Physiology of the central nervous system

Physiology of the

central nervous system

Motor division of the CNS

General function of the nervous system (NS)

Nervous system

-communication network of neurons that allows the organism to interact with the environment

(external, internal)

• main function: regulation of body functions • purpose: adaptation to changes - maintenance

of homeostasis and survival

http://www.alz.org/braintour/images/neuron_forest.jpg

1.

Peripheral NS

• Spinal cord

- conducts sensory information to the brain 

- conducts motor information to the effector organs  - serves as a simple regulatory centre (reflexes)

- damage of the spinal cord breaks down the connection between periphery and higher centres – serious consequences

• Brain

- subcortical level - control of involuntary and subconscious functions, emotions

- cortical level

- control of voluntary functions - control of lower levels of CNS

- brain cortex - site of cognitive (higher) functions:

memory, learning, thinking

Peripheral nervous system (PNS)

- transmits information from periphery to the central

nervous system and vice versa

Includes

-

cranial nerves

– originate in brain (I – XII)

-

spinal nerves

- originate in spinal cord

(C1, C2...Th,..L, S..)

Components

A. afferent (sensory) nervous system

(incoming to CNS)

B. efferent (motor) nervous system

(outgoing from CNS)

a. somatic nervous system

(transmit impulses to skeletal muscle)

b. autonomic nervous system

(transmit impulses to smooth muscles,

cardiac muscle and

I. sympathetic division (fight or flight)

Components of the nervous system (NS)

A/ sensory part:

- detection and transmission of stimuli

B/ information processing

– integration (combining several signals) – planning and implementation of motor

commands – perception

– thinking, learning

– memory – storage and retrieval of data – emotions

C/ motor part: determination of response and its transmission to effectors

Regulation of the motor functions

Spinal reflexes

- simple movements – reflex movements

Brainstem

– posture control, hand and eye movements

Cerebellum

- making movements „smooth“

- precisely regulates the sequence and duration of the elementary movements of each of these segments

Basal ganglia

- mainly caudate nucleus, the putamen, and the globus pallidus

- trigger well co-ordinated voluntary movements

- cognitive control of voluntary movements (e.g. running away from a dangerous animal)

Brain cortex – motor areas, decision about voluntary

Spinal cord

• white matter – nerve fibres

(afferent , efferent )

• grey matter – cell bodies

- dorsal root – sensory fibres

Neurons of the spinal grey matter

 a motor neurons

- their axons leave via ventral roots (and cranial nerves) and terminate in muscles (on the motor end-plate)

- discharge of a a-motor neurons causes a skeletal muscle contraction

- synapse with sensory neurons, interneurons, neurons descending from brain (pyramidal pathway)

 g motor neurons

- innervate muscle fibres of the muscle spindles (intrafusal muscle fibres) - control of muscle tone

- synapse with neurons

of extrapyramidal pathways

 interneurons

 propriospinal neurons - short fibres,

- synapse within the spinal cord - vertical connections

ventral dorsal

Knee jerk – the patellar reflex

- tap on the tendon of m. quadriceps femoris

Reflex

• one type of function of the NS

• definition: predictable, quick, stereotyped and involuntary response to stimulus • action that results from passing a nerve impulse over a reflex arc

Reflex arc – the basic circuit that

underlies a reflex:

1. sensory receptor – gathers stimuli

2. afferent nervous fibre – signal transduction into CNS

3. reflex (integration) centre – processes information

4. efferent nerve fibre – transduction of response 5. effector (muscle, gland)–

performance, response to the stimulus

Classification of reflexes – with respect to:

A/ Count of synapses:

1. Monosynaptic

= 1 synapse in reflex arc

2. Polysynaptic

= 2 or more synapses in reflex arc

Classification of reflexes – with respect to:

B/ Type of sensory receptor

Exteroceptive – receptor in the reflex arc is an exteroceptor (gathers stimuli from external environment)

