CHAPTER 9 & 10
9.1: Nervous system
Major aspects of the nervous system:
Sensory input: Gathering information from the environment
Integration and processing: Making decisions based on information
Motor output: Responding with actions
Functions of the nervous system:
Thinking
Movement
Managing internal body processes
Main cell types:
Neurons: Send messages using electrical impulses
Neuroglia: Support, protect, and nourish neurons
Neurotransmitters: Chemicals that carry signals between neurons or to other cells.
Nervous system and the endocrine system:
Controls hormone release to regulate body functions and keep balance (homeostasis).
9.2: Nervous system organization
General functions of the nervous system:
Sensory: Detects changes inside and outside the body
Integrative: Processes and interprets information
Motor: Responds with actions
Organs of the nervous system:
Central nervous system (CNS):
Includes the brain and spinal cord
Handles information processing and decision-making
Peripheral nervous system (PNS):
Consists of cranial and spinal nerves connecting CNS to the body
Divided into:
Sensory (afferent): Carries info to the CNS
Motor (efferent): Sends commands from CNS
Motor functions:
Somatic nervous system: Controls voluntary movements (skeletal muscles)
Autonomic nervous system: Controls involuntary functions (smooth muscles, cardiac muscles, glands)
General Functions of the Nervous System:
Sensory Function:
Sensory receptors detect changes inside or outside the body
Sensory neurons carry this information to the CNS
Integrative Function:
CNS coordinates and processes sensory information for decision-making
Motor Function:
CNS sends nerve impulses through motor neurons to effectors (muscles or glands) to respond
9.3: Neurons
Neuron (Nerve Cell) Structure:
Cell body (soma): Contains mitochondria, lysosomes, Golgi apparatus, Nissl bodies (similar to rough ER), neurofilaments, and a large nucleus with a nucleolus.
Dendrites: Short, branching extensions that conduct impulses toward the cell body; act as the receptive surface for communication.
Axon: Single extension that conducts impulses away from the cell body; starts from the axon hillock.
Myelin Sheath:
Covers larger axons, known as myelinated fibers.
Has gaps called nodes of Ranvier.
Increases the speed of nerve impulse conduction.
Structural Classification of Neurons:
Multipolar Neurons: Many dendrites, one axon; mostly in the CNS (interneurons and motor neurons).
Bipolar Neurons: One dendrite and one axon; found in special senses (eyes, nose, ears).
Unipolar Neurons: One process that splits into two; peripheral part has dendrites, central part leads to CNS; cell bodies in ganglia outside the CNS; these are sensory neurons.
Functional Classification of Neurons:
Sensory (Afferent) Neurons: Carry impulses from peripheral receptors to the CNS; usually unipolar, some are bipolar.
Interneurons (Association Neurons): Multipolar, located in the CNS, linking other neurons; cell bodies may cluster in nuclei.
Motor (Efferent) Neurons: Multipolar, send impulses from the CNS to muscles or glands (effectors).
9.4: Neuroglia
Neuroglia (Glial Cells):
Functions: Support, protect, insulate neurons, and fill spaces
Do not conduct nerve impulses
Types:
CNS Neuroglia:
Microglia: Act as phagocytes, remove debris, produce scar tissue
Oligodendrocytes: Form myelin sheath around CNS axons
Ependymal Cells: Produce cerebrospinal fluid
Types of Neuroglia:
CNS Neuroglia:
Astrocytes:
Provide structural support
Regulate nutrients and ion levels
Form the blood-brain barrier to protect brain tissue
PNS Neuroglia:
Schwann Cells: Create the myelin sheath around PNS axons
Satellite Cells: Offer protective covering for neuron cell bodies in the PNS
Regeneration of Neurons:
PNS Neurons: Can regenerate axons; Schwann cells' neurilemma guides growth to original connection.
CNS Neurons: Usually do not regenerate; oligodendrocytes lack neurilemma.
9.5: Charges inside a cell
Neuron Cell Membrane Polarity:
The inside of the neuron membrane is more negative than the outside due to unequal ion distribution.
Neurons and muscle cells are excitable, meaning they can respond to stimuli by changing their internal charge to a positive state.
This change in charge triggers events that allow neurons to communicate.
Membrane Potential: The charge inside a cell.
Resting Membrane Potential: The charge inside a cell at rest, typically about -70 mV in neurons.
Ion Distribution:
More sodium ions outside the cell than inside.
More potassium ions inside the cell than outside.
Large negatively charged ions and proteins are found inside the cell
Stimulation and Action Potential:
A neuron stays at rest until stimulated.
A stimulus can change the resting potential in either direction.
An excitatory stimulus opens chemically-gated Na⁺ channels, allowing Na⁺ ions to flow in, making the inside of the neuron less negative.
Threshold Stimulus: A stimulus strong enough to raise the potential from -70 mV to -55 mV (threshold potential).
Once threshold is reached, voltage-gated Na⁺ channels open, causing the potential to rise to about +30 mV, initiating an action potential.
The change from negative to positive inside the neuron is called depolarization, as both inside and outside become positive.
All-or-None Response:
An action potential either occurs or does not.
It happens when the charge reaches -55 mV.
All action potentials of a neuron are of the same strength.
Repolarization:
After reaching an action potential, the cell returns to resting potential (-70 mV).
Repolarization occurs through the outward flow of potassium ions through potassium channels.
Hyperpolarization:
After repolarization, the potential may dip below -70 mV.
Na+/K+ Pump:
Restores the balance by moving Na⁺ ions out of the cell and K⁺ ions back in.
9.6: Impulse conduction
Impulse Conduction:
An action potential at the trigger zone causes an electrical current to flow to adjacent regions of the axon membrane.
This local current spreads down the axon, stimulating the next region, and continues to the axon terminal.
This process is called impulse conduction.
Refractory Period:
The period during and after an action potential when another threshold stimulus will not trigger a new action potential.
It limits the frequency of action potentials.
