🪢 Lecture 3: Neurophysiology II

Organization of the CNS

Central Nervous System (CNS)
  • Brain
  • Spinal cord

Brain
  • Cerebrum (Forebrain)
  • Diencephalon
    • Thalamus
    • Hypothalamus
  • Cerebellum
  • Brain stem
    • Mesencephalon
    • Pons
    • Medulla oblongata

Tasks of the CNS

Receives Information
  • Receives a continuous flow of information
  • Information comes from outside and inside the body

Processes Information
  • Analyzes and interprets the information
  • Makes sense of internal and external information

Sends Commands
  • Sends commands to the body to:
    • Maintain function of vital systems and organs
    • Respond to the external environment

Complex Organization
  • Example → sequence of events required for a horse to jump a fence
  • Although very complex, the brain can be organized by:
    • Structural (anatomical) features
    • Function

Meninges and Cerebrospinal Fluid

Meninges
  • 3 layers surrounding the brain and spinal cord

Outer Meninx – Dura Mater
  • Thick and tough
  • Supports the brain
  • Inserted between the cerebral hemispheres
  • Forms the venous sinus
    • Collects most of the blood and cerebrospinal fluid (CSF) from the brain

Middle Meninx – Arachnoid
  • Middle layer of the meninges

Inner Meninx – Pia Mater
  • Tightly attached to the neural tissue

Subarachnoid Space
  • Located between the arachnoid and pia mater
  • Filled with cerebrospinal fluid (CSF)

Meninges and Venous Sinus

Meninges
  • Dura mater → outer layer
  • Arachnoid → middle layer
  • Pia mater → inner layer, tightly attached to brain tissue

Structures
  • Venous sinus → located within the dura mater
  • Falx cerebri → fold of dura mater between the cerebral hemispheres
  • Arachnoid villus → projects into the venous sinus
  • Subarachnoid space → between arachnoid and pia mater
    • Contains cerebrospinal fluid (CSF)
  • Brain → covered by the pia mater

Brain Ventricles and Cerebrospinal Fluid

Brain Structures
  • Right cerebral hemisphere
  • Cerebellum
  • Venous sinus
  • Arachnoid villus
  • Subarachnoid space → contains cerebrospinal fluid (CSF)

Ventricles and CSF Pathway
  • 3rd ventricle
  • Aqueduct → connects the 3rd and 4th ventricles
  • 4th ventricle
  • Central canal → continues from the 4th ventricle into the spinal cord

Cerebrospinal Fluid (CSF)

Formation
  • Formed by capillaries of specialized areas of the pia mater called the choroid plexus

Circulation
  • Circulates from the brain’s ventricles down to the central canal of the spinal cord
  • Diffuses from the 4th ventricle into the subarachnoid space
  • Circulates around the brain and spinal cord
  • Drained into the venous sinus before entering the blood circulation

Roles of CSF
  • Provides nutrients to the CNS
  • Protection → acts as a shock absorber
  • Maintains an ionic concentration ideal for optimal neuronal function

Blood–CSF Barrier
  • Located at the choroid plexus
  • Controls the uptake of nutrients and drugs into the CSF

Blood–Brain Barrier

What Can Pass Through
  • No cells or proteins can pass through
  • Low amino acid levels
  • Only liposoluble substances can diffuse
  • Example → gases

Cerebrum

Cerebral Cortex – Grey Matter
  • Most superficial layer of the cerebrum
  • Acquired late in evolution
  • Responsible for the conscious experience of sensory input
  • Site of high nervous association → integrates and connects information
  • Marked by a high degree of educability → ability to learn
  • Primary cortical areas have been mapped, mainly in humans

Cerebral Cortex – Functional Areas

Based on Anatomical Structure
  • Cerebral cortex can be organized based on anatomical structure

Based on Response to Stimulation
  • Can also be organized based on response to stimulation
  • Somesthetic → related to body sensations

Association Cortex
  • Areas without defined reactions to stimulation
  • Called the association cortex

Cerebrum – Brain Regions

Neocortex
  • Most complex part of the cerebrum

Rhinencephalon
  • Olfactory brain → involved in processing smell

Mesencephalon
  • Part of the midbrain
  • Processes visual information in birds

Cerebral Cortex – Body Representation

Somatosensory Cortex
  • Represents different body parts in the somatosensory cortex
  • Different areas of the body have different representations

Motor Cortex
  • Represents different body parts in the motor cortex
  • Different areas of the body have different representations

Rabbit and Cat
  • Equivalent body-part representations are found in the somatosensory cortex of both the rabbit and cat

