Introduction to the Nervous System

Nervous System Sections

1. Peripheral

2. Central

Peripheral Nervous System

comprises all neural tissue beyond the CNS (such as neurons that receive sensory information or that send information to muscles, and those that relay information to or from the spinal cord or the brain)

1. Autonomic

2. Somatic

Autonomic Peripheral Nervous System

controls self-regulated action of internal organs and glands

• sympathetic (arousing)

• parasympathetic (calming)

Contralateral

Opposite side of the brain

Ipsilateral

Same side of the brain

Unilateral

one side of the brain

Bilateral

both sides of the brain

Gray matter

neuronal cell bodies

White matter

axons, myelin, and glial cells

Axon tracts

occur within hemispheres, between hemispheres (commissure), and between cortical and sub-cortical regions (projection tracts)

Main commisure

corpus callosum —> massive white matter tract linking the two hemispheres

Major subdivisions of the brain

Brainstem

• medulla

• pons

• midbrain

brainstem houses cell bodies of most cranial nerves

Cranial nerves

receive sensory information from the head and neck and control their movement

• some are responsible for the neural control of internal organs

Medulla

• connects brain to spinal cord

  • all the ascending (sensory) and descending (motor) nerve fibers connecting the brain and spinal cord pass through the medulla

• controls many vital functions and reflexes such as respiration and heart rate

• home to part of a set of neurons known as the reticular activating system (RAS)

• directly superior to the spinal cord

• the region of the brain that contains many of the cell bodies of the 12 cranial nerves

• the region of the brain where most of the motor fibers cross from one side of the body to the other (—> right side of brain controls left side of body and vice versa)

Pons

• connective bridge —> connects the rest of the brain to the cerebellum and cranial nerves

• directly superior to the medulla and anterior to the cerebellum

• important center for control of certain types of eye movements and vestibular functions (e.g., balance)

• at the pons, information from both ears converges, allowing comparisons between the information received by each ear (thought to be important for localization of sounds)

• cell bodies of cranial nerves V-VIII

  • head and face sensations

  • motor control of eyes, face, mouth

  • secretion of saliva and tears

  • sensory roles in hearing and taste

Midbrain

• superior to the pons

• contains the nuclei of the cells that form some of the cranial nerves

• Orienting by sound and sight

  • two important structures on its dorsal side: the inferior colliculus and the superior colliculus

Inferior colliculus

a relay point for auditory information as it travels from the ear to the cortex

• involved in sound localization

• also contributes to reflexive movements of the head and eyes in response to sound

Superior Colliculus

allows us to perceive and orient toward large moving visual objects in the periphery

Cerebellum

• located posterior to the medulla

• important for regulation of muscle tone and guidance of motor activity

• also important for fluidity and precision in mental processes and the brain’s internal clock (lateral cerebellum)

Damage to the Cerebellum

damage interferes with precision of movement and disrupts balance, motor control, and equilibrium

• classic test used to detect damage: ask person to alternate between touching their nose, and then the doctor’s outstretched finger (path of hand will be imprecise and jagged)

• punch-drunk syndrome - a common manifestation of temporary disruption to the cerebellum, in which a person temporarily loses balance and coordination after sustaining a blow to the head

The Diencephalon

consists of the thalamus and the hypothalamus

Thalamus

Main relay station of the brain (for almost all sensory information coming into the cortex and almost all motor information leaving it)

• serves to reorganize information before it is sent elsewhere in the nervous system

  • the patterns of connections, both to and from the thalamus, are very specific —> one particular region of the thalamus receives information from just one sensory system and projects to only one particular region of the cortex

• “gateway to the cortex”

• all sensory pathways (except olfaction) make synaptic relays in the thalamus before continuing to the primary sensory receiving areas

• lateral geniculate nucleus and medial geniculate nucleus

Lateral geniculate nucleus

receives information from retina, sends axons to primary visual cortex

Medial geniculate nucleus

receives information from inner ear, sends axons to primary auditory cortex

Hypothalamus

• center for homeostasis (ex. provides signals telling the brain that it should drink/eat if thirsty/hungry)

• main link between nervous and endocrine system

• main site for control of hormone production

• it synthesizes and secretes neurohormones (releasing hormones)

• controls autonomic nervous system

• connected to master gland: pituitary gland

Pituitary Gland

• releases hormones in the bloodstream to influence other organs and tissues

• involved in thermoregulation, appetite, thirst, fatigue, circadian cycles, stress response, reproduction

• tightly linked with limbic system that plays major role in emotion regulation

Major Subcortical Systems

the basal ganglia and the limbic system

• located in regions below the cerebral cortex

Limbic system

amygdala and hippocampus

Amygdala

essential for emotional, especially fear processing

• implicated in the quick response to salient emotional information

Hippocampus

essential for conscious remembering

• formation of new long-term memories

Basal Ganglia

• groups of neurons that are central to motor functions

• damage results in motor deficits (tremor, slowness, changes in posture, etc.) generally characterized by involuntary movements

  • Parkinson’s or Huntington’s Disease

The Cerebral Cortex

• thin (1-3mm) layer, 2cm below scalp, that wraps surface of brain, that makes up 80% of the brain

