The Nervous System (March 12, 2025) (Presentation)

The Nervous System (NS)

• Every conscious action that occurs in your body is governed by the nervous system (NS)

  • Functions include:

    • Communication from one end of the body to another.

    • Collection and integration of internal and external stimuli.

    • Formulation of appropriate response.

• Most subconscious (or automatic) actions also are governed by the nervous system.

• The immediacy of the nervous system response sets it apart from other control systems of the body.

  • It is responsible for maintaining homeostasis.

  • The nervous system works in concert with the endocrine system.

The Nervous System Has Two Components

• Central Nervous System (CNS)

  • Composed of the brain and spinal cord.

  • Encased by the axial skeleton and covered by meninges.

  • Main integration system of the body.

• Peripheral Nervous System (PNS)

  • Composed of all neural tissue other than the brain and spinal cord.

  • Includes:

    • All the afferent and efferent neurons that extend from the CNS.

    • The autonomic, sensory, and somatic nerves of the body.

  

Central Nervous System (CNS) vs. Peripheral Nervous System (PNS)

• Sensory information enters the CNS, which analyzes it and then sends a motor response through the PNS to muscular or glandular tissue.

• Information reaches the CNS from the afferent division of the PNS.

  • The PNS picks up this information with one of three types of receptors.

    • Special senses (sight, hearing, taste, smell)

    • General sensory receptors (external temp., light, touch, pain)

    • Visceral receptors (proprioception, organ functions)

• Motor responses are sent through the efferent division of the PNS.

Two Divisions of the PNS

• The somatic division is involved in conscious movement.

  • It regulates voluntary movement of skeletal muscles.

• The autonomic division governs the body’s responses to changes in homeostasis with involuntary, subconscious reactions.

  • It regulates functions such as blood vessel diameter and stomach activity.

  • The autonomic nervous system has two subdivisions

    • Sympathetic NS

    • Parasympathetic NS

Autonomic NS

• Sympathetic NS

  • Includes nerves that control the body when it is actively moving and burning energy.

    • Fight or flight responses.

  • Nerves in the thoracic and lumbar regions of the spinal cord.

• Parasympathetic NS

  • Includes nerves responsible for digestion, energy storage and relaxation.

    • Rest and digest responses.

  • Nerves in the cranial and sacral regions of the spinal cord.

• Most body organs are innervated by both the Sympathetic NS and Parasympathetic NS.

  • To maintain homeostasis.

• SYMPATHETIC NS

  

• PARASYMPATHETIC NS

  

Sympathetic vs. Parasympathetic

Neurons

• The NS contains neurons and neuroglial cells.

• The neuron is the functional unit of the NS.

  • Functions include

    • Carrying sensory information to the brain.

    • Formulating responses.

    • Sending responses to organs.

  • Neurons arrange into bundles called nerves.

• Neuroglia are the supporting cells of nervous tissue.

  

Neuroglia

• PNS

  • Satellite cells

    • Regulate oxygen, carbon dioxide, nutrient, and neurotransmitter levels around ganglia.

  • Schwann cells

    • Surround axons in PNS, causing myelination of axons and faster impulse transmission, aid in repair after injury.

• CNS

  • Oligodendrocytes

    • Surround axons in CNS, causing myelination and providing structural support.

  • Astrocytes

    • Maintain blood-brain barrier; regulate nutrient, ion, and dissolved gas concentrations; absorb and recycle neurotransmitters; form scar tissue after injury.

  • Microglia

    • Clean up cellular debris and pathogens via phagocytosis.

  • Ependymal cells

    • Line ventricles and central canal of spinal cord, assist in cerebrospinal fluid production.

Neurons

• The three classes of neurons are based on function.

  • Sensory neurons detect conditions in the environment or body.

  • Motor neurons carry instruction to the body

  • Interneurons connect sensory and motor neurons.

  

The Reflex Arc

• Reflexes are rapid responses to sensory stimuli.

