Central Nervous System

BIO 231

Anatomy and Physiology I

Spinal Cord

– Located within the dorsal vertebral column

– From base of the brain to the pelvis

• In humans the posterior few segments are filled with nerves that supply the pelvis, buttocks & legs

– Gray matter around a central canal - butterfly shape

• Cell bodies, dendrites & synapses

– White matter surrounds gray matter

• Bundles of myelinated axons that connect different parts of the cord

Spinal Nerves - Sensory & motor

• Dorsal root - sensory - dorsal root ganglion (sensory cell bodies)

• Ventral root - motor

Spinal Cord Anatomy

Spinal Cord With Roots

Spinal Nerves

– 31 pairs

Lobes of the Brain

– Boundaries between not always clearly delineated - therefore just indicate general regions

• Frontal lobe

• Parietal lobe

• Occipital lobe

• Temporal lobe

Brain Architecture

– Front part of the brain increases in size & surface area from fish to mammals

– The surface thrown into folds - gyri (gyrus - singular)

– The grooves between the folds are sulci (sulcus - singular)

Cellular organization

• Tracts occur on the surface & deeper

• Gray matter - forms distinct collections of cell bodies - nuclei

• Cortex - opposite of spinal cord

– Outside - gray matter with white below (complex tangles of tracts below - with deep nuclei)

Human Brain - Lateral View

Human Brain - Ventral View

Mammalian Brain

• Forebrain - prosencephalon

– Telencephalon - cerebral hemispheres - higher functions, motor control & sensory processing

Diencephalon

• Thalamus - sensory relay

– Lateral geniculate - visual

– Medial geniculate - auditory

• Hypothalamus - homeostasis control center

• Pituitary gland - master endocrine gland

Diencephalon

Basal ganglia - Motor planning & Control (Distinct nuclei)

Limbic system - Emotions & Memory (distinct nuclei)

Midbrain - Mesencephalon

• Tectum - sensory processing center

– Superior colliculus - oculomotor reflexes (visual processing - lower vertebrate)

– Inferior colliculus - Auditory relay & processing

– Tegmentum - orientation reflexes & auditory

– Red nucleus - postural reflexes & motor control

– Substantia nigra - postural reflexes & motor control & linked with limbic system

Midbrain

Mammalian Brain

– Hindbrain - Rhombencephalon

• Metencephalon

– Cerebellum - coordination & learning

– Pons - control of respiration

– Medulla oblongata - control of respiration, heart rate, blood pressure, vomiting & coughing

Hindbrain

Central Core of the Brain

– Tube expands during development - forms the ventricles or chambers of the brain

– Fourth ventricle - chamber of the hindbrain

– Third ventricle - chamber of the forebrain

• Connected to the 4th ventricle by the cerebral aqueduct - passes through midbrain

– Right & left ventricles - expansions of the third ventricle in the telencephalon

Cerbrospinal Fluid (CSF)

• Clear colorless fluid containing ions & little protein fills the central canal of the spinal cord and the ventricles

• Also circulates over surface of the brain - acts as shock absorber

Ventricles of the Brain

External Covering of the Brain

– Meninges - three membranes

– Outermost - dura mater - tough inelastic bag surrounds the brain & spinal cord

– Middle most membrane - arachnoid membrane - appearance & consistency resembling a spider web

– Innermost - pia mater - thin membrane that adheres closely to the surface of the brain

– Pia mater is separated from the arachnoid by fluid filled subarachnoid space

Meninges of the Brain

Cerbrospinal Fluid - CSF

– Produced by chroid plexus in the walls of the ventricles of the cerebral hemispheres

– CSF flows from the paired ventricles then to the third & fourth ventricles

then to the central canal of the spinal cord

– CSF escapes into the subarachnoid space via small apertures near the base of the cerebellum