It ensures that the impulse is only transmitted in one direction, down the axon
Types of Impulse Conduction:
Continuous Conduction:
Occurs in unmyelinated axons.
Impulses are conducted sequentially along the entire length of the membrane.
Saltatory Conduction:
Occurs in myelinated axons.
The myelin sheath insulates the axon, preventing ion movement across the membrane.
Impulses "jump" from one Node of Ranvier to the next, where sodium and potassium channels are located.
Speed of Impulse Conduction:
Thick, myelinated motor axons conduct at 120 m/s.
Thin, unmyelinated sensory axons conduct at 0.5 m/s.
9.7: The synapse
Synapse: A junction between two communicating neurons.
Synaptic Cleft: The small gap between neurons where the impulse must be transmitted.
Presynaptic Neuron: The neuron sending the impulse.
Postsynaptic Neuron: The neuron receiving the impulse.
Synaptic Transmission: The process of neural communication across the synaptic cleft.
Neurotransmitters:
Chemical messengers stored in synaptic vesicles within the presynaptic neuron.
Released from the synaptic knob (expansion at the end of the presynaptic neuron) in response to a nerve impulse.
They diffuse across the synaptic cleft and bind to receptors on the postsynaptic neuron membrane
9.8: synaptic transmission
Excitatory Neurotransmitters:
Increase the entry of Na⁺ ions into the postsynaptic neuron.
Bring the membrane closer to threshold, making an action potential more likely.
Inhibitory Neurotransmitters:
Increase the flow of Cl⁻ ions into the neuron or K⁺ ions out of the neuron.
Make the inside of the neuron more negative, making an action potential less likely.
Summation:
The postsynaptic neuron receives inputs from many presynaptic neurons.
It sums the excitatory and inhibitory inputs to determine its response.
Neurotransmitters:
Over 100 neurotransmitters are produced in synaptic knobs and stored in synaptic vesicles.
Types of neurotransmitters include:
Acetylcholine
Monoamines
Amino acids
Neuropeptides
The action of a neurotransmitter depends on the type of receptors in the specific synapse.
Some neurons produce only one type of neurotransmitter, while others can produce two or three types
Neurotransmitter Recycling:
After acting on the postsynaptic cell, neurotransmitter effects must be stopped to prevent continuous stimulation.
Destruction or removal of neurotransmitters ensures they don't keep stimulating the postsynaptic neuron.
Enzymes in the synaptic cleft and on the postsynaptic membrane rapidly decompose neurotransmitters; for example, acetylcholinesterase breaks down acetylcholine.
Some neurotransmitters are taken back into the presynaptic neuron for reuse, a process called reuptake.
9.9: Impulse processing
Neuronal Pools:
Neurons in the CNS are organized into neuronal pools, which vary in size and work together.
Each pool receives input from afferent neurons and processes the information based on the pool’s characteristics.
Neurons in a pool can receive both excitatory and inhibitory input.
If the net input is excitatory but doesn't reach the threshold potential, no nerve impulse occurs.
If the net excitatory input reaches the threshold potential, a nerve impulse is generated.
Facilitation:
An increase in neurotransmitter release in response to repeated stimulation of an excitatory presynaptic neuron.
This response increases the likelihood that the postsynaptic neuron will reach threshold and generate an impulse.
Convergence:
Multiple fibers transmit nerve impulses to a single neuron within a pool.
This allows the neuron to summate impulses from different sources.
Divergence:
A single neuron transmits nerve impulses to several output fibers.
This amplifies the impulse, spreading it to multiple neurons.
9.10: Types of nerves
Nerve: A bundle of nerve fibers (axons) in the PNS.
Types of Nerves:
Sensory (Afferent) Nerves: Carry impulses to the CNS; their axons are called sensory fibers.
Motor (Efferent) Nerves: Carry impulses from the CNS to effectors (muscles or glands); their axons are called motor fibers.
Mixed Nerves: Carry both sensory and motor fibers; most nerves are of this type.
Connective Tissue Coverings:
Epineurium: The outer covering of a nerve.
Perineurium: The covering around fascicles (bundles of nerve fibers).
Endoneurium: The covering around individual nerve fibers (axons).
9.11: Neural pathways
Neural Pathways: The routes along which nerve impulses travel; the simplest pathway is a reflex arc.
Reflex Arc: Provides the basis for involuntary actions called reflexes.
Components of a Reflex Arc:
Sensory Receptor: Detects changes or stimuli.
Sensory Neuron: Carries information from the receptor toward the CNS.
Interneuron: Located in the CNS (reflex center), processes the information.
Motor Neuron: Carries the response (command) to the effectors.
Effector: A muscle or gland that responds to the stimulus and carries out the reflex action.
Reflexes: Automatic responses to stimuli (changes inside or outside the body) that help maintain homeostasis.
Reflexes control functions like heart rate, blood pressure, and automatic actions such as vomiting, sneezing, and swallowing.
Patellar (Knee-Jerk) Reflex: A simple reflex that involves only 2 neurons—a sensory and a motor neuron, with no interneuron.
Striking the patellar ligament stretches the quadriceps femoris muscle and tendon, activating stretch receptors.
Sensory neurons send impulses to the spinal cord, where they synapse with motor neurons.
The motor neurons send impulses to the quadriceps, causing the muscle to contract and the knee to extend.
This reflex helps maintain upright posture
Withdrawal Reflex: A protective reflex that occurs in response to painful stimuli, such as stepping on a tack or touching a hot stove.
Process:
Sensory Receptors detect pain and send messages through sensory neurons to the spinal cord.
Sensory neurons transmit impulses to interneurons in the spinal cord, where the information is processed.
Interneurons send motor commands to motor neurons.
Motor neurons stimulate flexor muscles to contract, pulling the body away from the painful stimulus.
Simultaneously, the extensor muscles are inhibited to prevent opposing movement.
A message is also sent to the brain for awareness of the pain.