Cerebrum – White Matter and Basal Nuclei

White Matter
  • Located beneath the cortex
  • Composed of myelinated nerve fibers → nerve fibers covered by a protective myelin sheath
  • Includes:
    • Association fibers → connect different parts of the same cortex
    • Commissural fibers → connect the two cerebral hemispheres
    • Projection fibers → connect the cortex to other brain structures and spinal cord

Basal Nuclei
  • Located deep within the cerebrum, close to the thalamus
  • Paired nuclei → groups of neurons found on both sides of the brain
  • Control complex semi-voluntary movements → movements that are partly automatic, such as walking and running
  • Work together with the cortex and cerebellum
  • In birds, also control voluntary movements

Cerebellum

Main Role
  • Not involved in consciousness or sensation
  • Main role → prevents distortion of intended movement
  • Helps make movements smooth and accurate

Information Received
  • Receives information from the inner ear → equilibrium/balance
  • Receives information from proprioceptive receptors → receptors in muscles, tendons, and joints that provide information about body position
  • Receives information from the cortex → visual and motor information

Movement Adjustment
  • Receives information about the status of the body and the intended movement
  • Makes adjustments for a smooth completion of movement
  • Corrective signals are sent through the thalamus to the cortex and spinal nuclei that command the muscles

Development
  • Well developed in animals that require precise adjustment for locomotion
  • Important for locomotion through air, ground, and water

Diencephalon

Diencephalon – “Interbrain”
  • Also called the interbrain
  • Includes the hypothalamus, thalamus, and epithalamus

Thalamus
  • Acts as a relay station for:
    • Sensory information
    • Information from the cerebellum and basal ganglia
    • Information going to and from the cerebral cortex

Epithalamus
  • Contains the olfactory correlation center
  • Contains the pineal gland

Hypothalamus
  • Produces neuropeptides that control the anterior pituitary
  • Produces neurohormones released by the posterior pituitary
  • Principal regulator of the autonomic nervous system
  • Along with the brain stem, it is the most important region for controlling homeostasis → maintaining stable internal conditions

Cerebrum – Medial Surface

Cerebral Hemispheres
  • The brain is divided into cerebral hemispheres
  • Right cerebral hemisphere shown from the medial surface

Gyri and Gyrus
  • Gyri → folds or raised areas of the cerebral cortex
  • Gyrus → singular form of gyri

Corpus Callosum
  • Structure connecting the two cerebral hemispheres

Diencephalon Structures
  • Thalamus
  • Hypothalamus
  • Pituitary gland

Brain Stem

Brain Stem
  • Includes nuclei that govern body functions through reflexes
  • Contains a large amount of white matter → acts as conveyers, carrying signals between brain regions and the body

Main Parts
  • Mesencephalon → midbrain
  • Pons
  • Medulla oblongata

Mesencephalon – Midbrain

Visual Reflex Center
  • Receives visual information
  • Controls eye movements for focus

Auditory Reflex Center
  • Responds to sounds
  • Helps control movements such as turning the head toward a noise

Birds
  • Well developed in birds
  • Important for processing visual information

Mammals
  • Most mammals have a very small midbrain
  • It is covered by the cortex

Pons and Medulla Oblongata

Postural Reflexes
  • Contain centers for postural reflexes
    • Hopping
    • Righting
    • Placing

Respiration
  • Control respiratory movements → breathing movements

Medulla Oblongata
  • Crucial for control of:
    • Heart activity
    • Blood pressure
    • Relative distribution of blood to organs

Cranial Nerves

General
  • 12 pairs of cranial nerves

Cranial Nerves I and II
  • I – Olfactory → purely sensory
  • II – Optic → purely sensory
  • Both remain in the brain

Cranial Nerves III–XII
  • Exit from the brain stem
  • Innervate structures of the head and neck
  • Most are mixed nerves → both sensory and motor

Exception – X: Vagus Nerve
  • Most widely distributed nerve throughout the body
  • Extends from the neck to the abdomen
  • Carries parasympathetic fibers to visceral structures in the thorax and abdomen

Cranial Nerves – I–XII

I – Olfactory Nerve
  • Smell

II – Optic Nerve
  • Vision

III – Oculomotor Nerve
  • Eye movement

IV – Trochlear Nerve
  • Eye movement

V – Trigeminal Nerve
  • Sensory and motor functions of the face

VI – Abducent Nerve
  • Eye movement

VII – Facial Nerve
  • Facial movement

VIII – Vestibulocochlear Nerve
  • Hearing and balance

IX – Glossopharyngeal Nerve
  • Sensory and motor functions involving the throat

X – Vagus Nerve
  • Parasympathetic control of visceral organs
  • Extends from the neck to the abdomen

XI – Accessory Nerve
  • Motor control of certain neck and shoulder muscles

XII – Hypoglossal Nerve
  • Motor control of the tongue

Spinal Cord – Vertebral Column

Vertebral Column
  • Protects and surrounds the spinal cord
  • Divided into cervical, thoracic, lumbar, sacral, and coccygeal regions
  • Number of vertebrae differs between species

