RBM1518 Human Physiology - Week 2, Session 4 Notes

Learning Objectives

• Describe how meninges, cerebrospinal fluid, and the blood brain barrier protect the CNS.

• List the major lobes and functional areas of the cerebral cortex.

• Describe the location of the diencephalon and name its subdivisions and functions.

• Identify the three major regions of the brain stem and note the functions of each area.

• Describe the general function of the basal nuclei (basal ganglia).

• Describe the location and function of the cerebellum.

• Locate the limbic system and the reticular formation and explain the role of each functional system.

• Define the role of spinal cord

• Distinguish between ascending and descending pathways

Organization of the Nervous System

• Central Nervous System (CNS): Brain and spinal cord

• Peripheral Nervous System (PNS): Afferent and efferent divisions

  • Afferent Division: Input to the CNS from the periphery

    • Sensory stimuli

    • Visceral stimuli (stimuli in the digestive tract)

  • Efferent Division: Output from the CNS to the periphery

    • Somatic Nervous System: Motor neurons that control skeletal muscles

    • Autonomic Nervous System: Controls smooth muscle, cardiac muscle, exocrine glands, and some endocrine glands

      • Sympathetic nervous system

      • Parasympathetic nervous system

      • Enteric nervous system (digestive organs only)

• Effector Organs: Made up of muscle and gland tissue

Key Concept: Central Nervous System

• Brain:

  • Control center of the nervous system

  • Occupies the cranial cavity

• Spinal Cord:

  • Continuous with the medulla oblongata (an inferior aspect of the brain stem)

  • Exits the skull through the foramen magnum (hole in the base of the skull)

  • Consists of 31 segments corresponding to the 31 pairs of spinal nerves

Covering of the Brain: Meninges and CSF

• The brain is protected by bone, meninges, and cerebrospinal fluid.

• Harmful substances are shielded from the brain by the blood-brain barrier.

• The blood-brain barrier (endothelial cells and associated astrocytes) maintains a stable environment in the brain.

Protective Structures of the Brain

Grey matter & white matter

• Grey matter contains neuron soma (cell body), synapses, dendrites, and non-
  myelinated axons.

• White matter contains mostly myelinated axons that implement global
  communication

• Nucleus: cell bodies imbedded in the white matter in the brain (e.g. basal nuclei)

Central Nervous System - Brain Regions and Organisation

The Cerebral Cortex

• Three types of functional areas

  • Sensory areas—conscious awareness of sensation

  • Association areas—integrate diverse information

  • Motor areas—control voluntary movement

• Each hemisphere concerned with contralateral (opposite) side of body

• Left hemisphere controls language, math, and logic

• Right hemisphere controls visual-spatial skills, intuition, emotion, and artistic and musical skills

• Hemispheres communicate almost instantaneously via fiber tracts and functional integration – corpus callosum

• Cerebral dominance: the hemisphere that is dominant for language

  • 90% of humans have left-sided dominance which results in right-handedness

  • In other 10%, roles of hemispheres are reversed

  

Cerebral Cortex - Hemispheres

*Left Cerebral Hemisphere
*Right Cerebral Hemisphere
*Longitudinal fissure

Left and Right Hemisphere

• Longitudinal fissure Cerebrum is divided into left and right hemisphere by the longitudinal fissure

• It is connected by interconnecting axons called corpus callosum

• Cortex of the hemispheres is wrinkled in order to increase area (can fit more neurons)

• Folds are called gyri; grooves are called sulci

Left Brain/Right Brain

• Left Brain: Logical, sequential, rational, analytical, objective, looks at parts

• Right Brain: Random, intuitive, holistic, synthesizing, subjective, looks at wholes

Brain Lobes

• Cerebral cortex is 3mm layer of gray matter with extensive folds to increase the surface area.

• Contains elevated ridges (gyri) and shallow grooves (sulci).

• Deep sulci divide the hemispheres into 5 lobes:

  • Frontal – voluntary motor function, planning, mood, social judgment, intellect, speech

  • Parietal – somatic sensory reception and integration

  • Temporal – hearing, smell, learning, memory

  • Occipital – visual centre

  • Insula – taste (gustatory cortex) deep to the temporal lobe

Brain Parts

Functional Areas of the Cerebral Cortex

• Frontal lobe: Prefrontal association cortex (planning for voluntary activity; decision making; personality traits), Premotor cortex (coordination of complex movements), Supplementary motor area (programming of complex movements - on inner surface-not visible), Broca's area (speech formation), Primary motor cortex (voluntary movement)

• Parietal lobe: Somatosensory cortex (somesthetic sensation and proprioception), Posterior parietal cortex (integration of somatosensory and visual input; important for complex movements), Parietal-temporal-occipital association cortex (integration of all sensory input; important in language), Central sulcus

• Occipital lobe: Primary visual cortex surrounded by higher-order visual cortex (sight)

• Temporal lobe: Primary auditory cortex surrounded by higher-order auditory cortex (hearing), Limbic association cortex (mostly on inner and bottom surface of temporal lobe; motivation and emotion; memory), Wernicke's area (speech understanding)