Function: This reflex helps prevent tissue damage by quickly responding to painful stimuli.
9.12: Meninges
Meninges: Three membranes that surround the brain and spinal cord, lying between the skull/vertebrae and the soft CNS tissues.
Dura Mater: Outermost layer.
Arachnoid Mater: Middle layer.
Pia Mater: Innermost layer.
Dura Mater:
Outermost layer of the meninges.
Made of tough, dense connective tissue, making it thick and protective.
Contains many blood vessels.
Forms the internal periosteum of the skull bones.
In some areas, it forms partitions between brain lobes and dural sinuses (which drain blood).
Around the spinal cord, the dura mater is separated from the vertebrae by the epidural space.
Arachnoid Mater:
The middle layer of the meninges.
A thin, web-like layer without blood vessels.
The subarachnoid space between the arachnoid and pia mater contains cerebrospinal fluid (CSF).
Pia Mater:
The innermost layer of the meninges.
A thin layer rich in blood vessels and nerves.
It is attached to the surface of the brain and spinal cord, following their contours.
9.13: Spinal cord
Spinal Cord:
Starts at the foramen magnum (base of the brain).
Extends down to the level of the intervertebral disc between the 1st and 2nd lumbar vertebrae.
Cervical Enlargement:
A thickened area near the top of the spinal cord.
Provides nerves to the upper limbs.
Lumbar Enlargement:
A thickened region near the bottom of the spinal cord.
Gives rise to nerves that serve the lower limbs.
Cauda Equina (Horse’s Tail):
A structure formed when the spinal cord tapers to a point.
Composed of spinal nerves from the lumbar and sacral areas
Structure of the Spinal Cord:
The spinal cord has 31 segments, each connected to a pair of spinal nerves.
Two grooves (anterior median fissure and posterior median sulcus) divide the cord into right and left halves.
Gray Matter:
Forms a butterfly-shaped core in the center of the spinal cord.
Contains interneurons and neuron cell bodies.
Sensory neuron cell bodies are in the posterior root ganglia outside the spinal cord.
The gray matter has posterior horns, anterior horns, and lateral horns.
White Matter:
Surrounds the gray matter and is made up of myelinated nerve fibers (nerve tracts).
The white matter is divided into three regions: anterior, lateral, and posterior funiculi (columns), each containing tracts of axons.
Central Canal:
Located in the center of the gray matter.
Contains cerebrospinal fluid (CSF).
Main Functions of the Spinal Cord:
Transmits impulses to and from the brain
Controls spinal reflexes
Tracts:
Ascending tracts: carry sensory information to the brain
Descending tracts: carry motor information from the brain to muscles or glands
Key Nerve Tracts:
Spinothalamic tracts: sensory info from the spinal cord to the thalamus
Corticospinal tracts: motor impulses from the cerebral cortex to the spinal cord
Extrapyramidal tracts: involved in balance and posture
Spinal Reflexes:
Controlled by reflex arcs through the spinal cord
9.14: The brain
The Brain:
Largest, most complex part of the nervous system
Contains 100 billion neurons and supporting neuroglia
Structure: gray matter outside, white matter inside
Main Parts of the Brain:
Cerebrum: largest part; handles higher mental functions, sensory, and motor functions
Diencephalon: processes sensory input and controls homeostasis
Cerebellum: coordinates muscle activity
Brainstem: regulates visceral activities and connects the nervous system parts
Cerebrum Structure:
Largest part of the mature brain
Consists of two mirror-image hemispheres
Corpus callosum: connects the two hemispheres
Surface Features:
Gyri: ridges
Sulci: shallow grooves
Fissures: deep grooves (longitudinal and transverse)
Lobes of the Cerebrum:
Frontal, Parietal, Temporal, Occipital: named after underlying bones
Insula: fifth lobe, deep in the lateral sulcus
Cerebral Structure:
Cerebral Cortex: thin outer layer of gray matter; contains 75% of neuron cell bodies
White Matter: beneath the cortex; made of myelinated fibers connecting the cortex to the nervous system
Functions of the Cerebrum:
Interpretation of sensory input
Initiation of voluntary muscle movements
Memory storage
Information integration for reasoning
Intelligence
Personality
Functional Areas of the Cerebral Cortex:
Divided into Sensory, Association, and Motor areas
Sensory Areas:
Cutaneous Senses: anterior parietal lobe
Visual Area: posterior occipital lobe
Auditory Area: posterior temporal lobe
Taste Area: base of central sulcus and insula
Smell Area: deep in temporal lobe
Sensory Fibers: cross over in the spinal cord or brainstem, causing the right side of the body to be interpreted by the left hemisphere
Association Areas of the Brain:
Analyze and interpret sensory impulses
Involved in reasoning, judgment, emotions, verbalizing ideas, and memory storage
Specific Association Areas:
Frontal Lobe: higher intellectual processes (planning, problem solving)
Parietal Lobe: understanding speech, choosing words
Occipital Lobe: analyzing visual patterns, integrating visual with other senses
Sensory-adjacent Areas: analyze sensory input
Key Areas:
General Interpretive Area: junction of parietal, temporal, occipital lobes; complex thought processing
Wernicke’s Area: in the temporal lobe, typically left side; understanding written and spoken language
Primary Motor Areas:
Located in the posterior frontal lobes, anterior to the central sulcus
Contains Pyramidal Cells (upper motor neurons) that connect to lower motor neurons targeting skeletal muscles
Motor control is crossed over in the brainstem, with the right hemisphere controlling the left side of the body
Specific Motor Areas:
Broca’s Area: in the frontal lobe, usually left side; controls speech muscles
Frontal Eye Field: in the frontal lobe; controls voluntary eye movements
Hemisphere Dominance:
Both hemispheres process sensory input and send motor impulses to the opposite side of the body
Left Hemisphere: dominant in most people for language (speech, writing, reading) and complex intellectual functions
Right Hemisphere: dominant in some individuals; specializes in nonverbal functions, body orientation, emotions, and intuitive thinking
Some people show equal dominance in both hemispheres