Proprioceptive – receptor in the reflex arc is an proprioceptor (gathers stimuli about position of the body)

Interoceptive – receptor in the reflex arc is an proprioceptor (gather stimuli from internal environment)

C/ Centre

Cranial (brain)

Spinal (spinal cord) Extracentral (ganglia

)

D/ Effector

Somatic – effector: skeletal muscle

Autonomic (visceral) – effector: cardiac muscle, smooth muscle, gland

E/ Origin

Inborn – related to locomotion, defence, food intake, sexual behaviour Acquired – develop during life

• involved in movement, muscle tone and posture regulation

• motor control

– in part conscious

– to major extent - subconscious reflex action – includes spinal reflexes

• coordination of movement depends on:

– discharges in the appropriate muscles (contraction)

– absence of discharges in inappropriate muscles (relaxation)

• both receptor and effector are in the muscle

• types of sensory receptors involved in

spinal reflexes:

– Golgi tendon organs

Monosynaptic reflexes

Muscle stretch reflexes (myotatic)

• spinal reflexes – refelex centre in the spinal cord

The muscle spindle

•a stretch receptor in a muscle

•a bundle of modified muscle fibres encased in a capsule = intrafusal fibres

•intrafusal fibres

– paralell with all muscle fibres (extrafusal) – fixed to connective tissue of the muscle – attached to a non-elastic central part

– a sensory nerve is wrapped around the non-elastic central part

•adequate stimulus for a muscle spindle:

muscle stretching

–passive stretching with a reflex hammer = stimulus that elicits a reflex

- active stretching – gamma motorneurons

•response – contraction of extrafusal muscle fibres of the same muscle

Operation of the muscle spindle

• resting state

• resting frequency of potentials

• if the muscle spindle is passively prolonged • the spacing between the coils of the

sensory nerve become more apart

• larger discharge rate – trnasmitted to the a -motoneuron – result: muscle contraction

• tapping on the muscle tendon (of different muscles)  passive prolongation of the muscle and also of the muscle spidle = stimulation(1)

• action potential is transmitted by afferent fibre (2)

• afferent fibre enters spinal cord through posterior roots

• afferent fibre synapses with efferent fibre (a – motoneuron) • synapse = integration centre (3) (information is processed)

• action potential is transmitted by efferent fibre (a – motoneuron) into muscles (via the motor endplate) (4)

• muscle contraction occurs (5-response) - this is seen as movement (flexion)

Spinal cord

• segments (levels) of the spinal cord contain regulatory circuits involved in control of the movements of a particular region of the body • muscle stretch reflexes have their inegration

centres in different segments of the spinal cord

Muscles in axial parts of the body limbs

(a motoneurons medial part) (a motoneurons -lateral part) = somatotopic organization

Examination of reflexes in a human

• basic examination in neurology

• indicates the function of reflex arc (and all its components)

• reflex - stereotypic reaction = predicted response

• normoreflexia – normal reflex response on a stimulus

• abnormal response indicates disorder in a part of reflex arc

Tyoes of abnormal results:

• hyperreflexia (or clonus) – hyperactive reflex

• hyporeflexia – diminished reflex

• areflexia – absence of response

Principle

• sensory receptors are stimulated

• the reflex response on stimulation is observed and evaluated

Procedure

• the patient is in sitting or lying position (depends on the reflex)

• sensory receptors are stimulated by a reflex hammer, cotton,

light, needle (depends on the reflex that we want to examine)

• examine following reflexes (see next slides)

Proprioceptive (deep tendon) reflexes

-

•

Masseter reflex (n V)

lower jaw

(mouth is moderately open) R: closing the mouth

•

Naso-palpebral reflex (n V,VII)

R: closing of eyelids

•

Biceps reflex (C5)

S: tap on lacerus fibrosus m. bic. brachii R: flexion of the forearm

•

Triceps reflex (C7)

•

Styloradial reflex (C7)

