Neurobiology 4 - Dr. Safiyyah Iqbal
Learning Outcomes and Recap
Learning Goals for Neurobiology Lecture 4
Familiarization with all components of the Peripheral Nervous System (PNS).
Understanding the concept of a spinal reflex and its core components.
Explaining and differentiating between the parasympathetic and sympathetic divisions of the Autonomic Nervous System (ANS).
Understanding that the Brain and Spinal Cord together constitute the Central Nervous System (CNS).
Recap of Key Neurobiology Concepts
Neurotransmitters are released at synapses between neurons.
Neurotransmitters exert either excitatory or inhibitory effects on the postsynaptic neuron.
The two primary forms of neural processing are convergence and divergence.
Overview of the Nervous System
Structural Parts of the Nervous System
Central Nervous System (CNS): Comprised of the brain and spinal cord. Its primary function is to receive information from the body and send out instructions.
Peripheral Nervous System (PNS): Comprised of all nerves and wiring outside of the CNS. It transmits messages from the brain to the rest of the body.
General Functions of the PNS
It transmits information both to and from the CNS.
It regulates vertebrate movement and the internal environment.
Sensory Neurons: These run from stimulus receptors to inform the CNS of various stimuli.
Motor Neurons: These run from the CNS to the muscles and glands (known as effectors) to initiate action.
Peripheral Nervous System (PNS) Divisions
Somatic Nervous System
Carries signals to and from skeletal muscles.
Primarily responds to external stimuli.
Governs consciously perceived sensations (voluntary control).
Produces excitation of skeletal muscle.
Structure: Uses a single neuron to connect the CNS directly to the organ/muscle.
Autonomic Nervous System (ANS)
Regulates the internal environment by controlling smooth muscle, cardiac muscle, and the organs of the digestive, cardiovascular, excretory, and endocrine systems.
Governs unconsciously perceived visceral sensations to maintain homeostasis.
Involves involuntary inhibition or excitation of smooth muscle, cardiac muscle, or glandular secretions.
Structure: Requires two neurons to connect the CNS to the organ.
Structural Anatomy of the PNS
Consists of left-right pairs of cranial and spinal nerves along with associated ganglia.
Cranial Nerves: There are pairs that connect directly with the brain.
Spinal Nerves: There are pairs.
These originate in the spinal cord and extend to the body below the head.
They contain both Somatic and Autonomic branches.
They possess both sensory and motor nerve fibers.
Spinal Nerve Roots
Dorsal Root: Contains sensory neurons carrying incoming signals to the spinal cord.
Ventral Root: Carries action potentials (AP) away from the spinal cord to effectors.
The Autonomic Division
General Characteristics
Regulates the internal environment and is generally involuntary and automatic.
First neurons are myelinated preganglionic neurons that lie within the CNS.
These preganglionic neurons make synaptic connections with unmyelinated postganglionic fibers.
The unmyelinated postganglionic fibers reach the effector organ.
Sympathetic Division
Function: Corresponds to arousal and energy generation; known as the "fight or flight" response.
Origin: Preganglionic motor neurons originate in the thoracic and lumbar regions of the spinal cord.
Neurotransmitter: Norepinephrine is the key neurotransmitter.
Physiological Effects:
Increases heart rate and respiration.
Raises blood pressure.
Dilates pupils.
Slows digestion, urine production, and saliva production.
Produces a unified response across all organs simultaneously.
Relaxes bladder muscles.
Stimulates secretion of epinephrine and norepinephrine.
Parasympathetic Division
Function: Corresponds to calming the body and returning to self-maintenance; known as the "rest and digest" response.
Origin: Preganglionic neurons originate in the brain or from the sacral region of the spinal cord.
Neurotransmitter: Acetylcholine is the key neurotransmitter.
Physiological Effects:
Lowers heart rate and respiration.
Increases digestion and stimulates digestive processes.
Permits/stimulates defecation and urination.
Constricts pupils.
Increases salivation.
Dilates blood vessels.
Contracts bladder muscles.
Homeostasis: The Sympathetic and Parasympathetic divisions work antagonistically to maintain physiological balance.
Somatic Division and Spinal Reflexes
Control Mechanisms
Conscious Control: Voluntary signals to and from skeletal muscles.
Reflex Control: Involuntary responses (Spinal Reflexes).
The Spinal Reflex
An involuntary response mediated primarily by the spinal cord and spinal nerves with minimal brain involvement.
Flexor (Withdrawal) Reflex: For example, withdrawing the right leg after a painful stimulus to the foot.
Crossed Extensor Reflex: Complements the flexor reflex; for example, extending the left leg to maintain balance when the right leg is withdrawn.
Stretch Reflex: For example, the knee-jerk reflex.
