Comprehensive Notes on Mammalian Nervous System: Structure and Higher Function

The Mammalian Nervous System: Structure and Higher Function

Concept 45.1: Functions Are Localized in the Nervous System

  • Vertebrate Nervous Systems:
    • Central Nervous System (CNS): Brain and spinal cord.
    • Peripheral Nervous System (PNS): Nerves connecting the CNS to body tissues and sensors.
    • Enteric Nervous System: Located in the gut.
  • Neuron: A nerve cell.
    • Nerve: A bundle of axons in the PNS that carries information. Some axons carry information to the CNS, while others carry information from the CNS to the body’s organs.
  • Peripheral Nervous System (PNS):
    • Afferent Portion: Carries sensory information from receptors to the CNS (conscious and unconscious).
    • Efferent Portion: Carries motor commands from the CNS to muscles and glands (conscious and unconscious).
  • CNS Development:
    • Develops from the neural tube in the embryo.
    • Anterior part develops into the hindbrain, midbrain, and forebrain.
    • The rest of the neural tube becomes the spinal cord.
    • Information flow follows paths emerging from the linear neural tube.
  • Midbrain: Integrates sensory information and coordinates motor responses.
  • Hindbrain: Develops into the medulla, pons, and cerebellum.
    • Medulla and pons control physiological functions like breathing.
  • Brainstem: Composed of the midbrain, medulla, and pons.
    • All information between the spinal cord and higher brain areas passes through the brainstem.
  • Cerebellum: Coordinates muscle activity and maintains balance.
  • Forebrain: Develops into two regions:
    • Diencephalon: Thalamus and hypothalamus.
    • Telencephalon (Cerebrum): Cerebral hemispheres with the cerebral cortex as the outer layer.
      • The cerebral cortex is a thin layer rich in cell bodies.
    • Telencephalization: Increase in size and complexity of the telencephalon during vertebrate evolution.
  • Spinal Cord:
    • Conducts information to and from the brain.
    • Integrates information from the PNS and issues motor commands (e.g., knee-jerk reflex, withdrawal reflex).
  • Brainstem and Cranial Nerves:
    • Regulates autonomic functions via 12 paired cranial nerves.
    • Examples: olfactory, optic, and auditory nerves.
    • Vagus nerve (cranial nerve X) communicates with many organs, including the heart and gut.
  • Nucleus: An anatomically distinct group of CNS neurons.
    • Brainstem nuclei are involved in regulating wakefulness and sleep.
    • Reticular Activating System: The core of the brainstem; activity promotes wakefulness.
  • Damage to the Brain:
    • Damage below the reticular activating system can cause paralysis but normal sleep-wake cycles.
    • Damage above the reticular activating system can result in coma.
  • Forebrain Core:
    • Thalamus: Communicates sensory information to the cerebral cortex.
    • Hypothalamus: Regulates homeostatic functions.
    • Limbic System: Surrounds the diencephalon; responsible for instincts, long-term memory formation, drives, sexual behavior, and emotions.
      • Amygdala: Involved in fear and fear memory.
      • Hippocampus: Transfers short-term memory to long-term memory.
  • Cerebral Cortex:
    • Folded into ridges (gyri) and valleys (sulci) to increase surface area within the skull.
  • Hemispheric Control:
    • Left hemisphere controls the right side of the body, while the right hemisphere controls the left side (except in the head).
    • Hemispheres are not symmetrical in function (e.g., language in the left hemisphere).
  • Cerebral Cortex Regions:
    • Association Cortex: Integrates sensory information or memories; involved in higher-order information processing.
  • Temporal Lobe:
    • Receives and processes auditory information.
    • Association areas identify and name objects.
    • Agnosias: Inability to identify objects.
    • Damage can cause inability to recognize faces or understand spoken language.
  • Frontal Lobe:
    • Divided from the parietal lobe by the central sulcus.
    • Primary Motor Cortex: Located in front of the central sulcus; controls muscles in specific body areas.
      • Body parts with fine motor control have disproportionate representation.
    • Association areas are involved with feeling and planning (executive function, personality).
      • Damage can lead to drastic personality changes (e.g., Phineas Gage).
  • Parietal Lobe:
    • Primary Somatosensory Cortex: Located just behind the central sulcus.
    • Receives touch and pressure information from the thalamus.
      • The entire body surface can be mapped; areas with high densities of mechanoreceptors have disproportionate representation.
    • Association functions: Attending to complex stimuli.
      • Damage to the right parietal lobe can cause contralateral neglect syndrome (inability to recognize stimuli from the left side).
      • Damage to the left parietal cortex does not cause the same degree of neglect of the right side.
  • Occipital Lobe:
    • Receives and processes visual information.
    • Association areas make sense of the visual world and translate visual experience into language.
  • Insular Cortex:
    • Receives a variety of afferent information.
    • Integrates physiological information to create a sensation of how the body “feels.”
  • Brain Size and Evolution:
    • Body size and brain size are correlated in vertebrates.
    • Higher primates fall above this regression line.
    • Humans have a large cerebral cortex, made larger by convolutions.
    • The proportion of the cortex devoted to integration of information is greatest in humans.