– In the subarachnoid space, CSF is absorbed into the blood

CSF Circulation

The Lower Brain - Brain Stem

– Medulla - from spinal cord

– Pons - from anterior medulla - bulging area - external surface ribbed

– Cranial nerves 5 through 12 arise from the brain stem

– Fiber tracts

• Pyramidal tracts - ventrally located - motor

– Corticospinal - cortex to spinal cord

– Spinocortical - spinal cord to cortex

The Lower Brain - Brain Stem

– Reticular system - loose network of of neurons - reticular activating system

• Modulate sensation of pain

• Modulate certain postural reflexes & muscle tone

• Control breathing & heart rate

• Regulate the level of brain arousal & in humans consciousness

• Receives massive sensory input

Human Medulla

The Cerebellum

– Part of the hindbrain - not brain stem

– Connected to pons via cerebral peduncles

• Represent thousands of fibers going into or out of the cerebellum

– A central point for motor organization. Yet it does not initiate movement and movement can be generated in the absence of it

The Cerebellum

– The cerebellum modulates or reorganizes motor commands - and by coordinating diverse signals, it obtains the maximum efficiency from them

– Lesions - produce disturbances in the coordination of limb and eye movements and disorders of muscle tone and posture

– Principal cell - Purkinje cell - projects to deep cerebellar nuclei

Cerebellum

The Midbrain

– Sits between hindbrain & forebrain

– Major thruway for axon tracts that connect the above two regions

– Tectum in lower vertebrates - visual processing

• In mammals - same area called superior collculus

– Inferior colliculus - dorsal surface of midbrain - auditory

– Tegmentum - below the aqueduct - largest midbrain area

• Red nucleus & substantia nigra - orientation reflexes

Human Midbrain

The Diencephalon

– Lower part of the forebrain

– Parts - Thalamus & hypothalamus

– Thalamus - major relay for sensory information

• Lateral geniculate nucleus -paired - visual input

• Medial geniculate nucleus - paired - auditory input

– Hypothalamus - regulates internal environment

• Initiate or suppress behaviors to maintain homeostasis

• Regulate pituitary gland - which regulates endocrine system

• Controls autonomic nervous system - to regulate homeostasis

The Telencephalon

– Upper part of the forebrain

– Half the brain volume

– Cerebral hemispheres - paired lobes - together called cerebrum

• Outer layers - cell bodies , dendrites & synapses - gray matter - cerebral cortex

• Hemispheres joined by corpus callosum

• Central sulcus - divides hemispheres into anterior & posterior halves

– Precentral gyrus - just ahead of the central sulcus - motor cortex

– Postcentral gyrus - sensory processing from body surface & muscles

The Telencephalon

– The basal ganglia nuclei clustered around the thalamus

• Functions as an entity partially devoted to sequencing individual motor programs into smooth series of actions

• Gate sensory influences into motor areas

• Regulate sensimotor interactions in a way that determines which sensory stimuli are use to initiate motor actions and which are disregarded

• Components

– Striatum (caudate + putamen)

– Globus pallidus

– Subthalamic nucleus - in diencephalon

– Substantia nigra - in midbrain

The Basal Ganglia

Autonomic Nervous System

– Sympathetic NS

• Parasympathetic NS

• Enteric NS

• Neurons of Autonomic Nervous System

– Preganglionic

• Cell bodies in spinal cord or brain stem

– Postganglionic

• Cell bodies in peripheral autonomic ganglia

– Visceral Afferents

Autonomic Nervous System

Peripheral Sympathetic NS

– Preganglionic - in thoracic & upper lumbar spinal cord

• Axons run in autonomic ganglia

• Postganglionic

– Axons in sympathetic chain or paravertebral ganglia

• Effector organs

– Blood vessels, hair, viscera, pupils, cardiac muscle, glands

Sympathetic NS

– Inhibitory effect on non sphincter muscle of viscera, digestive glands & SM of bronchi.

– All other effects excitatory!

Pattern of Sympathetic Connection

• Parasympathetic NS

– Preganglionic - in sacral cord and brain stem

• Vagus (CN X) - 75% of parasymp. outflow

– Postganglionic - short fibers

– All parasymp. innervated organs also sympathetically innervated

• e.g. bladder, rectum,GI tract, heart, lungs, lacrimal & salivary glands

Parasympathetic NS

– Not all Sympathetically. innervated structures are parasympathetically. innervated

– Exceptions

• Entire vascular system, adrenal medulla & pilomotor muscle - only sympathetically innervated.