•

Patellar reflex (knee jerk, L 2-4)

femoris

R: extension in knee joint

- the patient is sitting, and the examined limb is put over the knee of the other - if no response occurs, do the

Jendrassik manoeuver (abduce one hand from another)

•

Achilles tendon reflex (ankle

jerk, L5-S2)

S: tap on Achilles tendon R: extension of the foot

Achilles tendon reflex measurement

Principle:

-

thyroid hormones (thyroxin, triiodthyronine) influence the activity of

the central nervous system

-

examination of Achilles tendon reflex is an indirect indicator of thyroid

activity (Ankle jerk)

-

reflex time (stimulation – response) depends directly on the level of

thyroid hormones

-

normal reflex response – euthyrosis

-

weak response – hypothyrosis

Exteroceptive reflexes

- sensory receptors in skin or mucosa

Corneal reflex

(n V, VII)

S: touching the cornea with a piece of cotton R: blinking

(used in surgery-depth of anaesthesia, is the last to disappear in deepening anesthesia)

Abdominal reflexes

(Th 5 – Th 11)

• mesogastric • hypogastric

S: drawing of the abdominal wall with a needle

Plantar reflex

(L5-S2)

S: stimulation of external side of planta pedis with a needle (from heel to little finger and other fingers)

R: plantar flexion or the toe (or all fingers)

- if pyramidal pathway is impaired the response in plantar reflex is abnormal (this is called Babinski phenomenon)

R: extension of the toe

- normal in babies up to 6-8 months, later a sign of disorder - indicates abnormalities in pyramidal tract

Autonomic (vegetative) reflexes

- response – transmitted by autonomic fibres

• Pupilary reflex

S: illumination of an eye (use battery)

R: miosis – diameter of the pupil becomes narrow - consensual reaction – if one eye is illuminated,

miosis occur bilaterally

• Oculocardial reflex

S: moderate pressure on the eyeball

R: decrease in pulse frequency (by 5-10 per minute)

• Orthostatic reflex

S: sudden change position from lying to sitting R: increase in pulse frequency

• Clinostatic reflex

S: sudden change of position from sitting to lying R: decrease in pulse frequency

Alpha-gamma co - activation

• gamma motor neurons – adjust the sensitivity of the muscle spindle

• gamma – fibres – terminate in the polar parts of muscle spindle

• the stimulation by gamma fibres causes

contraction of muscle fibres in the polar region (periphery) of the muscle spindle

• the fibres in the centre (equatorial region) become prolonged

= stimulus – elicits a muscle stretch reflex that results in contraction of the muscle

• response - muscle contraction

• In this way the muscle tone is regulated

Gamma fibre

rest

Passive stretch (e.g. tapping with the reflex

hammer) – passive prolongation of the central part

Active stretch - intrafusal muscle contraction caused by gamma innervation – prolongation of the central part

Alpha-gamma co - activation

• a loop for regulation of the muscle tone • extrapyramidal pathways terminate on the -

gamma motor neurons in the spinal cord • gamma – fibres terminate in the polar parts

of muscle spindle

• the stimulation by gamma fibres causes contraction of muscle fibres in the polar region (periphery) of the muscle spindle • the fibres in the centre (equatorial region)

become prolonged

= stimulus – that elicits a muscle stretch reflex that results in contraction of the muscle

• response - muscle contraction

• In this way the muscle tone is regulated

Gamma - fibre

• a proprioceptive sensory receptor organ • located at the insertion of skeletal muscle

fibers into the tendons of skeletal muscle • made up of strands of collagen connected

at one end to the muscle and at the other with the tendon

• when the muscle contracts, the collagen fibrils are pulled tight, and this activates the Golgi tendon organ afferent

• it detects tension of the muscle

• synapses with interneurons that inhibit alpha-motoneurons

• response: relaxation of the muscle (prevents rupture)

1.Stretch reflex (myotatic) – monosynaptic

- receptor: muscle spindle (stimulus: passive prolongation)

- signal transmitted via afferent fibre - synapse in spinal cord to: - efferent fibre: alpha motoneuron (it gets stimulated)

- response: skeletal muscle contraction (the same muscle where the receptor is located)

2. Inverse myotatic reflex – polysynaptic

- receptor: Golgi tendon organ (can be stimulated by very strong contraction or stretching)

- signal transmitted via afferent nerve fibre and by interneurons in spinal cord to - efferent fibre: alpha motoneuron – it gets inhibited !!!

