.

    • Simultaneously, agonistic extensor muscles are inhibited; brain is informed for awareness.

  • Function:

    • Protective function serves to limit tissue damage.

9.12: Meninges

  • Definition:

    • Three membranes surrounding the brain and spinal cord situated between skull vertebrae and CNS tissues:

    • Dura Mater:

      • Outermost layer

      • Tough dense connective tissue; rich in blood vessels

      • Forms partitions between brain lobes and dural sinuses.

      • Separated from vertebrae by epidural space.

    • Arachnoid Mater:

      • Middle layer

      • Thin and web-like; avascular

      • Contains subarachnoid space filled with cerebrospinal fluid (CSF).

    • Pia Mater:

      • Innermost layer

      • Thin, vascular layer directly on the brain and spinal cord surfaces.

9.13: Spinal Cord

  • Spinal Cord Description:

    • Begins at brain's base at foramen magnum, extending downwards to the 1st and 2nd lumbar vertebrae intervertebral disc.

  • Enlargements:

    • Cervical Enlargement:

    • Supplies nerves to upper limbs.

    • Lumbar Enlargement:

    • Provides nerves serving lower limbs.

  • Cauda Equina:

    • Tangential spinal nerve structure in lumbar/sacral areas forming a 'horse’s tail'.

Structure of the Spinal Cord

  • Segments:

    • The spinal cord has 31 segments, each connecting to a corresponding pair of spinal nerves.

  • Gray and White Matter:

    • White matter (myelinated nerve fiber bundles) surrounds a butterfly-shaped core of gray matter (interneurons and cell bodies).

    • Cell bodies of entering sensory neurons are located in posterior root ganglia outside the spinal cord.

    • Gray matter contains anterior and posterior horns plus a lateral horn.

    • Central canal contains cerebrospinal fluid.

Functions of the Spinal Cord

  • Major Functions:

    • Transmitting impulses to/from the brain and housing spinal reflexes.

  • Tracts:

    • Ascending Tracts: Carry sensory information to the brain.

    • Descending Tracts: Carry motor information from the brain to muscles or glands.

    • Naming Convention: identifies origin/termination of fibers (e.g., Spinothalamic tracts from spinal cord to thalamus, Corticospinal tracts from cortex to spinal cord, and Extrapyramidal tracts for balance and posture).

9.14: The Brain

  • Overview of the Brain:

    • The brain is the largest and most intricate part of the nervous system containing approximately 100 billion multipolar neurons supported by neuroglia.

  • Structure:

    • Differing from spinal cord, brain’s gray matter is on the outside and white matter within.

  • Main Parts of the Brain:

    • Cerebrum: Largest portion, associated with higher mental/motor functions.

    • Diencephalon: Processes sensory input; controls homeostasis.

    • Cerebellum: Coordinates muscular activity.

    • Brainstem: Coordinates visceral activities; connects various CNS parts.

Structure of the Cerebrum

  • Cerebrum Definition:

    • The largest part of the mature brain, consisting of two hemispheres connected by the corpus callosum, a flat bundle of nerve fibers.

  • Surface Features:

    • Gyri: Ridges on the surface.

    • Sulci: Grooves on the surface.

    • Fissures: Deep grooves (longitudinal and transverse).

Structure of the Cerebrum 2

  • Lobes of the Cerebrum:

    • Four lobes named per underlying skull bones:

    • Frontal

    • Parietal

    • Temporal

    • Occipital

    • Insula:

    • A fifth lobe located deep in the lateral sulcus.

    • Cerebral Cortex:

    • Thin gray matter layer comprising 75% of neuron cell bodies.

    • White Matter:

    • Lies beneath the cortex, composed of myelinated nerve fibers aiding communication.

Functions of the Cerebrum

  • Higher Brain Functions Provided by Cerebrum:

    • Interpretation of sensory inputs.

    • Initiation of voluntary movements.

    • Memory storage.

    • Integration for reasoning.

    • Intelligence.

    • Personality.

Functional Areas of the Cerebral Cortex 1

  • Cortex Division:

    • Sensory areas interpret impulses:

    • Anterior parietal lobe (cutaneous senses).

    • Posterior occipital lobe (visual area).

    • Posterior temporal lobe (auditory area).

    • Base of central sulcus/insula (taste area).

    • Deep temporal lobe (smell area).

  • Sensory Processing:

    • Sensory fibers from the PNS cross at spinal cord or brainstem resulting in right-side impulses processed in left hemisphere.

Functional Areas of the Cerebral Cortex 2

  • Association Areas:

    • Analyze and interpret sensory input for reasoning, judgment, and memory processing:

    • Frontal lobe: higher intellectual processes.

    • Parietal lobe: understand speech/word choice.

    • Occipital lobe: analyze visual patterns.

    • General interpretive area: junction of parietal, temporal, and occipital lobes; aids complex processing.

    • Not all areas are bilateral; Wernicke’s area helps with language understanding, usually on the left.