Main Structures
  • Spinous process → dorsal projection of the vertebra
  • Vertebral body → main, weight-bearing part of the vertebra
  • Transverse process → lateral projection of the vertebra
  • Intervertebral disc → located between vertebrae
  • Intervertebral foramen → opening between vertebrae where spinal nerves pass

Orientation
  • Cranial → toward the head
  • Caudal → toward the tail
  • Dorsal → toward the back

Vertebral Column – Number of Vertebrae by Species

Horse
  • Cervical (C): 7
  • Thoracic (T): 18
  • Lumbar (L): 6
  • Sacral (S): 5
  • Coccygeal (Co): 15–20

Ox
  • C: 7
  • T: 13
  • L: 6
  • S: 5
  • Co: 18–20

Sheep
  • C: 7
  • T: 13
  • L: 6–7
  • S: 4
  • Co: 16–18

Goat
  • C: 7
  • T: 13
  • L: 7
  • S: 4
  • Co: 12

Hog
  • C: 7
  • T: 14–15
  • L: 6–7
  • S: 4
  • Co: 20–23

Dog
  • C: 7
  • T: 13
  • L: 7
  • S: 3
  • Co: 20–23

Chicken
  • C: 14
  • T: 7
  • L: 14
  • S: 6

Human
  • C: 7
  • T: 12
  • L: 5
  • S: 5
  • Co: 4

Vertebral Column – Regions and Vertebrae

Cervical (Neck)
  • 7 vertebrae

Thoracic (Chest)
  • 13 vertebrae

Lumbar (Low Back)
  • 7 vertebrae

Sacrum (Pelvic)
  • 3 vertebrae

Tail
  • Vertebrae extend into the tail

Intervertebral Disks
  • Vertebrae in this area have intervertebral disks

Spinal Cord – Cross Section

Central Canal
  • Located in the center of the spinal cord
  • Contains cerebrospinal fluid (CSF)

Dorsal Surface
  • Back/top side of the spinal cord

Dorsal Horn
  • Located on the dorsal side of the spinal cord

Ventral Horn
  • Located on the ventral side of the spinal cord
  • Ventral = belly/front side

Spinal Cord – White Matter and Grey Matter

White Matter
  • Contains myelinated nerve fibers → gives white appearance
  • Contains unmyelinated nerve fibers (axons)
  • Arranged in columns or “tracts” → bundles of nerve fibers

Grey Matter
  • Contains cell bodies and dendrites
  • Shaped like a butterfly

Dorsal Horn
  • Sensory → receives sensory information

Ventral Horn
  • Motor → controls skeletal muscles
  • Autonomic fibers → control automatic body functions

Middle Zone
  • Association → connects different areas of the spinal cord

Spinal Cord – Main Structures

Grey Matter
  • Contains cell bodies and dendrites

Dorsal Horn
  • Sensory → receives sensory information

Ventral Horn
  • Motor → controls skeletal muscles
  • Contains autonomic fibers → control automatic body functions

White Matter
  • Contains myelinated and unmyelinated nerve fibers (axons)
  • Arranged in columns or “tracts”

Dorsal Root Ganglion
  • Contains sensory nerve cell bodies

Spinal Cord
  • Part of the central nervous system

Spinal Nerve
  • Contains both sensory and motor nerve fibers

Ventral Root
  • Carries motor signals away from the spinal cord

Vertebra
  • Bone that surrounds and protects the spinal cord

Spinal Cord – Nerve Pathways

White Matter
  • Contains nerve fibers (axons)

Grey Matter
  • Contains cell bodies and dendrites

Dorsal Horn
  • Sensory → receives sensory information

Ventral Horn
  • Motor → controls skeletal muscles

Lateral Horn
  • Contains preganglionic autonomic neurons

Spinal Ganglion
  • Contains sensory neuron cell bodies

Sensory Neuron
  • Carries sensory information toward the spinal cord

Somatic Motor Neuron
  • Carries motor signals to skeletal muscles

Preganglionic Autonomic Neuron
  • Carries signals from the CNS to an autonomic ganglion

Spinal Nerves

Function
  • Entry and exit of neurons from the spinal cord

Formation
  • Formed when the dorsal and ventral roots join
  • Roots merge within the spinal canal

Number
  • Number corresponds to the vertebra they originate from

Exit
  • Emerge from the intervertebral foramina → openings between vertebrae

Spinal Segments
  • Each spinal segment receives sensory information from a section of the skin called a dermatome
  • Sends information to muscles called myotomes

Spinal Cord Injury
  • The organization of spinal nerves helps predict the effects of a spinal cord injury

FYI