Corpus Callosum:
Connects the two hemispheres
Allows the dominant hemisphere to control the motor cortex of the nondominant side and transfer sensory impulses from the nondominant side to the dominant side
Basal Nuclei (Basal Ganglia):
Masses of gray matter deep within the cerebral hemispheres
Composed of caudate nucleus, putamen, and globus pallidus
Produce dopamine, an inhibitory neurotransmitter
Relay motor impulses from the cerebrum and interact with the motor cortex, thalamus, and cerebellum to control motor activities
Facilitate voluntary movement
Altered activity can lead to Parkinson's disease and Huntington's disease
Ventricles:
Series of connected cavities within the cerebral hemispheres and brainstem
Continuous with the central canal of the spinal cord and the subarachnoid space
Filled with cerebrospinal fluid (CSF)
CSF Flow Path:
2 Lateral Ventricles
Interventricular Foramina
Third Ventricle
Cerebral Aqueduct
Fourth Ventricle (continuous with the central canal of the spinal cord and subarachnoid space)
Choroid Plexuses:
Masses of specialized capillaries from the pia mater
Found in all 4 ventricles
Secrete cerebrospinal fluid (CSF) into the ventricles (most CSF originates in the lateral ventricles)
CSF Circulation:
Circulates through ventricles and connecting passageways into the subarachnoid space
Reabsorbed back into the blood
CSF Functions:
Completely surrounds the brain and spinal cord
Nutritive and protective (cushioning) functions
Diencephalon:
Located between the cerebral hemispheres and above the midbrain
Surrounds the third ventricle
Mainly composed of gray matter
Main Parts:
Thalamus
Hypothalamus
Other Components:
Optic Tracts and Optic Chiasma
Infundibulum (connects pituitary gland to hypothalamus)
Posterior Pituitary
Mammillary Bodies
Pineal Gland
Functions of the Thalamus:
Sorts and directs sensory information to the cerebral cortex
Channels all sensory impulses except for smell
Produces general awareness of sensations like pain, touch, and temperature
Functions of the Hypothalamus:
Maintains homeostasis by regulating visceral activities and linking the endocrine system with the nervous system
Regulates:
Heart rate and arterial blood pressure
Body temperature, water and electrolyte balance, hunger, and body weight
Digestive tract movements and secretions
Sleep and wakefulness
Stimulates the posterior pituitary gland to secrete steroid hormones
Produces hormones that stimulate the anterior pituitary gland to secrete its hormones
The Limbic System:
Located in the area of the diencephalon; controls emotional experience and expression
Consists of several structures, including parts of the cerebral cortex, thalamus, hypothalamus, and basal nuclei (deep gray matter)
Produces feelings of fear, anger, pleasure, and sorrow, modifying behavior
Guides behavior by generating pleasant or unpleasant feelings about experiences, which can enhance the chance of survival
Brainstem:
Composed of:
Midbrain
Pons
Medulla Oblongata
Located at the base of the cerebrum
Connects the cerebrum, diencephalon, and cerebellum to the spinal cord
Midbrain:
Located between the diencephalon and pons
Contains myelinated nerve fiber bundles for conveying impulses to and from higher brain centers
Contains gray matter centers for auditory and visual reflexes
Main motor pathways between the cerebrum and lower nervous system parts
Pons:
Located between the midbrain and medulla oblongata
Transmits impulses to and from the medulla oblongata and cerebrum
Conducts impulses from the cerebrum to the cerebellum
Contains centers that regulate the rate and depth of breathing
Medulla Oblongata:
Transmits all ascending and descending impulses between the brain and spinal cord
Extends from the pons to the foramen magnum
Corticospinal tracts cross over in the pyramids of the medulla oblongata
Contains nuclei that control visceral functions:
Cardiac Center: alters heart rate
Vasomotor Center: controls vasoconstriction and vasodilation, helping regulate blood pressure
Respiratory Center: controls the rate and depth of breathing
Contains nuclei for reflexes such as coughing, sneezing, swallowing, and vomiting
Reticular Formation (Reticular Activating System):
Network of nerve fibers connecting gray matter masses scattered throughout the brainstem
Neurons connect the hypothalamus, basal nuclei, cerebellum, and cerebrum with major ascending and descending tracts
Decreased activity leads to sleep; increased activity leads to wakefulness
Injury to the reticular formation can result in a comatose state
Filters incoming sensory impulses
Cerebellum:
Located beneath the occipital lobes of the cerebrum, posterior to the brainstem
Composed of two hemispheres connected by the vermis
Cerebellar Cortex: thin layer of gray matter on the outside
Arbor Vitae: core of white matter
Communicates with other CNS parts through 3 pairs of tracts called cerebellar peduncles
Functions of the Cerebellum:
Integrates sensory information about body part positions
Coordinates skeletal muscle activity
Maintains posture
Ensures movement occurs as intended
9.15: Peripheral Nervous system
Peripheral Nervous System (PNS):
Connects the CNS to body parts
Includes cranial nerves (from the brain) and spinal nerves (from the spinal cord)
Contains sensory and motor divisions
Motor Division of the PNS:
Somatic Nervous System:
Connects the CNS to skeletal muscles and skin
Controls conscious activities
Autonomic Nervous System:
Connects the CNS to viscera (internal organs)
Controls subconscious activities
Cranial Nerves:
Twelve pairs of cranial nerves arise from the underside of the brain
Most are mixed nerves (contain both sensory and motor fibers), but some are sensory only, and others are primarily motor
The first pair arises from the cerebrum, the second pair from the thalamus, and the remaining pairs arise from the brainstem
The 12 pairs are designated by number and name, with numbers ordered from superior to inferior
Spinal Nerves:
31 pairs arise from the spinal cord
All except the first pair are mixed nerves
Grouped according to the level from which they arise:
8 pairs of cervical nerves
12 pairs of thoracic nerves
5 pairs of lumbar nerves
