Neuroscience: Spatial and Temporal Summation

Why Woodpeckers Don't Get Concussions

• Forces exerted are ~1000 x gravity, while humans typically only survive 100 x gravity.
• Thick neck muscles absorb some of the forces.
• Third inner eyelid prevents the birds’ eyeballs from popping out.
• The brain is surrounded by spongy bone with a large number of trabeculae.

Housekeeping

• Module 2 exam grades are posted on Friday afternoon.
• Module 3 content is posted on D2L.
• Remaining Module 3 content will be posted on D2L Friday.

Learning Objectives

• Spatial and temporal summation.
• Physiological events at the different segments of the neuron.
• Action potential and graph it.
• Propagation of an action potential in both unmyelinated and myelinated axons
• Events that occur when an action potential reaches the transmissive segment.

A Brief Review

• Excitatory postsynaptic potential (EPSP towards threshold).
• Inhibitory postsynaptic potential (IPSP away from threshold).

Spatial Summation

• Many synapses bring the membrane potential to threshold.
• An individual EPSP would not bring the membrane potential to threshold.

Temporal Summation

• One synapse rapidly fires to bring the membrane potential to threshold.
• Any individual EPSP in time would not bring the membrane potential to threshold.

Physiologic Events in the Neuron Segments

• Receptive Segment: Chemically gated ion channels (Na+, Cl-, K+).
• Initial Segment: Voltage-gated Na+ and K+ channels open.
• Conductive Segment: Propagation of action potential; voltage-gated Na+ and K+ channels cause action potential to travels.
• Transmissive Segment: Voltage-gated Na+/K+ and Ca2+ channels
• Na/K+ pump, K+ leak channels are the last 3 steps.

Action Potential

• Resting membrane potential (RMP): -70 mV.
• Voltage-gated Na+ channels open during depolarization.
• Voltage-gated K+ channels open during repolarization.

Conductive Segment

• The unstimulated axon has a resting membrane potential of -70 mV.
• Graded potentials reach the initial segment and are added together, moving from -70mV to -55 mV.
• Depolarization occurs when the threshold (-55 mV) is reached. Voltage-gated Na+ channels open, and Na+ enters rapidly, reversing the polarity from negative to positive (-55 mV → +30 mV).
• Repolarization: Voltage-gated Na+ channels close and voltage-gated K+ channels open. K+ moves out of the cell, and polarity reverses from positive to negative (+30 mV → -70 mV).
• Hyperpolarization occurs when voltage-gated K+ channels stay open longer than needed to reach the resting membrane potential, dropping below -70mv
• Voltage-gated K+ channels close, and the plasma membrane returns to resting conditions via Na+/K+ pumps (-80 mV → -70 mV).
• Amplitude and duration are relatively fixed.

Question

• Na+ into the cell in the receptive segment causes excitatory effect.

Question

• Opening a chloride (Cl-) channel causes inhibition.

Diffusion

• Diffusion is only effective over small distances.

Continuous versus Saltatory Conduction

• Continuous Conduction: Open/close doors down hallway, slow.
• Saltatory Conduction: Much faster than diffusion, preventing anything from entering/leaving axon (charge).
• Unmyelinated neurons are like a sprinkler hose releasing water continuously, resulting in a continuous conduction.
• Myelinated neurons are like sprinkle hoses but many of the holes are taped over so the water - ions/charge forced to leave at fewer regions spaced further apart, resulting in saltatory conduction

Transmissive Segment

• The action potential causes release of neurotransmitter.

Learning Objectives

• Use the directional terms to navigate around the anatomy of the brain.
• Describe the general anatomic features of the brain.
• Name the major gyri, sulci, and fissure of the brain.
• Describe the four major regions of the brain.
• Describe the composition and distribution of gray and white matter in the brain.
• Describe the concept of cerebral lateralization.
• Describe the general functions of the 5 lobes of the Cerebrum
• Describe the location and function of the association areas in the brain.
• Describe how the motor and sensory systems are mapped to the cerebrum.
• Describe the basic anatomy and physiology of the diencephalon.
• Describe the basic anatomy and physiology of the brain stem.
• Describe the basic anatomy and physiology of the cerebellum.