The Reflex Arc Structure
Stimulus is applied.
Sensory neurons carry the signal to the spinal cord via the dorsal root.
The cell body of the sensory neuron is located in the dorsal root ganglion.
Interneurons in the gray matter stimulate specific motor neurons on both sides of the body.
Signals are also sent to the brain.
Motor neurons carry the response signal via the ventral root to the effector muscles (e.g., the thigh).
Central Nervous System (CNS) Architecture
CNS Protection
Bone: The skull protects the brain, and the vertebrae protect the spinal cord.
Meninges: Three protective membranes.
Dura mater (outer layer).
Arachnoid (middle layer).
Pia mater (inner layer, adjacent to brain tissue).
Cerebrospinal Fluid (CSF): Bathes the brain and spinal cord, acting as a shock absorber. It is produced within the ventricles (Right, Left, Third, and Fourth ventricles) of the brain.
Blood-brain Barrier: Prevents the entry of certain chemicals and pathogens into the CNS.
The Spinal Cord
Acts as a "superhighway" for action potentials between the brain and body.
White Matter: Located in the outer portion; consists of myelinated ascending (sensory) and descending (motor) nerve tracts.
Grey Matter: Located in the center portion (butterfly shape); contains cell bodies and dendrites. Includes the dorsal horn and ventral horn.
Central Canal: Located in the center of the grey matter.
Brain Divisions and Functions
Hindbrain
Medulla Oblongata: Controls vital automatic functions. It house the Cardiovascular center (heart rate, blood pressure) and the Respiratory center.
Cerebellum: Coordinates basic body movement and stores/replicates sequences (e.g., using a keyboard).
Pons: Coordinates the flow of information between the cerebellum and higher brain centers; also aids in respiration.
Midbrain
Coordinates muscle groups and responses to sight and sound.
Coordinates movement of the head related to vision and hearing.
Forebrain
Hypothalamus: Responsible for the regulation of homeostasis.
Thalamus: Acts as a receiving, processing, and transfer center.
Limbic System: Contains pathways involved in emotions and memories.
Cerebrum: Responsible for language, decision making, and conscious thought. The Corpus Callosum connects the hemispheres.
Cerebral Cortex Functions
Handles memory storage, abstract thought, conscious awareness, and conscious control of skeletal muscle.
Occipital Lobe: Processes visual information.
Temporal Lobe: Interprets auditory information and comprehends spoken/written language.
Parietal Lobe: Receives and interprets sensory information from the skin.
Frontal Lobe: Initiates motor activity; responsible for speech and conscious thought.
Memory: All four lobes contribute to memory storage.
Disorders of the Nervous System
Trauma
Concussion: A blow to the head that disrupts electrical activity in the brain, possibly leading to unconsciousness or subdural hematoma.
Spinal Cord Injuries: Result in impairment of sensation and function below the site of injury.
Infections
Encephalitis: Inflammation of the brain, usually caused by a viral infection.
Meningitis: Inflammation of the meninges, caused by viral or bacterial infection.
Rabies: A viral disease spread by animal saliva/bites. The virus spreads from the bite to the brain via sensory neurons.
Degenerative and Transmission Disorders
Brain Tumours: Abnormal growths in or on the brain.
Epilepsy: Recurring episodes of abnormal electrical activity in the brain, known as seizures.
Parkinson’s Disease: Caused by the degeneration of dopamine-releasing neurons.
Alzheimer's Disease: The most common cause of dementia. Characterized by the accumulation of abnormal protein (beta amyloid), leading to progressive memory lapses and dementia.
Learning Goals for Neurobiology Lecture 4
Familiarization with all components of the Peripheral Nervous System (PNS), including its structural and functional aspects, to understand its critical role in transmitting information between the CNS and the rest of the body.
Understanding the concept of a spinal reflex, including the mechanism of reflex arcs and their significance in rapid response to stimuli, highlighting the implications for motor control and safety.
Explaining and differentiating between the parasympathetic and sympathetic divisions of the Autonomic Nervous System (ANS), including their respective pathways, neurotransmitters, and physiological effects, ensuring a clear understanding of homeostasis and stress responses.
Understanding that the Brain and Spinal Cord together constitute the Central Nervous System (CNS), appreciating the complexity and integration of neural processes that govern behavior and bodily functions.
Recap of Key Neurobiology Concepts
Neurotransmitters are released at synapses between neurons, playing crucial roles in signaling and communication across the nervous system.
Neurotransmitters exert either excitatory or inhibitory effects on the postsynaptic neuron, influencing whether the neuron will fire an action potential.
The two primary forms of neural processing are convergence and divergence, which allow for the integration of multiple signals and the distribution of neural signals to multiple targets, respectively.