Concept 45.2: Nervous System Functions Rely on Neural Circuits

  • Autonomic Nervous System (ANS):
    • Includes CNS and PNS components.
    • Controls involuntary functions.
    • Two divisions work in opposition:
      • Sympathetic: Fight or flight response.
      • Parasympathetic: Rest and digest.
  • Visual System Input Pathway:
    • Retinal ganglion cells collect information from photoreceptors.
    • Each cell has a receptive field of photoreceptors that receive light from a small area of the visual field.
    • Information from many photoreceptors is communicated to the brain as a single message.
  • Visual Cortex Neurons:
    • Also have receptive fields.
    • Action potentials from one retinal ganglion cell are received by hundreds of cortical neurons.
    • These neurons are responsive to different combinations of orientation, position, color, and patterns of light and dark.
  • Binocular Vision:
    • Humans have overlapping visual fields from two eyes.
    • Optic nerves from each eye join at the optic chiasm.
    • Half of the axons from each retina go to the opposite side of the brain.
  • Visual Cortex and Depth Perception:
    • The visual cortex sorts visual field information according to the eye of origin.
    • Binocular cells receive input from both eyes and interpret distance by measuring disparity.

Concept 45.3: Higher Brain Functions Involve Integration of Multiple Systems

  • Sleep:
    • Humans spend approximately a third of their lives sleeping.
    • Sleep researchers use electroencephalograms (EEGs) to record neuron activity.
  • Mammalian Sleep States (Non-Humans):
    • Slow-Wave Sleep: High-amplitude, slow-frequency waves in the EEG.
    • REM Sleep: Jerky movements of the eyeballs occur.
  • Human Sleep States:
    • Rapid-Eye-Movement (REM) Sleep: When dreams occur; the brain inhibits skeletal muscle activity.
    • Non-REM Sleep: Three stages, with stage 3 being deep, restorative, slow-wave sleep.
    • Experience 4-5 cycles of non-REM and REM sleep throughout the night.
  • Wakefulness:
    • Nuclei in the brainstem reticular-activating system are continuously active.
    • Neurotransmitters keep the resting potential of thalamus and cortex neurons near threshold.
  • Sleep Onset:
    • Activity slows in the brainstem nuclei, releasing less neurotransmitter.
    • Cells hyperpolarize and are less excitable.
    • Cells fire action potentials in bursts, synchronizing over broad areas, resulting in EEG slow-wave pattern of non-REM sleep.
  • Non-REM to REM Transition:
    • Brainstem nuclei become active again, and firing bursts cease.
    • Resting potentials of neurons return to near-threshold levels, allowing the cortex to process information.
    • Sensory and motor signals are inhibited, causing paralysis during REM sleep.
  • Language Lateralization:
    • Language ability primarily resides in the left cerebral hemisphere.
    • The hemispheres are connected by the corpus callosum.
      • If cut, knowledge from the right hemisphere cannot be expressed in language.
    • Damage to the left hemisphere can cause aphasia (inability to use or understand words).
  • Language Areas (Left Hemisphere):
    • Broca’s Area (Frontal Lobe): Essential for speech; damage allows reading and language understanding but impairs speech.
    • Wernicke’s Area (Temporal Lobe): Damage results in the inability to speak sensibly, and written or spoken language is not understood.
    • Angular Gyrus: near Wernicke’s area; integrates spoken and written language.
    • Normal language ability depends on information flow among these areas; damage to any area or pathway can cause aphasia.
  • Learning and Memory:
    • Learning: Modification of behavior by experience.
    • Memory: Ability to retain learned information.
    • Long-term memory requires long-lasting synaptic changes.
    • Long-Term Potentiation (LTP): High-frequency electrical stimulation makes circuits more sensitive to subsequent stimulation.
    • Long-Term Depression (LTD): Continuous, repetitive, low-level stimulation reduces responsiveness.
    • Memory involves interactions between several brain areas.
  • Types of Memory:
    • Immediate: Events happening now.
    • Short-Term: Lasts 10-15 minutes.
    • Long-Term: Lasts from days to a lifetime.
  • Hippocampus and Memory:
    • Knowledge of short-term to long-term memory transfer comes from studying individuals with limbic system damage (e.g., H.M.).
    • The hippocampus is essential for acquiring declarative memories.
  • Declarative Memory:
    • Involves people, places, things, and events.
    • Examples: spatial information in neural networks, replay of information in rats running mazes.
    • Replay also occurs during sleep, suggesting that sleep may be important for consolidating memories.
  • Consciousness:
    • Awareness of self, environment, and events.
    • Requires a perception of self integrated with physical, social, and past experiences.
    • Based on somatosensory information going to the CNS.
  • Insular Lobe Expansion:
    • In humans and great apes, the insular lobe is greatly expanded and communicates with brain areas involved in planning and decision-making.
    • Species with expanded insular lobes are the only ones that can recognize themselves in a mirror.