- response: muscle relaxation (the same muscle where the receptor is located) - function: protection against rupture of the muscle

3. Flexion reflex – polysynaptic

-stimulus causes response in both flexors and extensors of the same side: 1. activation of alpha motoneurons of the ipsilateral flexor muscles, 2. at the same time inhibits alpha motoneurons that supply antagonistic

extensor muscles

- example: when touching a hot object – contraction of flexors and relaxation of extensors causes removing the hand

4. Crossed extension reflex – polysynaptic a stimulus causes response on both sides in bots

extensors and flexors:

1.activation of alpha motoneurons of the ipsilateral flexor muscles

2.inhibits alpha motoneurons that supply antagonistic extensor muscles

- the opposite pattern occurs on contralateral side

3.flexors are inhibited

4.extensors are stimulated

- example: when a person steps on a nail, the leg that is stepping on the nail pulls away, while the other leg takes the weight of the whole body- reflex enhances postural support

Monosynaptic reflex arc

The cerebellum is important in balance and in

planning and executing voluntary movement.

Cerebellum

• an important control center for motor function – rate, range, force and direction of the movements

• contains as many neurons as the rest of the brain

1. vestibulocerebellum (archicerebellum) - regulates balance and eye movements

2. spinocerebellum (paleocerebellum) - regulates synergy of the body and limb movements + is able to elaborate proprioceptive input in order to anticipate the future position of the body

3. cerebrocerebellum (neocerebellum) involved in planning and initiation of movements

Examination of the cerebellum

Scanning speech

• Scanning speech refers to slow, slurred, monotonous, and irregular speech that is associated with dysarthria due to oral motor ataxia. Causes enunciation of individual syllables.

• Test: Ask the patient to say: “the British parliament”. In case of scanning speech it becomes “the Brit-tish Par-la-ment.”

Nystagmus

• Ocular findings are generally less prominent, but ipsilateral

gaze-evoked nystagmus are is seen with fast phase toward side of cerebellar lesion.

• Test: Ask the patient to follow your finger and observe presence of nystagmus.

Dysmetria (Finger to nose & finger to finger test)

• Limb ataxia is usually seen clinically as difficulty with coordinated tasks.

• Test: Ask patient to fully extend arm then touch nose or ask them to touch their nose then fully extend to touch your finger. You increase the difficulty of this test by adding resistance to the patient's movements or move your finger to different locations. Abnormality of this is called dysmetria.

Rebound phenomenon

• If the patient pulls on your hand and when you slip your hand out of their grasp, normally the antagonist muscles will contract and stop their arm from moving in the desired direction.

• In cerebellar disease this response is completely absent causing to limb to continue moving in the desired direction.

• Test: Have the patient pull on your hand and when they do, slip your hand out of their grasp. Observe the presence of rebound phenomenon.

(Be careful that you protect the patient from the unarrested movement causing them to strike themselves.)

Rapid alternating movements

• Common finding in cerebellar disease is dysdiadochokinesia (incoordination when performing rapid alternating movements).

• Test: Ask patient to place one hand over the next and have them flip one hand back and forth (pronate and supinate) as fast as possible

(alternatively you can ask the patient to quickly tap their foot on the floor as fast as possible).

Hypotonia

• Low muscle tone may be present in cerebellar diesases.

• Test: Perform the examination of the patellar reflex (knee jerk). “Pendular” knee jerk, leg keeps swinging after knee jerk more than 4 times (4 or less is normal).