5 pairs of sacral nerves
**1 pair of coccygeal nerves
Each spinal nerve arises from two roots:
Sensory posterior root
Motor anterior root
The posterior root contains a posterior root ganglion, housing the cell bodies of sensory neurons entering the spinal cord
The anterior and posterior roots unite to form a spinal nerve, which extends out of the vertebral canal through the intervertebral foramen
Spinal Nerve Plexuses:
Cervical Plexuses (C1 to C4):
Located on either side of the neck
Supply muscles and skin of the neck
Include the phrenic nerves, which control the diaphragm
Brachial Plexuses (C5 to T1):
Arise from lower cervical and upper thoracic nerves
Supply muscles and skin of the arms, forearms, and hands
Include the musculocutaneous, ulnar, median, radial, and axillary nerves
Lumbosacral Plexuses (L1 to S4):
Arise from the lower spinal cord
Supply muscles and skin of the lower abdomen, external genitalia, buttocks, and legs
Include the obturator, femoral, and sciatic nerves
Thoracic spinal nerves do not form plexuses but become intercostal nerves
Plexuses recombine spinal nerve axons, allowing axons from different spinal nerves to extend to the same part of the body through the same peripheral nerve
9.16: Autonomic Nervous System
Autonomic Nervous System (ANS):
Part of the PNS that operates continuously and independently, without conscious effort
Controls visceral motor functions of smooth muscle, cardiac muscle, and glands
Helps maintain homeostasis, responds to emotional stress, and prepares the body for strenuous activity
Regulates heart rate, blood pressure, breathing rate, and body temperature
General Characteristics of the ANS:
Autonomic activities are regulated by reflexes involving sensory receptors in the viscera and skin
Impulses are conducted to the brain or spinal cord, then motor impulses travel through cranial and spinal nerves, ganglia, and finally to effectors (muscles or glands)
Two Divisions of the ANS (with opposing effects):
Sympathetic Division:
Active in stress or emergency situations (fight or flight)
Parasympathetic Division:
Active under normal, restful conditions (rest and digest)
Autonomic Neurotransmitters
Preganglionic fibers in both the sympathetic and parasympathetic divisions release acetylcholine (cholinergic fibers).
Parasympathetic postganglionic fibers also release acetylcholine.
Sympathetic postganglionic fibers release norepinephrine (adrenergic fibers).
The sympathetic and parasympathetic divisions often have opposing effects on organs.
Some organs receive input from both divisions, while others, like blood vessels, are only controlled by the sympathetic division.
Control of Autonomic Activity
The autonomic nervous system is mainly controlled by control centers in the brain and spinal cord.
The limbic system and cerebral cortex can alter autonomic reactions through emotional influence.
10.1: Intro to the senses:
Senses are detected by sensory receptors that respond to environmental changes and send nerve impulses to the CNS for processing.
The body responds with feelings or sensations.
Categories of senses:
General senses: Widely distributed, simple structure (e.g., touch, pressure, temperature, pain).
Special senses: Complex sensory organs in the head (e.g., vision, hearing, smell, taste, balance).
10.2: Receptors, Sensations, and Perception
Types of sensory receptors:
Chemoreceptors: Sensitive to changes in chemical concentration.
Pain receptors: Detect tissue damage.
Thermoreceptors: Respond to temperature differences.
Mechanoreceptors: Respond to changes in pressure or movement.
Photoreceptors: Respond to light (found in the eye).
Sensation and Perception
Sensation: Occurs when receptors are stimulated and send impulses to the brain.
Perception: The conscious awareness of stimuli.
Projection: The brain sends the sensation back to its point of origin, allowing the person to pinpoint the area of stimulation.
The type of sensation depends on the region of the brain that receives the impulses.
Sensory adaptation
Brain must prioritize sensory impulses to avoid being overwhelmed by information.
can become less responsive to maintained stimuli (e.g., clothing on skin, persistent odors, ongoing sounds).
Sensory adaptation: The nervous system becomes less responsive to a maintained stimulus.
This occurs due to receptors becoming unresponsive or inhibited along the CNS pathway.
10.3: General senses
General senses:
Widespread, with receptors in the skin, muscles, joints, and viscera.
Senses of touch, pressure, temperature, and pain.
Receptors for touch and pressure:
Free nerve endings: In epithelial tissues, associated with itching and other sensations.
Tactile (Meissner's) corpuscles: Flattened connective tissue sheaths around nerve fibers; detect motion of objects on the skin; found in hairless areas (e.g., lips, fingertips, palms, soles, nipples).
Lamellated (Pacinian) corpuscles: Large structures in connective tissue; detect deep pressure; found in deep dermis and subcutaneous layer.
Temperature senses:
Warm receptors: Respond to temperatures between 25°C (77°F) and 45°C (113°F). Above this range, pain receptors are triggered, causing a burning sensation.
Cold receptors: Respond to temperatures between 10°C (50°F) and 20°C (68°F). Below this range, pain receptors are triggered, causing a freezing sensation.
Both warm and cold receptors adapt quickly, and after continuous stimulation for 1 minute, sensations start to fade.
Body position, movement, and stretch receptors:
Proprioception: The sense of body position and location in space.
Proprioceptors: Associated with skeletal muscle, prevent injury to muscles and tendons.
Muscle spindles: Bundles of special skeletal muscle fibers with sensory neuron fibers wrapped around them; monitor muscle contraction.
Golgi tendon organs: Located in tendons near muscle attachment; detect how much a tendon stretches during muscle contraction.
Sense of pain:
Pain receptors (nociceptors):
Some are free nerve endings that are triggered by tissue damage.
Overstimulation of other receptors (e.g., cold receptors) can also signal pain.
Nociceptors communicate with pain sensory neurons using Substance P (spinal cord) and Glutamate (brain).