  • The stimulus from a sensory neuron bypasses the brain and instead runs through an interneuron within the spinal cord.

  • The motor neuron then transmits an immediate response to the effector organ.

1. INTEGRATING CENTER: (one or more regions within the CNS that relay impulses from sensory to motor neurons)

2. SENSORY NEURON: (axon conducts impulses from receptor to integrating center)

3. SENSORY RECEPTOR: (responds to a stimulus by producing a generator or receptor potential)

4. EFFECTOR: (muscle or gland the responds to motor nerve impulses)

5. MOTOR NEURON: (axon conducts impulses from integrating center to effector)

6. iNTEGRATING CENTER: (one or more regions within the CNS that relay impulses from sensory to motor neurons)

Membrane Potential

• The membrane potential results from the difference in ion concentrations between the two sides of a cell membrane.

  • Measured as electrical differences.

• The membrane potential of a neuron during a typical nerve impulse alternates between -70 mV and +30 mV

  • At rest, the inner side of a neuron’s membrane is negatively charged relative to it’s outer side and its membrane potential is -70 mV.

  • Variations in ion concentration across the cell membrane allow neurons to generate action potentials.

  

Gates and Channels Control the Flow of Ions

• Ions cannot simply diffuse through the lipid bilayer of the cell membrane, they must ravel through channel proteins.

• Voltage-gated channels - Respond to trans-membrane voltage changes.

• Ligand-gated channels - Chemically regulated.

• Mechanically-regulated channels - Respond to distortions of the membrane surface.

Neurons Work Through Action Potentials

• An action potential is a brief change in electrical conditions at a neuron’s membrane.

  • When a neuron “fires”, the charge differential across the neuron’s membrane suddenly reverses polarity.

  • Action potentials are “all or nothing” events.

• IPSPs and EPSPs also influence the generation of action potentials.

1. Resting state: All voltage-gated Na⁺ and K⁺ channels are closed. Inactivation gate open.

2. Depolarizing phase: Depolarization to threshold opens Na⁺ channel activation gates. Na⁺ inflow further depolarizes the membrane, opening more Na⁺ channel activation gates.

3. Repolarizing phase: At +30 mV Na⁺ channel inactivation gates close and voltage-gated K⁺ channels open. Outflow of K⁺ causes repolarization.

4. Repolarization continues: K⁺ outflow restores resting membrane potential. Na⁺ channel inactivation gates open and return to resting state when K⁺ gate close.

Graded Responses Created Fine Neural Control

• Graded responses can be obtained by hyperpolarizing or depolarizing individual neural membranes.

• Excitatory postsynaptic potentials (EPSPs)

  • Cause slight depolarization of the neuron.

• Inhibitory postsynaptic potentials (IPSPs)

  • Hyperpolarize the neuron.

  • The membrane potential is further from the needed to generate an action potential, so a larger stimulus is required to begin an action potential.

Neurotransmitters

• Synapses separate one neuron from another.

• Neurotransmitters are chemicals used to transmit the nerve impulse from one cell to the next.

  • Diffuse across the synaptic cleft.

1. Action potential arrives.

2. Na⁺ channels open and depolarization causes Ca²⁺ channels to open.

3. Calcium causes synaptic vesicles to fuse with neuron membrane, dumping neurotransmitter acetylcholine into synapse.

4. Acetylcholine binds to ligand gated channel; receptor opens.

5. Na⁺ enters postsynaptic neuron and depolarizes cell causing action potential.

The Brain and Spinal Cord are Central to the NS

• The volume of the human brain is about 1250 cm³

• The human brain weights about 1,400 grams.

• The cerebrospinal fluid and meninges protect and nourish the CNS.

  • The meninges are a series of three consecutive tissue coverings between the nervous tissue and the bone.

  • The CSF functions to maintain uniform pressure within the brain and spinal cord.

Layers of the Meninges

• Dura mater

  • Tough connective tissue layer immediately beneath the skull.

• Arachnoid mater

  • A thin, fragile layer which resembles a spider web.