Parietal and Frontal Lobes

• The parietal lobes accomplish somatosensory processing

  • Somatosensory cortex – found in the postcentral gyrus

  • Sensations from the surface of the body, such as touch, pressure, heat, cold, and pain (and proprioception)

• The primary motor cortex – found in the precentral gyrus

  • located in the frontal lobes controls the skeletal muscles

  • Stimulation of different areas of the primary motor cortex brings about movement in different regions of the body

• Higher motor areas are also important in motor control

  • Supplementary motor area

  • Premotor cortex

  • Posterior parietal cortex

Brain has a body map

• The distorted human figure drawn to reflect the relative sensory space our body parts occupy on the cerebral cortex is called a homunculus

• Sensory homunculus is in the postcentral gyrus ( parietal lobe)

• Motor homunculus looks distorted and is placed in the precentral gyrus ( frontal lobe)

Cerebral Hemispheres: Lobes and Their Functions

Basal Nuclei

• Masses of gray matter found deep within the cortical white matter.

• Modify ongoing activity in motor pathways

• Modulate thalamus by inhibiting antagonistic and unnecessary movement.

• Help coordinate slow sustained contractions necessary for posture.

Parkinson's disease

*Degeneration of dopamine neurons in substantia nigra is hallmark sign of Parkinson's Disease

Diencephalon, Brainstem and Cerebellum

• Cerebral Cortex

  • Thalamus (medial)

  • Basal nuclei (lateral to thalamus)

  • Sensory perception

  • Voluntary control of movement

  • Language

  • Personality Traits

  • Sophisticated mental events, such as thinking, memory, decision making, creativity, and self-consciousness

• Basal Nuclei

  • Inhibition of muscle tone

  • Coordination of slow, sustained movements

  • Suppression of useless patterns of movement

• Thalamus

  • Relay station for all synaptic input

  • Crude awareness of sensation

  • Some degree of consciousness

  • Role in motor control

• Hypothalamus

  • Regulation of many homeostatic functions, such as temperature control, thirst, urine output, and food intake

  • Important link between nervous and endocrine systems

  • Extensive involvement with emotion and basic behavioral patterns

  • Role in sleep-wake cycle

• Cerebellum

  • Maintenance of balance

  • Enhancement of muscle tone

  • Coordination and planning of skilled voluntary muscle activity

• Brain Stem

  • Midbrain

  • Pons

  • Medulla - Spinal cord

  • Origin of majority of peripheral cranial nerves

  • Cardiovascular, respiratory, and digestive control centers

  • Regulation of muscle reflexes involved with equilibrium and posture

  • Reception and integration of all synaptic input from spinal cord; arousal and activation of cerebral cortex

  • Role in sleep-wake cycle

Limbic System: Emotional Brain

• Consists of a ring of forebrain structures that surround the brain stem Parts especially important in emotions, basic behavioural patterns, motivation.

• Amygdala – deals with anger, danger, and fear responses

• Hippocampus – converts new information into long-term memories.

• Emotional memory;

• Most limbic system outputs relay through Hypothalamus (visceral control) and Thalamus (gate to the cerebral cortex)

Cerebral Hemispheres: Lobes and their Functions - Lesions

Spinal Cord

• Conduction pathway between the body and the brain

• Bundle of nerve tissue within the vertebral canal

• Protected by vertebral column

• 42-45 cm long

• Extends from foramen magnum of skull to the level of L1/2 vertebra in adults

• Tapered inferior end: conus medularis

• Cervical and lumbar enlargements: thicker due to dense motor and sensory innervation of limbs

Spinal Cord Cross Section

• Grey Matter

  • Looks like the letter H or a butterfly

  • Contains the:

    • Dorsal horn

    • Ventral horn

    • Lateral Horn (in thoracolumbar region)

• White matter

  • Composed of predominantly myelinated, but also non-myelinated, fibres that allow communication between different segments/areas of the spinal cord, and the spinal cord and the brain

  • They run in 3 directions

    • Ascending

    • Descending

    • Transverse

White Matter of Spinal Cord

• Myelinated and non-myelinated nerve fibers allow communication between parts of spinal cord, and spinal cord andbrain Organized into tracts (bundles of nerve fibres with a similar function)

• Run in three directions:

  • Ascending: up to higher centers (sensory inputs); carry impulses to the brain

  • Descending: from brain to cord or lower cord levels (motor outputs); carry impulses from brain to effectors

  • Transverse: from one side to other (commissural fibers)

Ascending Pathways

• Conduct sensory pathways upward through a chain of three neurons:

  • First-order neuron ➢Conducts impulses from cutaneous receptors and proprioceptors ➢Branches diffusely as it enters spinal cord or medulla ➢Synapses with second-order neuron

  • Second-order neuron ➢Interneuron ➢Cell body in dorsal horn of spinal cord or medullary nuclei ➢Axons extend to thalamus or cerebellum

  • Third-order neuron ➢ Also an interneuron ➢ Cell bodies in thalamus ➢ Axon extends to somatosensory cortex ➢ No third-order neurons in cerebellum

Descending tracts

*Lateral corticospinal tract
*Ventral corticospinal tract