• divided into several lobes

• surface: gray matter

• the underlying tissue: white matter

• all higher cognitive functions involve the cerebral cortex

• convolution, or bump gyrus

• valleys between bumps = sulcus or fissures

• divided into two physically separated halves = cerebral hemispheres

The Four lobes of the cerebral cortex

Longitudinal Fissure

separates the right cerebral hemisphere from the left

• each hemisphere has a unique specialization in both cognitive and emotional functioning

Central Fissure

separates each hemisphere of the brain in an anterior-posterior dimension

• in general, areas of the brain in front of the central fissure are more involved in motor processing

• whereas, those behind are more involved in sensory processing

Sylvian (lateral) fissure

separates each hemisphere of the brain in the dorsal-ventral dimension

• the division is important because the area of the brain below the Sylvian fissure is the temporal lobe, which plays a key role in memory, emotion, and auditory processing

Occipital Lobe

Visual processing

• the remaining region of the brain (other than the other 3 lobes) behind the parieto-occipital sulcus

Temporal lobe

auditory processing, language, long-term memory, object recognition

• the area below the Sylvian fissure

Parietal Lobe

Somatosensory processing (pressure, texture, warmth, proprioception, nociception); integrates information from various sensory modalities

• the region directly behind the central fissure, but above the Sylvian fissure

Frontal lobe

all higher mental processes, executive functioning, cognitive control, motor planning, attention

• the area in front of the central fissure

Motor Cortex

Movement

(topographic correspondence between cortical regions and body surface with respect to somatosensory and motor processes)

Somatosensory Cortex

somatic sensation

(topographic correspondence between cortical regions and body surface with respect to somatosensory and motor processes)

Anterior

front of the brain

Posterior

back of the brain

Rostral

regions toward the front of the brain and towards the head

Caudal

regions toward the rear of the brain and towards the tail

Superior

top of the brain

Inferior

bottom of the brain

Dorsal

• top of the brain

• toward an animal’s back

Ventral

• bottom of the brain

• toward an animal’s stomach

Medial

areas in the middle or center of the brain

Lateral

areas that are toward the outside of the brain

Coronal

when the brain is sliced ear-to-ear to separate the front from the back

Horizontal (or axial/tranverse)

if the brain is sliced so that the top of the brain is separated from the bottom

Sagittal

the brain is cut so that the left side of the brain is separated from the right side

Midsagittal

a sagittal slice down the middle of the brain

Lateral

a sagittal slice that is more toward one side

Contralateral

the opposite side of the brain

• motor control occurs contralaterally

Ipsilateral

the same side of the brain

Unilateral

applies to only one side of the brain

Bilateral

applies to both sides of the brain

Proximal

Near the brain (or region)

Distal

far from the brain (or region)

• ex: distal muscles are in your far extremities (like your hands)

Central Nervous System

• brain and spinal cord

• very fragile —> entire CNS is encased in bone

  • spinal cord is enclosed within the spinal column

  • brain is enclosed within the skull

Cerebrospinal Fluid (CSF)

• between neurons and their bony encasement (CNS) —> brain essentially floats in CSF (makes it buoyant and cushions it)

• similar in composition to blood plasma

• also serves metabolic needs, allowing nutrients to reach neurons

Ventricles

the fluid-filled spaces that contain CSF

• most prominent are in the lateral ventricles

What are the 7 main subdivisions of the CNS?

1. spinal cord

2. the medulla

3. the cerebellum

4. the pons

5. the midbrain

6. the hypothalamus and thalamus (diencephalon)

7. the cerebral cortex

Spinal Cord

the portion of the nervous system through which most sensory neurons relay information to the brain, and through which motor commands from the brain are sent to the muscles

Spinal Column

the bony structure housing the spinal cord

• composed of vertebrae (sections)

  • at each vertebrae, sensory information enters the cord and motor information leaves it

• if the spinal cord were cut in cross-sections, two clumps of nerve cells, one located ventrally and another dorsally, would be prominent

  • cells in the dorsal section receive sensory information

  • cells in the ventral section are responsible for conveying motor commands to the muscles and for receiving input from the brain and from other regions of the spinal cord

Damage to the Spinal Cord

leaves a person without sensation in or motor control for all body areas that are connected to the brain by spinal cord segments distal to the point of injury

12 cranial nerves

• some are responsible for receipt of sensory information and motor control of the head

• other are responsible for the neural control of internal organs

Damage to the Medulla

damage can be fatal

• one common accompaniment of either diffuse or specific brain damage is swelling of the entire brain —> when this swelling puts enough pressure on the medulla to interfere with its functions, death can result

reticular activating system (RAS)

• receives input from the environment as well as the internal milieu of the body, and then project diffusely to many other regions of the brain

  • allows RAS to contribute to overall arousal and attention, as well as regulation of sleep-wake cycles

Visual Periphery

• plays a role in movement of the head and eyes in response to environmental stimuli —> with regards to large moving objects

Damage to the hypothalamus

Ventromedial region - eat more than is required to maintain a normal body weight —> obesity