Gait

• Test: Ask the patient to walk straight forward

• In cerebellar disorders – the gait is commonly wide based and staggering. They may fall to the side of the lesion.

Romberg’s test

• Whilst Rombergs test does not directly test for cerebellar ataxia, it

helps to differentiate cerebellar ataxia from sensory ataxia. In cerebellar ataxia the patient is likely to be unsteady on their feet even with the

eyes open.

• Test: Perform Romberg’s test by asking the patient to stand unaided with their eyes closed and heels together.

• If the patient sways (titubation) or loses balance then this test is positive. Stand near the patient in case they fall.

Regulation of the motor functions – Brain cortex

• Primary motor cortex - precentral gyrus in the frontal lobe

– Large neurons (pyramidal cells) allow conscious control of movement of individual

skeletal muscles

– The pyramidal cells' long axons form pyramidal (corticospinal) tracts

– Motor areas have been spatially mapped = somatotropy (motor homunculus)

• Premotor cortex - anterior to the precentral gyrus in the frontal lobe

– Regions controls learned motor skills that are repeated or patterned

– Also coordinates the movements of several muscles simultaneously and\or sequentially by sending activating

impulses to the primary motor cortex

• Broca's area - anterior to the premotor area

– Involved in directing motor speech

• Frontal eye field - anterior to the premotor cortex and superior to Broca's area

Volunary movement

• Multiple association areas of the cerebral cortex - motivation, ideas to produce motor activity

• Supplementary and premotor cortex – development of a motor plan

– Identification of the specific muscles that need to contract and their sequence

• Primary cortex cortex - upper motorneurons

• Spinal cord

Motor homunculus

At the primary motor cortex, motor representation of the body is arranged in an orderly manner. The amount of cortex devoted to any given body region is proportional to how richly innervated that region is (not to the body region's physical size).

Areas of the body with greater or more complex motor connections are represented as larger in the homunculus, the resulting image is that of a distorted human body, with disproportionately huge hands, lips, and face.

Lateralization of brain functions

Old concept

-Left hemisphere is dominant because it controls the dominant hand

Current concept

- generally, each hemisphere's structure is mirrored by the other side - despite strong similarities, the functions of each hemisphere differ

Left hemisphere functions

- verbal functions: speech, reading, writing

- langue – grammar, spelling - mathematics – analytical

thinking, counting and measurement

- logical thinking

- complex voluntary motor functions

- sequential approach

Right hemisphere functions

- complex sensory perception (music, dance) - site of spatial abilities

- fantasy, art

- visual memory - face recognition - language – intonation, accentuation - simultaneous, holistic approach

Males

– exhibit more lateralization

Females

Task: Assessment of motor handedness

• handedness - the preference of one hand over the other

• usually one hand is considered dominant

• most people are right-handed (90% of population)

• some individuals exhibit the ability to use both hands equally (ambidexterity) • preference for one hand is most likely the effect of brain lateralization

• handedness has a genetic basis, it is also influenced by socio-cultural pressures

- read the inventory questions and respond to each by filling a number into the box Please mark the box that best describes which hand

you use for the activity in question

Always Left (-50) Usually Left (-25) No Preference (0) Usually Right (25) Always Right (50) Writing (a letter) Throwing (a ball) Scissors Toothbrush

Knife (without fork) Spoon

Match (when striking) Computer mouse

Result:

- by making a sum of all numbers you get a score of Laterality Quotient (LQ) - evaluation: righthanded LQ: +400 to +200

lefthanded LQ: -400 to -200,

mixed handedness LQ: -200 to +200. Conclusion: Comment on your motor hand preference:

Always Left (-50) Usually Left (-25) No Preference (0) Usually Right (25) Always Right (50) Writing (a letter) Throwing (a ball) Scissors Toothbrush

Knife (without fork) Spoon

Match (when striking) Computer mouse