Tissue damage releases prostaglandins, which increase nociceptor sensitivity and pain intensity.
Aspirin and ibuprofen inhibit prostaglandin synthesis, reducing pain.
Morphine, heroin, and natural painkillers (endorphins, enkephalins) inhibit the release of Substance P.
Pain is beneficial to health, as it warns of potential harm to the body
Visceral pain:
Visceral pain receptors: The only receptors in the viscera that produce sensations.
Visceral pain receptors respond differently than surface tissue receptors; sometimes, damage doesn't produce pain, but stretch or spasms can cause strong pain.
Pain can be caused by mechanoreceptors, decreased blood flow (oxygen deprivation), or chemicals stimulating chemoreceptors.
Referred pain: Visceral pain felt as if it's coming from another body area due to shared nerve pathways between the skin and internal organs.
Example: Pain from the heart may be felt in the left shoulder or left arm.
Pain nerve fibers:
Fast (acute) pain fibers:
Myelinated fibers that carry impulses rapidly.
Associated with sharp pain that stops when the stimulus ends.
Typically sensed from the skin.
Slow (chronic) pain fibers:
Unmyelinated fibers that conduct impulses slowly.
Produce a dull, achy sensation that is hard to localize and continues after the stimulus ends.
Typically sensed from deep tissues.
Pain stimuli often trigger both fast and slow fibers, causing sharp pain followed by a dull, aching pain.
Pain pathways:
Pain impulses from the head reach the brain via sensory fibers of cranial nerves; all others travel through spinal nerves.
Pain impulses entering the spinal cord are processed in the gray matter of the posterior horn and then sent to the brain.
Most pain fibers terminate in the reticular formation, thalamus, or limbic system.
From there, neurons carry the information to the hypothalamus and cerebral cortex.
The limbic system provides the emotional response to pain.
The cerebral cortex identifies the pain source, determines its intensity, and formulates motor responses to the pain.
10.4: Special senses
Special senses are senses that have sensory receptors located in large, complex organs in the head.
Key special senses and their corresponding organs:
Smell: Detected by olfactory organs.
Taste: Detected by taste buds.
Hearing: Detected by structures in the ears.
Equilibrium (balance): Also detected by structures in the ears.
Sight: Detected by the eyes.
10.5: Sense of smell
Olfactory organs are masses of epithelium located in the roof of the nasal cavity and house olfactory receptor cells.
Olfactory receptors are chemoreceptors that detect and distinguish smells.
Located in bipolar neurons with cilia.
Supported by columnar epithelial cells.
Each neuron contains one type of olfactory receptor membrane protein.
Chemicals must dissolve in liquid to activate receptors.
Smell and taste work together to help in food selection.
Olfactory pathways describe how smell signals travel and are processed in the brain.
Stimulation of olfactory receptors: Fibers synapse in the olfactory bulbs near the crista galli of the ethmoid bone.
Cranial Nerve I (Olfactory Nerve): Formed by axons of olfactory receptor neurons.
Sensory impulses: Analyzed in the olfactory bulbs and sent to the temporal and frontal lobes for interpretation.
Some signals go to the limbic system, triggering emotional responses to odors.
Olfactory stimulation occurs when odors activate specific receptors in olfactory receptor cells.
Each odor stimulates specific receptors in the cell membranes of receptor cells.
This triggers an influx of sodium ions, causing depolarization; if the threshold is reached, an action potential is generated.
Hundreds of receptor types code for thousands of odors by transmitting signals in groups.
The brain decodes these signals into an olfactory code to interpret different smells.
Olfactory receptors adapt quickly, causing smells to fade rapidly.
10.6: sense of taste
Taste buds are spherical organs responsible for taste, each containing 50 to 100 taste cells.
Located mainly along the papillae of the tongue, and some are scattered in the mouth and pharynx.
Taste receptors:
Taste cells (gustatory cells) are modified epithelial cells that act as chemoreceptors and are replaced every 10 days.
Taste hairs protrude from openings called taste pores and are the sensitive parts of the cells.
Chemicals must dissolve in water (saliva) to be tasted.
The sense of taste involves specific membrane proteins that bind to chemicals in food.
Taste cells adapt quickly to stimuli.
Taste sensations are detected by different types of taste cells in the taste buds.
At least 5 types of taste cells exist, each most sensitive to a specific type of chemical stimulus, producing 5 primary tastes: sweet, sour, salty, bitter, and umami (savory).
Other possible sensations include alkaline and metallic.
Taste buds respond to one primary taste sensation, with distinct receptors for each.
All types of taste buds are found on various portions of the tongue.
Taste sensation adapts rapidly, similar to smell.
Taste pathways describe how taste signals are processed in the brain.
Taste impulses travel through the facial, glossopharyngeal, and vagus nerves to the medulla oblongata.
The impulses continue through the thalamus.
Finally, they are interpreted in the gustatory cortex in the parietal lobe of the cerebrum.
10.7: Sense of hearing
Sense of Hearing: The ear provides both the senses of hearing and equilibrium.
The ear has three portions: outer, middle, and inner.
Outer (External) Ear consists of:
Auricle (pinna): Collects sound.
External acoustic meatus (external auditory canal): An S-shaped tube that transports sound toward the eardrum.
Tympanic membrane (eardrum): Located at the end of the external acoustic meatus, it vibrates with sound waves.
Middle Ear: The middle ear, or tympanic cavity, is an air-filled space in the temporal bone. It contains three tiny bones called the auditory ossicles: malleus, incus, and stapes.
The tympanic membrane vibrates the malleus, which then vibrates the incus, and finally the stapes.
The stapes vibrate the fluid inside the oval window of the inner ear.
Vibrations in the fluid stimulate the hearing receptors in the inner ear.
The auditory ossicles transmit and amplify sound waves.
Auditory Tube: The auditory (eustachian) tube connects the middle ear to the nasopharynx.
It helps maintain equal air pressure on both sides of the eardrum, which is essential for normal hearing.