Brain Size and Neuron Count

• The adult human brain is about the size of a grapefruit and weights 3 pounds.

Early Brain Research

• Increased activity in the brain results in increased flow of blood to the brain to support the increased neural activity. The discovery was made by Italian physiologist Angelo Mosso

Basic Anatomical of the Brain

• Anterior: Front.
• Posterior: Back.
• Lateral: Away from middle.
• Medial: Toward the middle.
• Superior: Top.
• Inferior: Bottom.

Overview of Brain Anatomy

• The outer surface is folded to maximize surface area.
• Gyri: Ridges.
• Sulci: Depressions between ridges.
• Fissures: Deep sulci.

Basic Anatomy of the Brain

• Key Structures: Central sulcus and Longitudinal fissure.
• The hemispheres are termed left and right.

4 major regions of the brain

• Cerebrum (outer surface)
• Diencephalon
• Brain Stem
• Cerebellum

Cerebrum

• Cortex (gray).
• Medulla.
• Connection of communication on both sides

Gray and White Matter

• The cerebral cortex is primarily grey matter.

Question

• A deep depression in the cerebral cortex is called a fissure.

Cerebral Hemispheres

• Some localization but difficult to assign precise function to specific regions.
• Cerebral hemispheres receive sensory and project motor commands to the opposite side of the body.
• Mirror images with some specialization & different functions of each side of Brain
• Categorical region. Analytical and sequential (science + math = left).
• Representation: musical and artistic (shapes/colors = right.

Cerebrum

• Left and Right sections
• Frontal lobe
• Parietal Lobe
• Temporal Lobe
• Occipital Lobe

Cerebrum: Frontal Lobe

• Central Sulcus separates frontal from parietal
• Primary motor cortex
• Frontal lobe functions: Decisions, concentration, Planning, Personality, Verbal Communication
• Located Primary Motor Cortex functions: Voluntary Movement

Cerebrum: Parietal Lobe

• General Sensory Input sensory input from skin, joint
• The Cerebrum Parietal lobe the Post-central gyrus = posterior to the central sulcus
• Primary Somatosensory Cortex is located inside the Somatosensory Association Area

Motor and Sensory Homunculus

• Motor cortex (within precentral gyrus)
• medidPrimary somatosensory cortex (within postcentral gyrus)
• mediallateral

Cerebrum

• The front lobe is primarily involved in motor function and the primary motor cortex.

Sensory Function

• The Parietal area is primarily involved in the somatosensory cortex

Cerebrum: Temporal Lobe

• Temporal Lobe inferior to the lateral sulcus
• Sensory association areas in Temporal lobe
• Primary Auditory Cortex
• Primary Olfactory Cortex
• Involved in processing hearing, smell, and some visual memory
• Stores in auditory and visual memories

Cerebrum: Occipital Lobe

• The Occipital Lobe most posterior lobe
• Visual Association Area and located Primary Visual Cortex
• Responsible for processing visual inputs and coordinating eye movements

Cerebrum: Insular Lobe

• The location of the Insular Area deep to temporal lobe and lateral sulcus
• Association Area and has located the Primary Gustatory Cortex
• It has a system from CV system, visceral organs and Inputs of sensory memories like taste, hearing, and smell

Diencephalon

• Epithalamus: pineal gland.
• Thalamus.
• Hypothalamus involves the Autonomic Nervous System, Endocrine system Temperature, food intake.

Brain Stem

• The Stem connects the cerebellum and diencephalon to the spinal cord.
• The midbrain: relay center for motor and sensory pathways.
• The pons: motor and sensory tracts connect thebrain and spinal cord, nuclei involved in regulating respiration.
• Medulla Oblongata contains centers (nuclei) that regulate the function of thecardiovascular system and the respiratory system

Cerebellum

• Voluntary movement from primary motor cortex
• Assessment of voluntary movement
• Has Corrective feedback and feedback on movements functions