  • Cerebrospinal fluid flows between the strands of the arachnoid mater.

• Pia mater

  • The innermost layer, extremely thin and attached to neurons.

• Meningitis is an inflammation of these three layers.

  

Cerebrospinal Fluid (CSF)

• Provides a constant environment for the CNS.

• Its is continuously produced and absorbed, creating a constant flow.

  • Ventricles make CSF.

• CSF helps maintain the blood-brain barrier.

  • Permits passage of only certain ions and nutrients.

  

The Brain

• The brain has 4 main parts.

  • The brain stem.

  • The cerebellum.

  • The diencephalon.

  • The cerebrum.

  

The Brain Stem

• The oldest part of brain.

• Made up of the

  • Mid brain, medulla oblongata, and pons.

• Regulates

  • Heart rate, breathing, and blood pressure.

• Has reflex centers for

  • sneezing, coughing, hiccupping and swallowing.

  

The Cerebellum

• Focuses on muscles and movement.

  • Maintains muscle tone, posture and balance.

• Fine tunes conscious and unconscious movements directed by the cerebrum.

• Important in learning motor skills.

  • Riding a bike.

  • Taking notes in class.

• Important in proprioception.

  • Knowing where the body is in space.

  

The Diencephalon

• A relay center for sensory information and motor responses.

• Center for visual and auditory startle reflexes are located here.

• The thalamus and hypothalamus are also located here.

• The hypothalamus

  • Secretes hormones that control the pituitary gland.

  • Also regulates circadian rhythm, body temperature, heart rate, blood pressure and controls smooth muscle contraction.

• The thalamus is a relay station.

  • For incoming sensory information.

• The limbic system

  • Communicates with the anterior portion of thalamus.

  • Is also responsible for emotions.

  

The Cerebrum

• Largest portion of the brain.

• Central processing center where learning, remembering, and activity planning takes place.

  • Information is processed and integrated and appropriate responses are generated.

• Organized into four lobes.

  • Frontal, parietal, occipital, temporal.

• The surface of the cerebrum.

  • Covered in creases (sulci) and raised areas (gyri)

• The outer layer of cerebrum, called the cerebral cortex. is composed of gray matter.

  • Folded to provide larger surface area.

  • Contains neuronal cell bodies, dendrites, and non-myelinated axons.

  • Responsible for sensations, voluntary movements, and thoughts.

• The inner layer of the cerebrum consists of white matter that is composed of myelinated axons.

  • Myelinated axons are covered in lipids which allows for faster impulse transmission.

  • Information passed between areas of the brain via tracts of white matter.

• The left and right cerebral hemispheres are distinct.

  • The right analyzes sensory input, recognizes faces, and functions in spatial relationships.

  • The left includes general language interpretation and speech, controls writing and speaking, categorizes items and makes logical decisions.

• The cortex has motor areas, sensory areas, and association areas that integrate new information with stored information.

  • The primary motor area formulates voluntary motor commands.

• Association areas of the cerebral cortex integrate and coordinate information.

• The right and left sides of the brain connects through the transverse tracts of the corpous collosum.

• Special senses are integrated in specific areas of the cerebral cortex.

  • Visual interpretation occurs in the occipital lobe.

  • Auditory interpretation occurs in the temporal lobe.

  • Taste interpretation occurs in the parietal lobes.

The Cerebral Lobes and their Functions

• Posterior

  • Parietal Lobe:

    • Primary somatosensory area (postcentral gyrus)

    • Somatosensory association area

    • Common integrative area

    • Primary gustatory area.

  • Occipital Lobe:

    • Visual association area.

    • Primary visual area.

  • Temporal Lobe:

    • Auditory association area.

    • Primary auditory area.

• Anterior

  • Frontal Lobe:

    • Primary motor area (precentral gyrus)

    • Premotor area.

    • Frontal eye field area.

    • Broca’s speech area.

    • prefrontal cortex

    • Lateral cerebral sulcus.