Dorsal and Lateral regions - interfere with water intake

Gyrus

each convolution, or bump, of the brain is called a gyrus

• basically a giant sheath of neurons wrapped around the other brain structures

• serve to pack more brain tissue into a smaller space

• every brain has the same basic gyral pattern; but subtle individual variations exist

Sulcus

each valley between the bumps (or gyrus) in the cerebral cortex

• if its deep its called a fissure

Three major fissures in the brain

1. central fissure

2. sylvian (lateral) fissure

3. longitudinal fissure

• these three major fissures also divide each hemisphere into four major regions, or lobes

The four major lobes

1. frontal lobe

2. temporal lobe

3. parietal lobe

4. occipital lobe

Primary Sensory Cortex

the first region in the cortex to receive information about a particular sensory modality

Primary Motor Cortex

the region of the cortex that is the final exit point for neurons responsible for fine motor control of the body’s muscles

• in front of the central fissure in a long, narrow band called the motor strip

• body parts for which we have a large degree of fine motor control have a disproportionately larger area of brain tissue devoted to their control

General characteristics of organization shared between the primary sensory and motor cortexes

• all these brain areas are organized so that specific attributes of the physical world are “mapped” onto brain tissue (hair cells)

• these maps are distorted relative to the physical world, reflecting the density of receptors (or effectors) within a system

  • ex: higher density of receptors at focal point in vision —> much more of the primary visual cortex is devoted to processing visual info from central part of vision (than periphery)

• mapping of the world onto brain tissue occurs in an upside-down and backward manner for vision, touch, and motor control

Damage to the primary motor cortex

• leads to muscle weakness on the contralateral side of the body

  • ex: damage to dorsal regions of the motor strip —> weakness of the bottom part of the body

• massive destruction to the motor strip, along with damage to the basal ganglia (as often occurs after stroke) results in paralysis on the contralateral side of the body —> known as hemiplegia

Primary somatosensory cortex

portion of the cortex that receives information about tactile (touch) stimulation, proprioception, and pressure and pain sensations from internal organs and muscles

• map of the body onto the primary somatosensory cortex is inverted left-to-right and top-to-bottom

• distortion of body parts in the somatosensory map is proportional to the density of touch receptors (like motor homunculus)

Proprioception

the perception of the position of body parts and their movements

• located directly posterior to the central fissure

Hair cells in the cochlea of the ear

are differentially sensitive to sounds of different frequencies, which we perceive as tones of different pitch

How does tactile information reach the somatosensory cortex?

skin contains various nerve endings, or receptors, that are sensitive to different aspects of tactile information

• crude tactile information and pain and temperature information is sent to the cortex by neurons that synapse at dorsal regions of the spinal cord —> then carried to thalamus —> then to cortex

• information about fine touch and proprioception enters to spinal column but doesn’t synapse until the medulla —> from which point it crosses over and is carried to the thalamus —> then to the cortex

Damage to the somatosensory cortex

impairs fine discriminations of touch on the side of the body contralateral to the damaged cortex

• ex: can’t tell what material a cloth they are holding is

Primary Visual Cortex

• the first region of the cortex that processes visual information, more specifically patterns of light across different regions of space

• in the occipital lobe

How does information reach the primary visual cortex

• when you look straight ahead information to the right of the fixation (right visual field) projects to the left half of the retinas in both your eyes (and vice versa)

• except for information in the far periphery of the world, all visual information reaches both eyes (otherwise only detected by right if in right periphery and vice versa)

  • ultimately information from the right visual field is directed solely to the primary visual cortex of the left hemisphere (and vice versa)

Damage to the visual cortex

inability to perceive visual information

• homonymous hemianopsia, quadranopsia, and scotomas

Homonymous Hemianopsia

if the entire occipital cortex of only one hemisphere is damaged, no visual information can be detected in the contralateral visual field

Quadranopsia

when just the dorsal or ventral portion of the occipital cortex is damaged, in which case just one quadrant of the visual world is lost

Scotomas

when only small portions of the visual cortex are damaged

• particular regions of the visual field in which light-dark contrast cannot be detected

Auditory System

sensitive to sound (which is essentially pressure waves in the air)

• physical energy in sound waves —> vibrations in the eardrum + bones in the ear

• vibrations are transformed into pressure waves in a liquid in the cochlea, which contains hair cells that transduce pressure waves into a neural signal

• organized so that there are both ipsilateral and contralateral projections from the ear to the brain (auditory information received at the right ear projects to both the left and right hemispheres

Heschl’s Gyrus

• in superior portion of the posterior temporal lobe

• where primary auditory cortex is

Primary Auditory Cortex

• tonotopic —> lowest tones are processed rostrally and laterally and tones of increasing frequency are processed more caudally and medially

Tonotopic

organized according to the frequency of a tone, which we perceive as pitch

Damage to the primary auditory cortex

unilateral - does not preclude the ability to perceive sound because of the redundancy provided by both crossed and uncrossed connections in the auditory system

• alters sound threshold (the softest intensity that can be perceived (it becomes higher contralateral to the damaged hemisphere)

• ability to perceive location of a sound becomes poorer fir the contralateral side of space (partially because loudness of sound helps determine location)