Mucous membrane infections from the throat can travel up the auditory tube to the middle ear, potentially causing middle ear infections.
Inner (Internal) Ear: The part of the ear that includes the cochlea for hearing and the semicircular canals for balance.
The inner ear is a labyrinth, a network of chambers and tubes.
It has a membranous labyrinth inside a bony labyrinth in the temporal bone.
Perilymph is the fluid between the two labyrinths.
Endolymph is the fluid inside the membranous labyrinth.
The cochlea is responsible for hearing.
The semicircular canals are responsible for equilibrium (balance).
Cochlea: A spiral-shaped structure in the inner ear that contains three chambers involved in hearing.
The cochlea has 3 chambers: Scala vestibuli, Cochlear duct, and Scala tympani.
The oval window leads to the upper compartment, the scala vestibuli, which extends to the tip of the cochlea.
The scala tympani is the lower compartment, extending from the tip of the cochlea to the round window.
The cochlear duct is the middle compartment, located between the scala vestibuli and scala tympani.
The cochlear duct is separated from the scala vestibuli by the vestibular membrane and from the scala tympani by the basilar membrane.
Hearing Process in the Cochlea: Vibrations are converted into electrical signals that allow us to hear.
The spiral organ (organ of Corti), located on the basilar membrane, is the hearing receptor organ.
Hair cells in the spiral organ have hairs that extend into the endolymph of the cochlear duct.
The tectorial membrane lies above the hair cells and touches the tips of the hairs.
Vibrations from the basilar membrane cause hair cells to bend against the tectorial membrane, generating action potentials.
Vibrations in the inner ear fluids cause hair cells to bend in different regions, allowing us to hear different pitches (frequencies).
Action potentials travel along the cochlear branch of the vestibulocochlear nerve to the auditory cortex in the temporal lobe of the brain.
Measuring Sound Intensity: The level of sound intensity is measured in decibels (dB).
Decibels (dB) are a logarithmic scale for sound intensity.
0 dB is the least perceptible sound to the human ear, and 30 dB is 1,000 times as intense.
Common sound levels: Whisper (40 dB), Normal conversation (60-70 dB), Rock concert (120 dB).
Prolonged exposure to sounds above 85 dB can damage hearing receptors and cause permanent hearing loss.
Auditory Pathways: The process by which sound is interpreted by the brain.
Nerve fibers carry sound impulses to the auditory cortices in the temporal lobes for interpretation.
Some nerve fibers cross over, allowing both sides of the brain to interpret sounds from both ears.
Hearing loss can be conductive (due to interference with sound transmission) or sensorineural (due to damage to the cochlea, auditory nerve, or pathways).
Sound waves enter the external acoustic meatus.
Sound waves cause the eardrum to vibrate in response to the sound source.
The auditory ossicles (malleus, incus, stapes) amplify and transfer vibrations to the stapes.
The movement of the stapes at the oval window transfers vibrations to the perilymph in the scala vestibuli.
Vibrations pass through the vestibular membrane and enter the endolymph of the cochlear duct, causing movement of the basilar membrane.
Different frequencies of vibration in the basilar membrane stimulate specific receptor cells.
As receptor cells depolarize, their membrane becomes more permeable to calcium ions.
The inward diffusion of calcium causes neurotransmitter release from vesicles at the receptor cell's base.
The neurotransmitter stimulates dendrites of nearby sensory neurons.
Sensory impulses travel along fibers of the cochlear branch of the vestibulocochlear nerve.
Auditory cortices in the temporal lobes interpret the sensory impulses as sound.
10.8: Sense of Equilibrium
Equilibrium: The sense that helps maintain balance and body position.
Static equilibrium: Maintains the position of the head, posture, and balance when the head and body are still.
Dynamic equilibrium: Maintains balance when the head and body suddenly move or rotate.
Static Equilibrium: Maintains head position and balance when the body is still.
The organs of static equilibrium are located in the vestibule of the inner ear, between the cochlea and semicircular canals.
The membranous labyrinth in the vestibule contains two chambers: the utricle and saccule.
Each chamber contains an organ called a macula, which consists of:
Hair cells (sensory receptors),
Gelatinous material (which the hair cells' hairs project into),
Otoliths (calcium carbonate crystals embedded in the gelatinous mass).
Gravity or head movement causes the gelatin and otoliths to shift, bending the hair cells and generating action potentials.
Impulses are transmitted to the brain via the vestibular branch of the vestibulocochlear nerve, indicating head position.
Dynamic Equilibrium: Maintains balance during sudden head and body movements.
The organs of dynamic equilibrium are located inside the semicircular canals of the inner ear.
The three semicircular canals detect head motion and help balance the head and body during sudden movements.
The dynamic equilibrium organs are called cristae ampullaris, located in the ampulla of each semicircular canal.
The canals and cristae are positioned at right angles to each other.
Hair cells, the sensory receptors, extend into a dome-shaped gelatinous mass called the cupula.
Rapid movement of the head or body causes the semicircular canals to move, but the endolymph remains stationary.
This results in the bending of the cupula and the hair cells embedded within it.
The bending of the hair cells generates action potentials, which are sent to the brain.
Impulses travel to the brain via the vestibular branch of the vestibulocochlear nerve, signaling head position.
Proprioceptors (mechanoreceptors) in the joints and visual changes also assist in maintaining equilibrium.
10.9: Sense of sight
Eye and Accessory Organs: The eye detects vision, while accessory organs assist with protection and movement.
Eye: Contains visual receptors for the sense of vision.
Accessory organs of sight include the lacrimal apparatus, eyelids, and extrinsic muscles, which help protect and move the eye.
Eyelid:
Protects the eye from foreign objects.
Composed of 4 layers: skin (thinnest in the body), muscle, connective tissue, and conjunctiva.
Muscles:
Orbicularis oculi (closes eyelid)
Levator palpebrae superioris (raises eyelid).
Conjunctiva: A mucous membrane lining the inner eyelids and covering the anterior surface of the eyeball (except over the cornea).
Visual Accessory Organs: Assist with the protection, lubrication, and movement of the eye.
Lacrimal Apparatus:
Lacrimal gland produces tears to lubricate and cleanse the eye.
Tiny tubules release tears over the surface of the eye.
Two small ducts, the canaliculi, drain tears into the lacrimal sac, which then leads to the nasolacrimal duct and into the nasal cavity.
Tears contain lysozyme, an antibacterial enzyme.
Extrinsic Eye Muscles:
Attach to the sclera.
There are 6 extrinsic eye muscles that control eye movement in all directions.
Structure of the Eye: The eye is a fluid-filled sphere with three distinct layers.
Outer (Fibrous) Layer: The outermost layer, providing shape and protection. Includes the sclera and cornea.
Middle (Vascular) Layer: Contains blood vessels, providing nutrients and oxygen. Includes the choroid, ciliary body, and iris.
Inner (Nervous) Layer: The innermost layer, responsible for processing visual information. Includes the retina.
The spaces within the eye are filled with fluids that help maintain its shape and support its functions, including aqueous humor and vitreous humor.
Outer (Fibrous) Layer: Provides protection and shape to the eye.
Cornea: Transparent and makes up the anterior 1/6 of the outer layer. It helps focus light rays.
Sclera: White, making up the posterior 5/6 of the outer layer. It is mostly composed of collagen, providing protection and serving as an attachment point for the extrinsic eye muscles.
The optic nerve and blood vessels pierce the sclera at the posterior of the eye.
Middle (Vascular) Layer: Contains structures involved in nourishing the eye and regulating light.
Choroid Coat: Vascular and darkly pigmented. It nourishes the inner tissues of the eye and helps keep the interior dark.
Ciliary Body: Forms a ring around the front of the eye. It contains:
Ciliary processes: Secrete aqueous humor.
Ciliary muscles: Control the shape of the lens for focusing.
Suspensory ligaments: Hold the lens in place and help change its shape to focus on objects at different distances.
Accommodation: The process by which the lens adjusts its shape to focus on objects at different distances.
When the ciliary muscle relaxes, the suspensory ligaments pull outward, causing the lens to flatten for focusing on distant objects.
When the ciliary muscle contracts, the suspensory ligaments relax, allowing the lens to become more convex to focus on closer objects.
Inner Layer: Contains the retina and photoreceptor cells for vision.
The retina contains photoreceptors (visual receptor cells).
It is almost transparent and consists of several layers of cells.
The retina is continuous with the optic nerve at the back of the eye.
It forms the inner lining of the eye, extending forward almost to the ciliary body.
At the center of the retina is the macula lutea, which contains the fovea centralis – the point of sharpest vision.
Optic Disc: Located medial to the fovea centralis, it's where nerve fibers leave the eye to form the optic nerve. It's also called the blind spot because it lacks photoreceptors.
Posterior Cavity: The largest compartment of the eye, located between the lens, ciliary body, and retina.
The posterior cavity is filled with vitreous humor, which, along with collagenous fibers, forms the vitreous body.
Vitreous Body: Supports the internal parts of the eye and helps maintain its shape.
Light Refraction & Corrective Lenses: Refraction is the bending of light rays to focus images on the retina.
Refraction: The bending of light rays, which helps focus them on the fovea centralis of the retina.
Both the cornea and lens refract light rays to focus them on the retina, with the humors contributing slightly.
Emmetropia: Normal vision, where light focuses sharply on the retina due to a normal eye shape.
Myopia (Nearsightedness): Light rays focus in front of the retina and scatter, causing a blurry image. Corrected with a concave lens.
Hyperopia (Farsightedness): Light rays focus behind the retina and are not fully converged by the time they reach the retina. Corrected with a convex lens.
Photoreceptors: Visual receptor cells that are modified neurons, responsible for detecting light and enabling vision.
Rods:
More sensitive to light than cones, functioning in dim light.
Provide black and white vision.
Produce less precise images (general outlines) because their axon branches converge onto fewer nerve fibers.
There are many more rods than cones in the retina.
Cones:
Provide sharp images in bright light.
Enable color vision.
The fovea centralis contains densely packed cones but no rods.
Photopigments: Light-sensitive pigments in rods and cones that break down when they absorb light energy.
Rhodopsin (visual purple) is the light-sensitive pigment in rods.
When stimulated by light, rhodopsin decomposes into opsin (a protein) and retinal (produced from vitamin A).
The decomposition of rhodopsin activates an enzyme, which triggers changes in the rod cell membrane, generating a nerve impulse.
These nerve impulses travel to the visual cortex via the optic nerve, where they are interpreted as vision.
In bright light, most of the rhodopsin is broken down.
In dim light, rhodopsin regeneration occurs faster than its breakdown.
Cone Photopigments: In cones, the light-sensitive pigments consist of opsin proteins and retinal, similar to rods.
There are three sets of cones, each containing a different opsin protein.
The wavelength of light determines the color perceived, with each pigment being most sensitive to specific wavelengths of light:
Erythrolabe: Sensitive to red light.
Chlorolabe: Sensitive to green light.
Cyanolabe: Sensitive to blue light.
If all three sets of cones are stimulated, the color perceived is white; if none are stimulated, the color is black.
Color Blindness occurs when there is a lack of one or more cone pigments
Visual Pathways: The process by which visual information travels from the retina to the brain for interpretation.
The axons of retinal neurons leave the eyes to form the optic nerves.
Fibers from the medial half of each retina cross at the optic chiasma.
The pathway continues with fibers forming optic tracts, which proceed to the thalamus.
Just before reaching the thalamus, some nerve fibers enter nuclei responsible for visual reflexes.
From the thalamus, nerve pathways, known as optic radiations, lead to the visual cortex in the occipital lobes of the cerebrum.